Showing 1–50 of 50 results for author: Cubitt, R

Cascade of Spin Moiré Superlattices with In-Plane Field in Triangle Lattice Semimetal EuAg$_4$Sb$_2$

Authors: Paul M. Neves, Takashi Kurumaji, Joshua P. Wakefield, Chi Ian Jess Ip, Robert Cubitt, Satoru Hayami, Jonathan S. White, Joseph G. Checkelsky

Abstract: EuAg$_4$Sb$_2$ is a rhombohedral europium triangle lattice material that exhibits a rich phase diagram of spin moiré superlattices (SMS) and single-$q$ magnetic phases. In this paper, we characterize the incommensurate phases accessible with field applied in the plane with small angle neutron scattering (SANS). A variety of phases with unusual SANS patterns are accessible with magnetic field appli… ▽ More EuAg$_4$Sb$_2$ is a rhombohedral europium triangle lattice material that exhibits a rich phase diagram of spin moiré superlattices (SMS) and single-$q$ magnetic phases. In this paper, we characterize the incommensurate phases accessible with field applied in the plane with small angle neutron scattering (SANS). A variety of phases with unusual SANS patterns are accessible with magnetic field applied along the $a$ and $a^*$ directions. Many of these phases can be understood to be multi-$q$ phases. One phase in particular, ICM2b (ICM=incommensurate magnetic phase), is rather unconventional in that it is an anisotropic multi-$q$ phase that can rotate freely within the $ab$-plane, dependent on magnetic field direction and history. The stabilization of tunable multi-$q$ incommensurate spin textures \textit{via} in-plane field sets this class of materials apart from conventional skyrmion materials. We further identify that the propagation vectors of the in-plane phases have a significant commensuration with the diameter of the smallest pocket of the Fermi surface ($2k_{\text{F}}$). The multi/single-$q$ nature is also correlated with the enhancement of resistivity, suggesting that a gap opens in the electron bands at $q=2k_{\text{F}}$. We also compare with a phenomenological model of the phase diagram. The richness of phases revealed in this study hint at the frustrated nature of the incommensurate magnetism present in EuAg$_4$Sb$_2$ and motivate further probes of these phases and the origin of the stability of spin moiré superlattices. Finally, the coupling of the multi-$q$ nature and $q=2k_{\text{F}}$ commensuration condition reveals the key requirements for a strong SMS transport response. △ Less

Submitted 18 December, 2025; originally announced December 2025.

Comments: 11 pages, 5 figures, 1 table

Single-$q$ Cycloid and Double-$q$ Vortex Lattices in Layered Magnetic Semimetal EuAg$_4$Sb$_2$

Authors: Paul M. Neves, Takashi Kurumaji, Joshua P. Wakefield, Arno Hiess, Paul Steffens, Navid Qureshi, Robert Cubitt, Lisa M DeBeer-Schmitt, Johanna C. Palmstrom, Satoru Hayami, Marek Bartkowiak, Markus Zolliker, Jonathan S. White, Joseph G. Checkelsky

Abstract: Recently, a host of exotic magnetic textures such as topologically protected skyrmion lattices has been discovered in several bulk metallic lanthanide compounds. In addition to hosting skyrmion phases, a hallmark of this class of materials is the appearance of numerous spin textures characterized by a superposition of multi-$q$ magnetic modulations: spin moiré superlattices. The nuanced energy lan… ▽ More Recently, a host of exotic magnetic textures such as topologically protected skyrmion lattices has been discovered in several bulk metallic lanthanide compounds. In addition to hosting skyrmion phases, a hallmark of this class of materials is the appearance of numerous spin textures characterized by a superposition of multi-$q$ magnetic modulations: spin moiré superlattices. The nuanced energy landscape thus motivates detailed studies to understand the underlying interactions. Here, we comprehensively characterize and model the three zero-field magnetic textures present in one such material, EuAg$_4$Sb$_2$. Systematic symmetry breaking experiments using magnetic field and strain determine that the ground state incommensurate magnetic phase (ICM1) is single-$q$. In contrast, ICM2 and ICM3 are both double-$q$, \textit{i.e.}, spin moiré superlattices. Further, through application of polarized small angle neutron scattering and spherical neutron polarimetry, we demonstrate that ICM1 is a single-$q$ cycloid and ICM2 and ICM3 are double-$q$ vortex lattices, with Eu moments lying in the $ab$-plane in zero field and with a ferromagnetic component at finite field. Despite the quasi-2D nature of EuAg$_4$Sb$_2$, the modulations propagate out of the \textit{ab}-plane, leading to a shift of the spin texture between triangle lattice planes. Further, the ICM3 to ICM2 transition includes an unusual 45$^\circ$ rotation of the magnetic vortex lattice. Motivated by the coexistence of such drastically different phases in this compound, we conclude by developing a phenomenological model to understand the stability of these states. Our experimental probes and theoretical modeling definitively characterize three different and tunable phases in one material, and provide insight for the design of new topological spin-texture materials. △ Less

Submitted 18 December, 2025; originally announced December 2025.

Comments: 13 pages, 6 figures

Journal ref: Phys. Rev. X 16, 021054 (2026)

Single-ion anisotropy-stabilized short-period helimagnetism in frustrated chiral Co$_5$TeO$_8$

Authors: Priya R. Baral, Ravi Yadav, Victor Ukleev, Thomas LaGrange, Ivica Živković, Wen Hua Bi, Marek Bartkowiak, Robert Cubitt, Nina-Juliane Steinke, Vladimir Pomjakushin, Yurii Skourski, Henrik M. Rønnow, Oleg V. Yazyev, Arnaud Magrez, Jonathan S. White

Abstract: Chiral spin textures in magnetic insulators promise magneto-electric (ME) spintronics with orders-of-magnitude lower power consumption than metallic systems. However, realizing the short magnetic periods required for high-density device integration remains difficult, as conventional Dzyaloshinskii-Moriya interaction (DMI)-based mechanisms typically constrain spiral periods to tens of nanometers. W… ▽ More Chiral spin textures in magnetic insulators promise magneto-electric (ME) spintronics with orders-of-magnitude lower power consumption than metallic systems. However, realizing the short magnetic periods required for high-density device integration remains difficult, as conventional Dzyaloshinskii-Moriya interaction (DMI)-based mechanisms typically constrain spiral periods to tens of nanometers. While theory predicts that strong single-ion anisotropy (SIA) on frustrated lattices can stabilize complex non-coplanar textures, the potential for using this mechanism to engineer such compact textures remains largely unexplored. Here we report that a cubic chiral insulator Co$_5$TeO$_8$ provides an experimental example of this paradigm. Comprehensive neutron scattering and magnetometry reveal helimagnetic spirals with continuously tunable pitch of 5.7-10 nm embedded in a complex phase diagram spanning eight distinct phases. Capacitance anomalies throughout the phase diagram indicate magneto-electric coupling, pointing to the possibility of future $E$-field control of these textures. The temperature- and field-dependence of the helical wavevector strongly support a scenario in which site-dependent SIA provides the leading contribution to the selection of the helical period from a frustration-induced degenerate manifold. Consistent with this interpretation, $ab\,initio$ calculations place SIA approximately an order of magnitude above DMI, distinct from conventional helimagnets. Co$_5$TeO$_8$ thus offers an experimental realization of sub-10 nm helimagnetism and motivates a design principle for anisotropy-engineered correlated insulators. △ Less

Submitted 4 June, 2026; v1 submitted 17 December, 2025; originally announced December 2025.

Comments: Final version, significant changes in text and figures compared to v1

A Surface-confined Spiral State With the Double Period in the Cubic Chiral Helimagnet Cu$_2$OSeO$_3$

Authors: Priya R. Baral, Oleg I. Utesov, Samuel H. Moody, Matthew T. Littlehales, Pierluigi Gargiani, Manuel Valvidares, Robert Cubitt, Nina-Juliane Steinke, Chen Luo, Florin Radu, Arnaud Magrez, Jonathan S. White, Victor Ukleev

Abstract: The chiral magnetoelectric insulator Cu$_2$OSeO$_3$ hosts a rich and anisotropic magnetic phase diagram that includes helical, conical, field-polarised, tilted conical, and skyrmion lattice phases. Using resonant elastic x-ray scattering (REXS), we uncover a new spiral state confined to the surface of Cu$_2$OSeO$_3$. This surface-confined spiral state (SSS) displays a real-space pitch of $\sim$120… ▽ More The chiral magnetoelectric insulator Cu$_2$OSeO$_3$ hosts a rich and anisotropic magnetic phase diagram that includes helical, conical, field-polarised, tilted conical, and skyrmion lattice phases. Using resonant elastic x-ray scattering (REXS), we uncover a new spiral state confined to the surface of Cu$_2$OSeO$_3$. This surface-confined spiral state (SSS) displays a real-space pitch of $\sim$120 nm, which remarkably is twice the length of the incommensurate structures observed to-date in Cu$_2$OSeO$_3$. The SSS phase emerges at temperatures below 30~K when the magnetic field is applied between $3^\circ$ to $18^\circ$ away from the $\langle\text{110}\rangle$ crystallographic axes. Its surface localisation is demonstrated through a combination of REXS in reflection and transmission geometries, with complementary small-angle neutron scattering measurements suggesting its absence from the bulk. We attribute the stabilisation of the SSS to competing anisotropic interactions at the crystal surface. The discovery of a robust, surface-confined spiral paves the way for engineering energy-efficient, nanoscale spin-texture platforms for next-generation devices. △ Less

Submitted 30 April, 2026; v1 submitted 13 July, 2025; originally announced July 2025.

Comments: 9 pages, 4 figures

Helical spin dynamics in Cu$_2$OSeO$_3$ as measured with small-angle neutron scattering

Authors: Victor Ukleev, Priya R. Baral, Robert Cubitt, Nina-Juliane Steinke, Arnaud Magrez, Oleg I. Utesov

Abstract: The insulating chiral magnet Cu$_2$OSeO$_3$ exhibits a rich array of low-temperature magnetic phenomena, making it a prime candidate for the study of its spin dynamics. Using spin wave small-angle neutron scattering (SWSANS), we systematically investigated the temperature-dependent behavior of the helimagnon excitations in the field-polarized phase of Cu$_2$OSeO$_3$. Our measurements, spanning 5-5… ▽ More The insulating chiral magnet Cu$_2$OSeO$_3$ exhibits a rich array of low-temperature magnetic phenomena, making it a prime candidate for the study of its spin dynamics. Using spin wave small-angle neutron scattering (SWSANS), we systematically investigated the temperature-dependent behavior of the helimagnon excitations in the field-polarized phase of Cu$_2$OSeO$_3$. Our measurements, spanning 5-55 K, reveal the temperature evolution of spin-wave stiffness and damping constant with unprecedented resolution, facilitated by the insulating nature of Cu$_2$OSeO$_3$. These findings align with theoretical predictions and resolve discrepancies observed in previous studies, emphasizing the enhanced sensitivity of the SWSANS method. The results provide deeper insights into the fundamental magnetic properties of Cu$_2$OSeO$_3$, contributing to a broader understanding of chiral magnets. △ Less

Submitted 1 July, 2025; originally announced July 2025.

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

Skyrmion Lattice Manipulation with Electric Currents and Thermal Gradients in MnSi

Authors: N. Chalus, A. W. D. Leishman, R. M. Menezes, G. Longbons, U. Welp, W. -K. Kwok, J. S. White, M. Bartkowiak, R. Cubitt, Y. Liu, E. D. Bauer, M. Janoschek, M. V. Milosevic, M. R. Eskildsen

Abstract: The skyrmion lattice (SkL) in MnSi was studied using small-angle neutron scattering and under the influence of a radial electric current in a Corbino geometry. In response to the applied current, the SkL undergoes an angular reorientation with respect to the MnSi crystal lattice. The reorientation is non-monotonic with increasing current, with the SkL rotating first in one direction and then the o… ▽ More The skyrmion lattice (SkL) in MnSi was studied using small-angle neutron scattering and under the influence of a radial electric current in a Corbino geometry. In response to the applied current, the SkL undergoes an angular reorientation with respect to the MnSi crystal lattice. The reorientation is non-monotonic with increasing current, with the SkL rotating first in one direction and then the other. The SkL reorientation was studied at different sample locations and found to depend on the local current density as inferred from a finite element analysis. The non-monotonic response indicates the presence of two competing effects on the SkL, most likely due to the presence of both radial electric and thermal currents. Such a scenario is supported by micromagnetic simulations, which show how these effects can act constructively or destructively to drive the SkL rotation, depending on the direction of the electric current. In addition, the simulations also suggest how the direction of the skyrmion flow may affect the SkL orientation. △ Less

Submitted 20 January, 2025; v1 submitted 9 December, 2024; originally announced December 2024.

Comments: Minor revisions

Journal ref: Phys. Rev. B 111, 064410 (2025)

Two characteristic contributions to the superconducting state of 2$H$-NbSe$_2$

Authors: A. Alshemi, E. M. Forgan, A. Hiess, R. Cubitt, J. S. White, K. Schmalzl, E. Blackburn

Abstract: Multiband superconductivity arises when multiple electronic bands contribute to the formation of the superconducting state, allowing distinct pairing interactions and gap structures. Here, we present field- and temperature-dependent data on the vortex lattice structure in 2$H$-NbSe$_2$ as a contribution to the ongoing debate on the nature of the superconductivity in this material. The field-depend… ▽ More Multiband superconductivity arises when multiple electronic bands contribute to the formation of the superconducting state, allowing distinct pairing interactions and gap structures. Here, we present field- and temperature-dependent data on the vortex lattice structure in 2$H$-NbSe$_2$ as a contribution to the ongoing debate on the nature of the superconductivity in this material. The field-dependent data clearly show that there are two distinct superconducting bands, and the contribution of one of them to the vortex lattice signal is completely suppressed for magnetic fields above $\sim$ 0.8 T, well below $B\mathrm{_{c2}}$. By combining the temperature and field scans, we can deduce that there is a moderate degree of interband coupling. From the observed temperature dependences, we find that at low field and zero temperature, the two gaps in temperature units are 13.1 $\pm$ 0.2 and 6.5 $\pm$ 0.3 K ($Δ_{0}$ = 1.88 and 0.94 $k\mathrm{_{B}} T\mathrm{_{c}} $); the band with the larger gap gives just under two-thirds of the superfluid density. The penetration depth extrapolated to zero field and zero temperature is 160 $\pm$ 2 nm. △ Less

Submitted 6 May, 2025; v1 submitted 26 November, 2024; originally announced November 2024.

Comments: 7 Pages, 3 Figuers - Revised verion for final submission

Investigating the superconducting state of 2$H$-NbS$_2$ as seen by the vortex lattice

Authors: A. Alshemi, E. Campillo, E. M. Forgan, R. Cubitt, M. Abdel-Hafiez, E. Blackburn

Abstract: 2$H$-NbS$_2$ is a classic example of an anisotropic multi-band superconductor, with significant recent work focussing on the interesting responses seen when high magnetic fields are applied precisely parallel to the hexagonal niobium planes. It is often contrasted with its sister compound 2$H$-NbSe$_2$ because they have similar onset temperatures for superconductivity, but 2$H$-NbS$_2$ has no char… ▽ More 2$H$-NbS$_2$ is a classic example of an anisotropic multi-band superconductor, with significant recent work focussing on the interesting responses seen when high magnetic fields are applied precisely parallel to the hexagonal niobium planes. It is often contrasted with its sister compound 2$H$-NbSe$_2$ because they have similar onset temperatures for superconductivity, but 2$H$-NbS$_2$ has no charge density wave whereas in 2$H$-NbSe$_2$ the charge density wave order couples strongly to the superconductivity. Using small-angle neutron scattering, a bulk-sensitive probe, we have studied the vortex lattice and how it responds to the underlying superconducting anisotropy. This is done by controlling the orientation of the field with respect to the Nb planes. The superconducting anisotropy, $Γ_{ac} = 7.07 \pm 0.2$, is found to be field independent over the range measured (0.15 to 1.25 T), and the magnetic field distribution as a function of the applied magnetic field is found to be in excellent quantitative agreement with anisotropic London theory modified with a core-size cut-off correction, providing the first complete validation of this model. We find values of $λ_{ab} = 141.9 \pm 1.5 $ nm for the in-plane London penetration depth, and $λ_{c} \sim$ 1 \textmu m for the out-of-plane response. The field-independence indicates that we are primarily sampling the larger of the two gaps generating the superconductivity in this material. △ Less

Submitted 29 April, 2024; originally announced April 2024.

Comments: 13 pges, 8 figuers

Observation by SANS and PNR of pure Néel-type domain wall profiles and skyrmion suppression below room temperature in magnetic [Pt/CoFeB/Ru]$_{10}$ multilayers

Authors: Victor Ukleev, Fernando Ajejas, Anton Devishvili, Alexei Vorobiev, Nina-Juliane Steinke, Robert Cubitt, Chen Luo, Radu-Marius Abrudan, Florin Radu, Vincent Cros, Nicolas Reyren, Jonathan S. White

Abstract: We report investigations of the magnetic textures in periodic [Pt(1 nm)/(CoFeB(0.8 nm)/Ru(1.4 nm)]$_{10}$ multilayers using polarised neutron reflectometry (PNR) and small-angle neutron scattering (SANS). The multilayers are known to host skyrmions stabilized by Dzyaloshinskii-Moriya interactions induced by broken inversion symmetry and spin-orbit coupling at the asymmetric interfaces. From depth-… ▽ More We report investigations of the magnetic textures in periodic [Pt(1 nm)/(CoFeB(0.8 nm)/Ru(1.4 nm)]$_{10}$ multilayers using polarised neutron reflectometry (PNR) and small-angle neutron scattering (SANS). The multilayers are known to host skyrmions stabilized by Dzyaloshinskii-Moriya interactions induced by broken inversion symmetry and spin-orbit coupling at the asymmetric interfaces. From depth-dependent PNR measurements, we observe well-defined structural features, and obtain the layer-resolved magnetization profiles. The in-plane magnetization of the CoFeB layers calculated from fitting of the PNR profiles is found to be in excellent agreement with magnetometry data. Using SANS as a bulk probe of the entire multilayer, we observe long-period magnetic stripe domains and skyrmion ensembles with full orientational disorder at room temperature. No sign of skyrmions is found below 250\,K, which we suggest is due to an increase of a effective magnetic anisotropy in the CoFeB layer on cooling that suppresses skyrmion stability. Using polarised SANS at room temperature, we prove the existence of pure Néel-type windings in both stripe domain and skyrmion regimes. No Bloch-type winding admixture, i.e. an indication for hybrid windings, is detected within the measurement sensitivity, in good agreement with expectations according to our micromagnetic modelling of the multilayers. Our findings using neutron techniques offer valuable microscopic insights into the rich magnetic behavior of skyrmion-hosting multilayers, which are essential for the advancement of future skyrmion-based spintronic devices. △ Less

Submitted 29 January, 2024; v1 submitted 10 November, 2023; originally announced November 2023.

Journal ref: Science and Technology of Advanced Materials, 25:1, 2315015, 2024

Tilted Spirals and Low Temperature Skyrmions in Cu2OSeO3

Authors: M. Crisanti, A. O. Leonov, R. Cubitt, A. Labh, H. Wilhelm, Marcus P. Schmidt, C. Pappas

Abstract: The bulk helimagnet Cu2OSeO3 represents a unique example in the family of B20 cubic helimagnets exhibiting a tilted spiral and skyrmion phase at low temperatures when the magnetic field is applied along the easy <001> crystallographic direction. Here we present a systematic study of the stability and ordering of these low temperature magnetic states. We focus our attention on the temperature and f… ▽ More The bulk helimagnet Cu2OSeO3 represents a unique example in the family of B20 cubic helimagnets exhibiting a tilted spiral and skyrmion phase at low temperatures when the magnetic field is applied along the easy <001> crystallographic direction. Here we present a systematic study of the stability and ordering of these low temperature magnetic states. We focus our attention on the temperature and field dependencies of the tilted spiral state that we observe persisting up to above T =35 K, i.e. up to higher temperatures than reported so far. We discuss these results in the frame of the phenomenological theory introduced by Dzyaloshinskii in an attempt to reach a quantitative description of the experimental findings. We find that the anisotropy constants, which are the drivers behind the observed behaviour, exhibit a pronounced temperature dependence. This explains the differences in the behaviour observed at high temperatures (above T = 18 K), where the cubic anisotropy is weak, and at low temperatures (below T = 18 K), where a strong cubic anisotropy induces an abrupt appearance of the tilted spirals out of the conical state and enhances the stability of skyrmions. △ Less

Submitted 18 July, 2023; originally announced July 2023.

Journal ref: Physical Review RESEARCH 5, 033033 (2023)

Adherence of the rotating vortex lattice in the noncentrosymmetric superconductor Ru$_{7}$B$_{3}$ to the London model

Authors: A. S. Cameron, Y. V. Tymoshenko, P. Y. Portnichenko, A. S. Sukhanov, M. Ciomaga Hatnean, D. McK. Paul, G. Balakrishnan, R. Cubitt, D. S. Inosov

Abstract: The noncentrosymmetric superconductor Ru$_7$B$_3$ has in previous studies demonstrated remarkably unusual behaviour in its vortex lattice, where the nearest neighbour directions of the vortices dissociate from the crystal lattice and instead show a complex field-history dependence, and the vortex lattice rotates as the field is changed. In this study, we look at the vortex lattice form factor of R… ▽ More The noncentrosymmetric superconductor Ru$_7$B$_3$ has in previous studies demonstrated remarkably unusual behaviour in its vortex lattice, where the nearest neighbour directions of the vortices dissociate from the crystal lattice and instead show a complex field-history dependence, and the vortex lattice rotates as the field is changed. In this study, we look at the vortex lattice form factor of Ru$_7$B$_3$ during this field-history dependence, to check for deviations from established models, such as the London model. We find that the data is well described by the anisotropic London model, which is in accordance with theoretical predictions that the alterations to the structure of the vortices due to broken inversion symmetry should be small. From this, we also extract values for the penetration depth and coherence length. △ Less

Submitted 29 September, 2022; originally announced September 2022.

Journal ref: J. Phys.: Condens. Matter 35, 425602 (2023)

Probing superconducting order in overdoped Ca$_{x}$Y$_{1-x}$Ba$_{2}$Cu$_{3}$O$_{7}$ by neutron diffraction measurements of the vortex lattice

Authors: A. S. Cameron, E. Campillo, A. Alshemi, M. Bartkowiak, L. Shen, H. Kawano-Furukawa, A. T. Holmes, O. Prokhnenko, A. Gazizulina, J. S. White, R. Cubitt, N. -J. Steinke, C. D. Dewhurst, A. Erb, E. M. Forgan, E. Blackburn

Abstract: We present small angle neutron scattering studies of the magnetic vortex lattice (VL) in Ca$_{0.04}$Y$_{0.96}$Ba$_{2}$Cu$_{3}$O$_{7}$ up to a field of 16.7 T, and Ca$_{0.15}$Y$_{0.85}$Ba$_{2}$Cu$_{3}$O$_{7}$ up to 25 T. We find that the series of vortex lattice structure transitions have shifted down in field relative to those reported for the undoped compound. We attribute this mainly to the weak… ▽ More We present small angle neutron scattering studies of the magnetic vortex lattice (VL) in Ca$_{0.04}$Y$_{0.96}$Ba$_{2}$Cu$_{3}$O$_{7}$ up to a field of 16.7 T, and Ca$_{0.15}$Y$_{0.85}$Ba$_{2}$Cu$_{3}$O$_{7}$ up to 25 T. We find that the series of vortex lattice structure transitions have shifted down in field relative to those reported for the undoped compound. We attribute this mainly to the weakening of the 1-D superconductivity in the Cu-O chains by the disorder introduced by doping. The hole doping by calcium is also expected to alter the Fermi velocity and it reduces the upper critical field of the system. The high-field structure of the vortex lattice is similar to recent measurements on the parent compound in fields of 25~T, which indicates that the fundamental \textit{d}-wave nature of the superconducting gap is unchanged by calcium doping. This is corroborated by the temperature dependence of the VL form factor which also shows the same \textit{d}-wave behaviour as observed in other cuprates. We also find evidence of Pauli paramagnetic effects in the field dependence of the VL form factor. △ Less

Submitted 13 August, 2022; originally announced August 2022.

Comments: 9 pages, 6 figures

Spin wave stiffness and damping in a frustrated chiral helimagnet Co$_8$Zn$_8$Mn$_4$ as measured by small-angle neutron scattering

Authors: V. Ukleev, K. A. Pschenichnyi, O. Utesov, K. Karube, Mühlbauer, R. Cubitt, Y. Tokura, Y. Taguchi, J. S. White, S. V. Grigoriev

Abstract: Multiple intriguing low temperature phenomena have recently been discovered in the family of chiral cubic Co-Zn-Mn compounds with $β-$Mn-type structure. In particular, Co$_8$Zn$_8$Mn$_4$ displays a reduction of the helical spiral pitch on cooling, along with lattice shape transformations of metastable skyrmions and the manifestation of peculiar magnetic textures due to strong magnetocrystalline an… ▽ More Multiple intriguing low temperature phenomena have recently been discovered in the family of chiral cubic Co-Zn-Mn compounds with $β-$Mn-type structure. In particular, Co$_8$Zn$_8$Mn$_4$ displays a reduction of the helical spiral pitch on cooling, along with lattice shape transformations of metastable skyrmions and the manifestation of peculiar magnetic textures due to strong magnetocrystalline anisotropy. Here we report on temperature-dependent measurements of helimagnon excitations in the field polarized regime Co$_8$Zn$_8$Mn$_4$ using the spin wave small-angle neutron scattering (SWSANS) technique. By applying a new analytical expression to interpret the data, quantitative estimates for both spin wave stiffness and damping are extracted across a wide temperature range between 70 K and 250 K. We speculate that their non-trivial temperature-dependencies arise due to the effects of magnetic frustration arising from Mn magnetic moments, which is further reflected in continuous variations of both exchange and Dzyaloshinskii-Moriya interactions. △ Less

Submitted 22 June, 2022; originally announced June 2022.

Comments: 10 pages, 4 figures. Accepted to Physical Review Research

Journal ref: Phys. Rev. Research 4, 023239, 2022

Tuning topological spin textures in size-tailored chiral magnet insulator particles

Authors: Priya R. Baral, Victor Ukleev, Thomas LaGrange, Robert Cubitt, Ivica Zivkovic, Henrik M. Ronnow, Jonathan S. White, Arnaud Magrez

Abstract: Topological spin textures such as skyrmions hold high potential for use as magnetically active elements in diverse near-future applications. While skyrmions in metallic multilayers attract great attention in this context, unleashing the myriad potential of skyrmions for various applications requires the discovery and customization of alternative host system paradigms. Here we developed and applied… ▽ More Topological spin textures such as skyrmions hold high potential for use as magnetically active elements in diverse near-future applications. While skyrmions in metallic multilayers attract great attention in this context, unleashing the myriad potential of skyrmions for various applications requires the discovery and customization of alternative host system paradigms. Here we developed and applied a chemical method to synthesize octahedral particles of the chiral insulating skyrmion host Cu2OSeO3 with both narrow size distribution, and tailored dimensions approaching the nanoscale. Combining magnetometry and neutron scattering experiments with micromagnetic simulations, we show that the bulk phase diagram of Cu2OSeO3 changes dramatically below octahedral heights of 400 nm. Further particle size-dependent regimes are identified where various topological spin textures such as skyrmions, merons and bobbers can stabilize, prior to a lower critical octahedral height of approx. 190 nm below which no topological spin texture is found stable. These findings suggest conditions under which sparse topological spin textures confined to chiral magnet nanoparticles can be stable, and provide fresh potential for insulator-based application paradigms. △ Less

Submitted 21 June, 2022; originally announced June 2022.

Singlet-triplet mixing in the order parameter of the noncentrosymmetric superconductor Ru$_{7}$B$_{3}$

Authors: A. S. Cameron, Y. S. Yerin, Y. V. Tymoshenko, P. Y. Portnichenko, A. S. Sukhanov, M. Ciomaga Hatnean, D. McK. Paul, G. Balakrishnan, R. Cubitt, A. Heinemann, D. S. Inosov

Abstract: One of the key effects which is predicted to arise in superconductors without a centre of inversion is the mixing of singlet and triplet order parameters, which are no longer good quantum numbers on their own due to parity. We have probed the gap structure in the noncentrosymmetric superconductor Ru$_7$B$_3$, through small-angle neutron diffraction from the vortex lattice, in order to search for t… ▽ More One of the key effects which is predicted to arise in superconductors without a centre of inversion is the mixing of singlet and triplet order parameters, which are no longer good quantum numbers on their own due to parity. We have probed the gap structure in the noncentrosymmetric superconductor Ru$_7$B$_3$, through small-angle neutron diffraction from the vortex lattice, in order to search for the proposed mixed order parameter. We find that the measured temperature dependence of the vortex-lattice form factor is well characterised by a model constructed to describe the effects of broken inversion symmetry on the superconducting state, indicating the presence of a mixed singlet-triplet gap and confirming the theoretical predictions. △ Less

Submitted 8 February, 2022; originally announced February 2022.

Journal ref: Phys. Rev. B 105, 094519 (2022)

Selection rules and dynamic magnetoelectric effect of the spin waves in multiferroic BiFeO$_3$

Authors: D. G. Farkas, D. Szaller, I. Kézsmárki, U. Nagel, T. Rõõm, L. Peedu, J. Viirok, J. S. White, R. Cubitt, T. Ito, R. S. Fishman, S. Bordács

Abstract: We report the magnetic field dependence of the THz absorption and non-reciprocal directional dichroism spectra of BiFeO$_3$ measured on the three principal crystal cuts for fields applied along the three principal directions of each cut. From the systematic study of the light polarization dependence we deduced the optical selection rules of the spin-wave excitations. Our THz data, combined with sm… ▽ More We report the magnetic field dependence of the THz absorption and non-reciprocal directional dichroism spectra of BiFeO$_3$ measured on the three principal crystal cuts for fields applied along the three principal directions of each cut. From the systematic study of the light polarization dependence we deduced the optical selection rules of the spin-wave excitations. Our THz data, combined with small-angle neutron scattering results showed that i) an in-plane magnetic field rotates the $\mathbf{q}$ vectors of the cycloids perpendicular to the magnetic field, and ii) the selection rules are mostly determined by the orientation of the $\mathbf{q}$ vector with respect to the electromagnetic fields. We observed a magnetic field history dependent change in the strength and the frequency of the spin-wave modes, which we attributed to the change of the orientation and the length of the cycloidal $\mathbf{q}$ vector, respectively. Finally, we compared our experimental data with the results of linear spin-wave theory that reproduces the magnetic field dependence of the spin-wave frequencies and most of the selection rules, from which we identified the spin-polarization coupling terms relevant for the optical magnetoelectric effect. △ Less

Submitted 30 September, 2021; originally announced September 2021.

Field-induced vortex-like textures as a probe of the critical line in reentrant spin glasses

Authors: N. Martin, L. J. Bannenberg, M. Deutsch, C. Pappas, G. Chaboussant, R. Cubitt, I. Mirebeau

Abstract: We study the evolution of the low-temperature field-induced magnetic defects observed under an applied magnetic field in a series of frustrated amorphous ferromagnets (Fe$_{1-x}$Mn$_{x}$)$_{75}$P$_{16}$B$_{3}$Al$_{3}$ (a-FeMn). Combining small-angle neutron scattering and Monte Carlo simulations, we show that the morphology of these defects resemble that of quasi-bidimensional spin vortices. They… ▽ More We study the evolution of the low-temperature field-induced magnetic defects observed under an applied magnetic field in a series of frustrated amorphous ferromagnets (Fe$_{1-x}$Mn$_{x}$)$_{75}$P$_{16}$B$_{3}$Al$_{3}$ (a-FeMn). Combining small-angle neutron scattering and Monte Carlo simulations, we show that the morphology of these defects resemble that of quasi-bidimensional spin vortices. They are observed in the reentrant spin-glass (RSG) phase, up to the critical concentration $x_{\rm C} \approx 0.36$ which separates the RSG and "true" spin glass (SG) within the low temperature part of the magnetic phase diagram of a-FeMn. These vortices systematically decrease in size with increasing magnetic field or decreasing the average exchange interaction, and they finally disappear in the SG sample ($x = 0.41$), being replaced by field-induced correlations over finite length scales. We argue that the study of these nanoscopic defects could be used to probe the nature of the critical line between the RSG and SG phases. △ Less

Submitted 18 May, 2021; originally announced May 2021.

Comments: 23 pages, 14 figures, includes supplement

Current-induced self-organisation of mixed superconducting states

Authors: X. S. Brems, S. Mühlbauer, W. Y. Córdoba-Camacho, A. A. Shanenko, A. Vagov, R. Cubitt

Abstract: Small-angle neutron scattering is used in combination with transport measurements to investigate the current-induced effects on the morphology of the intermediate mixed state domains in the intertype superconductor niobium. We report the robust self-organisation of the vortex lattice domains to elongated parallel stripes perpendicular to the applied current in a steady-state. The experimental resu… ▽ More Small-angle neutron scattering is used in combination with transport measurements to investigate the current-induced effects on the morphology of the intermediate mixed state domains in the intertype superconductor niobium. We report the robust self-organisation of the vortex lattice domains to elongated parallel stripes perpendicular to the applied current in a steady-state. The experimental results for the formation of the superstructure are supported by theoretical calculations, which highlight important details of the vortex matter evolution. The investigation demonstrates a mechanism of a spontaneous pattern formation that is closely related to the universal physics governing the intermediate mixed state in low-$κ$ superconductors. △ Less

Submitted 16 April, 2021; originally announced April 2021.

Comments: 15 pages, 9 figures

Reversible ordering and disordering of the vortex lattice in UPt3

Authors: K. E. Avers, S. J. Kuhn, A. W. D. Leishman, W. J. Gannon, L. DeBeer-Schmitt, C. D. Dewhurst, D. Honecker, R. Cubitt, W. P. Halperin, M. R. Eskildsen

Abstract: When studied by small-angle neutron scattering the vortex lattice (VL) in UPt3 undergoes a gradual disordering as a function of time due to 235U fission. This temporarily heats regions of the sample above the critical temperature, where, upon re-cooling, the vortices remain in a quenched vortex glass state. The disordering rate is proportional to the magnetic field, suggesting that it is governed… ▽ More When studied by small-angle neutron scattering the vortex lattice (VL) in UPt3 undergoes a gradual disordering as a function of time due to 235U fission. This temporarily heats regions of the sample above the critical temperature, where, upon re-cooling, the vortices remain in a quenched vortex glass state. The disordering rate is proportional to the magnetic field, suggesting that it is governed by collective VL properties such as the elastic moduli. An ordered VL can be re-formed by applying a small field oscillation, showing that the fission does not cause significant radiation damage to the UPt3 crystals, even after long exposure. △ Less

Submitted 6 May, 2022; v1 submitted 17 March, 2021; originally announced March 2021.

Comments: Multiple revisions

Journal ref: Phys. Rev. B 105, 184512 (2022)

Metastable skyrmion lattices governed by magnetic disorder and anisotropy in $β$-Mn-type chiral magnets

Authors: K. Karube, J. S. White, V. Ukleev, C. D. Dewhurst, R. Cubitt, A. Kikkawa, Y. Tokunaga, H. M. Rønnow, Y. Tokura, Y. Taguchi

Abstract: Magnetic skyrmions are vortex-like topological spin textures often observed in structurally chiral magnets with Dzyaloshinskii-Moriya interaction. Among them, Co-Zn-Mn alloys with a $β$-Mn-type chiral structure host skyrmions above room temperature. In this system, it has recently been found that skyrmions persist over a wide temperature and magnetic field region as a long-lived metastable state,… ▽ More Magnetic skyrmions are vortex-like topological spin textures often observed in structurally chiral magnets with Dzyaloshinskii-Moriya interaction. Among them, Co-Zn-Mn alloys with a $β$-Mn-type chiral structure host skyrmions above room temperature. In this system, it has recently been found that skyrmions persist over a wide temperature and magnetic field region as a long-lived metastable state, and that the skyrmion lattice transforms from a triangular lattice to a square one. To obtain perspective on chiral magnetism in Co-Zn-Mn alloys and clarify how various properties related to the skyrmion vary with the composition, we performed systematic studies on Co$_{10}$Zn$_{10}$, Co$_9$Zn$_9$Mn$_2$, Co$_8$Zn$_8$Mn$_4$ and Co$_7$Zn$_7$Mn$_6$ in terms of magnetic susceptibility and small-angle neutron scattering measurements. The robust metastable skyrmions with extremely long lifetime are commonly observed in all the compounds. On the other hand, preferred orientation of a helimagnetic propagation vector and its temperature dependence dramatically change upon varying the Mn concentration. The robustness of the metastable skyrmions in these materials is attributed to topological nature of the skyrmions as affected by structural and magnetic disorder. Magnetocrystalline anisotropy as well as magnetic disorder due to the frustrated Mn spins play crucial roles in giving rise to the observed change in helical states and corresponding skyrmion lattice form. △ Less

Submitted 11 August, 2020; originally announced August 2020.

Comments: 70 pages, 19 figures

Journal ref: Physical Review B 102, 064408 (2020)

Pinning of Helimagnetic Phase Transitions in Zn-Substituted Skyrmion Host Cu$_2$OSeO$_3$

Authors: M. T. Birch, S. H. Moody, M. N. Wilson, M. Crisanti, O. Bewley, A. Štefančič, G. Balakrishnan, R. Fan, P. Steadman, D. Alba Venero, R. Cubitt, P. D. Hatton

Abstract: Magnetic skyrmions are nano-sized topological spin textures stabilized by a delicate balance of magnetic energy terms. The chemical substitution of the underlying crystal structure of skyrmion-hosting materials offers a route to manipulate these energy contributions, but also introduces additional effects such as disorder and pinning. While the effects of doping and disorder have been well studied… ▽ More Magnetic skyrmions are nano-sized topological spin textures stabilized by a delicate balance of magnetic energy terms. The chemical substitution of the underlying crystal structure of skyrmion-hosting materials offers a route to manipulate these energy contributions, but also introduces additional effects such as disorder and pinning. While the effects of doping and disorder have been well studied in B20 metallic materials such as Fe$_{1-x}$Co$_x$Si and Mn$_{1-x}$Fe$_x$Si, the consequences of chemical substitution in the magnetoelectric insulator Cu$_2$OSeO$_3$ have not been fully explored. In this work, we utilize a combination of AC magnetometry and small angle neutron scattering to investigate the magnetic phase transition dynamics in pristine and Zn-substituted Cu$_2$OSeO$_3$. The results demonstrate that the first order helical-conical phase transition exhibits two thermally separated behavioural regimes: at high temperatures, the helimagnetic domains transform by large-scale, continuous rotations, while at low temperatures, the two phases coexist. Remarkably, the effects of pinning in the substituted sample are less prevalent at low temperatures, compared to high temperatures, despite the reduction of available thermal activation energy. We attribute this behaviour to the large, temperature-dependent, cubic anisotropy unique to Cu$_2$OSeO$_3$, which becomes strong enough to overcome the pinning energy at low temperatures. Consideration and further exploration of these effects will be crucial when engineering skyrmion materials towards future applications. △ Less

Submitted 18 June, 2020; originally announced June 2020.

Journal ref: Phys. Rev. B 102, 104424 (2020)

Development of the magnetism in the solid solution of the candidate Weyl semimetals Ce$_x$Pr$_{1-x}$AlGe

Authors: Pascal Puphal, Sarah Krebber, Emmanuelle Suard, Robert Cubitt, Chennan Wang, Tian Shang, Victor Ukleev, Jonathan S. White, Ekaterina Pomjakushina

Abstract: We investigate the macroscopic and microscopic physical properties of the solid solution of Ce$_{1-x}$Pr$_{x}$AlGe. The series tunes from CeAlGe with its multi-$\vec{k}$ structure and a major Moment in the ab-plane, to PrAlGe with an easy-c-axis ferromagnetic ground state co-existing with a low density of nanoscale textured magnetic Domain walls. Using AC-, DC-susceptiblity, resistivity, specific… ▽ More We investigate the macroscopic and microscopic physical properties of the solid solution of Ce$_{1-x}$Pr$_{x}$AlGe. The series tunes from CeAlGe with its multi-$\vec{k}$ structure and a major Moment in the ab-plane, to PrAlGe with an easy-c-axis ferromagnetic ground state co-existing with a low density of nanoscale textured magnetic Domain walls. Using AC-, DC-susceptiblity, resistivity, specific heat, muon spin relaxation/rotation and neutron scattering we analyze the magnetic ground state of the series. We provide further evidence supporting our previous claim for spin-glass like properties in pure PrAlGe. With introduction of Pr to CeAlGe the finite magnetic field required to stabilize the topological multi-$\vec{k}$ magnetic phase for $x=0$ becomes suppressed. The crossover between the two end-member ground states occurs in the vicinity of $x=0.3$, a region where we further anticipate the field-induced topological magnetic phase for $x < 0.3$ to become the zero field ground state. △ Less

Submitted 13 February, 2020; originally announced February 2020.

Journal ref: Phys. Rev. B 101, 214416 (2020)

Macroscopic Manifestation of Domain-wall Magnetism and Magnetoelectric Effect in a Néel-type Skyrmion Host

Authors: K. Geirhos, B. Gross, B. G. Szigeti, A. Mehlin, S. Philipp, J. S. White, R. Cubitt, S. Widmann, S. Ghara, P. Lunkenheimer, V. Tsurkan, A. O. Leonov, S. Bordács, M. Poggio, I. Kézsmárki

Abstract: We report a magnetic state in GaV$_4$Se$_8$ which emerges exclusively in samples with mesoscale polar domains and not in polar mono-domain crystals. Its onset is accompanied with a sharp anomaly in the magnetic susceptibility and the magnetic torque, distinct from other anomalies observed also in polar mono-domain samples upon transitions between the cycloidal, the Néel-type skyrmion lattice and t… ▽ More We report a magnetic state in GaV$_4$Se$_8$ which emerges exclusively in samples with mesoscale polar domains and not in polar mono-domain crystals. Its onset is accompanied with a sharp anomaly in the magnetic susceptibility and the magnetic torque, distinct from other anomalies observed also in polar mono-domain samples upon transitions between the cycloidal, the Néel-type skyrmion lattice and the ferromagnetic states. We ascribe this additional transition to the formation of magnetic textures localized at structural domain walls, where the magnetic interactions change stepwise and spin textures with different spiral planes, hosted by neighbouring domains, need to be matched. A clear anomaly in the magneto-current indicates that the domain-wall-confined magnetic states also have strong contributions to the magnetoelectric response. We expect polar domain walls to commonly host such confined magnetic edge states, especially in materials with long wavelength magnetic order. △ Less

Submitted 22 January, 2020; originally announced January 2020.

Journal ref: npj Quantum Mater. 5, 44 (2020)

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

Magnetic Guinier law

Authors: A. Michels, A. Malyeyev, I. Titov, D. Honecker, R. Cubitt, E. Blackburn, K. Suzuki

Abstract: Small-angle scattering of x-rays and neutrons is a routine method for the determination of nanoparticle sizes. The so-called Guinier law represents the low-q approximation for the small-angle scattering curve from an assembly of particles. The Guinier law has originally been derived for nonmagnetic particle-matrix-type systems, and it is successfully employed for the estimation of particle sizes i… ▽ More Small-angle scattering of x-rays and neutrons is a routine method for the determination of nanoparticle sizes. The so-called Guinier law represents the low-q approximation for the small-angle scattering curve from an assembly of particles. The Guinier law has originally been derived for nonmagnetic particle-matrix-type systems, and it is successfully employed for the estimation of particle sizes in various scientific domains (e.g., soft matter physics, biology, colloidal chemistry, materials science). An important prerequisite for it to apply is the presence of a discontinuous interface separating particles and matrix. Here, we introduce the Guinier law for the case of magnetic small-angle neutron scattering (SANS) and experimentally demonstrate its applicability for the example of nanocrystalline cobalt. It is well-known that the magnetic microstructure of nanocrystalline ferromagnets is highly nonuniform on the nanometer length scale and characterized by a spectrum of continuously varying long-wavelength magnetization fluctuations, i.e., these systems do not manifest sharp interfaces in their magnetization profile. The magnetic Guinier radius depends on the applied magnetic field, on the magnetic interactions (exchange, magnetostatics), and on the magnetic anisotropy-field radius, which characterizes the size over which the magnetic anisotropy field is coherently aligned into the same direction. In contrast to the nonmagnetic conventional Guinier law, the magnetic version can be applied to fully dense random-anisotropy-type ferromagnets. △ Less

Submitted 8 November, 2019; v1 submitted 27 September, 2019; originally announced September 2019.

Journal ref: IUCrJ 7, 136-142 (2020)

Unveiling the vortex glass phase in the surface and volume of a type-II superconductor

Authors: Jazmín Aragón Sánchez, Raúl Cortés Maldonado, Néstor René Cejas Bolecek, Gonzalo Rumi, Pablo Pedrazzini, Moira I. Dolz, Gladys Nieva, Cornelis J. van der Beek, Marcin Konczykowski, C. D. Dewhurst, R. Cubitt, Alejandro B. Kolton, Alain Pautrat, Yanina Fasano

Abstract: Order-disorder transitions between glassy phases are quite common in nature and yet a comprehensive survey of the microscopic structural changes remains elusive since the scale of the constituents is tiny and in most cases few of them take part in the transformation. Vortex matter in type-II superconductors is a model system where some of the experimental challenges inherent to this general questi… ▽ More Order-disorder transitions between glassy phases are quite common in nature and yet a comprehensive survey of the microscopic structural changes remains elusive since the scale of the constituents is tiny and in most cases few of them take part in the transformation. Vortex matter in type-II superconductors is a model system where some of the experimental challenges inherent to this general question can be tackled by adequately choosing the host superconducting sample. For instance, Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8 + δ}$ is a type-II superconductor with weak point disorder that presents a transition between two glassy phases on increasing the constituents' (vortices) density. At low vortex densities, the impact of disorder produces the nucleation of a glassy yet quasi-crystalline phase, the Bragg glass. For high vortex densities the stable phase, coined as $\textit{vortex glass}$, was proposed to be disordered, but its structural properties have remained elusive up to now. Here we answer this question by combining surface and bulk vortex imaging techniques, and show that the vortex glass is neither a messy nor a hexatic phase: in the plane of vortices it presents large crystallites with positional correlations growing algebraically and short-ranged orientational order. However, no dramatic change in the correlation length along the direction of vortices is observed on traversing the order-disorder transformation. △ Less

Submitted 16 May, 2019; originally announced May 2019.

Journal ref: Commun Phys 2, 143 (2019)

Oriented Three-Dimensional Magnetic Biskyrmion in MnNiGa Bulk Crystals

Authors: Xiyang Li, Shilei Zhang, Hang Li, Diego Alba Venero, Jonathan S White, Robert Cubitt, Qingzhen Huang, Jie Chen, Lunhua He, Gerrit van der Laan, Wenhong Wang, Thorsten Hesjedal, Fangwei Wang

Abstract: A biskyrmion consists of two bound, topologically stable skyrmion spin textures. These coffee-bean-shaped objects have been observed in real-space in thin plates using Lorentz transmission electron microscopy (LTEM). From LTEM imaging alone, it is not clear whether biskyrmions are surface-confined objects, or, analogously to skyrmions in non-centrosymmetric helimagnets, three-dimensional tube-like… ▽ More A biskyrmion consists of two bound, topologically stable skyrmion spin textures. These coffee-bean-shaped objects have been observed in real-space in thin plates using Lorentz transmission electron microscopy (LTEM). From LTEM imaging alone, it is not clear whether biskyrmions are surface-confined objects, or, analogously to skyrmions in non-centrosymmetric helimagnets, three-dimensional tube-like structures in bulk sample. Here, we investigate the biskyrmion form factor in single- and polycrystalline MnNiGa samples using small angle neutron scattering (SANS). We find that biskyrmions are not long-range ordered, not even in single-crystals. Surprisingly all of the disordered biskyrmions have their in-plane symmetry axis aligned along certain directions, governed by the magnetocrystalline anisotropy. This anisotropic nature of biskyrmions may be further exploited to encode information. △ Less

Submitted 25 February, 2019; originally announced February 2019.

Measuring the Formation Energy Barrier of Skyrmions in Zinc Substituted Cu$_2$OSeO$_3$

Authors: M. N. Wilson, M. Crisanti, C. Barker, A. Štefančič, J. S. White, M. T. Birch, G. Balakrishnan, R. Cubitt, P. D. Hatton

Abstract: We report small angle neutron scattering (SANS) measurements of the skyrmion lattice in (Cu$_{0.976}$Zn$_{0.024}$)$_2$OSeO$_3$ under the application of an electric field. These measurements show an expansion of the skyrmion lattice stability region with electric field similar to that seen in pristine Cu$_2$OSeO$_3$. Furthermore, using time-resolved SANS, we observe the slow formation of skyrmions… ▽ More We report small angle neutron scattering (SANS) measurements of the skyrmion lattice in (Cu$_{0.976}$Zn$_{0.024}$)$_2$OSeO$_3$ under the application of an electric field. These measurements show an expansion of the skyrmion lattice stability region with electric field similar to that seen in pristine Cu$_2$OSeO$_3$. Furthermore, using time-resolved SANS, we observe the slow formation of skyrmions after an electric or magnetic field is applied, which has not been observed in pristine Cu$_2$OSeO$_3$ crystals. The measured formation times are dramatically longer than the corresponding skyrmion destruction times after the external field is removed, and increase exponentially from 100~s at 52.5~K to 10,000~s at 51.5~K. This thermally activated behaviour indicates an energy barrier for skyrmion formation of 1.57(2)~eV, the size of which demonstrates the huge cost for creating these complex chiral objects. △ Less

Submitted 27 March, 2019; v1 submitted 31 January, 2019; originally announced January 2019.

Comments: 8 pages, 8 figures

Journal ref: Phys. Rev. B 99, 174421 (2019)

Disordered skyrmion phase stabilized by magnetic frustration in a chiral magnet

Authors: K. Karube, J. S. White, D. Morikawa, C. D. Dewhurst, R. Cubitt, A. Kikkawa, X. Z. Yu, Y. Tokunaga, T. Arima, H. M. Rønnow, Y. Tokura, Y. Taguchi

Abstract: Magnetic skyrmions are vortex-like topological spin textures often observed to form a triangular-lattice skyrmion crystal in structurally chiral magnets with Dzyaloshinskii-Moriya interaction. Recently $β$-Mn structure-type Co-Zn-Mn alloys were identified as a new class of chiral magnet to host such skyrmion crystal phases, while $β$-Mn itself is known as hosting an elemental geometrically frustra… ▽ More Magnetic skyrmions are vortex-like topological spin textures often observed to form a triangular-lattice skyrmion crystal in structurally chiral magnets with Dzyaloshinskii-Moriya interaction. Recently $β$-Mn structure-type Co-Zn-Mn alloys were identified as a new class of chiral magnet to host such skyrmion crystal phases, while $β$-Mn itself is known as hosting an elemental geometrically frustrated spin liquid. Here we report the intermediate composition system Co$_7$Zn$_7$Mn$_6$ to be a unique host of two disconnected, thermal-equilibrium topological skyrmion phases; one is a conventional skyrmion crystal phase stabilized by thermal fluctuations and restricted to exist just below the magnetic transition temperature $T_\mathrm{c}$, and the other is a novel three-dimensionally disordered skyrmion phase that is stable well below $T_\mathrm{c}$. The stability of this new disordered skyrmion phase is due to a cooperative interplay between the chiral magnetism with Dzyaloshinskii-Moriya interaction and the frustrated magnetism inherent to $β$-Mn. △ Less

Submitted 6 November, 2018; originally announced November 2018.

Comments: 57 pages, 16 figures

Journal ref: Compositio Math. 155 (2019) 2263-2295

Metamagnetic texture in a polar antiferromagnet

Authors: D. A. Sokolov, N. Kikugawa, T. Helm, H. Borrmann, U. Burkhardt, R. Cubitt, E. Ressouche, M. Bleuel, K. Kummer, A. P. Mackenzie, U. K. Rößler

Abstract: The notion of a simple ordered state implies homogeneity. If the order is established by a broken symmetry, elementary Landau theory of phase transitions shows that only one symmetry mode describes this state. Precisely at points of phase coexistence domain states formed of large regions of different phases can be stabilized by long range interactions. In uniaxial antiferromagnets the so-called me… ▽ More The notion of a simple ordered state implies homogeneity. If the order is established by a broken symmetry, elementary Landau theory of phase transitions shows that only one symmetry mode describes this state. Precisely at points of phase coexistence domain states formed of large regions of different phases can be stabilized by long range interactions. In uniaxial antiferromagnets the so-called metamagnetism is an example of such a behavior, when an antiferromagnetic and field-induced spin-polarized paramagnetic/ferromagnetic state co-exist at a jump-like transition in the magnetic phase diagram. Here, combining experiment with theoretical analysis, we show that a different type of mixed state between antiferromagnetism and ferromagnetism can be created in certain acentric materials. In the small-angle neutron scattering experiments we observe a field-driven spin-state in the layered antiferromagnet Ca3Ru2O7, which is modulated on a scale between 8 and 20 nm and has both antiferromagnetic and ferromagnetic parts. We call this state a metamagnetic texture and explain its appearance by the chiral twisting effects of the asymmetric Dzyaloshinskii-Moriya (DM) exchange. The observation can be understood as an extraordinary coexistence, in one thermodynamic state, of spin orders belonging to different symmetries. Experimentally, the complex nature of this metamagnetic state is demonstrated by measurements of anomalies in electronic transport which reflect the spin-polarization in the metamagnetic texture, determination of the magnetic orbital moments, which supports the existence of strong spin-orbit effects, a pre-requisite for the mechanism of twisted magnetic states in this material. △ Less

Submitted 12 November, 2019; v1 submitted 15 October, 2018; originally announced October 2018.

Comments: Supplementary materials included

Journal ref: Nat. Phys. 15, 671-677 (2019)

Rotation of the magnetic vortex lattice in Ru7B3 driven by the effects of broken time-reversal and inversion symmetry

Authors: A. S. Cameron, Y. S. Yerin, Y. V. Tymoshenko, P. Y. Portnichenko, A. S. Sukhanov, M. Ciomaga Hatnean, D. McK. Paul, G. Balakrishnan, R. Cubitt, D. S. Inosov

Abstract: We observe a hysteretic reorientation of the magnetic vortex lattice in the noncentrosymmetric superconductor Ru7B3, with the change in orientation driven by altering magnetic field below Tc. Normally a vortex lattice chooses either a single or degenerate set of orientations with respect to a crystal lattice at any given field or temperature, a behavior well described by prevailing phenomenologica… ▽ More We observe a hysteretic reorientation of the magnetic vortex lattice in the noncentrosymmetric superconductor Ru7B3, with the change in orientation driven by altering magnetic field below Tc. Normally a vortex lattice chooses either a single or degenerate set of orientations with respect to a crystal lattice at any given field or temperature, a behavior well described by prevailing phenomenological and microscopic theories. Here, in the absence of any typical VL structural transition, we observe a continuous rotation of the vortex lattice which exhibits a pronounced hysteresis and is driven by a change in magnetic field. We propose that this rotation is related to the spontaneous magnetic fields present in the superconducting phase, which are evidenced by the observation of time-reversal symmetry breaking, and the physics of broken inversion symmetry. Finally, we develop a model from the Ginzburg-Landau approach which shows that the coupling of these to the vortex lattice orientation can result in the rotation we observe. △ Less

Submitted 9 October, 2018; originally announced October 2018.

Journal ref: Phys. Rev. B 100, 024518 (2019)

Increased lifetime of metastable skyrmions by doping

Authors: M. T. Birch, R. Takagi, S. Seki, M. N. Wilson, F. Kagawa, A. Štefančič, G. Balakrishnan, R. Fan, P. Steadman, C. J. Ottley, M. Crisanti, R. Cubitt, T. Lancaster, Y. Tokura, P. D. Hatton

Abstract: Previous observations of metastable magnetic skyrmions have shown that close to the equilibrium pocket the metastable state has a short lifetime, and therefore rapid cooling is required to generate a significant skyrmion population at low temperatures. Here, we report that the lifetime of metastable skyrmions in Cu$_2$OSeO$_3$ is extended by a factor of 50 with the introduction of only 2.5% zinc d… ▽ More Previous observations of metastable magnetic skyrmions have shown that close to the equilibrium pocket the metastable state has a short lifetime, and therefore rapid cooling is required to generate a significant skyrmion population at low temperatures. Here, we report that the lifetime of metastable skyrmions in Cu$_2$OSeO$_3$ is extended by a factor of 50 with the introduction of only 2.5% zinc doping, allowing over 50% of the population to survive when field-cooling at a rate of just 1 K/min. Our systematic study suggests that the lifetime enhancement is due to the removal of spins by the non-magnetic dopant, which entropically limits the number of skyrmion decay pathways. We expect that doping can be exploited to control the lifetime of the metastable SkL state in other chiral magnets, offering a method of engineering skyrmion materials towards application in future devices. △ Less

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

Comments: 9 Pages, 7 Figures, submission in progress

Journal ref: Phys. Rev. B 100, 014425 (2019)

Unconventional superconductivity in the nickel-chalcogenide superconductor, TlNi$_2$Se$_2$

Authors: Erik Jellyman, Philippa Jefferies, Stephen Pollard, Ted Forgan, Elizabeth Blackburn, Emma Campillo, Alex Holmes, Robert Cubitt, Jorge Gavilano, Hangdong Wang, Jianhua Du, Minghu Fang

Abstract: We present the results of a study of the vortex lattice (VL) of the nickel chalcogenide superconductor TlNi2Se2, using small angle neutron scattering. This superconductor has the same crystal symmetry as the iron arsenide materials. Previous work points to it being a two-gap superconductor, with an unknown pairing mechanism. No structural transitions in the vortex lattice are seen in the phase dia… ▽ More We present the results of a study of the vortex lattice (VL) of the nickel chalcogenide superconductor TlNi2Se2, using small angle neutron scattering. This superconductor has the same crystal symmetry as the iron arsenide materials. Previous work points to it being a two-gap superconductor, with an unknown pairing mechanism. No structural transitions in the vortex lattice are seen in the phase diagram, arguing against d-wave gap symmetry. Empirical fits of the temperature-dependence of the form factor and penetration depth rule out a simple s-wave model, supporting the presence of nodes in the gap function. The variation of the VL opening angle with field is consistent with earlier reports of of multiple gaps. △ Less

Submitted 19 February, 2020; v1 submitted 9 August, 2018; originally announced August 2018.

Journal ref: Phys. Rev. B 101, 134523 (2020)

Spin Textures induced by Quenched Disorder in a Reentrant Spin Glass: Vortices versus "Frustrated" Skyrmions

Authors: I. Mirebeau, N. Martin, M. Deutsch, L. J. Bannenberg, C. Pappas, G. Chaboussant, R. Cubitt, C. Decorse, A. O. Leonov

Abstract: Reentrant spin glasses are frustrated disordered ferromagnets developing vortex-like textures under an applied magnetic field. Our study of a Ni$_{0.81}$Mn$_{0.19}$ single crystal by small angle neutron scattering clarifies their internal structure and shows that these textures are randomly distributed. Spin components transverse to the magnetic field rotate over length scales of 3-15 nm in the ex… ▽ More Reentrant spin glasses are frustrated disordered ferromagnets developing vortex-like textures under an applied magnetic field. Our study of a Ni$_{0.81}$Mn$_{0.19}$ single crystal by small angle neutron scattering clarifies their internal structure and shows that these textures are randomly distributed. Spin components transverse to the magnetic field rotate over length scales of 3-15 nm in the explored field range, decreasing as field increases according to a scaling law. Monte-Carlo simulations reveal that the internal structure of the vortices is strongly distorted and differs from that assumed for "frustrated" skyrmions, built upon a competition between symmetric exchange interactions. Isolated vortices have small non-integer topological charge. The vortices keep an anisotropic shape on a 3 dimensional lattice, recalling "croutons" in a "ferromagnetic soup". Their size and number can be tuned independently by the magnetic field and concentration x (or heat treatment), respectively. This opens an original route to understand and control the influence of quenched disorder in systems hosting non trivial spin textures. △ Less

Submitted 3 July, 2018; originally announced July 2018.

Comments: 23 pages, 15 figures, includes supplement

Journal ref: Physical Review B 98, 014420 (2018)

Magnetic field control of cycloidal domains and electric polarization in multiferroic BiFeO$_3$

Authors: S. Bordács, D. G. Farkas, J. S. White, R. Cubitt, L. DeBeer-Schmitt, T. Ito, I. Kézsmárki

Abstract: The magnetic field induced rearrangement of the cycloidal spin structure in ferroelectric mono-domain single crystals of the room-temperature multiferroic BiFeO$_3$ is studied using small-angle neutron scattering (SANS). The cycloid propagation vectors are observed to rotate when magnetic fields applied perpendicular to the rhombohedral (polar) axis exceed a pinning threshold value of $\sim$5\,T.… ▽ More The magnetic field induced rearrangement of the cycloidal spin structure in ferroelectric mono-domain single crystals of the room-temperature multiferroic BiFeO$_3$ is studied using small-angle neutron scattering (SANS). The cycloid propagation vectors are observed to rotate when magnetic fields applied perpendicular to the rhombohedral (polar) axis exceed a pinning threshold value of $\sim$5\,T. In light of these experimental results, a phenomenological model is proposed that captures the rearrangement of the cycloidal domains, and we revisit the microscopic origin of the magnetoelectric effect. A new coupling between the magnetic anisotropy and the polarization is proposed that explains the recently discovered magnetoelectric polarization to the rhombohedral axis. △ Less

Submitted 29 October, 2017; originally announced October 2017.

Journal ref: Phys. Rev. Lett. 120, 147203 (2018)

Cycloidally modulated magnetic order stabilized by thermal fluctuations in the Néel-type skyrmion host GaV$_4$S$_8$

Authors: J. S. White, Á. Butykai, R. Cubitt, D. Honecker, C. D. Dewhurst, L. F. Kiss, V. Tsurkan, S. Bordács

Abstract: We report small-angle neutron scattering studies of the lacunar spinel GaV$_4$S$_8$, which reveal the long-wavelength magnetic states to be cycloidally modulated. This provides direct support for the formation of Néel-type skyrmions recently claimed to exist in this compound. In striking contrast with all other bulk skyrmion host materials, upon cooling the modulated magnetic states transform into… ▽ More We report small-angle neutron scattering studies of the lacunar spinel GaV$_4$S$_8$, which reveal the long-wavelength magnetic states to be cycloidally modulated. This provides direct support for the formation of Néel-type skyrmions recently claimed to exist in this compound. In striking contrast with all other bulk skyrmion host materials, upon cooling the modulated magnetic states transform into a ferromagnetic state. These results indicate all of the modulated states in GaV$_4$S$_8$, including the skyrmion state, gain their stability from thermal fluctuations, while at lower temperature the ferromagnetic state emerges in accord with the strong easy-axis magnetic anisotropy. In the vicinity of the transition between the ferromagnetic and modulated states, both a phase coexistence and a soliton-like state are also evidenced by our study. △ Less

Submitted 12 April, 2017; originally announced April 2017.

Journal ref: Phys. Rev. B 97, 020401 (2018)

Magnetic-field-induced nonlocal effects on the vortex interactions in twin-free YBa2Cu3O7

Authors: J. S. White, R. W. Heslop, A. T. Holmes, E. M. Forgan, V. Hinkov, N. Egetenmeyer, J. L. Gavilano, M. Laver, C. D. Dewhurst, R. Cubitt, A. Erb

Abstract: The vortex lattice (VL) in the high-kappa superconductor YBa2Cu3O7, at 2 K and with the magnetic field parallel to the crystal c-axis, undergoes a sequence of transitions between different structures as a function of applied magnetic field. However, from structural studies alone, it is not possible to determine precisely the system anisotropy that governs the transitions between different structur… ▽ More The vortex lattice (VL) in the high-kappa superconductor YBa2Cu3O7, at 2 K and with the magnetic field parallel to the crystal c-axis, undergoes a sequence of transitions between different structures as a function of applied magnetic field. However, from structural studies alone, it is not possible to determine precisely the system anisotropy that governs the transitions between different structures. To address this question, here we report new small-angle neutron scattering measurements of both the VL structure at higher temperatures, and the field- and temperature-dependence of the VL form factor. Our measurements demonstrate how the influence of anisotropy on the VL, which in theory can be parameterized as nonlocal corrections, becomes progressively important with increasing magnetic field, and suppressed by increasing the temperature towards Tc. The data indicate that nonlocality due to different anisotropies play important roles in determining the VL properties. △ Less

Submitted 10 August, 2011; originally announced August 2011.

Comments: To appear in Phys. Rev. B

Journal ref: Phys. Rev. B 84, 104519 (2011)

The Absence of Vortex Lattice Melting in a Conventional Superconductor

Authors: C. J. Bowell, R. J. Lycett, M. Laver, C. D. Dewhurst, R. Cubitt, E. M. Forgan

Abstract: The state of the vortex lattice extremely close to the superconducting to normal transition in an applied magnetic field is investigated in high purity niobium. We observe that thermal fluctuations of the order parameter broaden the superconducting to normal transition into a crossover but no sign of a first order vortex lattice melting transition is detected in measurements of the heat capacity o… ▽ More The state of the vortex lattice extremely close to the superconducting to normal transition in an applied magnetic field is investigated in high purity niobium. We observe that thermal fluctuations of the order parameter broaden the superconducting to normal transition into a crossover but no sign of a first order vortex lattice melting transition is detected in measurements of the heat capacity or the small angle neutron scattering (SANS) intensity. Direct observation of the vortices via SANS always finds a well ordered vortex lattice. The fluctuation broadening is considered in terms of the Lowest Landau Level theory of critical fluctuations and scaling is found to occur over a large H_{c2}(T) range. △ Less

Submitted 14 October, 2010; originally announced October 2010.

Journal ref: PhysRevB.82.144508 2010

The pairing state in KFe2As2 studied by measurements of the magnetic vortex lattice

Authors: H. Kawano-Furukawa, C. J. Bowell, J. S. White, R. W. Heslop, A. S. Cameron, E. M. Forgan, K. Kihou, C. H. Lee, A. Iyo, H. Eisaki, T. Saito, H. Fukazawa, Y. Kohori, R. Cubitt, C. D. Dewhurst, J. L. Gavilano, M. Zolliker

Abstract: Understanding the mechanism and symmetry of electron pairing in iron-based superconductors represents an important challenge in condensed matter physics [1-3]. The observation of magnetic flux lines - "vortices" - in a superconductor can contribute to this issue, because the spatial variation of magnetic field reflects the pairing. Unlike many other iron pnictides, our KFe2As2 crystals have very w… ▽ More Understanding the mechanism and symmetry of electron pairing in iron-based superconductors represents an important challenge in condensed matter physics [1-3]. The observation of magnetic flux lines - "vortices" - in a superconductor can contribute to this issue, because the spatial variation of magnetic field reflects the pairing. Unlike many other iron pnictides, our KFe2As2 crystals have very weak vortex pinning, allowing small-angle-neutron-scattering (SANS) observations of the intrinsic vortex lattice (VL). We observe nearly isotropic hexagonal packing of vortices, without VL-symmetry transitions up to high fields along the fourfold c-axis of the crystals, indicating rather small anisotropy of the superconducting properties around this axis. This rules out gap nodes parallel to the c-axis, and thus d-wave and also anisotropic s-wave pairing [2, 3]. The strong temperature-dependence of the intensity down to T<<Tc indicates either widely different full gaps on different Fermi surface sheets, or nodal lines perpendicular to the axis. △ Less

Submitted 8 October, 2010; v1 submitted 24 May, 2010; originally announced May 2010.

Comments: 13 pages, 3 figures

Measuring the Penetration Depth Anisotropy in MgB$_2$ using Small-Angle Neutron Scattering

Authors: D. Pal, L. DeBeer-Schmitt, T. Bera, R. Cubitt, C. D. Dewhurst, J. Jun, N. D. Zhigadlo, J. Karpinski, V. G. Kogan, M. R. Eskildsen

Abstract: Using small-angle neutron scattering we have measured the misalignment between an applied field of 4 kOe and the flux-line lattice in MgB$_2$, as the field is rotated away from the c axis by an angle $θ$. The measurements, performed at 4.9 K, showed the vortices canting towards the c axis for all field orientations. Using a two-band/two-gap model to calculate the magnetization we are able to fit… ▽ More Using small-angle neutron scattering we have measured the misalignment between an applied field of 4 kOe and the flux-line lattice in MgB$_2$, as the field is rotated away from the c axis by an angle $θ$. The measurements, performed at 4.9 K, showed the vortices canting towards the c axis for all field orientations. Using a two-band/two-gap model to calculate the magnetization we are able to fit our results yielding a penetration depth anisotropy, $\glam = 1.1 \pm 0.1$. △ Less

Submitted 13 February, 2006; v1 submitted 15 November, 2005; originally announced November 2005.

Comments: 4figures

Journal ref: Phys. Rev. B 73, 012513 (2006)

Square vortex lattice at anomalously low magnetic fields in electron-doped Nd$_{1.85}$Ce$_{0.15}$CuO$_{4}$

Authors: R. Gilardi, J. Mesot, S. P. Brown, E. M. Forgan, A. Drew, S. L. Lee, R. Cubitt, C. D. Dewhurst, T. Uefuji, K. Yamada

Abstract: We report here on the first direct observations of the vortex lattice in the bulk of electron-doped Nd$_{1.85}$Ce$_{0.15}$CuO$_{4}$ single crystals. Using small angle neutron scattering, we have observed a square vortex lattice with the nearest-neighbors oriented at 45$^{\circ}$ from the Cu-O bond direction, which is consistent with theories based on the d-wave superconducting gap. However, the… ▽ More We report here on the first direct observations of the vortex lattice in the bulk of electron-doped Nd$_{1.85}$Ce$_{0.15}$CuO$_{4}$ single crystals. Using small angle neutron scattering, we have observed a square vortex lattice with the nearest-neighbors oriented at 45$^{\circ}$ from the Cu-O bond direction, which is consistent with theories based on the d-wave superconducting gap. However, the square symmetry persists down to unusually low magnetic fields. Moreover, the diffracted intensity from the vortex lattice is found to decrease rapidly with increasing magnetic field. △ Less

Submitted 11 October, 2004; originally announced October 2004.

Comments: 4 pages, 4 Figures, accepted for publication in Phys. Rev. Lett

Journal ref: Phys. Rev. Lett. 93 (2004) 217001

Swelling of phospholipid floating bilayers: the effect of chain length

Authors: Giovanna Fragneto, Thierry Charitat, Edith Bellet-Amalric, Robert Cubitt, Francois Graner

Abstract: The equilibrium distance between two lipid bilayers stable in bulk water and in proximity of a substrate was investigated. Samples consisted of a homogeneous lipid bilayer, floating near an identical bilayer deposited on the hydrophilic surface of a silicon single crystal. Lipids were saturated di-acyl phosphocholines, with the number of carbon atoms per chain, n, varying from 16 to 20. The aver… ▽ More The equilibrium distance between two lipid bilayers stable in bulk water and in proximity of a substrate was investigated. Samples consisted of a homogeneous lipid bilayer, floating near an identical bilayer deposited on the hydrophilic surface of a silicon single crystal. Lipids were saturated di-acyl phosphocholines, with the number of carbon atoms per chain, n, varying from 16 to 20. The average and r.m.s. positions of the floating bilayer were determined by means of neutron specular reflectivity. Samples were prepared at room temperature (i.e. with the lipids in the gel phase) and measurements performed at various temperatures so that the whole region of transition from gel to fluid phase was explored. Data have been interpreted in terms of competition between the interbilayer potential and membrane fluctuations and used to estimate the bending rigidity of the bilayer. △ Less

Submitted 26 June, 2003; originally announced June 2003.

Transition from Type-I to Type-II Superconducting Behaviour with Temperature observed by $μ$SR and SANS

Authors: C. M. Aegerter, H. Keller, S. L. Lee, C. Ager, F. Y. Ogrin, R. Cubitt, E. M. Forgan, W. J. Nutall, P. G. Kealey, S. H. Lloyd, S. T. Johnson, T. M. Riseman, M. P. Nutley

Abstract: We investigate the superconducting behaviour of Bi doped Pb. Pure lead shows type-I behaviour entering an intermediate state in a magnetic field. High dopings of Bi ($>$3%) lead to type-II behaviour showing a mixed state, where the magnetic field penetrates the superconductor in the form of a flux lattice. At intermediate doping, the sample shows both type-I and type-II behaviour depending on th… ▽ More We investigate the superconducting behaviour of Bi doped Pb. Pure lead shows type-I behaviour entering an intermediate state in a magnetic field. High dopings of Bi ($>$3%) lead to type-II behaviour showing a mixed state, where the magnetic field penetrates the superconductor in the form of a flux lattice. At intermediate doping, the sample shows both type-I and type-II behaviour depending on the temperature. This arises because the Ginzburg-Landau parameter $κ$ passes through its critical value of $1/\sqrt{2}$ with temperature. △ Less

Submitted 26 May, 2003; originally announced May 2003.

Comments: 8 pages, 7 figures

Observation of vortex lattice melting in large untwinned YBa$_2$Cu$_3$O$_{7-x}$ single crystals

Authors: C. M. Aegerter, H. Keller, S. H. Lloyd, P. G. Kealey, E. M. Forgan, S. T. Johnson, T. M. Riseman, R. Cubitt, S. L. Lee, C. Ager, D. McK. Paul, I. M. Savic, M. Yethiraj, S. Tajima, A. Rykov

Abstract: We present a study of the vortex lattice in untwinned YBa$_2$Cu$_3$O$_{7-x}$ crystals, using a combination of muon spin rotation and neutron small angle scattering measurements. Both methods show a very sharp melting temperature consistent with a first order transition. The dependence of the melting temperature on the angle of the field with respect to the crystallographic c-axis is studied. The… ▽ More We present a study of the vortex lattice in untwinned YBa$_2$Cu$_3$O$_{7-x}$ crystals, using a combination of muon spin rotation and neutron small angle scattering measurements. Both methods show a very sharp melting temperature consistent with a first order transition. The dependence of the melting temperature on the angle of the field with respect to the crystallographic c-axis is studied. The results are compared to thermal measurements. △ Less

Submitted 26 May, 2003; originally announced May 2003.

Comments: 8 pages, 9 figures

Direct evidence for an intrinsic square vortex lattice in the overdoped high-Tc superconductor La1.83Sr0.17CuO4+d

Authors: R. Gilardi, J. Mesot, A. Drew, U. Divakar, S. L. Lee, E. M. Forgan, O. Zaharko, K. Conder, V. K. Aswal, C. D. Dewhurst, R. Cubitt, N. Momono, M. Oda

Abstract: We report here the first direct observations of a well ordered vortex lattice in the bulk of a La(2-x)Sr(x)CuO(4+d) (La214) single crystal (slightly overdoped, x=0.17). Our small angle neutron scattering investigation of the mixed phase reveals a crossover from triangular to square coordination with increasing magnetic field. The existence of an intrinsic square vortex lattice has never been obs… ▽ More We report here the first direct observations of a well ordered vortex lattice in the bulk of a La(2-x)Sr(x)CuO(4+d) (La214) single crystal (slightly overdoped, x=0.17). Our small angle neutron scattering investigation of the mixed phase reveals a crossover from triangular to square coordination with increasing magnetic field. The existence of an intrinsic square vortex lattice has never been observed in high-temperature superconductors (HTSC), and is indicative of the coupling of the vortex lattice to a source of anisotropy, such as those provided by a d-wave order parameter or the presence of stripes. △ Less

Submitted 12 April, 2002; originally announced April 2002.

Comments: 4 pages, 3 figures (eps, 2 color), to appear in Phys. Rev. Lett

Journal ref: Phys. Rev. Lett. 88, 217003 (2002)

A Bragg glass phase in the vortex lattice of a type II superconductor

Authors: T. Klein, I. Joumard, S. Blanchard, J. Marcus, R. Cubitt, T. Giamarchi, P. Le Doussal

Abstract: Although crystals are usually quite stable, they are sensitive to a disordered environment: even an infinitesimal amount of impurities can lead to the destruction of the crystalline order. The resulting state of matter has been a longstanding puzzle. Until recently it was believed to be an amorphous state in which the crystal would break into crystallites. But a different theory predicts the exi… ▽ More Although crystals are usually quite stable, they are sensitive to a disordered environment: even an infinitesimal amount of impurities can lead to the destruction of the crystalline order. The resulting state of matter has been a longstanding puzzle. Until recently it was believed to be an amorphous state in which the crystal would break into crystallites. But a different theory predicts the existence of a novel phase of matter: the so-called Bragg glass, which is a glass and yet nearly as ordered as a perfect crystal. The lattice of vortices that can contain magnetic flux in type II superconductors provide a good system to investigate these ideas. Here we show that neutron diffraction data of the vortex lattice in type II superconductors provides unambiguous evidence for a weak, power-law decay of the crystalline order characteristic of a Bragg glass. The theory also predicts accurately the electrical transport properties of superconductors; it naturally explains the observed phase transition and the dramatic jumps in the critical current associated with the melting of the Bragg glass. Moreover the model explains experiments as diverse as X-ray scattering in disordered liquid crystals and conductivity of electronic crystals. △ Less

Submitted 1 October, 2001; originally announced October 2001.

Comments: 9 pages, 4 figures

Journal ref: Nature 413, 404 (2001)

A reconstruction from small-angle neutron scattering measurements of the real space magnetic field distribution in the mixed state of Sr$_2$RuO$_4$

Authors: P. G. Kealey, T. M. Riseman, E. M. Forgan, L. M. Galvin, A. P. Mackenzie, S. L. Lee, D. McK. Paul, R. Cubitt, D. F. Agterberg, R. Heeb, Z. Q. Mao, Y. Maeno

Abstract: We have measured the diffracted neutron scattering intensities from the square magnetic flux lattice in the perovskite superconductor Sr$_2$RuO$_4$, which is thought to exhibit p-wave pairing with a two-component order parameter. The relative intensities of different Bragg reflections over a wide range of field and temperature have been shown to be inconsistent with a single component Ginzburg-L… ▽ More We have measured the diffracted neutron scattering intensities from the square magnetic flux lattice in the perovskite superconductor Sr$_2$RuO$_4$, which is thought to exhibit p-wave pairing with a two-component order parameter. The relative intensities of different Bragg reflections over a wide range of field and temperature have been shown to be inconsistent with a single component Ginzburg-Landau theory but qualitatively agree with a two component p-wave Ginzburg-Landau theory. △ Less

Submitted 8 February, 2000; originally announced February 2000.

Comments: PRL Format (4 pages) with 7 figures and two tables

Vortex lattice structures and pairing symmetry in Sr2RuO4

Authors: D. F. Agterberg, R. Heeb, P. G. Kealey, T. M. Riseman, E. M. Forgan, A. P. Mackenzie, L. M. Galvin, R. S. Perry, S. L. Lee, D. McK. Paul, R. Cubitt, Z. Q. Mao, S. Akima, Y. Maeno

Abstract: Recent experimental results indicate that superconductivity in Sr2RuO4 is described by the p-wave E_u representation of the D_{4h} point group. Results on the vortex lattice structures for this representation are presented. The theoretical results are compared with experiment. Recent experimental results indicate that superconductivity in Sr2RuO4 is described by the p-wave E_u representation of the D_{4h} point group. Results on the vortex lattice structures for this representation are presented. The theoretical results are compared with experiment. △ Less

Submitted 28 January, 2000; originally announced January 2000.

Comments: 4 pages, 3 figures, M2S-HTSC-VI proceedings

Flux-Line Lattice Structures in Untwinned YBa2Cu3O

Authors: S. T. Johnson, E. M. Forgan, S. H. Lloyd, C. M. Aegerter, S. L. Lee, R. Cubitt, P. G. Kealey, C. Ager, S. Tajima, A. Rykov, D. McK. Paul

Abstract: A small angle neutron scattering study of the flux-line lattice in a large single crystal of untwinned YBa2Cu3O is presented. In fields parallel to the c-axis, diffraction spots are observed corresponding to four orientations of a hexagonal lattice, distorted by the a-b anisotropy. A value for the anisotropy, the penetration depth ratio, of 1.18(2) was obtained. The high quality of the data is s… ▽ More A small angle neutron scattering study of the flux-line lattice in a large single crystal of untwinned YBa2Cu3O is presented. In fields parallel to the c-axis, diffraction spots are observed corresponding to four orientations of a hexagonal lattice, distorted by the a-b anisotropy. A value for the anisotropy, the penetration depth ratio, of 1.18(2) was obtained. The high quality of the data is such that second order diffraction is observed, indicating a well ordered FLL. With the field at 33 degrees to c a field dependent re-orientation of the lattice is observed around 3T. △ Less

Submitted 17 February, 1999; v1 submitted 15 April, 1998; originally announced April 1998.

Comments: 4 pages, 4 figures