Showing 1–50 of 70 results for author: Keller, L

Predicting Magic from Very Few Measurements

Authors: J. M. Varela, L. L. Keller, A. de Oliveira Junior, D. A. Moreira, R. Chaves, R. A. Macêdo

Abstract: The nonstabilizerness of quantum states is a necessary resource for universal quantum computation, yet its characterization is notoriously demanding. Quantifying nonstabilizerness typically requires an exponential number of measurements and a doubly exponential classical post-processing cost to evaluate its standard monotones. In this work, we show that nonstabilizerness is, to a large extent, in… ▽ More The nonstabilizerness of quantum states is a necessary resource for universal quantum computation, yet its characterization is notoriously demanding. Quantifying nonstabilizerness typically requires an exponential number of measurements and a doubly exponential classical post-processing cost to evaluate its standard monotones. In this work, we show that nonstabilizerness is, to a large extent, in the eyes of the beholder: it can be witnessed and quantified using any set of $m$ $n$-qubit Pauli measurements, provided the set contains anti-commuting pairs. We introduce a general framework that projects the stabilizer polytope onto the subspace defined by these observables and provide an algorithm that estimates magic from Pauli expectation values with runtime exponential in the number of measurements $m$ and polynomial in the number of qubits $n$. By relating the problem to a stabilizer-restricted variant of the quantum marginal problem, we also prove that deciding membership in the corresponding reduced stabilizer polytope is NP-hard. In particular, unless $\mathrm{P} = \mathrm{NP}$, no algorithm polynomial in $m$ can solve the problem in full generality, thus establishing fundamental complexity-theoretic limitations. Finally, we employ our framework to compute nonstabilizerness in different Hamiltonian ground states, demonstrating the practical performance of our method in regimes beyond the reach of existing techniques. △ Less

Submitted 21 February, 2026; originally announced February 2026.

Comments: 20 pages, 3 figures

Anisotropic Band-Split Magnetism in Magnetostrictive CoFe$_2$O$_4$

Authors: Harry Lane, Guratinder Kaur, Masahiro Kawamata, Yusuke Nambu, Lukas Keller, Russell A. Ewings, David J. Voneshen, Travis J. Williams, Helen C. Walker, Dwight Viehland, Peter M. Gehring, Chris Stock

Abstract: Single crystal spinel CoFe$_2$O$_4$ exhibits the largest room-temperature saturation magnetostriction among non-rare-earth compounds and a high Curie temperature ($T_c \sim 780$ K), properties that are critical to a wide range of industrial and medical applications. Neutron spectroscopy reveals a large band splitting ($\sim$ 60 meV) between two ferrimagnetic magnon branches, which is driven by sit… ▽ More Single crystal spinel CoFe$_2$O$_4$ exhibits the largest room-temperature saturation magnetostriction among non-rare-earth compounds and a high Curie temperature ($T_c \sim 780$ K), properties that are critical to a wide range of industrial and medical applications. Neutron spectroscopy reveals a large band splitting ($\sim$ 60 meV) between two ferrimagnetic magnon branches, which is driven by site mixing between Co$^{2+}$ and Fe$^{3+}$ cations, and a significantly weaker magnetocrystalline anisotropy ($\sim$ 3 meV). Central to this behavior is the competition between extremely large mismatched molecular fields on the tetrahedral $A$-site and octahedral $B$-site sublattices and the single-ion anisotropy on the $B$-site. This creates a strong energetic anisotropy that locks the magnetic moment within each structural domain in place. As a result of these differing energy scales, switching structural domains is energetically favored over a global spin reorientation under applied magnetic fields, and this is what amplifies the magnetostrictive nature of CoFe$_2$O$_4$. △ Less

Submitted 17 December, 2025; originally announced December 2025.

Comments: (19 pages; 9 figures)

Journal ref: Adv.Funct.Mater. 2025, e16830

Wrinkling in Sheets with Nonuniform Growth and Bending Rigidity

Authors: I. Levin, S. L. Keller

Abstract: Thin elastic sheets bend easily, leading to mechanical instabilities such as wrinkling. Here, we investigate wrinkles at edges of bi-strips, which consist of two thin sheets, one that swells and one that does not, joined side-by-side. It is well known that when bending rigidity is uniform across an isolated bi-strip, swelling results in axisymmetric shapes like a wine bottle: two cylinders of diff… ▽ More Thin elastic sheets bend easily, leading to mechanical instabilities such as wrinkling. Here, we investigate wrinkles at edges of bi-strips, which consist of two thin sheets, one that swells and one that does not, joined side-by-side. It is well known that when bending rigidity is uniform across an isolated bi-strip, swelling results in axisymmetric shapes like a wine bottle: two cylinders of different radii are joined by a smooth transition zone. However, when the bending rigidity of the swollen sheet differs from that of the non-swollen sheet, purely axisymmetric shapes are no longer energetically favorable, and wrinkles arise. When the bending rigidity of the non-swollen sheet is essentially infinite, the wrinkles coarsen with distance from the transition zone such that dimensionless wavelengths and widths are related by $\tildeλ \propto \tilde{w}^{2/3}$. If the bending rigidity of the non-swollen sheet is non-infinite (but~still significantly larger than that of the swollen sheet), then the non-swollen sheet assumes a non-infinite radius of curvature, $R_0$. We find that the wrinkles in this system extend a critical distance, $w_C$, beyond the junction of the two strips and that $w_C \propto R_0$. Local undulations of wrinkles are favorable in this system because they decrease the overall bending energy by allowing the non-swollen sheet to have a larger radius of curvature than would otherwise be dictated by its reference geometry. Our results are relevant to a wide range of sheets that experience non-uniform growth, whether in natural systems such as plants or in synthetic systems such as designed, responsive materials. △ Less

Submitted 14 November, 2025; originally announced November 2025.

Hidden frustration in the triangular-lattice antiferromagnet NdCd3P3

Authors: Juan R. Chamorro, Steven J. Gomez Alvarado, Dibyata Rout, Sarah Schwarz, Allen Scheie, Ganesh Pokharel, Alexander I. Kolesnikov, Lukas Keller, Stephen D. Wilson

Abstract: We report a study of the magnetic ground state and crystal electric field (CEF) scheme in the triangular-lattice antiferromagnet NdCd$_3$P$_3$. Combined neutron scattering, magnetization, and heat capacity measurements demonstrate that the Nd$^{3+}$ moments occupying the triangular lattice in this material harbor hidden signs of frustration not detected in typical Curie-Weiss-based parameterizatio… ▽ More We report a study of the magnetic ground state and crystal electric field (CEF) scheme in the triangular-lattice antiferromagnet NdCd$_3$P$_3$. Combined neutron scattering, magnetization, and heat capacity measurements demonstrate that the Nd$^{3+}$ moments occupying the triangular lattice in this material harbor hidden signs of frustration not detected in typical Curie-Weiss-based parameterization of the frustration index ($f=Θ_{CW} / T_N$). This is evidenced by a zero-field splitting of the Kramers' ground state and first excited state doublets at temperatures far in excess of $T_N$ as well as signatures of low-energy fluctuations for $T>>T_N$. A suppression of the zero-field ordered moment relative to its field saturation value is observed, and the impacts of this magnetic frustration as well as the coexisting bond frustration in the CdP honeycomb network on the physical properties of NdCd$_3$P$_3$ are discussed. △ Less

Submitted 9 September, 2025; originally announced September 2025.

Incommensurate magnetic order arising from frustrated interchain interactions in the spin-1/2 chain compound AgCuVO$_4$

Authors: A. Hromov, A. Zorko, M. Gomilšek, I. Puente Orench, L. Keller, T. Shiroka, A. Prokofiev, M. Pregelj

Abstract: Quantum spin chains with competing interactions offer a platform where low dimensionality and frustration--both acting to suppress magnetic order--intersect. We studied magnetic ordering in the spin-1/2 chain compound AgCuVO$_4$ using muon spin spectroscopy and neutron diffraction. Long-range magnetic order emerges at $T_N = 2.0(1)$ K, which is $\sim$1/200 of the dominant intrachain coupling $J$ a… ▽ More Quantum spin chains with competing interactions offer a platform where low dimensionality and frustration--both acting to suppress magnetic order--intersect. We studied magnetic ordering in the spin-1/2 chain compound AgCuVO$_4$ using muon spin spectroscopy and neutron diffraction. Long-range magnetic order emerges at $T_N = 2.0(1)$ K, which is $\sim$1/200 of the dominant intrachain coupling $J$ and $\sim$1/15 of the interchain interactions. The collinear incommensurate amplitude-modulated magnetic structure features a reduced ordered moment of 0.13(3) $μ_\mathrm{B}$, confined to the $ab$ plane and modulated along the $c$ axis--perpendicular to the spin chains--indicating frustrated interchain couplings. The low $T_N$, small moment, and incommensurate order highlight strong frustration, positioning AgCuVO$_4$ as a model system for exploring frustration in quantum spin chains. △ Less

Submitted 22 October, 2025; v1 submitted 25 July, 2025; originally announced July 2025.

Comments: 6 pages, 4 figures

Journal ref: Phys. Rev. B 112, 134433 (2025)

Cluster Spin Glass State in Ba$_3$Sb$_{1+x}$Co$_{2-x}$O$_{9-δ}$: Cation Disorder and Mixed-Valence Co Dimers

Authors: Anzar Ali, Guratinder Kaur, Lukas Keller, Masahiko Isobe

Abstract: We investigate the structural, magnetic, and thermodynamic properties of \BSCO\ ($x$ = 0.04, $δ$ = 0.54), a hexagonal perovskite featuring face-sharing CoO$_6$ octahedra that forms Co dimers. DC and AC magnetization measurements reveal a frequency-dependent spin-freezing transition consistent with glassy dynamics. AC susceptibility fits best to the Vogel-Fulcher model, indicating collective freezi… ▽ More We investigate the structural, magnetic, and thermodynamic properties of \BSCO\ ($x$ = 0.04, $δ$ = 0.54), a hexagonal perovskite featuring face-sharing CoO$_6$ octahedra that forms Co dimers. DC and AC magnetization measurements reveal a frequency-dependent spin-freezing transition consistent with glassy dynamics. AC susceptibility fits best to the Vogel-Fulcher model, indicating collective freezing of interacting spin clusters. Isothermal magnetization follows the Langevin function, suggesting finite-sized magnetic clusters rather than isolated paramagnetic moments. Non-equilibrium dynamics, evidenced by thermoremanent magnetization and memory effects, further support a spin-glass-like state. Heat capacity shows no sharp anomalies, and neutron powder diffraction confirms the absence of magnetic Bragg peaks down to 1.5~K, ruling out long-range magnetic order. Rietveld refinement reveals significant Co/Sb intersite disorder ($\sim$~30\pct) and oxygen non-stoichiometry, introducing exchange randomness and frustration that drive the spin-glass-like behavior. Electrical resistivity exhibits Arrhenius-type temperature dependence with an activation energy of 0.173~eV, consistent with semiconducting behavior. Temperature-dependent X-ray diffraction shows no structural phase transitions, confirming that the spin-glass-like state is not lattice-driven. Our results establish \BSCO\ as a cluster spin-glass candidate, where Co dimers, disorder, and geometric frustration prevent long-range order, leading to slow spin dynamics. These findings highlight the role of cation disorder and oxygen vacancies in stabilizing unconventional magnetic states in cobalt-based hexagonal perovskites. △ Less

Submitted 24 June, 2025; originally announced June 2025.

Report number: Phys. Rev. B 112, 024409

Pressure evolution of coplanar antiferromagnetism in heavy-fermion Ce$_{2}$CoAl$_{7}$Ge$_{4}$

Authors: M. O. Ajeesh, A. O. Scheie, Yu Liu, L. Keller, S. M. Thomas, P. F. S. Rosa, E. D. Bauer

Abstract: Ce$_{2}$$M$Al$_{7}$Ge$_{4}$ ($M=$ Co, Ir, Ni or Pd) are heavy-fermion materials and host a variety of ground states ranging from magnetism to non-Fermi liquid behavior. The Co, Ir, and Ni members of the series undergo magnetic ordering with decreasing transition temperatures. In contrast, the Pd compound does not magnetically order down to 0.4 K and shows non-Fermi liquid behavior, suggesting prox… ▽ More Ce$_{2}$$M$Al$_{7}$Ge$_{4}$ ($M=$ Co, Ir, Ni or Pd) are heavy-fermion materials and host a variety of ground states ranging from magnetism to non-Fermi liquid behavior. The Co, Ir, and Ni members of the series undergo magnetic ordering with decreasing transition temperatures. In contrast, the Pd compound does not magnetically order down to 0.4 K and shows non-Fermi liquid behavior, suggesting proximity to a magnetic quantum critical point. Among the series, Ce$_{2}$CoAl$_{7}$Ge$_{4}$ orders antiferromagnetically below $T_N=1.9$ K along with heavy-fermion behavior below 15 K. We report the magnetic structure of the antiferromagnetic phase in Ce$_{2}$CoAl$_{7}$Ge$_{4}$ and the evolution of the magnetic transition under external pressure. Rietveld refinement of the neutron diffraction data suggests a coplanar antiferromagnetic structure with a wave vector $k = (1,1,1)$ and an ordered moment of $0.383 \pm 0.018 \> μ_B$ in the antiferromagnetic phase. Electrical resistivity and AC calorimetry measurements under hydrostatic pressure reveal a suppression of the antiferromagnetic transition toward zero temperature around $p=1.1$ GPa. However, there is no evidence of non-Fermi liquid behavior associated with the suppression of magnetism by pressure, unlike the effect of transition-metal substitution. △ Less

Submitted 21 May, 2025; originally announced May 2025.

Comments: 7 pages, 7 figures

Unraveling Spin Density Wave Order in Layered Nickelates $\mathrm{La_3Ni_2O_7}$ and $\mathrm{La_2PrNi_2O_7}$ via Neutron Diffraction

Authors: Igor Plokhikh, Thomas J. Hicken, Lukas Keller, Vladimir Pomjakushin, Samuel H. Moody, Pascale Foury-Leylekian, Jonas J. Krieger, Hubertus Luetkens, Zurab Guguchia, Rustem Khasanov, Dariusz Jakub Gawryluk

Abstract: The discovery of pressure-induced superconductivity in two- and three-layer Ruddlesden-Popper nickelates has generated significant interest in these materials as a platform for unconventional superconductivity. While their ground state exhibits magnetism, a direct determination of their magnetic structure remains elusive. Understanding this aspect is crucial, as magnetism may play a role in the pa… ▽ More The discovery of pressure-induced superconductivity in two- and three-layer Ruddlesden-Popper nickelates has generated significant interest in these materials as a platform for unconventional superconductivity. While their ground state exhibits magnetism, a direct determination of their magnetic structure remains elusive. Understanding this aspect is crucial, as magnetism may play a role in the pairing mechanism of superconductivity in these compounds. We resolve the magnetic structures of the bilayer (2222) polymorphs of La3Ni2O7 and La2PrNi2O7 using neutron powder diffraction (NPD) and muon-spin rotation/relaxation (muSR). Magnetic neutron scattering appears below approximately 150 K in both compounds and is observed at the (qx, 1/2, 0) position, with qx = 0 and 1/2 for La3Ni2O7 and qx = 0 for La2PrNi2O7. Within a single layer, alternating low (0.05 - 0.075 muB) and high (0.66 muB) magnetic moment stripes form. These layers stack antiferromagnetically along the c-direction to form bilayers. The presence of two propagation vectors (qx = 0 and 1/2) in undoped La3Ni2O7 suggests the coexistence of two magnetic stacking polymorphs within a single crystallographic phase. The muSR spectra further confirm these magnetic structures. Our findings provide a detailed understanding of the magnetic ground state in bilayer nickelates, offering insights into possible precursor states that may influence the emergence of superconductivity in these materials. △ Less

Submitted 7 March, 2025; originally announced March 2025.

Comments: 14 pages, 6 figures

Effect of Pressure and Oxygen-Isotope Substitution on Density-Wave Transitions in La$_4$Ni$_3$O$_{10}$

Authors: Rustem Khasanov, Vahid Sazgari, Thomas J. Hicken, Igor Plokhikh, Marisa Medarde, Ekaterina Pomjakushina, Lukas Keller, Vladimir Pomjakushin, Marek Bartkowiak, Szymon Królak, Michał J. Winiarski, Alexander Steppke, Jonas A. Krieger, Hubertus Luetkens, Tomasz Klimczuk, Christof W. Schneider, Dariusz J. Gawryluk, Zurab Guguchia

Abstract: Understanding the interplay between magnetism and superconductivity in nickelate systems is a key objective in condensed matter physics. Here, we present a systematic muon-spin rotation/relaxation ($μ$SR) and resistivity study of the trilayer Ruddlesden-Popper nickelate La$_4$Ni$_3$O$_{10}$ under ambient and applied pressure, combined with oxygen-isotope substitution. At ambient pressure, two inco… ▽ More Understanding the interplay between magnetism and superconductivity in nickelate systems is a key objective in condensed matter physics. Here, we present a systematic muon-spin rotation/relaxation ($μ$SR) and resistivity study of the trilayer Ruddlesden-Popper nickelate La$_4$Ni$_3$O$_{10}$ under ambient and applied pressure, combined with oxygen-isotope substitution. At ambient pressure, two incommensurate spin-density-wave (SDW) transitions are identified at $T_{SDW}\simeq132$ K and $T^\ast\simeq80-90$ K. Comparison of the internal magnetic fields with dipole-field calculations reveals a magnetic structure consistent with antiferromagnetically coupled SDW order on the outer two Ni layers, with smaller moments on the inner layer. Above $T^\ast$, the moments lie mainly in the $ab$ plane, whereas below this temperature they develop a $c$-axis component. The internal fields at the muon stopping sites appear abruptly at $T_{SDW}$, suggesting a first-order-like SDW transition closely linked to the charge-density-wave (CDW) order occurring at the same temperature ($T_{SDW}=T_{CDW}$). Under pressure, all transition temperatures -- $T_{SDW}$, $T^\ast$, and $T_{CDW}$ -- are suppressed at a nearly uniform rate of $\simeq-13$ K/GPa. This contrasts with bilayer La$_3$Ni$_2$O$_7$, where pressure enhances the separation between the SDW and CDW transitions. Oxygen-isotope substitution ($^{16}$O $\rightarrow$ $^{18}$O) shifts $T_{CDW}$ to higher values. The isotope effect on $T_{SDW}$ and $T^\ast$ differs markedly: when CDW and SDW are intertwined, a notable isotope effect is observed on $T_{SDW}$, yielding nearly identical isotope shifts for $T_{CDW}$ and $T_{SDW}$, whereas no isotope effect is detected at $T^\ast$, where the SDW transition occurs independently of the CDW. △ Less

Submitted 9 March, 2026; v1 submitted 6 March, 2025; originally announced March 2025.

Comments: 18 pages, 10 figures

Journal ref: Phys. Rev. Research 8, 013249 (2026)

Fluctuation-driven topological Hall effect in room-temperature itinerant helimagnet Fe3Ga4

Authors: Priya R. Baral, Victor Ukleev, Ivica Živković, Youngro Lee, Fabio Orlandi, Pascal Manuel, Yurii Skourski, Lukas Keller, Anne Stunault, J. Alberto Rodríguez-Velamazán, Robert Cubitt, Arnaud Magrez, Jonathan S. White, Igor I. Mazin, Oksana Zaharko

Abstract: The topological Hall effect (THE) is a hallmark of a non-trivial geometric spin arrangement in a magnetic metal, originating from a finite scalar spin chirality (SSC). The associated Berry phase is often a consequence of non-coplanar magnetic structures identified by multiple k-vectors. For single-k magnetic structures however with zero SSC, the emergence of a finite topological Hall signal presen… ▽ More The topological Hall effect (THE) is a hallmark of a non-trivial geometric spin arrangement in a magnetic metal, originating from a finite scalar spin chirality (SSC). The associated Berry phase is often a consequence of non-coplanar magnetic structures identified by multiple k-vectors. For single-k magnetic structures however with zero SSC, the emergence of a finite topological Hall signal presents a conceptual challenge. Here, we report that a fluctuation-driven mechanism involving chiral magnons is responsible for the observed THE in a low-symmetry compound, monoclinic Fe3Ga4. Through neutron scattering experiments, we discovered several nontrivial magnetic phases in this system. In our focus is the helical spiral phase at room temperature, which transforms into a transverse conical state in applied magnetic field, supporting a significant THE signal up to and above room temperature. Our work offers a fresh perspective in the search for novel materials with intertwined topological magnetic and transport properties. △ Less

Submitted 18 February, 2025; originally announced February 2025.

Comments: 20 pages, 4 figures

Structure and Magnetic Properties of a Maple Leaf antiferromagnet Ho$_3$ScO$_6$

Authors: C. Aguilar-Maldonado, R. Feyerherm, K. Prokeš, L. Keller, B. Lake

Abstract: Ho$_3$ScO$_6$ harbours a frustrated Maple Leaf Lattice (MLL). It crystalizes in the Mg$_3$TeO$_6$-type structure, and has a centrosymmetric trigonal space group (R$\bar{3}$). This system contains stacked layers of magnetic rings along the c-axis consisting of six magnetic Ho$^{3+}$ ions forming Ho hexagons, which are connected into a 2-dimensional network by equilateral and isosceles triangles to… ▽ More Ho$_3$ScO$_6$ harbours a frustrated Maple Leaf Lattice (MLL). It crystalizes in the Mg$_3$TeO$_6$-type structure, and has a centrosymmetric trigonal space group (R$\bar{3}$). This system contains stacked layers of magnetic rings along the c-axis consisting of six magnetic Ho$^{3+}$ ions forming Ho hexagons, which are connected into a 2-dimensional network by equilateral and isosceles triangles to form a rare example of a MLL. Long range magnetic order is reached below $T_N=4.1$ K with a 120$^\circ$ spin arrangement on the equilateral triangles resulting in a positive vector chirality ground state configuration. △ Less

Submitted 28 October, 2024; v1 submitted 22 October, 2024; originally announced October 2024.

Asymmetric Fluid Flow in Helical Pipes Inspired by Shark Intestines

Authors: Ido Levin, Naroa Sadaba, Alshakim Nelson, Sarah L. Keller

Abstract: Unlike human intestines, which are long, hollow tubes, the intestines of sharks and rays contain interior helical structures surrounding a cylindrical hole. One function of these structures may be to create asymmetric flow, favoring passage of fluid down the digestive tract, from anterior to posterior. Here, we design and 3D print biomimetic models of shark intestines, in both rigid and deformable… ▽ More Unlike human intestines, which are long, hollow tubes, the intestines of sharks and rays contain interior helical structures surrounding a cylindrical hole. One function of these structures may be to create asymmetric flow, favoring passage of fluid down the digestive tract, from anterior to posterior. Here, we design and 3D print biomimetic models of shark intestines, in both rigid and deformable materials. We use the rigid models to test which physical parameters of the interior helices (the pitch, the hole radius, the tilt angle, and the number of turns) yield the largest flow asymmetries. These asymmetries exceed those of traditional Tesla valves, structures specifically designed to create flow asymmetry without any moving parts. When we print the biomimetic models in elastomeric materials so that flow can couple to the structure's shape, flow asymmetry is significantly amplified; it is 7-fold larger in deformable structures than in rigid structures. Last, we 3D-print deformable versions of the intestine of a dogfish shark, based on a tomogram of a biological sample. This biomimic produces flow asymmetry comparable to traditional Tesla valves. The ability to influence the direction of a flow through a structure has applications in biological tissues and artificial devices across many scales, from large industrial pipelines to small microfluidic devices. △ Less

Submitted 10 July, 2024; originally announced July 2024.

Comments: 7 pages, 4 figures, SI (10 S. figures)

Journal ref: Proc. Natl. Acad. Sci. U.S.A. 121 (40) e2406481121 (2024)

Continuum excitations in a spin-supersolid on a triangular lattice

Authors: M. Zhu, V. Romerio, N. Steiger, S. D. Nabi, N. Murai, S. Ohira-Kawamura, K. Yu. Povarov, Y. Skourski, R. Sibille, L. Keller, Z. Yan, S. Gvasaliya, A. Zheludev

Abstract: Magnetic, thermodynamic, neutron diffraction and inelastic neutron scattering are used to study spin correlations in the easy-axis XXZ triangular lattice magnet K2Co(SeO3)2. Despite the presence of quasi-2D "supersolid" magnetic order, the low-energy excitation spectrum contains no sharp modes and is instead a broad and structured multi-particle continuum. Applying a weak magnetic field drives the… ▽ More Magnetic, thermodynamic, neutron diffraction and inelastic neutron scattering are used to study spin correlations in the easy-axis XXZ triangular lattice magnet K2Co(SeO3)2. Despite the presence of quasi-2D "supersolid" magnetic order, the low-energy excitation spectrum contains no sharp modes and is instead a broad and structured multi-particle continuum. Applying a weak magnetic field drives the system into an m = 1/3 fractional magnetization plateau phase and restores sharp spin wave modes. To some extent, the behavior at zero field can be understood in terms of spin wave decay. However, the presence of clear excitation minima at the M-points of the Brillouin zone suggest that the spinon language may provide a more adequate description, and signals a possible proximity to a Dirac spin liquid state. △ Less

Submitted 20 August, 2024; v1 submitted 29 January, 2024; originally announced January 2024.

Journal ref: Phys. Rev. Lett. 133, 186704 (2024)

Freezing of short-range ordered antiferromagnetic clusters in the CrFeTi$_2$O$_7$ system

Authors: Arun Kumar, Soumendra Nath Panja, Lukas Keller, Sunil Nair

Abstract: We report on the CrFeTi$_2$O$_7$ (CFTO) system using a combination of x-ray diffraction, dc magnetization, ac susceptibility, specific heat and neutron diffraction measurements. CFTO is seen to crystallize in a monoclinic $P21/a$ symmetry. It shows a glassy freezing at $T{_f}$ ~ 22 K, characterized by the observation of bifurcation between ZFC and FC $χ$ (T) curves, frequency dispersion across… ▽ More We report on the CrFeTi$_2$O$_7$ (CFTO) system using a combination of x-ray diffraction, dc magnetization, ac susceptibility, specific heat and neutron diffraction measurements. CFTO is seen to crystallize in a monoclinic $P21/a$ symmetry. It shows a glassy freezing at $T{_f}$ ~ 22 K, characterized by the observation of bifurcation between ZFC and FC $χ$ (T) curves, frequency dispersion across $T{_f}$ in ac susceptibility, appearance of non-zero remanent magnetization and coercivity below $T{_f}$, very slow relaxation of iso-thermal remanent magnetization with time and a linear temperature dependence of magnetic contribution to specific heat $C{_m}$ below $T{_f}$. The microscopic neutron diffraction analysis of CFTO not only confirms the absence of long-range antiferromagnetic ordering but also exhibits diffuse scattering due to the presence of shortrange ordered antiferromagnetically correlated spin clusters. △ Less

Submitted 15 January, 2024; originally announced January 2024.

Magnetic structure and phase diagram of the Heisenberg-Ising spin chain antiferromagnetic PbCo$_{2}$V$_{2}$O$_{8}$

Authors: K. Puzniak, C. Aguilar-Maldonado, R. Feyerherm, K. Prokeš, A. T. M. N. Islam, Y. Skourski, L. Keller, B. Lake

Abstract: The effective spin-1/2 antiferromagnetic Heisenberg-Ising chain materials, ACo$_2$V$_2$O$_8$, A = Sr, Ba, are a rich source of exotic fundamental phenomena and have been investigated for their model magnetic properties both in zero and non-zero magnetic fields. Here we investigate a new member of the family, namely PbCo$_2$V$_2$O$_8$. We synthesize powder and single crystal samples of PbCo$_2$V… ▽ More The effective spin-1/2 antiferromagnetic Heisenberg-Ising chain materials, ACo$_2$V$_2$O$_8$, A = Sr, Ba, are a rich source of exotic fundamental phenomena and have been investigated for their model magnetic properties both in zero and non-zero magnetic fields. Here we investigate a new member of the family, namely PbCo$_2$V$_2$O$_8$. We synthesize powder and single crystal samples of PbCo$_2$V$_2$O$_8$ and determine its magnetic structure using neutron diffraction. Furthermore, the magnetic field/temperature phase diagrams for magnetic field applied along the c, a, and [110] crystallographic directions in the tetragonal unit cell are determined via magnetization and heat capacity measurements. A complex series of phases and quantum phase transitions are discovered that depend strongly on both the magnitude and direction of the field. Our results show that \pcvo is an effective spin-1/2 antiferromagnetic Heisenberg-Ising chain with properties that are in general comparable to those of SrCo$_2$V$_2$O$_8$ and BaCo$_2$V$_2$O$_8$. One interesting departure from the results of these related compounds, is however, the discovery of a new field-induced phase for the field direction $H\|$[110] which has not been previously observed. △ Less

Submitted 28 September, 2023; originally announced September 2023.

Revisiting the antiferromagnetic structure of $\rm Tb_{14}Ag_{51}$. The importance of distinguishing alternative symmetries for a multidimensional order parameter

Authors: V. Pomjakushin, J. M. Perez-Mato, P. Fischer, L. Keller, W. Sikora

Abstract: We revisit the antiferromagnetic structure of $\rm Tb_{14}Ag_{51}$ [P. Fischer., $et\, al$. (2005). PRB, 72 134413] with the propagation vector $[{1\over3},{1\over3},{0}] $ and parent space group $P6/m$ using both magnetic symmetry and irreducible representation arguments. We have found a new magnetic structure under the hexagonal Shubnikov magnetic space group $P\bar{6}'$, which fits much better… ▽ More We revisit the antiferromagnetic structure of $\rm Tb_{14}Ag_{51}$ [P. Fischer., $et\, al$. (2005). PRB, 72 134413] with the propagation vector $[{1\over3},{1\over3},{0}] $ and parent space group $P6/m$ using both magnetic symmetry and irreducible representation arguments. We have found a new magnetic structure under the hexagonal Shubnikov magnetic space group $P\bar{6}'$, which fits much better the experimental data. This new solution was obtained by constraining the spin arrangement to one of the three possible magnetic space groups of maximal symmetry that can be realised by a magnetic ordering transforming according to the 4-dimensional physically irreducible representation that is known to be relevant in this magnetic phase. The refined model, parameterised under $P\bar{6}'$, implicitly includes the presence of a third harmonic with the propagation vector at the gamma point $[0,0,0]$, which has an important weight in the final result. The structure consists of 13 symmetry-independent Tb magnetic moments with the same size of $8.48(2)μ_B$, propagating cycloidally in the $ab$-plane. The modulation has a substantial deviation from being purely sinusoidal due to the contribution of the mentioned third harmonic. △ Less

Submitted 17 January, 2023; originally announced January 2023.

Comments: 4 pages, 1 table, 7 figures

Journal ref: Acta Cryst. (2022). B78, 172-178

Disorder driven cluster glass state in a geometrically frustrated hexagonal perovskite

Authors: Shruti Chakravarty, Øystein Slagtern Fjellvåg, Arpan Bhattacharyya, Lukas Keller, Sunil Nair

Abstract: We report the observation of cluster glass-like properties in a double perovskite ruthenate Ba$_2$CoRuO$_6$ through structural (neutron and synchrotron X-ray diffraction), magnetic and transport measurements. The system exhibits classic glassy characteristics like a frequency dependence in ac Susceptibility, aging and memory effects along with persistance of short-range correlations upto room temp… ▽ More We report the observation of cluster glass-like properties in a double perovskite ruthenate Ba$_2$CoRuO$_6$ through structural (neutron and synchrotron X-ray diffraction), magnetic and transport measurements. The system exhibits classic glassy characteristics like a frequency dependence in ac Susceptibility, aging and memory effects along with persistance of short-range correlations upto room temperature. The significant ($\sim30\%$) anti-site disorder on the dimer sites, coupled with the inherent geometrical frustration, allows a variety of exchange (both antiferro- and ferromagnetic) interactions to be distributed randomly across the lattice. On cooling, locally dominant interactions cause spins to nucleate and form local, short-range ordered clusters which grow in size until a global freezing occurs at about $T_f \sim 43K$. △ Less

Submitted 11 April, 2023; v1 submitted 13 September, 2022; originally announced September 2022.

Interplay of magnetic order and ferroelasticity in the spin-orbit coupled antiferromagnet K$_2$ReCl$_6$

Authors: Alexandre Bertin, Tusharkanti Dey, Daniel Brüning, Dmitry Gorkov, Kevin Jenni, Astin Krause, Petra Becker, Ladislav Bohaty, Daniel Khomskii, Vladimir Pomjakushin, Lukas Keller, Markus Braden, Thomas Lorenz

Abstract: The magnetic and structural phase transitions occurring in K$_2$ReCl$_6$ were studied by macroscopic and microscopic techniques. Structural phase transitions associated with rotations of the ReCl$_6$ octahedra lower the symmetry from cubic to monoclinic, form ferroelastic domains, and are visible in susceptibility, specific heat and thermal expansion measurements. In the antiferromagnetically orde… ▽ More The magnetic and structural phase transitions occurring in K$_2$ReCl$_6$ were studied by macroscopic and microscopic techniques. Structural phase transitions associated with rotations of the ReCl$_6$ octahedra lower the symmetry from cubic to monoclinic, form ferroelastic domains, and are visible in susceptibility, specific heat and thermal expansion measurements. In the antiferromagnetically ordered state slightly below $T_{\rm N}$=12\,K these domains can be rearranged by a magnetic field inducing a relative elongation of the polydomain crystal parallel to the field of 0.6\%. At zero field the magnetic structure in K$_2$ReCl$_6$ does not exhibit a weak ferromagnetic component, but at large magnetic field a distinct magnetic structure with a finite weak ferromagnetic component is stabilized. High magnetic fields rearrange the domains in the crystal to align the weak ferromagnetic moment parallel to the field. The altered domain structure with the crystal elongation is abruptly suppressed at lower temperature but persists upon heating to well above $T_{\rm N}$. However, heating above the lowest structural phase transition and successive cooling restore the initial shape. △ Less

Submitted 8 March, 2024; v1 submitted 22 July, 2022; originally announced July 2022.

Comments: 14 pages, 10 figures; In this revised version of our manuscript, neutron powder diffraction data have been included and the discussion of the multidomain structure has been extended

Journal ref: Phys. Rev. B 109, 094409 (2024)

Role of Dy on the magnetic properties of orthorhombic DyFeO3

Authors: Banani Biswas, Veronica F. Michel, Oystein S. Fjellvag, Gesara Bimashofer, Max Dobeli, Michal Jambor, Lukas Keller, Elisabeth Muller, Victor Ukleev, Ekaterina V. Pomjakushina, Deepak Singh, Uwe Stuhr, Carlos A. F. Vaz, Thomas Lippert, Christof W. Schneider

Abstract: Orthoferrites are a class of magnetic materials with a magnetic ordering temperature above 600 K, predominant G-type antiferromagnetic ordering of the Fe-spin system and, depending on the rare-earth ion, a spin reorientation of the Fe spin taking place at lower temperatures. DyFeO3 is of particular interest since the spin reorientation is classified as a Morin transition with the transition temper… ▽ More Orthoferrites are a class of magnetic materials with a magnetic ordering temperature above 600 K, predominant G-type antiferromagnetic ordering of the Fe-spin system and, depending on the rare-earth ion, a spin reorientation of the Fe spin taking place at lower temperatures. DyFeO3 is of particular interest since the spin reorientation is classified as a Morin transition with the transition temperature depending strongly on the Dy-Fe interaction. Here, we report a detailed study of the magnetic and structural properties of microcrystalline DyFeO3 powder and bulk single crystal using neutron diffraction and magnetometry between 1.5 and 450 K. We find that, while the magnetic properties of the single crystal are largely as expected, the powder shows strongly modified magnetic properties, including a modified spin reorientation and a smaller Dy-Fe interaction energy of the order of 10 μeV. Subtle structural differences between powder and single crystal show that they belong to distinct magnetic space groups. In addition, the Dy ordering at 2 K in the powder is incommensurate, with a modulation vector of 0.0173(5) c*, corresponding to a periodicity of ~58 unit cells. △ Less

Submitted 13 July, 2022; originally announced July 2022.

Comments: contains supplementary material

Journal ref: Published in Phys. Rev. Materials 6, 074401 (2022)

Nanocriticality in the magnetic phase transition of CoO nanoparticles

Authors: Machteld E. Kamminga, Jonas Okkels Birk, Jari í Hjøllum, Henrik Jacobsen, Jakob Lass, Thorbjørn L. Koch, Niels B. Christensen, Christof Niedermayer, Lukas Keller, Luise Theil Kuhn, Elisabeth T. Ulrikkeholm, Erik Brok, Cathrine Frandsen, Kim Lefmann

Abstract: The universal theory of critical phase transitions describes the critical behavior at second-order phase transitions in infinitely large systems. With the increased contemporary interest in nanoscale materials, we investigated CoO nanoparticles by means of neutron scattering and found how the theory of critical phenomena breaks down in the nanoscale regime. Using CoO as a model system, we have ide… ▽ More The universal theory of critical phase transitions describes the critical behavior at second-order phase transitions in infinitely large systems. With the increased contemporary interest in nanoscale materials, we investigated CoO nanoparticles by means of neutron scattering and found how the theory of critical phenomena breaks down in the nanoscale regime. Using CoO as a model system, we have identified a size-dependent nanocritical temperature region close to the antiferromagnetic phase transition where the magnetic correlation length of the nanoparticles converges to a constant value, which is significantly smaller than that of the saturated state found at low temperatures. This is in clear contrast to the divergence around $T_{\rm N}$ observed for bulk systems. Our findings of nanocriticality in the magnetic phase transition is of great importance for the understanding of phase transitions at the nanoscale. △ Less

Submitted 30 May, 2022; v1 submitted 27 May, 2022; originally announced May 2022.

Magnetic order and exchange couplings in the frustrated diamond lattice antiferromagnet MnSc$_2$Se$_4$

Authors: K. Guratinder, V. Tsurkan, L. Prodan, L. Keller, J. P. Embs, F. Juranyi, M. Medarde, Ch. Rüegg, O. Zaharko

Abstract: We report the magnetic properties of $A$-site spinel compound MnSc$_2$Se$_4$. The macroscopic magnetic measurements uncovers successive magnetic transitions at $T_{\rm{N1}}$= 2.04 K, followed by two further transitions at $T_{\rm{N2}}$=1.8 K and $T_{\rm{N3}}$=1.6 K. Neutron powder diffraction reveals that both, $T_{\rm{N2}} < T < T_{\rm{N1}}$ and $T <T_{\rm{N3}}$, orders are associated with the pr… ▽ More We report the magnetic properties of $A$-site spinel compound MnSc$_2$Se$_4$. The macroscopic magnetic measurements uncovers successive magnetic transitions at $T_{\rm{N1}}$= 2.04 K, followed by two further transitions at $T_{\rm{N2}}$=1.8 K and $T_{\rm{N3}}$=1.6 K. Neutron powder diffraction reveals that both, $T_{\rm{N2}} < T < T_{\rm{N1}}$ and $T <T_{\rm{N3}}$, orders are associated with the propagation vector $k$=(3/4 3/4 0), while the magnetic structures are collinear amplitude modulated and helical, respectively. Using neutron powder spectroscopy we demonstrated the effect of substitution of S by Se on the magnetic exchange. The energy range of the spin-wave excitations is supressed due to the chemical pressure of the $X$- ion in MnSc$_2X_4$ ($X$=S, Se) spinels. △ Less

Submitted 1 April, 2022; originally announced April 2022.

Comments: Accepted in Phys. Rev. B

Magnetism of the 2D honeycomb layered Na2Ni2TeO6 compound driven by intermediate Na-layer crystal-structure

Authors: A. K. Bera, S. M. Yusuf, L. Keller, F. Yokaichiya, J. R. Stewart

Abstract: The microscopic spin-spin correlations in the 2D layered spin-1 honeycomb lattice compound Na2Ni2TeO6 have been investigated by neutron diffraction and inelastic neutron scattering. The honeycomb lattice of spin-1 Ni2+ ions, within the crystallographic ab planes, are well separated along the c axis by an intermediate Na layer whose crystal structure contains chiral nuclear density distributions of… ▽ More The microscopic spin-spin correlations in the 2D layered spin-1 honeycomb lattice compound Na2Ni2TeO6 have been investigated by neutron diffraction and inelastic neutron scattering. The honeycomb lattice of spin-1 Ni2+ ions, within the crystallographic ab planes, are well separated along the c axis by an intermediate Na layer whose crystal structure contains chiral nuclear density distributions of Na ions. The chirality of the alternating Na layers is opposite. Such alternating chirality of the Na layer dictates the magnetic periodicity along the c axis where an up-up-down-down spin arrangement of the in-plane zigzag AFM structure is found. Besides, the above described commensurate (CM) zigzag AFM order state is found to coexist with an incommensurate (ICM) AFM state below the TN ~ 27.5 K. The ICM state is found to appear at much higher temperature ~ 50 K and persists down to lowest measured temperature of 1.7 K. Our reverse Monte Carlo (RMC) analysis divulges a two dimensional (2D) magnetic correlations (within the ab plane) of the ICM AFM state over the entire temperature range 1.7-50 K. Further, the spin-Hamiltonian has been determined by carrying out inelastic neutron scattering experiments and subsequent linear spin-wave theory analysis which reveals the presence of competing inplane exchange interactions up to 3 rd nearest neighbours consistent with the zigzag AFM ground state, and weak interplanar interaction as well as a weak single-ion-anisotropy. The values of the exchange constants yield that Na2Ni2TeO6 is situated well inside the zigzag AFM phase (spans over a wide ranges of J2/J1 and J3/J1 values) in the theoretical phase diagram. The present study, thus, provides a detailed microscopic understanding of the magnetic correlations and divulges the intertwining magneto-structural correlations. △ Less

Submitted 16 February, 2022; originally announced February 2022.

Comments: 24 pages, 14 figures,

Journal ref: Phys. Rev. B 105, 014410 (2022)

Magnetic correlations in triangular antiferromagnet FeGa$_2$S$_4$

Authors: K. Guratinder, M. Schmidt, H. C. Walker, R. Bewley, M. Wörle, D. Cabra, S. A. Osorio, M. Villalba, A. K. Madsen, L. Keller, A. Wildes, P. Puphal, A. Cervellino, Ch. Rüegg, O. Zaharko

Abstract: The crystal structure and magnetic correlations in triangular antiferromagnet FeGa$_2$S$_4$ are studied by x-ray diffraction, magnetic susceptibility, neutron diffraction and neutron inelastic scattering. We report significant mixing at the cation sites and disentangle magnetic properties dominated by major and minor magnetic sites. The magnetic short-range correlations at 0.77 Å$^{-1}$ correspond… ▽ More The crystal structure and magnetic correlations in triangular antiferromagnet FeGa$_2$S$_4$ are studied by x-ray diffraction, magnetic susceptibility, neutron diffraction and neutron inelastic scattering. We report significant mixing at the cation sites and disentangle magnetic properties dominated by major and minor magnetic sites. The magnetic short-range correlations at 0.77 Å$^{-1}$ correspond to the major sites and being static at base temperature they evolve into dynamic correlations around 30 - 50 K. The minor sites contribute to the magnetic peak at 0.6 Å$^{-1}$, which vanishes at 5.5 K. Our analytical studies of triangular lattice models with bilinear and biquadratic terms provide the ratios between exchanges for the proposed ordering vectors. The modelling of the inelastic neutron spectrum within linear spin wave theory results in the set of exchange couplings $J_1=1.7$\,meV, $J_2=0.9$\,meV, $J_3=0.8$\,meV for the bilinear Heisenberg Hamiltonian. However, not all features of the excitation spectrum are explained with this model. △ Less

Submitted 14 July, 2021; originally announced July 2021.

Comments: Accepted in Phys. Rev. B

Magnetic order in the quasi-one-dimensional Ising system RbCoCl$_3$

Authors: N. P. Hänni, D. Sheptyakov, M. Mena, E. Hirtenlechner, L. Keller, U. Stuhr, L. -P. Regnault, M. Medarde, A. Cervellino, Ch. Rüegg, B. Normand, K. W. Krämer

Abstract: The dynamical properties of free and bound domain-wall excitations in Ising-chain materials have recently become the focus of intense research interest. New materials and spectrometers have made it possible to control the environment of coupled Ising chains by both effective internal and applied external fields, which can be both longitudinal and transverse, and thus to demonstrate how the resulti… ▽ More The dynamical properties of free and bound domain-wall excitations in Ising-chain materials have recently become the focus of intense research interest. New materials and spectrometers have made it possible to control the environment of coupled Ising chains by both effective internal and applied external fields, which can be both longitudinal and transverse, and thus to demonstrate how the resulting magnetic phase transitions and the nature of the associated excited states obey fundamental symmetry properties. In RbCoCl$_3$, the weakly coupled Ising chains form a triangular lattice whose frustrated geometry and magnetic ordering transitions at low temperature open new possibilities for the Ising-chain environment. We have investigated the structure and magnetism in RbCoCl$_3$ by high-resolution x-ray diffraction and neutron scattering measurements on powder and single crystal samples between 1.5 K and 300 K. Upon cooling, the Co$^{2+}$ spins develop one-dimensional antiferromagnetic correlations along the chain axis ($c$-axis) below 90 K. Below the first Néel temperature, $T_{N1}$ = 28 K, a partial 3D magnetic order sets in, with propagation vector ${\vec k}_1$ = (1/3,1/3,1), the moments aligned along the $c$-axis and every third chain uncorrelated from its neighbours. Only below a second magnetic phase transition at $T_{N2}$ = 13 K does the system achieve a fully ordered state, with two additional propagation vectors: ${\vec k}_2$ = (0,0,1) establishes a "honeycomb" $c$-axis order, in which 1/3 of the chains are subject to a strong effective mean field due to their neighbours whereas 2/3 experience no net field, while ${\vec k}_3$ = (1/2,0,1) governs a small, staggered in-plane ordered moment. We conclude that RbCoCl$_3$ is an excellent material to study the physics of Ising chains in a wide variety of temperature-controlled environments. △ Less

Submitted 1 February, 2021; originally announced February 2021.

Comments: 23+3 pages, 10+2 figures, 6+4 tables

Journal ref: Phys. Rev. B 103, 094424 (2021)

Structural disorder and magnetic correlations driven by oxygen doping in Nd2NiO4+d (d ~ 0.11)

Authors: Sumit Ranjan Maity, Monica Ceretti, Lukas Keller, Jürg Schefer, Tian Shang, Ekaterina Pomjakushina, Martin Meven, Denis Sheptyakov, Antonio Cervellino, Werner Paulus

Abstract: We investigated the influence of oxygen over-stoichiometry on apical oxygen disorder and magnetic correlations in Nd2NiO4+d (d~0.11) in the temperature range of 2-300 K by means of synchrotron x-ray powder diffraction, neutron single crystal and powder diffraction studies, combined with macroscopic magnetic measurements. In the investigated temperature range, the compound crystalizes in a tetragon… ▽ More We investigated the influence of oxygen over-stoichiometry on apical oxygen disorder and magnetic correlations in Nd2NiO4+d (d~0.11) in the temperature range of 2-300 K by means of synchrotron x-ray powder diffraction, neutron single crystal and powder diffraction studies, combined with macroscopic magnetic measurements. In the investigated temperature range, the compound crystalizes in a tetragonal commensurate structure with the P42/ncm space group with excess oxygen atoms occupy the 4b (3/4 1/4 1/4) interstitial sites, coordinated by four apical oxygen atoms. Large and anisotropic thermal displacement parameters are found for equatorial and apical oxygen atoms, which are strongly reduced on an absolute scale compared to the Nd2NiO4.23 phase. Maximum Entropy analysis of the neutron single crystal diffraction data uncovered anharmonic contributions to the displacement parameters of the apical oxygen atoms, toward the nearest vacant 4b interstitial site, related to the phonon assisted oxygen diffusion mechanism. Macroscopic magnetization measurements and neutron powder diffraction studies reveal long-range antiferromagnetic ordering of the Ni-sublattice at TN ~ 53 K with a weak ferromagnetic component along the c-axis, while the long-range magnetic ordering of the Nd-sublattice occurs below 10 K. Temperature dependent neutron diffraction patterns show the appearance of a commensurate magnetic order at TN with the propagation vector k = (100) and the emergence of an additional incommensurate phase below 30 K, while both phases coexist at 2 K. The commensurate magnetic structure is best described by the P42/nc`m` Shubnikov space group. Refined magnetic moments of the Ni and Nd-sites at 2 K are 1.144(76) muB and 1.632(52) muB respectively. A possible origin of the incommensurate phase is discussed and a tentative magnetic phase diagram is proposed. △ Less

Submitted 13 January, 2021; originally announced January 2021.

Journal ref: Physical Review MATERIALS 3, 083604 (2019)

Interdependent scaling of long-range oxygen and magnetic ordering in non-stoichiometric Nd${}_2$NiO${}_{4.10}$

Authors: Sumit Ranjan Maity, Monica Ceretti, Lukas Keller, Jürg Schefer, Martin Meven, Ekaterina Pomjakushina, Werner Paulus

Abstract: Hole doping in Nd${}_{2}$NiO${}_{4.00}$ can be either achieved by substituting the trivalent Nd atoms by bivalent alkaline earth metals or by oxygen doping, yielding Nd${}_{2}$NiO${}_{4+δ}$. In this study, we investigated the interplay between oxygen and spin ordering for a low oxygen doping concentration i.e. Nd${}_{2}$NiO${}_{4.10}$. Although the extra oxygen doping level remains rather modest w… ▽ More Hole doping in Nd${}_{2}$NiO${}_{4.00}$ can be either achieved by substituting the trivalent Nd atoms by bivalent alkaline earth metals or by oxygen doping, yielding Nd${}_{2}$NiO${}_{4+δ}$. In this study, we investigated the interplay between oxygen and spin ordering for a low oxygen doping concentration i.e. Nd${}_{2}$NiO${}_{4.10}$. Although the extra oxygen doping level remains rather modest with only one out of 20 possible interstitial tetrahedral lattice sites occupied, we observed by single crystal neutron diffraction the presence of a complex 3D modulated structure related to oxygen ordering already at ambient, the modulation vectors being $\pm$2/13\textit{\textbf{a*}}$\pm$3/13\textit{\textbf{b*}}, $\pm$3/13\textit{\textbf{b*}}$\pm$2/13\textit{\textbf{b*}} and $\pm$1/5\textit{\textbf{a*}}$\pm$1/2\textit{\textbf{c*}} and satellite reflections up to fourth order. Temperature dependent neutron diffraction studies indicate the coexistence of oxygen and magnetic ordering below T${}_{N}$ $\simeq$ 48 K, the wave vector of the Ni sublattice being \textbf{\textit{k}}=(100). In addition, magnetic satellite reflections adapt exactly the same modulation vectors as found for the oxygen ordering, evidencing a unique coexistence of 3D modulated ordering for spin and oxygen ordering in Nd${}_{2}$NiO${}_{4.10}$. Temperature dependent measurements of magnetic intensities suggest two magnetic phase transitions below 48 K and 20 K, indicating two distinct onsets of magnetic ordering for the Ni and Nd sublattice, respectively. △ Less

Submitted 12 January, 2021; originally announced January 2021.

Comments: 9 pages, 5 figures, supplemetary material linked in the paper

Journal ref: Physical Review MATERIALS 5, 014401 (2021)

Magnetic and structural properties of Ni-substituted magnetoelectric Co$_4$Nb$_2$O$_9$

Authors: Hadi Papi, Virgile Yves Favre, Hossein Ahmadvand, Mojtaba Alaei, Mohammad Khondabi, Denis Sheptyakov, Lukas Keller, Parviz Kameli, Ivica Zivkovic, Henrik M. Rønnow

Abstract: The magnetic and structural properties of polycrystalline Co$_{4-x}$ Ni$_x$ Nb$_2$ O$_9$ (x=1,2) have been investigated by neutron powder diffraction, magnetization and heat capacity measurements, and density functional theory (DFT) calculations. For x=1, the compound crystallizes in the trigonal P$\bar{3}$c1 space group. Below T$_N$ = 31 K it develops a weakly non-collinear antiferromagnetig stru… ▽ More The magnetic and structural properties of polycrystalline Co$_{4-x}$ Ni$_x$ Nb$_2$ O$_9$ (x=1,2) have been investigated by neutron powder diffraction, magnetization and heat capacity measurements, and density functional theory (DFT) calculations. For x=1, the compound crystallizes in the trigonal P$\bar{3}$c1 space group. Below T$_N$ = 31 K it develops a weakly non-collinear antiferromagnetig structure with magnetic moments in the ab-plane. The compound with x=2 has crystal structure of the orthorhombic Pbcn space group and shows a hard ferrimagnetic behavior below T$_C$ =47 K. For this compound a weakly non-collinear ferrimagnetic structure with two possible configurations in ab plane was derived from ND study. By calculating magnetic anisotropy energy via DFT, the ground-state magnetic configuration was determined for this compound. The heat capacity study in magnetic fields up to 140 kOe provide further information on the magnetic structure of the compounds. △ Less

Submitted 23 October, 2020; originally announced October 2020.

Comments: Published in Physical Review B, 8 pages, 8 figures

Journal ref: Phys.Rev. B 100, 2019, 134408

Magnetism and anomalous transport in the Weyl semimetal PrAlGe: Possible route to axial gauge fields

Authors: D. Destraz, L. Das, S. S. Tsirkin, Y. Xu, T. Neupert, J. Chang, A. Schilling, A. G. Grushin, J. Kohlbrecher, L. Keller, P. Puphal, E. Pomjakushina, J. S. White

Abstract: In magnetic Weyl semimetals, where magnetism breaks time-reversal symmetry, large magnetically sensitive anomalous transport responses are anticipated that could be useful for topological spintronics. The identification of new magnetic Weyl semimetals is therefore in high demand, particularly since in these systems Weyl node configurations may be easily modified using magnetic fields. Here we expl… ▽ More In magnetic Weyl semimetals, where magnetism breaks time-reversal symmetry, large magnetically sensitive anomalous transport responses are anticipated that could be useful for topological spintronics. The identification of new magnetic Weyl semimetals is therefore in high demand, particularly since in these systems Weyl node configurations may be easily modified using magnetic fields. Here we explore experimentally the magnetic semimetal PrAlGe, and unveil a direct correspondence between easy-axis Pr ferromagnetism and anomalous Hall and Nernst effects. With sizes of both the anomalous Hall conductivity and Nernst effect in good quantitative agreement with first principles calculations, we identify PrAlGe as a system where magnetic fields can connect directly to Weyl nodes via the Pr magnetization. Furthermore, we find the predominantly easy-axis ferromagnetic ground state co-exists with a low density of nanoscale textured magnetic domain walls. We describe how such nanoscale magnetic textures could serve as a local platform for tunable axial gauge fields of Weyl fermions. △ Less

Submitted 4 August, 2020; originally announced August 2020.

Comments: 42 pages, 5 figures

Journal ref: npj Quantum Materials 5, 5 (2020)

Neutron powder diffraction study of NaMn$_2$O$_4$ and Li$_{0.92}$Mn$_2$O$_4$: New insights on spin-charge-orbital ordering

Authors: N. Matsubara, E. Nocerino, K. Kamazawa, O. K. Forslund, Y. Sassa, L. Keller, V. V. Sikolenko, V. Pomjakushin, H. Sakurai, J. Sugiyama, M. Månsson

Abstract: The high-pressure synthesized quasi-one-dimensional compounds NaMn$_2$O$_4$ and Li$_{0.92}$Mn$_2$O$_4$ are both antiferromagnetic insulators, and here their atomic and magnetic structures were investigated using neutron powder diffraction. The present crystal structural analyses of NaMn2O4 reveal that Mn3+/Mn4+ charge-ordering state exist even at low temperature (down to 1.5 K). It is evident from… ▽ More The high-pressure synthesized quasi-one-dimensional compounds NaMn$_2$O$_4$ and Li$_{0.92}$Mn$_2$O$_4$ are both antiferromagnetic insulators, and here their atomic and magnetic structures were investigated using neutron powder diffraction. The present crystal structural analyses of NaMn2O4 reveal that Mn3+/Mn4+ charge-ordering state exist even at low temperature (down to 1.5 K). It is evident from one of the Mn sites shows a strongly distorted Mn3+ octahedra due to the Jahn-Teller effect. Above TN = 39 K, a two-dimensional short-range correlation is observed, as indicated by an asymmetric diffuse scattering. Below TN, two antiferromagnetic transitions are observed (i) a commensurate long-range Mn3+ spin ordering below 39 K, and (ii) an incommensurate Mn4+ spin ordering below 10 K. The commensurate magnetic structure (kC = 0.5, -0.5, 0.5) follows the magnetic anisotropy of the local easy axes of Mn3+, while the incommensurate one shows a spin-density-wave order with kIC = (0,0,0.216). For Li$_{0.92}$Mn$_2$O$_4$, on the other hand, absence of a long-range spin ordered state down to 1.5 K is confirmed. △ Less

Submitted 30 June, 2020; originally announced June 2020.

Comments: 11 pages, 8 figures

Journal ref: Phys. Rev. Research 2, 043143 (2020)

Observation of plaquette fluctuations in the spin-1/2 honeycomb lattice

Authors: C. Wessler, B. Roessli, K. W. Krämer, B. Delley, O. Waldmann, L. Keller, D. Cheptiakov, H. B. Braun, M. Kenzelmann

Abstract: Quantum spin liquids are materials that feature quantum entangled spin correlations and avoid magnetic long-range order at T = 0 K. Particularly interesting are two-dimensional honeycomb spin lattices where a plethora of exotic quantum spin liquids have been predicted. Here, we experimentally study an effective S=1/2 Heisenberg honeycomb lattice with competing nearest and next-nearest neighbor int… ▽ More Quantum spin liquids are materials that feature quantum entangled spin correlations and avoid magnetic long-range order at T = 0 K. Particularly interesting are two-dimensional honeycomb spin lattices where a plethora of exotic quantum spin liquids have been predicted. Here, we experimentally study an effective S=1/2 Heisenberg honeycomb lattice with competing nearest and next-nearest neighbor interactions. We demonstrate that YbBr$_3$ avoids order down to at least T=100 mK and features a dynamic spin-spin correlation function with broad continuum scattering typical of quantum spin liquids near a quantum critical point. The continuum in the spin spectrum is consistent with plaquette type fluctuations predicted by theory. Our study is the experimental demonstration that strong quantum fluctuations can exist on the honeycomb lattice even in the absence of Kitaev-type interactions, and opens a new perspective on quantum spin liquids. △ Less

Submitted 7 May, 2020; v1 submitted 5 March, 2020; originally announced March 2020.

Comments: 32 pages, 7 Figures

Journal ref: npj Quantum Mater. 5, 85 (2020)

Mixed system $Cs_3Cu_3Cl_{8-x}Br_xOH$ with weakly connected Cu-triangles

Authors: Natalija van Well, Michael Bolte, Claudio Eisele, Lukas Keller, Jürg Schefer, Sander van Smaalen

Abstract: To study the relationship between the properties of low-dimensional spin systems with weakly coupled Cu-triangles and their crystal structure, single crystals of $Cs_3Cu_3Cl_8OH$ (1) and the new $Cs_3Cu_3Cl_{7.6}Br_{0.4}OH$ (2) were grown. Both compounds are isostructural and crystallize in a monoclinic structure with space group $P2_1/c$. The magnetic susceptibility of (1) shows a maximum at… ▽ More To study the relationship between the properties of low-dimensional spin systems with weakly coupled Cu-triangles and their crystal structure, single crystals of $Cs_3Cu_3Cl_8OH$ (1) and the new $Cs_3Cu_3Cl_{7.6}Br_{0.4}OH$ (2) were grown. Both compounds are isostructural and crystallize in a monoclinic structure with space group $P2_1/c$. The magnetic susceptibility of (1) shows a maximum at $2.23 K$ and of (2) at $2.70 K$, which are attributed to antiferromagnetic phase transitions. Furthermore, the magnetization along the $b$-axis at $1.9 K$ for both compounds shows a spin-flop transition into a new antiferromagnetic phase. This transition occurs at $0.61 T$ for (1) and at $2.0 T$ for (2). The antiferromagnetic order can be suppressed by a magnetic field $B_{C1}= 1.1 T$ for (1) and $B_{C2}= 1.2 T$ for (2). First single crystal neutron diffraction measured on (1) at different temperatures reveals the magnetic signal on the top of the nuclear reflection at (-1 0 0). Its magnetic ordering temperature was found to be at $T_{N1}= 2.12(3) K$. △ Less

Submitted 28 February, 2020; originally announced February 2020.

Comments: Accepted Paper

Journal ref: Journal of Physics and Chemistry of Solids 140C (2020) 109386

Topological Magnetic Phase in the Candidate Weyl Semimetal CeAlGe

Authors: P. Puphal, V. Pomjakushin, N. Kanazawa, V. Ukleev, D. J. Gawryluk, J. Ma, M. Naamneh, N. C. Plumb, L. Keller, R. Cubitt, E. Pomjakushina, J. S. White

Abstract: We report the discovery of topological magnetism in the candidate magnetic Weyl semimetal CeAlGe. Using neutron scattering we find this system to host several incommensurate, square-coordinated multi-$\vec{k}$ magnetic phases below $T_{\rm{N}}$. The topological properties of a phase stable at intermediate magnetic fields parallel to the $c$-axis are suggested by observation of a topological Hall e… ▽ More We report the discovery of topological magnetism in the candidate magnetic Weyl semimetal CeAlGe. Using neutron scattering we find this system to host several incommensurate, square-coordinated multi-$\vec{k}$ magnetic phases below $T_{\rm{N}}$. The topological properties of a phase stable at intermediate magnetic fields parallel to the $c$-axis are suggested by observation of a topological Hall effect. Our findings highlight CeAlGe as an exceptional system for exploiting the interplay between the nontrivial topologies of the magnetization in real space and Weyl nodes in momentum space. △ Less

Submitted 18 January, 2020; originally announced January 2020.

Comments: 6 pages, 5 figures

Journal ref: Phys. Rev. Lett. 124, 017202 (2020) - fully Open Access

A quantum liquid of magnetic octupoles on the pyrochlore lattice

Authors: Romain Sibille, Nicolas Gauthier, Elsa Lhotel, Victor Porée, Vladimir Pomjakushin, Russell A. Ewings, Toby G. Perring, Jacques Ollivier, Andrew Wildes, Clemens Ritter, Thomas C. Hansen, David A. Keen, Gøran J. Nilsen, Lukas Keller, Sylvain Petit, Tom Fennell

Abstract: Spin liquids are highly correlated yet disordered states formed by the entanglement of magnetic dipoles$^1$. Theories typically define such states using gauge fields and deconfined quasiparticle excitations that emerge from a simple rule governing the local ground state of a frustrated magnet. For example, the '2-in-2-out' ice rule for dipole moments on a tetrahedron can lead to a quantum spin ice… ▽ More Spin liquids are highly correlated yet disordered states formed by the entanglement of magnetic dipoles$^1$. Theories typically define such states using gauge fields and deconfined quasiparticle excitations that emerge from a simple rule governing the local ground state of a frustrated magnet. For example, the '2-in-2-out' ice rule for dipole moments on a tetrahedron can lead to a quantum spin ice in rare-earth pyrochlores - a state described by a lattice gauge theory of quantum electrodynamics$^{2-4}$. However, f-electron ions often carry multipole degrees of freedom of higher rank than dipoles, leading to intriguing behaviours and 'hidden' orders$^{5-6}$. Here we show that the correlated ground state of a Ce$^{3+}$-based pyrochlore, Ce$_2$Sn$_2$O$_7$, is a quantum liquid of magnetic octupoles. Our neutron scattering results are consistent with the formation of a fluid-like state of matter, but the intensity distribution is weighted to larger scattering vectors, which indicates that the correlated degrees of freedom have a more complex magnetization density than that typical of magnetic dipoles in a spin liquid. The temperature evolution of the bulk properties in the correlated regime below 1 Kelvin is well reproduced using a model of dipole-octupole doublets on a pyrochlore lattice$^{7-8}$. The nature and strength of the octupole-octupole couplings, together with the existence of a continuum of excitations attributed to spinons, provides further evidence for a quantum ice of octupoles governed by a '2-plus-2-minus' rule. Our work identifies Ce$_2$Sn$_2$O$_7$ as a unique example of a material where frustrated multipoles form a 'hidden' topological order, thus generalizing observations on quantum spin liquids to multipolar phases that can support novel types of emergent fields and excitations. △ Less

Submitted 2 December, 2019; originally announced December 2019.

Journal ref: Nature Physics (2020)

Tunable Berry Curvature Through Magnetic Phase Competition in a Topological Kagome Magnet

Authors: Z. Guguchia, J. Verezhak, D. Gawryluk, S. S. Tsirkin, J. -X. Yin, I. Belopolski, H. Zhou, G. Simutis, S. -S. Zhang, T. A. Cochran, G. Chang, E. Pomjakushina, L. Keller, Z. Skrzeczkowska, Q. Wang, H. C. Lei, R. Khasanov, A. Amato, S. Jia, T. Neupert, H. Luetkens, M. Z. Hasan

Abstract: Magnetic topological phases of quantum matter are an emerging frontier in physics and material science. Along these lines, several kagome magnets have appeared as the most promising platforms. However, the magnetic nature of these materials in the presence of topological state remains an unsolved issue. Here, we explore magnetic correlations in the kagome magnet Co_3Sn_2S_2. Using muon spin-rotati… ▽ More Magnetic topological phases of quantum matter are an emerging frontier in physics and material science. Along these lines, several kagome magnets have appeared as the most promising platforms. However, the magnetic nature of these materials in the presence of topological state remains an unsolved issue. Here, we explore magnetic correlations in the kagome magnet Co_3Sn_2S_2. Using muon spin-rotation, we present evidence for competing magnetic orders in the kagome lattice of this compound. Our results show that while the sample exhibits an out-of-plane ferromagnetic ground state, an in-plane antiferromagnetic state appears at temperatures above 90 K, eventually attaining a volume fraction of 80% around 170 K, before reaching a non-magnetic state. Strikingly, the reduction of the anomalous Hall conductivity above 90 K linearly follows the disappearance of the volume fraction of the ferromagnetic state. We further show that the competition of these magnetic phases is tunable through applying either an external magnetic field or hydrostatic pressure. Our results taken together suggest the thermal and quantum tuning of Berry curvature field via external tuning of magnetic order. Our study shows that Co_3Sn_2S_2 is a rare example where the magnetic competition drives the thermodynamic evolution of the Berry curvature field, thus tuning its topological state. △ Less

Submitted 19 April, 2019; originally announced April 2019.

Comments: 11 pages, 5 figures, Supplementary Information is available upon request

Journal ref: Nature Communications 11, 559 (2020)

Putative spin-nematic phase in BaCdVO(PO$_{4}$)$_{2}$

Authors: M. Skoulatos, F. Rucker, G. J. Nilsen, A. Bertin, E. Pomjakushina, J. Ollivier, A. Schneidewind, R. Georgii, O. Zaharko, L. Keller, Ch. Rüegg, C. Pfleiderer, B. Schmidt, N. Shannon, A. Kriele, A. Senyshyn, A. Smerald

Abstract: We report neutron scattering and AC magnetic susceptibility measurements of the 2D spin-1/2 frustrated magnet BaCdVO(PO$_{4}$)$_{2}$. At temperatures well below $T_{\sf N}\approx 1K$, we show that only 34 % of the spin moment orders in an up-up-down-down strip structure. Dominant magnetic diffuse scattering and comparison to published $μ$sr measurements indicates that the remaining 66 % is fluctua… ▽ More We report neutron scattering and AC magnetic susceptibility measurements of the 2D spin-1/2 frustrated magnet BaCdVO(PO$_{4}$)$_{2}$. At temperatures well below $T_{\sf N}\approx 1K$, we show that only 34 % of the spin moment orders in an up-up-down-down strip structure. Dominant magnetic diffuse scattering and comparison to published $μ$sr measurements indicates that the remaining 66 % is fluctuating. This demonstrates the presence of strong frustration, associated with competing ferromagnetic and antiferromagnetic interactions, and points to a subtle ordering mechanism driven by magnon interactions. On applying magnetic field, we find that at $T=0.1$ K the magnetic order vanishes at 3.8 T, whereas magnetic saturation is reached only above 4.5 T. We argue that the putative high-field phase is a realization of the long-sought bond-spin-nematic state. △ Less

Submitted 29 March, 2019; originally announced March 2019.

Journal ref: Phys. Rev. B 100, 014405 (2019)

Estimating the Effective Elasticity Properties of a Diamond/$β$-SiC Composite Thin Film by 3D Reconstruction and Numerical Homogenization

Authors: Bernhard Eidel, Ajinkya Gote, Marius Ruby, Lorenz Holzer, Lukas Keller, Xin Jiang

Abstract: The main aim of the present work is to estimate the effective elastic stiffnesses of a two-phase diamond/$β$-SiC composite thin film that is fabricated by chemical vapor deposition. The parameters of linear elasticity are determined by numerical homogenization. The database is sparse since for the 3D volume of interest only two micrographs displaying the phase distributions in perpendicular planes… ▽ More The main aim of the present work is to estimate the effective elastic stiffnesses of a two-phase diamond/$β$-SiC composite thin film that is fabricated by chemical vapor deposition. The parameters of linear elasticity are determined by numerical homogenization. The database is sparse since for the 3D volume of interest only two micrographs displaying the phase distributions in perpendicular planes are available; micrographs each of a cross-section and the surface of the thin film. A representative volume element (RVE) is reconstructed by an optimization software and by means of identified material symmetries in 2D of the specimen. The elastic homogenization results indicate that the two-phase diamond/$β$-SiC composite exhibits the behavior of transverse isotropy, for which the set of six independent material parameters is identified. △ Less

Submitted 18 December, 2018; originally announced December 2018.

MSC Class: 74E05; 74E10; 74E30; 74G15; 74K35; 74Q05;

Distortion mode anomalies in bulk PrNiO$_3$ illustrating the potential of symmetry-adapted distortion mode analysis for the study of phase transitions

Authors: D. J. Gawryluk, Y. M. Klein, T. Shang, D. Sheptyakov, L. Keller, N. Casati, J. Rodríguez-Carvajal, Ph. Lacorre, M. T. Fernández-Díaz, M. Medarde

Abstract: The origin of the metal-to-insulator transition in RNiO3 perovskites with trivalent 4f ion has challenged the condensed matter research community for almost three decades. A drawback for progress in this direction has been the lack of studies combining physical properties and accurate structural data covering the full nickelate phase diagram. Here we focus on a small region close to the itinerant… ▽ More The origin of the metal-to-insulator transition in RNiO3 perovskites with trivalent 4f ion has challenged the condensed matter research community for almost three decades. A drawback for progress in this direction has been the lack of studies combining physical properties and accurate structural data covering the full nickelate phase diagram. Here we focus on a small region close to the itinerant limit (R = Pr, 1.5K < T < 300K), where we investigate the gap opening and the simultaneous emergence of charge order in PrNiO3. We combine electric resistance, magnetization, and heat capacity measurements with high resolution neutron and synchrotron x-ray powder diffraction data that, in contrast to previous studies, we analyze in terms of symmetry-adapted distortion modes. Such analysis allows to identify the contribution of the different modes to the global distortion in a broad temperature range. Moreover, it shows that the structural changes at the MIT, traditionally described in terms of the evolution of the interatomic distances and angles, appear as abrupt increases of all nonzero mode amplitudes at TMIT = TN ~130K accompanied by the appearance of new modes below this temperature. A further interesting observation is the existence of a nearly perfect linear correlation between the amplitude of the breathing mode associated to the charge order and the staggered magnetization below the MIT. Our data also uncover a previously unnoticed anomaly at T* ~60K (~0.4 x TMIT), clearly visible in the electrical resistance, lattice parameters and some mode amplitudes. Since phase coexistence is only observed in a small temperature region around TMIT (~10K), these observations suggest the existence of a hidden symmetry in the insulating phase. We discuss some possible origins, among them the theoretically predicted existence of polar distortions induced by the non-centrosymmetric magnetic order. △ Less

Submitted 7 November, 2019; v1 submitted 28 September, 2018; originally announced September 2018.

Comments: 18 pages, 14 figures

Journal ref: Phys. Rev. B 100, 205137 (2019)

Pauling entropy, metastability and equilibrium in Dy$_2$Ti$_2$O$_7$ spin ice

Authors: S. R. Giblin, M. Twengström, L. Bovo, M. Ruminy, M. Bartkowiak, P. Manuel, J. C. Andresen, D. Prabhakaran, G. Balakrishnan, E. Pomjakushina, C. Paulsen, E. Lhotel, L. Keller, M. Frontzek, S. C. Capelli, O. Zaharko, P. A. McClarty, S. T. Bramwell, P. Henelius, T. Fennell

Abstract: Determining the fate of the Pauling entropy in the classical spin ice material Dy$_2$Ti$_2$O$_7$ with respect to the third law of thermodynamics has become an important test case for understanding the existence and stability of ice-rule states in general. The standard model of spin ice - the dipolar spin ice model - predicts an ordering transition at $T\approx 0.15$ K, but recent experiments by Po… ▽ More Determining the fate of the Pauling entropy in the classical spin ice material Dy$_2$Ti$_2$O$_7$ with respect to the third law of thermodynamics has become an important test case for understanding the existence and stability of ice-rule states in general. The standard model of spin ice - the dipolar spin ice model - predicts an ordering transition at $T\approx 0.15$ K, but recent experiments by Pomaranski $et\ al.$ suggest an entropy recovery over long time scales at temperatures as high as $0.5$ K, much too high to be compatible with theory. Using neutron scattering and specific heat measurements at low temperatures and with long time scales ($0.35$ K$/10^6$ s and $0.5$ K$/10^5$ s respectively) on several isotopically enriched samples we find no evidence of a reduction of ice-rule correlations or spin entropy. High-resolution simulations of the neutron structure factor show that the spin correlations remain well described by the dipolar spin ice model at all temperatures. Further, by careful consideration of hyperfine contributions, we conclude that the original entropy measurements of Ramirez $et\ al.$ are, after all, essentially correct: the short-time relaxation method used in that study gives a reasonably accurate estimate of the equilibrium spin ice entropy due to a cancellation of contributions. △ Less

Submitted 24 April, 2018; originally announced April 2018.

Comments: 9 pages, 7 figures

MSC Class: 82-08 ACM Class: I.6.1; I.6.5

Journal ref: Phys. Rev. Lett. 121, 067202 (2018)

Revisiting the Magnetic Structure and Charge Ordering in La$_{1/3}$Sr$_{2/3}$FeO$_3$ by Neutron Powder Diffraction and Mössbauer Spectroscopy

Authors: F. Li, V. Pomjakushin, T. Mazet, R. Sibille, B. Malaman, R. Yadav, L. Keller, M. Medarde, K. Conder, E. Pomjakushina

Abstract: The magnetic ordering of La$_{1/3}$Sr$_{2/3}$FeO$_3$ perovskite has been studied by neutron powder diffraction and $^{57}$Fe Mössbauer spectroscopy down to 2 K. From symmetry analysis, a chiral helical model and a collinear model are proposed to describe the magnetic structure. Both are commensurate, with propagation vector k = (0,0,1) in R-3c space group. In the former model, the magnetic moments… ▽ More The magnetic ordering of La$_{1/3}$Sr$_{2/3}$FeO$_3$ perovskite has been studied by neutron powder diffraction and $^{57}$Fe Mössbauer spectroscopy down to 2 K. From symmetry analysis, a chiral helical model and a collinear model are proposed to describe the magnetic structure. Both are commensurate, with propagation vector k = (0,0,1) in R-3c space group. In the former model, the magnetic moments of Fe adopt the magnetic space group P3$_{2}$21 and have helical and antiferromagnetic ordering propagating along the c axis. The model allows only one Fe site, with a magnetic moment of 3.46(2) $μ_{\rm{B}}$ at 2 K. In the latter model, the magnetic moments of iron ions adopt the magnetic space group C2/c or C2'/c' and are aligned collinearly. The model allows the presence of two inequivalent Fe sites with magnetic moments of amplitude 3.26(3) $μ_{\rm{B}}$ and 3.67(2) $μ_{\rm{B}}$, respectively. The neutron diffraction pattern is equally well fitted by either model. The Mössbauer spectroscopy study suggests a single charge state Fe$^{3.66+}$ above the magnetic transition and a charge disproportionation into Fe$^{(3.66-ζ)+}$ and Fe$^{(3.66+2ζ)+}$ below the magnetic transition. The compatibility of the magnetic structure models with the Mössbauer spectroscopy results is discussed. △ Less

Submitted 17 May, 2018; v1 submitted 23 February, 2018; originally announced February 2018.

Journal ref: Phys. Rev. B 97, 174417 (2018)

Direct-write of free-form 3D nanostructures with controlled magnetic frustration

Authors: Lukas Keller, Mohanad K. I. Al Mamoori, Jonathan Pieper, Christian Gspan, Irina Stockem, Christian Schröder, Sven Barth, Robert Winkler, Harald Plank, Merlin Pohlit, Jens Müller, Michael Huth

Abstract: Building nanotechnological analogues of naturally occurring magnetic structures has proven to be a powerful approach to studying topics like geometry-induced magnetic frustration and to provide model systems for statistical physics. Moreover, it practically allows to engineer novel physical properties by realizing artificial lattice geometries that are not accessible via natural crystallization or… ▽ More Building nanotechnological analogues of naturally occurring magnetic structures has proven to be a powerful approach to studying topics like geometry-induced magnetic frustration and to provide model systems for statistical physics. Moreover, it practically allows to engineer novel physical properties by realizing artificial lattice geometries that are not accessible via natural crystallization or chemical synthesis. This has been accomplished with great success in the field of two-dimensional artificial spin ice systems with important branches reaching into the field of magnetic logic devices. Although first proposals have been made to advance into three dimensions (3D), established nanofabrication pathways based on electron beam lithography have not been adapted to obtain free-form 3D nanostructures. Here we demonstrate the direct-write fabrication of freestanding ferromagnetic 3D nano-architectures with full control over the degree of magnetic frustration. By employing micro-Hall sensing, we have determined the magnetic stray field generated by our free-form structures in an externally applied magnetic field and we have performed micromagnetic and macro-spin simulations to deduce the spatial magnetization profiles in the structures and analyze their switching behavior. Furthermore we show that the magnetic 3D elements can be combined with other 3D elements of different chemical composition and intrinsic material properties. △ Less

Submitted 18 September, 2017; originally announced September 2017.

Dipolar spin ice states with fast monopole hopping rate in CdEr$_2$X$_4$ (X = Se, S)

Authors: Shang Gao, Oksana Zaharko, Vladimir Tsurkan, Lilian Prodan, Edward Riordan, Jorge Lago, Björn Fåk, Andrew Wildes, Marek M. Koza, Clemens Ritter, Peter Fouquet, Lukas Keller, Emmanuel Canévet, Marisa Medarde, Jakob Blomgren, Christer Johansson, Sean R. Giblin, Stanislav Vrtnik, Jože Luzar, Alois Loidl, Christian Rüegg, Tom Fennell

Abstract: Excitations in a spin ice behave as magnetic monopoles, and their population and mobility control the dynamics of a spin ice at low temperature. CdEr$_2$Se$_4$ is reported to have the Pauling entropy characteristic of a spin ice, but its dynamics are three-orders of magnitude faster than the canonical spin ice Dy$_2$Ti$_2$O$_7$. In this letter we use diffuse neutron scattering to show that both Cd… ▽ More Excitations in a spin ice behave as magnetic monopoles, and their population and mobility control the dynamics of a spin ice at low temperature. CdEr$_2$Se$_4$ is reported to have the Pauling entropy characteristic of a spin ice, but its dynamics are three-orders of magnitude faster than the canonical spin ice Dy$_2$Ti$_2$O$_7$. In this letter we use diffuse neutron scattering to show that both CdEr$_2$Se$_4$ and CdEr$_2$S$_4$ support a dipolar spin ice state -- the host phase for a Coulomb gas of emergent magnetic monopoles. These Coulomb gases have similar parameters to that in Dy$_2$Ti$_2$O$_7$, i.e. dilute and uncorrelated, so cannot provide three-orders faster dynamics through a larger monopole population alone. We investigate the monopole dynamics using ac susceptometry and neutron spin echo spectroscopy, and verify the crystal electric field Hamiltonian of the Er$^{3+}$ ions using inelastic neutron scattering. A quantitative calculation of the monopole hopping rate using our Coulomb gas and crystal electric field parameters shows that the fast dynamics in CdEr$_2$X$_4$ (X = Se, S) are primarily due to much faster monopole hopping. Our work suggests that CdEr$_2$X$_4$ offer the possibility to study alternative spin ice ground states and dynamics, with equilibration possible at much lower temperatures than the rare earth pyrochlore examples. △ Less

Submitted 20 March, 2018; v1 submitted 30 May, 2017; originally announced May 2017.

Comments: 11 pages, 9 figures, accepted for publication in Physical Review Letters

Journal ref: Phys. Rev. Lett. 120, 137201 (2018)

Non-collinear antiferromagnetism of coupled spins and pseudospins in the double perovskite La2CuIrO6

Authors: Kaustuv Manna, R. Sarkar, S. Fuchs, Y. A. Onykiienko, A. K. Bera, G. Aslan Cansever, S. Kamusella, A. Maljuk, C. G. F. Blum, L. T. Corredor, A. U. B. Wolter, S. M. Yusuf, M. Frontzek, L. Keller, M. Iakovleva, E. Vavilova, H. -J. Grafe, V. Kataev, H. -H. Klauss, D. S. Inosov, S. Wurmehl, B. Büchner

Abstract: We report the structural, magnetic and thermodynamic properties of the double perovskite compound La2CuIrO6 from X-ray, neutron diffraction, neutron depolarization, dc magnetization, ac susceptibility, specific heat, muon-spin-relaxation (uSR), electron-spin-resonance (ESR) and nuclear magnetic resonance (NMR) measurements. Below ~113 K, short-range spin-spin correlations occur within the Cu2+ sub… ▽ More We report the structural, magnetic and thermodynamic properties of the double perovskite compound La2CuIrO6 from X-ray, neutron diffraction, neutron depolarization, dc magnetization, ac susceptibility, specific heat, muon-spin-relaxation (uSR), electron-spin-resonance (ESR) and nuclear magnetic resonance (NMR) measurements. Below ~113 K, short-range spin-spin correlations occur within the Cu2+ sublattice. With decreasing temperature, the Ir4+ sublattice progressively involves in the correlation process. Below T = 74 K, the magnetic sublattices of Cu (spin s = 1/2) and Ir (pseudospin j = 1/2) in La2CuIrO6 are strongly coupled and exhibit an antiferromagnetic phase transition into a non-collinear magnetic structure accompanied by a small uncompensated transverse moment. A weak anomaly in ac-susceptibility as well as in the NMR and μSR spin lattice relaxation rates at 54 K is interpreted as a cooperative ordering of the transverse moments which is influenced by the strong spin-orbit coupled 5d ion Ir4+. We argue that the rich magnetic behaviour observed in La2CuIrO6 is related to complex magnetic interactions between the strongly correlated spin-only 3d ions with the strongly spin-orbit coupled 5d transition ions where a combination of the spin-orbit coupling and the low-symmetry of the crystal lattice plays a special role for the spin structure in the magnetically ordered state. △ Less

Submitted 6 October, 2016; v1 submitted 26 August, 2016; originally announced August 2016.

Comments: 15 Pages, 18 figures, To appear in PRB

Journal ref: Phys. Rev. B 94 (2016) 144437

Long-range and short-range magnetic correlations, and microscopic origin of net magnetization in the spin-1 trimer chain compound CaNi3P4O14

Authors: A. K. Bera, S. M. Yusuf, Amit Kumar, M. Majumder, K. Ghoshray, L. Keller

Abstract: Spin-spin correlations and microscopic origin of net magnetization in the spin-1 trimer chain compound CaNi3P4O14 have been investigated by powder neutron diffraction. The present study reveals a 3D long-range magnetic ordering below 16 K where the magnetic structure consists of ferromagnetic trimers that are coupled ferromagnetically along the spin-chain. The moment components along the a and c a… ▽ More Spin-spin correlations and microscopic origin of net magnetization in the spin-1 trimer chain compound CaNi3P4O14 have been investigated by powder neutron diffraction. The present study reveals a 3D long-range magnetic ordering below 16 K where the magnetic structure consists of ferromagnetic trimers that are coupled ferromagnetically along the spin-chain. The moment components along the a and c axes arrange antiferromagnetically. Our study establishes that the uncompensated moment components along the b axis result in a net magnetization per unit cell. The magnetic structure, determined in the present study, is in agreement with the results of recent first principles calculation; however, it is in contrast to a fascinating experimental prediction of ferrimagnetic ordering based on the periodicity of the exchange interactions in CaNi3P4O14. Our study also confirms the presence of broad diffuse magnetic scattering, due to 1D short-range spin-spin correlations, over a wide temperature range below ~50 K down to a temperature well below the Tc. Total neutron scattering analysis by the RMC method reveals that the dominating spin-spin correlation above Tc is ferromagnetic and along the b axis. The nearest neighbour spin-spin correlations along the a and c axes are found to be weakly antiferromagnetic. The nature of the trimer spin structure of the short-range state is similar to that of the 3D long-range ordered state. The present investigation of microscopic nature of the magnetic ground state also explains the condition required for the 1/3 magnetization plateau to be observed in the trimer spin-chains. In spite of the S=1 trimer chain system, the present compound CaNi3P4O14 is found to be a good realization of 3D magnet below the Tc=16 K with full ordered moment values of ~2 mu_B/Ni2+ (1.98 and 1.96 mu_B/Ni2+ for two Ni sites, respectively) at 1.5 K. △ Less

Submitted 26 May, 2016; originally announced May 2016.

Comments: 10 pages, 8 figures

Journal ref: Phys. Rev. B 93, 184409 (2016)

Commensurate and incommensurate magnetic order in spin-1 chains stacked on the triangular lattice in Li$_2$NiW$_2$O$_8$

Authors: K. M. Ranjith, R. Nath, M. Majumder, D. Kasinathan, M. Skoulatos, L. Keller, Y. Skourski, M. Baenitz, A. A. Tsirlin

Abstract: We report thermodynamic properties, magnetic ground state, and microscopic magnetic model of the spin-1 frustrated antiferromaget Li$_{2}$NiW$_{2}$O$_{8}$ showing successive transitions at $T_{\rm N1}\simeq 18$ K and $T_{\rm N2}\simeq 12.5$ K in zero field. Nuclear magnetic resonance and neutron diffraction reveal collinear and commensurate magnetic order with the propagation vector… ▽ More We report thermodynamic properties, magnetic ground state, and microscopic magnetic model of the spin-1 frustrated antiferromaget Li$_{2}$NiW$_{2}$O$_{8}$ showing successive transitions at $T_{\rm N1}\simeq 18$ K and $T_{\rm N2}\simeq 12.5$ K in zero field. Nuclear magnetic resonance and neutron diffraction reveal collinear and commensurate magnetic order with the propagation vector $\mathbf k=(\frac12,0,\frac12)$ below $T_{\rm N2}$. The ordered moment of 1.8 $μ_B$ at 1.5 K is directed along $[0.89(9),-0.10(5),-0.49(6)]$ and matches the magnetic easy axis of spin-1 Ni$^{2+}$ ions, which is determined by the scissor-like distortion of the NiO$_6$ octahedra. Incommensurate magnetic order, presumably of spin-density-wave type, is observed in the region between $T_{\rm N2}$ and $T_{\rm N1}$. Density-functional band-structure calculations put forward a three-dimensional spin lattice with spin-1 chains running along the $[01\bar 1]$ direction and stacked on a spatially anisotropic triangular lattice in the $ab$ plane. We show that the collinear magnetic order in Li$_2$NiW$_2$O$_8$ is incompatible with the triangular lattice geometry and thus driven by a pronounced easy-axis single-ion anisotropy of Ni$^{2+}$. △ Less

Submitted 13 July, 2016; v1 submitted 6 March, 2016; originally announced March 2016.

Comments: Published version: 12+ pages, 11 figures, 3 tables

Journal ref: Phys. Rev. B 94, 014415 (2016)

A novel type of splayed ferromagnetic order observed in Yb2Ti2O7

Authors: A. Yaouanc, P. Dalmas de Reotier, L. Keller, B. Roessli, A. Forget

Abstract: The pyrochlore insulator Yb2Ti2O7 has attracted the attention of experimentalists and theoreticians alike for about 15 years. Conflicting neutron diffraction data on the possible existence of magnetic Bragg reflections at low temperature have been published. Here we report the observation of magnetic Bragg reflections by neutron powder diffraction at 60 mK. The magnetic diffraction pattern is anal… ▽ More The pyrochlore insulator Yb2Ti2O7 has attracted the attention of experimentalists and theoreticians alike for about 15 years. Conflicting neutron diffraction data on the possible existence of magnetic Bragg reflections at low temperature have been published. Here we report the observation of magnetic Bragg reflections by neutron powder diffraction at 60 mK. The magnetic diffraction pattern is analyzed using representation theory. We find Yb2Ti2O7 to be a splayed ferromagnet as reported for Yb2Sn2O7, a sibling compound with also dominating ferromagnetic interactions as inferred from the positive Curie-Weiss temperature. However, the configuration of the magnetic moment components perpendicular to the easy axis is of the all-in--all-out type in Yb2Ti2O7 while it is two-in--two-out in Yb2Sn2O7. An overall experimental picture of the magnetic properties emerges. △ Less

Submitted 24 August, 2016; v1 submitted 5 February, 2016; originally announced February 2016.

Comments: Revised version accepted for publication. To appear shortly in J. Phys.: Condens. Matter

Journal ref: J. Phys.: Condens. Matter 28, 426002 (2016)

Distinct Magnetic Phases in Structurally Uniform SrCoO$_{3-y}$

Authors: Z. H. Zhu, F. J. Rueckert, J. I. Budnick, W. A. Hines, Ch. Niedermayer, L. Keller, H. Luetkens, B. Dabrowski, S. Kolesnik, B. O. Wells

Abstract: Two magnetic phase transitions have been noted for SrCoO$_{3-y}$ for near-stoichiometric oxygen concentrations (small y). Using muon spin rotation and neutron scattering experiments, we have established that the two transitions represent separate, spatially distinct magnetic phases that coexist in a two-phase equilibrium mixture. The two phases most likely represent areas of the sample with differ… ▽ More Two magnetic phase transitions have been noted for SrCoO$_{3-y}$ for near-stoichiometric oxygen concentrations (small y). Using muon spin rotation and neutron scattering experiments, we have established that the two transitions represent separate, spatially distinct magnetic phases that coexist in a two-phase equilibrium mixture. The two phases most likely represent areas of the sample with different effective valence charge density. Further, the phases exist over regions with a length scale intermediate between nanoscale charge inhomogeneity and systems such as manganites or super-oxygenated cuprates with large length scale phase separation. △ Less

Submitted 15 April, 2016; v1 submitted 26 October, 2015; originally announced October 2015.

Comments: 5 pages, 4 figures

Journal ref: Phys. Rev. B 93, 224412 (2016)

Magnetic structure of the spin-1/2 frustrated quasi-one-dimensional antiferromagnet Cu3Mo2O9: Appearance of a partial disordered state

Authors: Masashi Hase, Haruhiko Kuroe, Vladimir Yu. Pomjakushin, Lukas Keller, Ryo Tamura, Noriki Terada, Yoshitaka Matsushita, Andreas Doenni, Tomoyuki Sekine

Abstract: We investigated the crystal and magnetic structures of the spin-1/2 frustrated antiferromagnet Cu3Mo2O9 in which the spin system consists of antiferromagnetic chains and dimers. The space group at room temperature has been reported to be orthorhombic Pnma (No. 62). We infer that the space group above TN = 7.9 K is monoclinic P2_1/m (No. 11) from the observation of reflections forbidden in Pnma in… ▽ More We investigated the crystal and magnetic structures of the spin-1/2 frustrated antiferromagnet Cu3Mo2O9 in which the spin system consists of antiferromagnetic chains and dimers. The space group at room temperature has been reported to be orthorhombic Pnma (No. 62). We infer that the space group above TN = 7.9 K is monoclinic P2_1/m (No. 11) from the observation of reflections forbidden in Pnma in x-ray powder diffraction experiments at room temperature. We determined the magnetic structure of Cu3Mo2O9 in neutron powder diffraction experiments. Magnetic moments on dimer sites lie in the ac planes. The magnitudes are 0.50 - 0.74 mu_B. Moments on chain sites may exist but the magnitudes are very small. The magnetic structure indicates that a partial disordered state is realized. We consider the origin of the magnetic structure, weak ferromagnetism, and electric polarization. △ Less

Submitted 29 June, 2015; originally announced June 2015.

Comments: 8 pages, 5 figures, 2 tables

Crystal structure and phonon softening in Ca3Ir4Sn13

Authors: D. G. Mazzone, S. Gerber, J. L. Gavilano, R. Sibille, M. Medarde, B. Delley, M. Ramakrishnan, M. Neugebauer, L. P. Regnault, D. Chernyshov, A. Piovano, T. M. Fernandez-Diaz, L. Keller, A. Cervellino, E. Pomjakushina, K. Conder, M. Kenzelmann

Abstract: We investigated the crystal structure and lattice excitations of the ternary intermetallic stannide Ca3Ir4Sn13 using neutron and x-ray scattering techniques. For T > T* ~ 38 K the x-ray diffraction data can be satisfactorily refined using the space group Pm-3n. Below T* the crystal structure is modulated with a propagation vector of q = (1/2, 1/2, 0). This may arise from a merohedral twinning in w… ▽ More We investigated the crystal structure and lattice excitations of the ternary intermetallic stannide Ca3Ir4Sn13 using neutron and x-ray scattering techniques. For T > T* ~ 38 K the x-ray diffraction data can be satisfactorily refined using the space group Pm-3n. Below T* the crystal structure is modulated with a propagation vector of q = (1/2, 1/2, 0). This may arise from a merohedral twinning in which three tetragonal domains overlap to mimic a higher symmetry, or from a doubling of the cubic unit cell. Neutron diffraction and neutron spectroscopy results show that the structural transition at T* is of a second-order, and that it is well described by mean-field theory. Inelastic neutron scattering data point towards a displacive structural transition at T* arising from the softening of a low-energy phonon mode with an energy gap of Delta(120 K) = 1.05 meV. Using density functional theory the soft phonon mode is identified as a 'breathing' mode of the Sn12 icosahedra and is consistent with the thermal ellipsoids of the Sn2 atoms found by single crystal diffraction data. △ Less

Submitted 1 July, 2015; v1 submitted 27 March, 2015; originally announced March 2015.

Journal ref: Phys. Rev. B 92, 024101 (2015)

Incommensurate magnetic structure, Fe/Cu chemical disorder and magnetic interactions in the high-temperature multiferroic YBaCuFeO5

Authors: M. Morin, A. Scaramucci, M. Bartkowiak, E. Pomjakushina, G. Deng, D. Sheptyakov, L. Keller, J. Rodriguez-Carvajal, N. A. Spaldin, M. Kenzelmann, K. Conder, M. Medarde

Abstract: Motivated by the recent observations of incommensurate magnetic order and electric polarization in YBaCuFeO5 up to temperatures TN2 as high as 230K [1,2] we report here for the first time a model for the incommensurate magnetic structure of this material that we complement with ab-initio calculations of the magnetic exchange parameters. Using neutron powder diffraction we show that the appearance… ▽ More Motivated by the recent observations of incommensurate magnetic order and electric polarization in YBaCuFeO5 up to temperatures TN2 as high as 230K [1,2] we report here for the first time a model for the incommensurate magnetic structure of this material that we complement with ab-initio calculations of the magnetic exchange parameters. Using neutron powder diffraction we show that the appearance of polarization below TN2 is accompanied by the replacement of the high temperature collinear magnetic order by a circular inclined spiral with propagation vector ki=(1/2, 1/2, 1/2+-q). Moreover, we find that the polarization approximately scales with the modulus of the magnetic modulation vector q down to the lowest temperature investigated (T=3K). Further, we observe occupational Fe/Cu disorder in the FeO5-CuO5 bipyramids, although a preferential occupation of such units by Fe-Cu pairs is supported by the observed magnetic order and by density functional calculations. We calculate exchange coupling constants for different Fe/Cu distributions and show that, for those containing Fe-Cu dimers, the resulting magnetic order is compatible with the experimentally observed collinear magnetic structure (kc=(1/2, 1/2, 1/2), TN2 > T > TN1 = 440K). Based on these results we discuss possible origins for the incommensurate modulation and its coupling with ferroelectricity. △ Less

Submitted 8 January, 2015; originally announced January 2015.

Comments: 16 pages, 11 Figures, Accepted for publication in Physical Review B (8 Jan 2015)

Journal ref: Phys. Rev. B 91, 064408 (2015)

Understanding the multiple magnetic structures of the intermetallic compound NdMn1.4Co0.6Si2

Authors: Madhumita Halder, A. K. Bera, Amit Kumar, L. Keller, S. M. Yusuf

Abstract: Magnetic phases for the intermetallic compound NdMn1.4Co0.6Si2 have been investigated at various temperatures by dc magnetization, neutron diffraction and neutron depolarization. Our study shows multiple magnetic phase transitions with temperature (T) over 1.5-300 K. In agreement with dc-magnetization and neutron depolarization results, the temperature dependence of the neutron diffraction pattern… ▽ More Magnetic phases for the intermetallic compound NdMn1.4Co0.6Si2 have been investigated at various temperatures by dc magnetization, neutron diffraction and neutron depolarization. Our study shows multiple magnetic phase transitions with temperature (T) over 1.5-300 K. In agreement with dc-magnetization and neutron depolarization results, the temperature dependence of the neutron diffraction patterns shows five distinct regions with different magnetic phases. These temperature regions are (i) T >= 215 K, (ii) 215 K > T >= 50 K, (iii) 50 K > T >= 40 K, (iv) 40 K > T > 15 K, and (v) T =< 15 K. The corresponding magnetic structures are paramagnetic, commensurate collinear antiferromagnetic (AFM-I), incommensurate AFM (AFM-II), mixed ferromagnetic and AFM (FM+AFM-II), and incommensurate AFM (AFM-II), respectively. △ Less

Submitted 22 December, 2014; originally announced December 2014.

Comments: 26 pages, 10 figures

Journal ref: Journal of Alloys and Compounds 592, 86 (2014)