Showing 1–50 of 59 results for author: Radu, F

Vector-field control and emergent basal-plane anisotropy of magnetic textures in noncentrosymmetric (Fe$_{0.63}$Ni$_{0.3}$Pd$_{0.07}$)$_3$P

Authors: Victor Ukleev, Oleg I. Utesov, Lorenzo Ubilla, Chen Luo, Radu-Marius Abrudan, Peter Wild, Holger Kropf, Moritz Winter, Sebastian Schneider, Alexander Tahn, Bernd Rellinghaus, Tim A. Butcher, Simone Finizio, Sebastian Wintz, Markus Weigand, Max T. Birch, Yoshinori Tokura, Yasujiro Taguchi, Kosuke Karube, Florin Radu

Abstract: (Fe$_{0.63}$Ni$_{0.3}$Pd$_{0.07}$)$_3$P is a room-temperature magnet with $S_4$ symmetry that hosts a rich variety of topological spin textures. Here, we report a combined resonant small-angle x-ray scattering and ptychography study of (Fe$_{0.63}$Ni$_{0.3}$Pd$_{0.07}$)$_3$P in vector magnetic fields over a broad temperature range. We demonstrate deterministic vector-field control of magnetic stri… ▽ More (Fe$_{0.63}$Ni$_{0.3}$Pd$_{0.07}$)$_3$P is a room-temperature magnet with $S_4$ symmetry that hosts a rich variety of topological spin textures. Here, we report a combined resonant small-angle x-ray scattering and ptychography study of (Fe$_{0.63}$Ni$_{0.3}$Pd$_{0.07}$)$_3$P in vector magnetic fields over a broad temperature range. We demonstrate deterministic vector-field control of magnetic stripe domains, where in-plane fields continuously rotate their orientation via a transition from a chiral stripe to an achiral fan configuration. Furthermore, at 50 K and below, the stripe orientation becomes metastably pinned and retains its field-trained direction. While the magnitude of the wavevector is nearly isotropic within the basal plane at room temperature, a pronounced temperature evolution of anisotropic interactions emerges upon cooling. In particular, non-trivial anisotropy axes develop at 20-50 K reflecting the combined effects of magnetocrystalline anisotropy, anisotropic exchange, and Dzyaloshinskii-Moriya interaction (DMI), whose effective orientation is found to rotate with temperature. These results establish (Fe$_{0.63}$Ni$_{0.3}$Pd$_{0.07}$)$_3$P as a model system for vector-field control of chiral spin textures and reveal a previously unrecognized temperature-driven evolution of the effective DMI landscape in a noncentrosymmetric magnet. △ Less

Submitted 7 August, 2026; v1 submitted 4 August, 2026; originally announced August 2026.

Proximity-Induced Skyrmion Stabilization at the Cu2OSeO3/Bi2Se3 Interface

Authors: Sina Mehboodi, Victor Ukleev, Chen Luo, Radu-Marius Abrudan, Jie Xiao, Ronny Golnak, Florin Radu, Matthias Kronseder, Christian H. Back, Aisha Aqeel

Abstract: We investigate proximity-induced magnetic interactions at the interface between the topological insulator Bi2Se3 and the chiral magnetic insulator Cu2OSeO3, with particular focus on the low temperature skyrmion phase. Broadband ferromagnetic resonance spectroscopy reveals enhanced stability of noncollinear spin textures in the Cu2OSeO3/Bi2Se3 heterostructure compared with bare Cu2OSeO3. In additio… ▽ More We investigate proximity-induced magnetic interactions at the interface between the topological insulator Bi2Se3 and the chiral magnetic insulator Cu2OSeO3, with particular focus on the low temperature skyrmion phase. Broadband ferromagnetic resonance spectroscopy reveals enhanced stability of noncollinear spin textures in the Cu2OSeO3/Bi2Se3 heterostructure compared with bare Cu2OSeO3. In addition to an extra resonance mode in the tilted conical phase that is absent in bare Cu2OSeO3, field cycling resolves two counterclockwise skyrmion resonance branches separated by approximately 238 MHz, consistent with the coexistence of a bulk skyrmion lattice and an interfacial skyrmion phase stabilized by proximity-induced exchange coupling and enhanced interfacial Dzyaloshinskii-Moriya interactions. The finite frequency separation indicates that the two skyrmion phases occupy distinct magnetic energy landscapes while retaining similar resonance character. Resonant elastic x-ray scattering measurements further confirm that the interfacial skyrmion phase spans a broader magnetic-field range than the bulk phase, demonstrating enhanced stability and ordering of topological spin textures at the interface. These findings establish interface engineering as a promising route for extending the stability regime of skyrmion and tilted-conical phases in topological-magnetic heterostructures. △ Less

Submitted 30 June, 2026; originally announced June 2026.

Quasi-1D Spin Textures: From Chiral Soliton Lattice to Fan State

Authors: M. Winter, A. Pignedoli, A. S. Sukhanov, M. Azhar, A. Tahn, B. Achinuq, J. R. Bollard, V. Ukleev, C. Luo, F. Radu, S. Wintz, M. Weigand, A. Mistonov, P. Vir, J. Geck, C. Felser, G. van der Laan, T. Hesjedal, K. Everschor-Sitte, B. Rellinghaus, M. C. Rahn

Abstract: In most helimagnets, an applied magnetic field aligns the propagation direction of a helical spin texture with the field, resulting in a conical state and obscuring the unwinding process. Here, we access a complementary regime in the anisotropic chiral magnet Mn$_{1.4}$PtSn, where crystal symmetry constrains the propagation direction of the spin modulation. Using resonant elastic X-ray scattering… ▽ More In most helimagnets, an applied magnetic field aligns the propagation direction of a helical spin texture with the field, resulting in a conical state and obscuring the unwinding process. Here, we access a complementary regime in the anisotropic chiral magnet Mn$_{1.4}$PtSn, where crystal symmetry constrains the propagation direction of the spin modulation. Using resonant elastic X-ray scattering in a vector magnet, we track the evolution of quasi-one-dimensional spin textures that propagate along a chiral crystallographic axis while the magnetic field is applied perpendicular to this direction. Together with micromagnetic simulations, our measurements reveal a transformation from the zero-field $π$-chiral soliton lattice into a fan-like state. In this state, the propagation direction remains transverse to the applied field, while the spins oscillate about the field direction. During magnetization, the modulation length decreases continuously with the field and approaches the field-polarized state. Simulations indicate that magnetostatic interactions in finite samples play a key role in stabilizing this behavior. Our results provide evidence for a fan-like regime in a chiral magnet and highlight how field orientation can be used to control one-dimensional spin textures. △ Less

Submitted 18 June, 2026; originally announced June 2026.

Comments: 9 pages, 6 figures; with Supplemental Material (4 pages) appended; a video is included as an ancillary file

Local multiferroic ordering at room temperature in collinear magnetoelectric antiferromagnets induced by flexo-Zeeman coupling

Authors: Paulina J. Prusik, Igor Veremchuk, Florin Radu, Andrey N. Anisimov, Pavlo Makushko, Georgy V. Astakhov, René Hübner, Alexander Edström, Jürgen Fassbender, Fabian Ganss, Massimiliano Stengel, Kirill D. Belashchenko, Denys Makarov, Oleksandr V. Pylypovskyi

Abstract: Spin-driven multiferroicity attracts significant interest due to its tunability and inherently strong magnetoelectric coupling. While this mechanism induces sizeable electric polarization, it typically occurs at low temperatures and in complex materials. In the simple oxide, the magnetoelectric antiferromagnet Cr$_2$O$_3$, we experimentally demonstrate the existence of specific domain walls that a… ▽ More Spin-driven multiferroicity attracts significant interest due to its tunability and inherently strong magnetoelectric coupling. While this mechanism induces sizeable electric polarization, it typically occurs at low temperatures and in complex materials. In the simple oxide, the magnetoelectric antiferromagnet Cr$_2$O$_3$, we experimentally demonstrate the existence of specific domain walls that act as room-temperature multiferroic regions. This behavior stems from an anisotropic crystal-symmetry-dependent mechanism of exchange origin, which is applicable to a broad class of bipartite antiferromagnets. The key signature is the magnetization occurring at antiferromagnetic textures, driven by the flexo-Zeeman interaction. These findings establish a foundation for exploring high-temperature spin-driven multiferroicity for magnetoelectric spin-orbit memory and logic applications. △ Less

Submitted 3 June, 2026; originally announced June 2026.

Atomically-sharp magnetic soliton in the square-net lattice EuRhAl$_{4}$Si$_{2}$

Authors: Kevin Allen, Juba Bouaziz, Yichen Zhang, Kai Du, Sanu Mishra, Gustav Bihlmayer, Yiqing Hao, Victor Ukleev, Chen Luo, Florin Radu, Yuxiang Gao, Marta Zonno, Sergey Gorovikov, Christopher Lane, Jian-Xin Zhu, Huibo Cao, Sang-Wook Cheong, Ming Yi, Stefan Blügel, Emilia Morosan

Abstract: Topological spin textures are hallmark manifestations of competing interactions in magnetic matter. Their effective description by nonlinear field theories reflects an energetic frustration that destabilizes uniform order while selecting finite-size, topologically nontrivial configurations as stationary states. Among the most extreme realizations are atomically-sharp domain wall excitations, namel… ▽ More Topological spin textures are hallmark manifestations of competing interactions in magnetic matter. Their effective description by nonlinear field theories reflects an energetic frustration that destabilizes uniform order while selecting finite-size, topologically nontrivial configurations as stationary states. Among the most extreme realizations are atomically-sharp domain wall excitations, namely one-dimensional (1D) magnetic solitons, which represent the ultimate scaling limit of magnetic textures. Such solitons may emerge in magnetic systems where effective exchange interactions compete directly with uniaxial magnetic anisotropy. Here we show that the square-net rare earth compound EuRhAl$_{4}$Si$_{2}$ realizes a very susceptible regime where the magnetic anisotropy competes with highly frustrated exchange interactions stabilizing a rare ferrimagnetic $\uparrow\uparrow\downarrow$ state that, under applied magnetic field, supports the formation of atomically-sharp soliton defects. We confirm the bulk response of the 1D magnetic solitons via magnetization and electrical transport measurements. We establish both the zero- and in-field $\uparrow\uparrow\downarrow$ order via neutron diffraction, while magnetic force microscopy visualizes its real-space evolution into a stripe-like array. To elucidate the microscopic origin of the soliton, we relate the Ruderman-Kittel-Kasuya-Yosida (RKKY)-driven exchange interactions and the magnetic anisotropy through density functional theory, and we construct an effective 1D $J_{1}$-$J_{2}$-$K$ model whose atomistic spin dynamics simulations reproduce the observed soliton states as a function of external field. Our results demonstrate that EuRhAl$_{4}$Si$_{2}$ hosts atomically-sharp, field-driven 1D magnetic solitons, providing a new platform for studying 1D topological excitations at the atomic length scale. △ Less

Submitted 10 February, 2026; originally announced February 2026.

The 2026 Skyrmionics Roadmap

Authors: Sabri Koraltan, Claas Abert, Manfred Albrecht, Maria Azhar, Christian Back, Hélène Béa, Max T. Birch, Stefan Blügel, Olivier Boulle, Felix Büttner, Ping Che, Vincent Cros, Emily Darwin, Louise Desplat, Claire Donnelly, Haifeng Du, Karin Everschor-Sitte, Amalio Fernández-Pacheco, Simone Finizio, Giovanni Finocchio, Markus Garst, Raphael Gruber, Dirk Grundler, Satoru Hayami, Thorsten Hesjedal , et al. (42 additional authors not shown)

Abstract: Magnetic skyrmions and related topological spin textures have emerged as a central topic in condensed-matter physics, combining fundamental significance with potential for transformative applications in spintronics, magnonics, and beyond. Over the past decade, advances in material platforms, imaging techniques, theoretical modeling, and device concepts have established skyrmionics as a rapidly exp… ▽ More Magnetic skyrmions and related topological spin textures have emerged as a central topic in condensed-matter physics, combining fundamental significance with potential for transformative applications in spintronics, magnonics, and beyond. Over the past decade, advances in material platforms, imaging techniques, theoretical modeling, and device concepts have established skyrmionics as a rapidly expanding field. At the same time, challenges remain in stabilizing, controlling, and integrating such textures into functional architectures, while novel phenomena such as antiskyrmions, higher-order skyrmions, hopfions, and antiferromagnetic textures arise. The 2026 Skyrmionics Roadmap represents a collective effort of many authors, providing a comprehensive perspective on the current state-of-the-art and the outlook for the coming years. In 33 focused sections, each co-authored by two researchers, we chart progress in theory and modeling, material systems, skyrmion dynamics, and skyrmion technologies. By offering a consolidated vision, this Roadmap aims to guide both fundamental research and application-driven efforts, accelerating the transition of skyrmionics from conceptual breakthroughs toward practical technologies. △ Less

Submitted 23 January, 2026; originally announced January 2026.

Comments: 244 Pages, 33 Sections, 52 Figures

Spontaneous nonreciprocal transport in a gate-tunable ferromagnetic Rashba 2-dimensional electron gas

Authors: Gabriel Lazrak, Radu Abrudan, Borge Göbel, David Hrabovsky, Chen Luo, Victor Ukleev, Srijani Mallik, Luis M. Vicente-Arche, Florin Radu, Sergio Valencia, Annika Johansson, Agnès Barthélémy, Manuel Bibes

Abstract: The broken inversion symmetry at interfaces of complex oxides gives rise to emergent phenomena, including ferromagnetism and Rashba spin-orbit coupling (SOC), which profoundly influence the electronic structure by entangling spin and momentum. While the interplay between Rashba SOC and ferromagnetism is theoretically intriguing, its experimental manifestations remain largely unexplored. Here, we d… ▽ More The broken inversion symmetry at interfaces of complex oxides gives rise to emergent phenomena, including ferromagnetism and Rashba spin-orbit coupling (SOC), which profoundly influence the electronic structure by entangling spin and momentum. While the interplay between Rashba SOC and ferromagnetism is theoretically intriguing, its experimental manifestations remain largely unexplored. Here, we demonstrate that ferromagnetic 2DEGs at SrTiO$_3$-based interfaces exhibit spontaneous nonreciprocal transport - a distinctive hallmark of Rashba ferromagnets - even in the absence of an external magnetic field. This nonreciprocal response, along with clear signatures of ferromagnetism such as anisotropic magnetoresistance and the anomalous Hall effect (AHE), is strongly tunable by gate voltage. Remarkably, the AHE not only varies in amplitude but even reverses sign, reflecting a subtle interplay between Fermi level position and Berry curvature distribution. These results establish SrTiO$_3$ 2DEGs as a model platform for studying Rashba ferromagnetism and demonstrate active control over transport phenomena in time- and inversion-symmetry-broken systems, paving the way for gate-tunable spintronic devices. △ Less

Submitted 21 August, 2025; originally announced August 2025.

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

Above-room-temperature ferromagnetism in large-area epitaxial Fe3GaTe2/graphene van der Waals heterostructures

Authors: Tauqir Shinwari, Kacho Imtiyaz Ali Khan, Hua Lv, Atekelte Abebe Kassa, Frans Munnik, Simon Josephy, Achim Trampert, Victor Ukleev, Chen Luo, Florin Radu, Jens Herfort, Michael Hanke, Joao Marcelo Jordao Lopes

Abstract: Fe3GaTe2 (FGaT), a two-dimensional (2D) layered ferromagnetic metal, exhibits a high Curie temperature (TC) ~ 360 K along with strong perpendicular magnetic anisotropy (PMA), making it a promising material candidate for next-generation energy-efficient magnetic devices. However, the vast majority of studies on FGaT to date have been limited to millimeter-sized bulk crystals and exfoliated flakes,… ▽ More Fe3GaTe2 (FGaT), a two-dimensional (2D) layered ferromagnetic metal, exhibits a high Curie temperature (TC) ~ 360 K along with strong perpendicular magnetic anisotropy (PMA), making it a promising material candidate for next-generation energy-efficient magnetic devices. However, the vast majority of studies on FGaT to date have been limited to millimeter-sized bulk crystals and exfoliated flakes, which are unsuitable for practical applications and integration into device processing. Also, its combination with other 2D materials to form van der Waals heterostructures has only been achieved by flake stacking. Consequently, the controlled large-scale growth of FGaT and related heterostructures remains largely unexplored. In this work, we demonstrate a breakthrough in the high-quality, large-scale growth of epitaxial FGaT thin films on single-crystalline graphene/SiC templates using molecular beam epitaxy. Structural characterization confirms the high crystalline quality of the continuous FGaT/graphene van der Waals heterostructures. Temperature-dependent magnetization and anomalous Hall measurements reveal robust PMA with an enhanced TC well above room temperature, reaching up to 400 K. Furthermore, X-ray absorption and X-ray magnetic circular dichroism spectra provide insight into the spin and orbital magnetic moment contributions, further validating the high TC and robust PMA. These findings are highly significant for the future development of high-performance spintronic devices based on 2D heterostructures, with potential applications in next-generation data storage, logic processing and quantum technologies. △ Less

Submitted 9 May, 2025; originally announced May 2025.

Journal ref: ACS Nano 2025

Observation of distorted tilted conical phase at the surface of a bulk chiral magnet with resonant elastic x-ray scattering

Authors: S. Mehboodi, V. Ukleev, C. Luo, R. Abrudan, F. Radu, C. H. Back, A. Aqeel

Abstract: We report on various magnetic configurations including spirals and skyrmions at the surface of the magnetic insulator Cu$_2$OSeO$_3$ at low temperatures with a magnetic field applied along <100> using resonant elastic X-ray scattering (REXS). We observe a well-ordered surface state referred to as a distorted tilted conical spiral (TC) phase over a wide range of magnetic fields. The distorted TC ph… ▽ More We report on various magnetic configurations including spirals and skyrmions at the surface of the magnetic insulator Cu$_2$OSeO$_3$ at low temperatures with a magnetic field applied along <100> using resonant elastic X-ray scattering (REXS). We observe a well-ordered surface state referred to as a distorted tilted conical spiral (TC) phase over a wide range of magnetic fields. The distorted TC phase shows characteristic higher harmonic magnetic satellites in the REXS reciprocal space maps. Skyrmions emerge following static magnetic field cycling and appear to coexist with the distorted TC phase. Our results indicate that this phase represents a distinct and stable surface state that does not disappear with field cycling and persists until the field strength is increased sufficiently to create the field-polarized state. △ Less

Submitted 20 December, 2024; originally announced December 2024.

Report number: 26

Competing anisotropies in the chiral cubic magnet Co$_8$Zn$_8$Mn$_4$ unveiled by resonant x-ray magnetic scattering

Authors: Victor Ukleev, Oleg I. Utesov, Chen Luo, Florin Radu, Sebastian Wintz, Markus Weigand, Simone Finizio, Moritz Winter, Alexander Tahn, Bernd Rellinghaus, Kosuke Karube, Yoshinori Tokura, Yasujiro Taguchi, Jonathan S. White

Abstract: The cubic $β$-Mn-type alloy Co$_8$Zn$_8$Mn$_4$ is a chiral helimagnet that exhibits a peculiar temperature-dependent behavior in the spiral pitch, which decreases from 130 nm at room temperature to 70 nm below 20 K. Notably, this shortening is also accompanied by a structural transition of the metastable skyrmion texture, transforming from a hexagonal lattice to a square lattice of elongated skyrm… ▽ More The cubic $β$-Mn-type alloy Co$_8$Zn$_8$Mn$_4$ is a chiral helimagnet that exhibits a peculiar temperature-dependent behavior in the spiral pitch, which decreases from 130 nm at room temperature to 70 nm below 20 K. Notably, this shortening is also accompanied by a structural transition of the metastable skyrmion texture, transforming from a hexagonal lattice to a square lattice of elongated skyrmions. The underlying mechanism of these transformations remain unknown, with interactions potentially involved including temperature-dependent Dzyaloshinskii-Moriya interaction, magnetocrystalline anisotropy, and exchange anisotropy. Here, x-ray resonant magnetic small-angle scattering in vectorial magnetic fields was employed to investigate the temperature dependence of the anisotropic properties of the helical phase in Co$_8$Zn$_8$Mn$_4$. Our results reveal quantitatively that the magnitude of the anisotropic exchange interaction increases by a factor of 4 on cooling from room temperature to 20 K, leading to a 5% variation in the helical pitch within the (001) plane at 20 K. While anisotropic exchange interaction contributes to the shortening of the spiral pitch, its magnitude is insufficient to explain the variation in the spiral periodicity from room to low temperatures. Finally, we demonstrate that magnetocrystalline and exchange anisotropies compete, favoring different orientations of the helical vector in the ground state. △ Less

Submitted 25 April, 2024; v1 submitted 22 April, 2024; originally announced April 2024.

Comments: 8 pages, 4 figures

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

Transient grating spectroscopy on a DyCo$_5$ thin film with femtosecond extreme ultraviolet pulses

Authors: Victor Ukleev, Ludmila Leroy, Riccardo Mincigrucci, Dario Deangelis, Danny Fainozzi, Nupur Ninad Khatu, Ettore Paltanin, Laura Foglia, Filippo Bencivenga, Chen Luo, Florian Ruske, Florin Radu, Cristian Svetina, Urs Staub

Abstract: Surface acoustic waves (SAWs) are excited by femtosecond extreme ultraviolet (EUV) transient gratings (TGs) in a room-temperature ferrimagnetic DyCo$_5$ alloy. TGs are generated by crossing a pair of EUV pulses from a free electron laser (FEL) with the wavelength of 20.8\,nm matching the Co $M$-edge, resulting in a SAW wavelength of $Λ=44$\,nm. Using the pump-probe transient grating scheme in a re… ▽ More Surface acoustic waves (SAWs) are excited by femtosecond extreme ultraviolet (EUV) transient gratings (TGs) in a room-temperature ferrimagnetic DyCo$_5$ alloy. TGs are generated by crossing a pair of EUV pulses from a free electron laser (FEL) with the wavelength of 20.8\,nm matching the Co $M$-edge, resulting in a SAW wavelength of $Λ=44$\,nm. Using the pump-probe transient grating scheme in a reflection geometry the excited SAWs could be followed in the time range of -10 to 100\,ps in the thin film. Coherent generation of TGs by ultrafast EUV pulses allows to excite SAW in any material and to investigate their couplings to other dynamics such as spin waves and orbital dynamics. In contrast, we encountered challenges in detecting electronic and magnetic signals, potentially due to the dominance of the larger SAW signal and the weakened reflection signal from underlying layers. A potential solution for the latter challenge involves employing soft X-ray probes, albeit introducing additional complexities associated with the required grazing incidence geometry. △ Less

Submitted 13 February, 2024; v1 submitted 2 November, 2023; originally announced November 2023.

Journal ref: Structural Dynamics, 11, 025101, 2024

Direct observation of the exchange anisotropy in the helimagnetic insulator Cu$_2$OSeO$_3$

Authors: Priya R. Baral, Oleg I. Utesov, Chen Luo, Florin Radu, Arnaud Magrez, Jonathan S. White, Victor Ukleev

Abstract: The helical magnetic structures of cubic chiral systems are well-explained by the competition among Heisenberg exchange, Dzyaloshinskii-Moriya interaction, cubic anisotropy, and anisotropic exchange interaction (AEI). Recently, the role of the latter has been argued theoretically to be crucial for the low-temperature phase diagram of the cubic chiral magnet Cu$_2$OSeO$_3$, which features tilted co… ▽ More The helical magnetic structures of cubic chiral systems are well-explained by the competition among Heisenberg exchange, Dzyaloshinskii-Moriya interaction, cubic anisotropy, and anisotropic exchange interaction (AEI). Recently, the role of the latter has been argued theoretically to be crucial for the low-temperature phase diagram of the cubic chiral magnet Cu$_2$OSeO$_3$, which features tilted conical and disordered skyrmion states for a specific orientation of the applied magnetic field ($μ_0 \vec{\mathrm{H}} \parallel [001]$). In this study, we exploit transmission resonant x-ray scattering ($t-$REXS) in vector magnetic fields to directly quantify the strength of the AEI in Cu$_2$OSeO$_3$, and measure its temperature dependence. We find that the AEI continuously increases below 50\,K, resulting in a conical spiral pitch variation of $10\%$ in the (001) plane. Our results contribute to establishing the interaction space that supports tilted cone and low-temperature skyrmion state formation, facilitating the goals for both a quantitative description and eventual design of the diverse spiral states existing amongst chiral magnets. △ Less

Submitted 6 June, 2023; originally announced June 2023.

Journal ref: Phys. Rev. Research 5, L032019 (2023)

Direct observation of Néel-type skyrmions and domain walls in a ferrimagnetic DyCo$_3$ thin film

Authors: Chen Luo, Kai Chen, Victor Ukleev, Sebastian Wintz, Markus Weigand, Radu-Marius Abrudan, Karel Prokeš, Florin Radu

Abstract: Isolated magnetic skyrmions are stable, topologically protected spin textures that are at the forefront of research interests today due to their potential applications in information technology. A distinct class of skyrmion hosts are rare earth - transition metal (RE-TM) ferrimagnetic materials. To date, the nature and the control of basic traits of skyrmions in these materials are not fully under… ▽ More Isolated magnetic skyrmions are stable, topologically protected spin textures that are at the forefront of research interests today due to their potential applications in information technology. A distinct class of skyrmion hosts are rare earth - transition metal (RE-TM) ferrimagnetic materials. To date, the nature and the control of basic traits of skyrmions in these materials are not fully understood. We show that for an archetypal ferrimagnetic material DyCo$_3$ that exhibits a strong perpendicular anisotropy, the ferrimagnetic skyrmion size can be tuned by an external magnetic field. Moreover, by taking advantage of the high spatial resolution of scanning transmission X-ray microscopy (STXM) and utilizing a large x-ray magnetic linear dichroism (XMLD) contrast that occurs naturally at the RE resonant edges, we resolve the nature of the magnetic domain walls of ferrimagnetic skyrmions. We demonstrate that through this method one can easily discriminate between Bloch and Néel type domain walls for each individual skyrmion. For all isolated ferrimagnetic skyrmions, we observe that the domain walls are of Néel-type. This key information is corroborated with results of micromagnetic simulations and allows us to conclude on the nature of the Dzyaloshinskii-Moriya interaction (DMI) which concurs to the stabilisation of skyrmions in this ferrimagnetic system. Establishing that an intrinsic DMI occurs in RE-TM materials will also be beneficial towards a deeper understanding of chiral spin texture control in ferrimagnetic materials. △ Less

Submitted 10 August, 2023; v1 submitted 26 April, 2023; originally announced April 2023.

Comments: 38 pages, 16 figures

Epitaxial monolayers of magnetic 2D semiconductor FeBr$_{2}$ grown on Au(111)

Authors: S. E. Hadjadj, C. González-Orellana, J. Lawrence, D. Bikaljević, M. Peña-Díaz, P. Gargiani, L. Aballe, J. Naumann, M. Á. Niño, M. Foerster, S. Ruiz-Gómez, S. Thakur, I. Kumberg, J. Taylor, J. Hayes, J. Torres, C. Luo, F. Radu, D. G. de Oteyza, W. Kuch, J. I. Pascual, C. Rogero, M. Ilyn

Abstract: Magnetic two-dimensional (2D) semiconductors have attracted a lot of attention because modern preparation techniques are capable of providing single crystal films of these materials with precise control of thickness down to the single-layer limit. It opens up a way to study rich variety of electronic and magnetic phenomena with promising routes towards potential applications. We have investigated… ▽ More Magnetic two-dimensional (2D) semiconductors have attracted a lot of attention because modern preparation techniques are capable of providing single crystal films of these materials with precise control of thickness down to the single-layer limit. It opens up a way to study rich variety of electronic and magnetic phenomena with promising routes towards potential applications. We have investigated the initial stages of epitaxial growth of the magnetic van der Waals semiconductor FeBr\textsubscript{2} on a single-crystal Au(111) substrate by means of low-temperature scanning tunneling microscopy, low-energy electron diffraction, x-ray photoemission spectroscopy, low-energy electron emission microscopy and x-ray photoemission electron microscopy. Magnetic properties of the one- and two-layer thick films were measured via x-ray absorption spectroscopy/x-ray magnetic circular dichroism. Our findings show a striking difference in the magnetic behaviour of the single layer of FeBr\textsubscript{2} and its bulk counterpart, which can be attributed to the modifications in the crystal structure due to the interaction with the substrate. △ Less

Submitted 19 September, 2023; v1 submitted 24 April, 2023; originally announced April 2023.

Comments: 20 pages, 17 figures,

Journal ref: Chem. Mater. 35, 23, (2023)

Evidence of The Anomalous Fluctuating Magnetic State by Pressure Driven 4f Valence Change in EuNiGe$_3$

Authors: K. Chen, C. Luo, Y. Zhao F. Baudelet, A. Maurya, A. Thamizhavel, U. K. Rößler, D. Makarov, F. Radu

Abstract: In rare-earth compounds with valence fluctuation, the proximity of the 4f level to the Fermi energy leads to instabilities of the charge configuration and the magnetic moment. Here, we provide direct experimental evidence for an induced magnetic polarization of the Eu$^{3+}$ atomic shell with J=0, due to intra-atomic exchange and spin-orbital coupling interactions with Eu$^{2+}$ atomic shell. By a… ▽ More In rare-earth compounds with valence fluctuation, the proximity of the 4f level to the Fermi energy leads to instabilities of the charge configuration and the magnetic moment. Here, we provide direct experimental evidence for an induced magnetic polarization of the Eu$^{3+}$ atomic shell with J=0, due to intra-atomic exchange and spin-orbital coupling interactions with Eu$^{2+}$ atomic shell. By applying external pressure, a transition from antiferromagnetic to a fluctuating behavior in a EuNiGe$_3$ single crystals is probed. Magnetic polarization is observed for both valence states of Eu$^{2+}$ and Eu$^{3+}$ across the entire pressure range. The anomalous magnetism is discussed in terms of a homogeneous intermediate valence state where frustrated Dzyaloshinskii-Moriya couplings are enhanced by the onset of spin-orbital interaction and engender a chiral spin-liquid-like precursor. △ Less

Submitted 29 January, 2023; originally announced January 2023.

Journal ref: J. Phys. Chem. Lett. 2023, 14, 1000-1006

Chiral surface spin textures in Cu$_2$OSeO$_3$ unveiled by soft x-ray scattering in specular reflection geometry

Authors: V. Ukleev, C. Luo, R. Abrudan, A. Aqeel, C. H. Back, F. Radu

Abstract: Resonant elastic soft x-ray magnetic scattering (XRMS) is a powerful tool to explore long-periodic spin textures in single crystals. However, due to the limited momentum transfer range imposed by long wavelengths of photons in the soft x-ray region, Bragg diffraction is restricted to crystals with the large lattice parameters. Alternatively, small angle x-ray scattering has been involved in the so… ▽ More Resonant elastic soft x-ray magnetic scattering (XRMS) is a powerful tool to explore long-periodic spin textures in single crystals. However, due to the limited momentum transfer range imposed by long wavelengths of photons in the soft x-ray region, Bragg diffraction is restricted to crystals with the large lattice parameters. Alternatively, small angle x-ray scattering has been involved in the soft energy x-ray range which, however, brings in difficulties with the sample preparation that involves focused ion beam milling to thin down the crystal to below a few hundred nm thickness. We show how to circumvent these restrictions by using XRMS in specular reflection from a sub-nanometer smooth crystal surface. The method allows observing diffraction peaks from the helical and conical spin modulations at the surface of a Cu$_2$OSeO$_3$ single crystal and probing their corresponding chirality as contributions to the dichroic scattered intensity. The results suggest a promising way to carry out XRMS studies on plethora of noncentrosymmetric systems hitherto unexplored with soft x-rays due to the absence of the commensurate Bragg peaks in the available momentum transfer range. △ Less

Submitted 3 July, 2022; originally announced July 2022.

Comments: 11 pages, 5 figures

Laser induced ultrafast Gd 4f spin dynamics in Co100-xGdx alloys by means of time-resolved XMCD

Authors: T. Ferté, M. Beens, G. Malinowski, K. Holldack, R. Abrudan, F. Radu, T. Kachel, M. Hehn, C. Boeglin, B. Koopmans, N. Bergeard

Abstract: We have studied the laser induced ultrafast quenching of Gd 4f magnetic order in ferrimagnetic Co100-xGdx alloys to highlight the role of the inter-atomic exchange coupling. We have taken advantage of the ultrashort soft X-ray pulses deliver by the femtoslicing beamline at the BESSY II synchrotron radiation source at the Helmholtz-Zentrum Berlin to perform element- and time-resolved X-ray Magnetic… ▽ More We have studied the laser induced ultrafast quenching of Gd 4f magnetic order in ferrimagnetic Co100-xGdx alloys to highlight the role of the inter-atomic exchange coupling. We have taken advantage of the ultrashort soft X-ray pulses deliver by the femtoslicing beamline at the BESSY II synchrotron radiation source at the Helmholtz-Zentrum Berlin to perform element- and time-resolved X-ray Magnetic Circular Dichroism spectroscopy.Our results show that the laser induced quenching of Gd 4f magnetic order occurs on very different time-scales for the Co72Gd28, the Co77Gd23 and the Co79Gd21 alloys. Most of the magnetic moment losses occur within the first picosecond (ps) while the electron distribution is strongly out of equilibrium. After the equilibration of the electrons and lattice temperatures (t > 1 ps), the magnetic losses occur on slower rates that depend on the alloy composition: increasing the Co composition speeds up the demagnetization of Gd 4f sublattice. The strength of the inter-atomic exchange coupling which depends on composition, determines the efficiency of the angular momentum flow from the Gd 4f spin towards the lattice. Our results are in qualitative agreements with the predictions of the microscopic three temperatures model for ferrimagnetic alloys. △ Less

Submitted 21 June, 2022; originally announced June 2022.

Comments: 8 pages, 6 figures

Megahertz-rate Ultrafast X-ray Scattering and Holographic Imaging at the European XFEL

Authors: Nanna Zhou Hagström, Michael Schneider, Nico Kerber, Alexander Yaroslavtsev, Erick Burgos Parra, Marijan Beg, Martin Lang, Christian M. Günther, Boris Seng, Fabian Kammerbauer, Horia Popescu, Matteo Pancaldi, Kumar Neeraj, Debanjan Polley, Rahul Jangid, Stjepan B. Hrkac, Sheena K. K. Patel, Sergei Ovcharenko, Diego Turenne, Dmitriy Ksenzov, Christine Boeglin, Igor Pronin, Marina Baidakova, Clemens von Korff Schmising, Martin Borchert , et al. (75 additional authors not shown)

Abstract: The advent of X-ray free-electron lasers (XFELs) has revolutionized fundamental science, from atomic to condensed matter physics, from chemistry to biology, giving researchers access to X-rays with unprecedented brightness, coherence, and pulse duration. All XFEL facilities built until recently provided X-ray pulses at a relatively low repetition rate, with limited data statistics. Here, we presen… ▽ More The advent of X-ray free-electron lasers (XFELs) has revolutionized fundamental science, from atomic to condensed matter physics, from chemistry to biology, giving researchers access to X-rays with unprecedented brightness, coherence, and pulse duration. All XFEL facilities built until recently provided X-ray pulses at a relatively low repetition rate, with limited data statistics. Here, we present the results from the first megahertz repetition rate X-ray scattering experiments at the Spectroscopy and Coherent Scattering (SCS) instrument of the European XFEL. We illustrate the experimental capabilities that the SCS instrument offers, resulting from the operation at MHz repetition rates and the availability of the novel DSSC 2D imaging detector. Time-resolved magnetic X-ray scattering and holographic imaging experiments in solid state samples were chosen as representative, providing an ideal test-bed for operation at megahertz rates. Our results are relevant and applicable to any other non-destructive XFEL experiments in the soft X-ray range. △ Less

Submitted 20 January, 2022; v1 submitted 17 January, 2022; originally announced January 2022.

Comments: 13 pages, 5 figures. Supplementary Information as ancillary file

Journal ref: J. Synchrotron Rad. (2022), 29

Search for enhanced magnetism at the interface between Bi$_2$Se$_3$ and EuSe

Authors: K. Prokeš, Chen Luo, H. Ryll, E. Schierle, D. Marchenko, E. Weschke, F. Radu, R. Abrudan, V. V. Volobuev, G. Springholz, O. Rader

Abstract: Enhanced magnetism has recently been reported for the topological-insulator/ferromagnet interface Bi$_2$Se$_3$/EuS with Curie temperatures claimed to be raised above room temperature from the bulk EuS value of 16 K. Here we investigate the analogous interface Bi$_2$Se$_3$/EuSe. EuSe is a low-temperature layered ferrimagnet that is particularly sensitive to external perturbations. We find that supe… ▽ More Enhanced magnetism has recently been reported for the topological-insulator/ferromagnet interface Bi$_2$Se$_3$/EuS with Curie temperatures claimed to be raised above room temperature from the bulk EuS value of 16 K. Here we investigate the analogous interface Bi$_2$Se$_3$/EuSe. EuSe is a low-temperature layered ferrimagnet that is particularly sensitive to external perturbations. We find that superconducting quantum interference device (SQUID) magnetometry of Bi$_2$Se$_3$/EuSe heterostructures reveals precisely the magnetic phase diagram known from EuSe, including the ferrimagnetic phase below 5 K, without any apparent changes from the bulk behavior. Choosing a temperature of 10 K to search for magnetic enhancement, we determine an upper limit for a possible magnetic coercive field of 3 mT. Using interface sensitive x-ray absorption spectroscopy we verify the magnetic divalent configuration of the Eu at the interface without contamination by Eu3+, and by x-ray magnetic circular dichroism (XMCD) we confirm at the interface the magnetic hysteresis obtained by SQUID. XMCD data obtained at 10 K in a magnetic field of 6 T indicate a magnetic spin moment of mz,spin = 7 $μ$B/Eu$^{2+}$, in good agreement with the SQUID data and the expected theoretical moment of Eu2+. Subsequent XMCD measurements in zero field show, however, that sizable remanent magnetization is absent at the interface for temperatures down to about 10 K. △ Less

Submitted 24 August, 2021; originally announced August 2021.

Comments: 11 figures, 35 references

Journal ref: Phys. Rev. B 103, (2021) 115438

Direct imaging of chiral domain walls and Néel-type skyrmionium in ferrimagnetic alloys

Authors: Boris Seng, Daniel Schönke, Javier Yeste, Robert M. Reeve, Nico Kerber, Daniel Lacour, Jean-Loïs Bello, Nicolas Bergeard, Fabian Kammerbauer, Mona Bhukta, Tom Ferté, Christine Boeglin, Florin Radu, Radu Abrudan, Torsten Kachel, Stéphane Mangin, Michel Hehn, Mathias Kläui

Abstract: The evolution of chiral spin structures is studied in ferrimagnet Ta/Ir/Fe/GdFeCo/Pt multilayers as a function of temperature using scanning electron microscopy with polarization analysis (SEMPA). The GdFeCo ferrimagnet exhibits pure right-hand Néel-type domain wall (DW) spin textures over a large temperature range. This indicates the presence of a negative Dzyaloshinskii-Moriya interaction (DMI)… ▽ More The evolution of chiral spin structures is studied in ferrimagnet Ta/Ir/Fe/GdFeCo/Pt multilayers as a function of temperature using scanning electron microscopy with polarization analysis (SEMPA). The GdFeCo ferrimagnet exhibits pure right-hand Néel-type domain wall (DW) spin textures over a large temperature range. This indicates the presence of a negative Dzyaloshinskii-Moriya interaction (DMI) that can originate from both the top Fe/Pt and the Co/Pt interfaces. From measurements of the DW width, as well as complementary magnetic characterization, the exchange stiffness as a function of temperature is ascertained. The exchange stiffness is surprisingly mostly constant, which is explained by theoretical predictions. Beyond single skyrmions, we find by direct imaging a pure Néel-type skyrmionium, which due to the absence of a skyrmion Hall angle is a promising topological spin structure to enable high impact potential applications in the next generation of spintronic devices. △ Less

Submitted 21 July, 2021; v1 submitted 26 February, 2021; originally announced February 2021.

Frequency-independent terahertz anomalous Hall effect in DyCo$_{5}$, Co$_{32}$Fe$_{68}$ and Gd$_{27}$Fe$_{73}$ thin films from DC to 40 THz

Authors: Tom S. Seifert, Ulrike Martens, Florin Radu, Mirkow Ribow, Marco Berritta, Lukas Nádvorník, Ronald Starke, Tomas Jungwirth, Martin Wolf, Ilie Radu, Markus Münzenberg, Peter M. Oppeneer, Georg Woltersdorf, Tobias Kampfrath

Abstract: The anomalous Hall effect (AHE) is a fundamental spintronic charge-to-charge-current conversion phenomenon and closely related to spin-to-charge-current conversion by the spin Hall effect. Future high-speed spintronic devices will crucially rely on such conversion effects at terahertz (THz) frequencies. Here, we reveal that the AHE remains operative from DC up to 40 THz with a flat frequency respo… ▽ More The anomalous Hall effect (AHE) is a fundamental spintronic charge-to-charge-current conversion phenomenon and closely related to spin-to-charge-current conversion by the spin Hall effect. Future high-speed spintronic devices will crucially rely on such conversion effects at terahertz (THz) frequencies. Here, we reveal that the AHE remains operative from DC up to 40 THz with a flat frequency response in thin films of three technologically relevant magnetic materials: DyCo$_{5}$, Co$_{32}$Fe$_{68}$ and Gd$_{27}$Fe$_{73}$. We measure the frequency-dependent conductivity-tensor elements $σ_{xx}$ and $σ_{yx}$ and find good agreement with DC measurements. Our experimental findings are fully consistent with ab-initio calculations of $σ_{yx}$ for CoFe and highlight the role of the large Drude scattering rate (~100 THz) of metal thin films, which smears out any sharp spectral features of the THz AHE. Finally, we find that the intrinsic contribution to the THz AHE dominates over the extrinsic mechanisms for the Co$_{32}$Fe$_{68}$ sample. The results imply that the AHE and related effects such as the spin Hall effect are highly promising ingredients of future THz spintronic devices reliably operating from DC to 40 THz and beyond. △ Less

Submitted 4 March, 2021; v1 submitted 3 November, 2020; originally announced November 2020.

Journal ref: Advanced Materials 2021, 2007398

Magnetization relaxation and search for the magnetic gap in bulk-insulating V-doped (Bi, Sb)$_2$Te$_3$

Authors: E. Golias, E. Weschke, T. Flanagan, E. Schierle, A. Richardella, E. D. L. Rienks, P. S. Mandal, A. Varykhalov, J. Sánchez-Barriga, F. Radu, N. Samarth, O. Rader

Abstract: V-doped (Bi,Sb)$_2$Te$_3$ has a ten times higher magnetic coercivity than its Cr-doped counterpart and therefore is believed to be a superior system for the quantum anomalous Hall effect (QAHE). The QAHE requires the opening of a magnetic band gap at the Dirac point. We do not find this gap by angle-resolved photoelectron spectroscopy down to 1 K. By x-ray magnetic circular dichroism (XMCD) we dir… ▽ More V-doped (Bi,Sb)$_2$Te$_3$ has a ten times higher magnetic coercivity than its Cr-doped counterpart and therefore is believed to be a superior system for the quantum anomalous Hall effect (QAHE). The QAHE requires the opening of a magnetic band gap at the Dirac point. We do not find this gap by angle-resolved photoelectron spectroscopy down to 1 K. By x-ray magnetic circular dichroism (XMCD) we directly probe the magnetism at the V site and in zerofield. Hysteresis curves of the XMCD signal show a strong dependence of the coercivity on the ramping velocity of the magnetic field. The XMCD signal decays on a time scale of minutes which we conclude contributes to the absence of a detectable magnetic gap at the Dirac point. △ Less

Submitted 14 October, 2020; originally announced October 2020.

Comments: 12 pages, 3 figures

Observation of compact ferrimagnetic skyrmions in DyCo$_3$ film

Authors: K. Chen, D. Lott, A. Philippi-Kobs, M. Weigand, C. Luo, F. Radu

Abstract: Owing to the experimental discovery of magnetic skyrmions stabilized by the Dzyaloshinskii-Moriya and/or dipolar interactions in thin films, there is a recent upsurge of interest in magnetic skyrmions with antiferromagnetic spins in order to overcome the fundamental limitations inherent with skyrmions in ferromagnetic materials. Here, we report on the observation of compact ferrimagnetic skyrmions… ▽ More Owing to the experimental discovery of magnetic skyrmions stabilized by the Dzyaloshinskii-Moriya and/or dipolar interactions in thin films, there is a recent upsurge of interest in magnetic skyrmions with antiferromagnetic spins in order to overcome the fundamental limitations inherent with skyrmions in ferromagnetic materials. Here, we report on the observation of compact ferrimagnetic skyrmions for the class of amorphous alloys consisting of 4f rare-earth and 3d transition-metal elements with perpendicular magnetic anisotropy, using a DyCo$_3$ film, that are identified by combining x-ray magnetic scattering, scanning transmission x-ray microscopy, and Hall transport technique. These skyrmions, with antiparallel aligned Dy and Co magnetic moments and a characteristic core radius of about 40~nm, are formed during the nucleation and annihilation of the magnetic maze-like domain pattern exhibiting a topological Hall effect contribution. Our findings provide a promising route for fundamental research in the field of ferrimagnetic/antiferromagnetic spintronics towards practical applications. △ Less

Submitted 3 August, 2020; originally announced August 2020.

Comments: 7 pages, 4 figures

Intrinsic 2D-XY ferromagnetism in a van der Waals monolayer

Authors: Amilcar Bedoya-Pinto, Jing-Rong Ji, Avanindra Pandeya, Pierluigi Gargiani, Manuel Valvidares, Paolo Sessi, Florin Radu, Kai Chang, Stuart Parkin

Abstract: Long before the recent fascination with two-dimensional materials, the critical behaviour and universality scaling of phase transitions in low-dimensional systems has been a topic of great interest. Particularly intriguing is the case of long-range magnetic order in two dimensions, once considered to be excluded in systems with continuous symmetry by the Hohenberg-Mermin-Wagner theorem. While an o… ▽ More Long before the recent fascination with two-dimensional materials, the critical behaviour and universality scaling of phase transitions in low-dimensional systems has been a topic of great interest. Particularly intriguing is the case of long-range magnetic order in two dimensions, once considered to be excluded in systems with continuous symmetry by the Hohenberg-Mermin-Wagner theorem. While an out-of-plane anisotropy has been shown to stabilize 2D magnetic order, this proof has remained elusive for a 2D magnet with in-plane rotational symmetry. Here, we construct a nearly ideal easy-plane system, a CrCl3 monolayer grown on Graphene/6H-SiC (0001), and unambiguously demonstrate robust in-plane ferromagnetic ordering with a critical scaling behaviour characteristic of a 2D-XY system. These observations suggest the first realization of a finite-size Berezinskii-Kosterlitz-Thouless (BKT) phase transition in a large-area, quasi-freestanding, van der Waals monolayer magnet with a XY universality class; and further constitute an ideal platform to study exotic phenomena like superfluid spin transport or 2D topological in-plane spin textures -- such as merons. △ Less

Submitted 16 February, 2021; v1 submitted 13 June, 2020; originally announced June 2020.

Journal ref: Science, Vol 374, Issue 6567, pp. 616-620 (2021)

Charge transfer induced interfacial ferromagnetism in La$_{0.7}$Sr$_{0.3}$MnO$_3$/NdNiO$_3$

Authors: K. Chen, C. Luo, B. B. Chen, R. M. Abrudan, G. Koster, S. K. Mishra, F. Radu

Abstract: Charge transfer induced interfacial ferromagnetism and its impact on the exchange bias effect in La$_{0.7}$Sr$_{0.3}$MnO$_3$/NdNiO$_3$ correlated oxide heterostructures were investigated by soft x-ray absorption and x-ray magnetic circular dichroism spectra in a temperature range from 10 to 300 K. Besides the antiferromagnetic Ni$_3^+$ cations which are naturally part of the NdNiO$_3$ layer, Ni… ▽ More Charge transfer induced interfacial ferromagnetism and its impact on the exchange bias effect in La$_{0.7}$Sr$_{0.3}$MnO$_3$/NdNiO$_3$ correlated oxide heterostructures were investigated by soft x-ray absorption and x-ray magnetic circular dichroism spectra in a temperature range from 10 to 300 K. Besides the antiferromagnetic Ni$_3^+$ cations which are naturally part of the NdNiO$_3$ layer, Ni$_2^+$ ions are formed at the interface due to a charge transfer mechanism involving the Mn element of the adjacent layer. They exhibit a ferromagnetic behavior due to the exchange coupling to the Mn$_4^+$ ions in the La$_{0.7}$Sr$_{0.3}$MnO$_3$ layer. This can be seen as detrimental to the strength of the unidirectional anisotropy since a significant part of the interface does not contribute to the pinning of the ferromagnetic layer. By analyzing the line shape changes of the x-ray absorption at the Ni L$_{2,3}$ edges, the metal-insulator transition of the NdNiO$_3$ layer is resolved in an element specific manner. This phase transition is initiated at about 120 K, way above the paramagnetic to antiferromagnetic transition of NdNiO$_3$ layer which measured to be 50 K. Exchange bias and enhanced coercive fields were observed after field cooling the sample through the Neel temperature of the NdNiO$_3$ layer. Different from La$_{0.7}$Sr$_{0.3}$MnO$_3$/LaNiO$_3$, the exchange bias observed in La$_{0.7}$Sr$_{0.3}$MnO$_3$/NdNiO$_3$ is due to the antiferromagnetism of NdNiO$_3$ and the frustration at the interface. These results suggest that reducing the interfacial orbital hybridization may be used as a tunable parameter for the strength of the exchange bias effect in all-oxide heterostructures which exhibit a charge transfer mechanism. △ Less

Submitted 28 April, 2020; originally announced April 2020.

Comments: 8 pages, 5 figures

Anomalous and topological Hall effects in epitaxial thin films of the noncollinear antiferromagnet Mn$_{3}$Sn

Authors: James M. Taylor, Anastasios Markou, Edouard Lesne, Pranava Keerthi Sivakumar, Chen Luo, Florin Radu, Peter Werner, Claudia Felser, Stuart S. P. Parkin

Abstract: Noncollinear antiferromagnets with a D0$_{19}$ (space group = 194, P6$_{3}$/mmc) hexagonal structure have garnered much attention for their potential applications in topological spintronics. Here, we report the deposition of continuous epitaxial thin films of such a material, Mn$_{3}$Sn, and characterize their crystal structure using a combination of x-ray diffraction and transmission electron mic… ▽ More Noncollinear antiferromagnets with a D0$_{19}$ (space group = 194, P6$_{3}$/mmc) hexagonal structure have garnered much attention for their potential applications in topological spintronics. Here, we report the deposition of continuous epitaxial thin films of such a material, Mn$_{3}$Sn, and characterize their crystal structure using a combination of x-ray diffraction and transmission electron microscopy. Growth of Mn$_{3}$Sn films with both (0001) c-axis orientation and (40$\bar{4}$3) texture is achieved. In the latter case, the thin films exhibit a small uncompensated Mn moment in the basal plane, quantified via magnetometry and x-ray magnetic circular dichroism experiments. This cannot account for the large anomalous Hall effect simultaneously observed in these films, even at room temperature, with magnitude $σ_{\mathrm{xy}}$ ($μ_{0}H$ = 0 T) = 21 $\mathrmΩ^{-1}\mathrm{cm}^{-1}$ and coercive field $μ_{0}H_{\mathrm{C}}$ = 1.3 T. We attribute the origin of this anomalous Hall effect to momentum-space Berry curvature arising from the symmetry-breaking inverse triangular spin structure of Mn$_{3}$Sn. Upon cooling through the transition to a glassy ferromagnetic state at around 50 K, a peak in the Hall resistivity close to the coercive field indicates the onset of a topological Hall effect contribution, due to the emergence of a scalar spin chirality generating a real-space Berry phase. We demonstrate that the polarity of this topological Hall effect, and hence the chiral-nature of the noncoplanar magnetic structure driving it, can be controlled using different field cooling conditions. △ Less

Submitted 12 December, 2019; v1 submitted 9 October, 2019; originally announced October 2019.

Journal ref: Phys. Rev. B 101, 094404 (2020)

Ferromagnetic resonance with magnetic phase selectivity by means of resonant elastic x-ray scattering on a chiral magnet

Authors: Simon Pöllath, Aisha Aqeel, Andreas Bauer, Chen Luo, Hanjo Ryll, Florin Radu, Christian Pfleiderer, Georg Woltersdorf, Christian H. Back

Abstract: Cubic chiral magnets, such as Cu$_{2}$OSeO$_{3}$, exhibit a variety of non-collinear spin textures, including a trigonal lattice of spin whirls, so-called skyrmions. Using magnetic resonant elastic x-ray scattering (REXS) on a crystalline Bragg peak and its magnetic satellites while exciting the sample with magnetic fields at GHz frequencies, we probe the ferromagnetic resonance modes of these spi… ▽ More Cubic chiral magnets, such as Cu$_{2}$OSeO$_{3}$, exhibit a variety of non-collinear spin textures, including a trigonal lattice of spin whirls, so-called skyrmions. Using magnetic resonant elastic x-ray scattering (REXS) on a crystalline Bragg peak and its magnetic satellites while exciting the sample with magnetic fields at GHz frequencies, we probe the ferromagnetic resonance modes of these spin textures by means of the scattered intensity. Most notably, the three eigenmodes of the skyrmion lattice are detected with large sensitivity. As this novel technique, which we label REXS-FMR, is carried out at distinct positions in reciprocal space, it allows to distinguish contributions originating from different magnetic states, providing information on the precise character, weight and mode mixing as a prerequisite of tailored excitations for applications. △ Less

Submitted 20 September, 2019; v1 submitted 18 September, 2019; originally announced September 2019.

Comments: 18 pages, main text with appended supplementary material, 3 main figures, 1 supplementary figure

Magnetic and electrical transport signatures of uncompensated moments in epitaxial thin films of the non-collinear antiferromagnet Mn$_{3}$Ir

Authors: James M. Taylor, Edouard Lesne, Anastasios Markou, Fasil Kidane Dejene, Pranava Keerthi Sivakumar, Simon Pöllath, Kumari Gaurav Rana, Neeraj Kumar, Chen Luo, Hanjo Ryll, Florin Radu, Florian Kronast, Peter Werner, Christian H. Back, Claudia Felser, Stuart S. P. Parkin

Abstract: Non-collinear antiferromagnets, with either an L1$_{2}$ cubic crystal lattice (e.g. Mn$_{3}$Ir and Mn$_{3}$Pt) or a D0$_{19}$ hexagonal structure (e.g. Mn$_{3}$Sn and Mn$_{3}$Ge), exhibit a number of novel phenomena of interest to topological spintronics. Amongst the cubic systems, for example, tetragonally distorted Mn$_{3}$Pt exhibits an intrinsic anomalous Hall effect (AHE). However, Mn$_{3}$Pt… ▽ More Non-collinear antiferromagnets, with either an L1$_{2}$ cubic crystal lattice (e.g. Mn$_{3}$Ir and Mn$_{3}$Pt) or a D0$_{19}$ hexagonal structure (e.g. Mn$_{3}$Sn and Mn$_{3}$Ge), exhibit a number of novel phenomena of interest to topological spintronics. Amongst the cubic systems, for example, tetragonally distorted Mn$_{3}$Pt exhibits an intrinsic anomalous Hall effect (AHE). However, Mn$_{3}$Pt only enters a non-collinear magnetic phase close to the stoichiometric composition and at suitably large thicknesses. Therefore, we turn our attention to Mn$_{3}$Ir, the material of choice for use in exchange bias heterostructures. In this paper, we investigate the magnetic and electrical transport properties of epitaxially grown, face-centered-cubic $γ$-Mn$_{3}$Ir thin films with (111) crystal orientation. Relaxed films of 10 nm thickness exhibit an ordinary Hall effect, with a hole-type carrier concentration of (2.24 $\pm$ 0.08) $\times$ 10$^{23}$ cm$^{-3}$. On the other hand, TEM characterization demonstrates that ultrathin 3 nm films grow with significant in-plane tensile strain. This may explain a small remanent moment, observed at low temperatures, shown by XMCD spectroscopy to arise from uncompensated Mn spins. Of the order 0.02 $μ_{B}$ / atom, this dominates electrical transport behavior, leading to a small AHE and negative magnetoresistance. These results are discussed in terms of crystal microstructure and chiral domain behavior, with spatially resolved XML(C)D-PEEM supporting the conclusion that small antiferromagnetic domains, < 20 nm in size, of differing chirality account for the absence of observed Berry curvature driven magnetotransport effects. △ Less

Submitted 9 April, 2019; originally announced April 2019.

Journal ref: Appl. Phys. Lett. 115, 062403 (2019)

X-ray magnetic linear dichroism as a probe for non-collinear magnetic state in ferrimagnetic single layer exchange bias systems

Authors: Chen Luo, Hanjo Ryll, Christian H. Back, Florin Radu

Abstract: Ferrimagnetic alloys are extensively studied for their unique magnetic properties leading to possible applications in perpendicular magnetic recording, due to their deterministic ultrafast switching and heat assisted magnetic recording capabilities. On a prototype ferrimagnetic alloy we demonstrate fascinating properties that occur close to a critical temperature where the magnetization is vanishi… ▽ More Ferrimagnetic alloys are extensively studied for their unique magnetic properties leading to possible applications in perpendicular magnetic recording, due to their deterministic ultrafast switching and heat assisted magnetic recording capabilities. On a prototype ferrimagnetic alloy we demonstrate fascinating properties that occur close to a critical temperature where the magnetization is vanishing, just as in an antiferromagnet. From the X-ray magnetic circular dichroism measurements, an anomalous 'wing shape' hysteresis loop is observed slightly above the compensation temperature. This bears the characteristics of an intrinsic exchange bias effect, referred to as atomic exchange bias. We further exploit the X-ray magnetic linear dichroism (XMLD) contrast for probing non-collinear states which allows us to discriminate between two main reversal mechanisms, namely perpendicular domain wall formation versus spin-flop transition. Ultimately, we analyze the elemental magnetic moments for the surface and the bulk parts, separately, which allows to identify in the phase diagram the temperature window where this effect takes place. Moreover, we suggests that this effect is a general phenomenon in ferrimagnetic thin films which may also contribue to the understanding of the mechanism behind the all optical switching effect. △ Less

Submitted 13 November, 2018; originally announced November 2018.

Comments: 20 pages, 9 figures

Polarized neutron channeling in weakly magnetic films

Authors: S. V. Kozhevnikov, V. D. Zhaketov, T. Keller, Yu. N. Khaydukov, F. Ott, F. Radu

Abstract: We describe a new sensitive method for the investigation of weakly magnetic films placed inside a tri-layer planar waveguide. Polarized neutrons tunnel into the waveguide through the surface, channel along the layers and are emitted from the end face as a narrow and slightly divergent microbeam. Polarization analysis permits to detect very small magnetization in the order of a few 10 Gauss. The ma… ▽ More We describe a new sensitive method for the investigation of weakly magnetic films placed inside a tri-layer planar waveguide. Polarized neutrons tunnel into the waveguide through the surface, channel along the layers and are emitted from the end face as a narrow and slightly divergent microbeam. Polarization analysis permits to detect very small magnetization in the order of a few 10 Gauss. The magnetic film containing the rare-earth element Tb was investigated using both fixed wavelength and time-of-flight polarized neutron reflectometers. The experimental results are presented and discussed. △ Less

Submitted 16 October, 2018; originally announced October 2018.

Comments: Submitted to NIM A

Divergence of neutron microbeams from planar waveguides

Authors: S. V. Kozhevnikov, V. D. Zhaketov, T. Keller, Yu. N. Khaydukov, F. Ott, F. Radu

Abstract: Neutron planar waveguides are focusing devices generating a narrow neutron beam of submicron width. Such a neutron microbeam can be used for the investigation of local microstructures with high spatial resolution. The essential parameter of the microbeam is its angular width. The main contribution to the microbeam angular divergence is Fraunhofer diffraction on a narrow slit. We review and discuss… ▽ More Neutron planar waveguides are focusing devices generating a narrow neutron beam of submicron width. Such a neutron microbeam can be used for the investigation of local microstructures with high spatial resolution. The essential parameter of the microbeam is its angular width. The main contribution to the microbeam angular divergence is Fraunhofer diffraction on a narrow slit. We review and discuss various ways to characterize the angular divergence of the neutron microbeam using time-of-flight and fixed wavelength reflectometers. △ Less

Submitted 15 October, 2018; originally announced October 2018.

Comments: 17 pages, 16 figures. Submitted to NIM A

Magnetic-field-induced incommensurate to collinear spin order transition in NiBr$_{2}$

Authors: S. Babu, K. Prokes, Y. K. Huang, F. Radu, S. K. Mishra

Abstract: The triangular spin lattice of NiBr$_{2}$ is a canonical example of a frustrated helimagnet that shows a temperature-driven phase transition from a collinear commensurate antiferromagnetic structure to an incommensurate spin helix on cooling. Employing neutron diffraction, bulk magnetization, and magnetic susceptibility measurements, we have studied the f\hspace*{.5pt}ield-induced magnetic states… ▽ More The triangular spin lattice of NiBr$_{2}$ is a canonical example of a frustrated helimagnet that shows a temperature-driven phase transition from a collinear commensurate antiferromagnetic structure to an incommensurate spin helix on cooling. Employing neutron diffraction, bulk magnetization, and magnetic susceptibility measurements, we have studied the f\hspace*{.5pt}ield-induced magnetic states of the NiBr$_{2}$ single crystal. Experimental f\hspace*{.5pt}indings enable us to recapitalize the driving forces of the spin spiral ordering in the triangular spin-lattice systems, in general. Neutron diffraction data conf\hspace*{.5pt}irms, at low temperature below T$_{\rm m}$ = 22.8(1) K, the presence of diffraction satellites characteristic of an incommensurate magnetic state, which are symmetrically arranged around main magnetic reflections that evolve just below T$_{\rm N}$ = 44.0(1) K. Interestingly, a f\hspace*{.5pt}ield-induced transition from the incommensurate to commensurate spin phase has been demonstrated that enforces spin helix to restore the high temperature compensated antiferromagnetic structure. This spin reorientation can be described as a spin-flop transition in the (\hbox{$a$--$b$}) basal plane of a triangular spin lattice system. These f\hspace*{.5pt}indings offer a new pathway to control the spin helix in incommensurate phases that are currently considered having high technical implications in the next-generation data storage devices. △ Less

Submitted 16 October, 2018; v1 submitted 11 October, 2018; originally announced October 2018.

Comments: 28 pages, 11 figures

Report number: 019910JAP

Journal ref: 2019

Neutron channeling in a nonmagnetic planar waveguide

Authors: S. V. Kozhevnikov, T. Keller, Yu. N. Khaydukov, F. Ott, F. Radu

Abstract: We investigate neutron propagation in a middle layer of a planar waveguide which is a tri-layer thin film. A narrow divergent microbeam emitted from the end face of the film is registered. The neutron channeling length is experimentally measured as a function of the guiding channel width. Experimental results are compared with calculations. We investigate neutron propagation in a middle layer of a planar waveguide which is a tri-layer thin film. A narrow divergent microbeam emitted from the end face of the film is registered. The neutron channeling length is experimentally measured as a function of the guiding channel width. Experimental results are compared with calculations. △ Less

Submitted 10 August, 2018; originally announced August 2018.

Comments: 11 pages, 12 figures, submitted to JETP

Experimental determination of exchange constants in antiferromagnetic Mn$_2$Au

Authors: A. A. Sapozhnik, C. Luo, H. Ryll, F. Radu, M. Jourdan, H. Zabel, H. -J. Elmers

Abstract: Mn$_2$Au is an important antiferromagnetic (AF) material for spintronics applications. Due to its very high Néel temperature of about 1500 K, some of the basic properties are difficult to explore, such as the AF susceptibility and the exchange constants. Experimental determination of these properties is further complicated in thin films by unavoidable presence of uncompensated and quasiloose spins… ▽ More Mn$_2$Au is an important antiferromagnetic (AF) material for spintronics applications. Due to its very high Néel temperature of about 1500 K, some of the basic properties are difficult to explore, such as the AF susceptibility and the exchange constants. Experimental determination of these properties is further complicated in thin films by unavoidable presence of uncompensated and quasiloose spins on antisites and at interfaces. Using x-ray magnetic circular dichroism (XMCD), we have measured the spin and orbital contribution to the susceptibility in the direction perpendicular to the in-plane magnetic moments of a Mn$_2$Au(001) film and in fields up to 8 T. By performing these measurements at a low temperature of 7 K and at room temperature, we were able to separate the loose spin contribution from the susceptibility of AF coupled spins. The value of the AF exchange constant obtained with this method for a 10 nm thick Mn$_2$Au(001) film equals to (24 $\pm$ 5) meV. △ Less

Submitted 9 March, 2018; originally announced March 2018.

The complete micromagnetic characterization of asymmetrically sandwiched ferromagnetic films

Authors: M. Kopte, U. K. Rößler, R. Schäfer, T. Kosub, A. Kákay, O. Volkov, H. Fuchs, E. Y. Vedmedenko, F. Radu, O. G. Schmidt, J. Lindner, J. Faßbender, D. Makarov

Abstract: The magnetic properties of ferromagnetic thin films down to the nanoscale are ruled by the exchange stiffness, anisotropies and the effects of magnetic fields. As surfaces break inversion symmetry, an additional effective chiral exchange is omnipresent in any magnetic nanostructure. These so-called Dzyaloshinskii-Moriya interactions (DMI) affect all inhomogeneous magnetic states either subtly or s… ▽ More The magnetic properties of ferromagnetic thin films down to the nanoscale are ruled by the exchange stiffness, anisotropies and the effects of magnetic fields. As surfaces break inversion symmetry, an additional effective chiral exchange is omnipresent in any magnetic nanostructure. These so-called Dzyaloshinskii-Moriya interactions (DMI) affect all inhomogeneous magnetic states either subtly or spectacularly. E.g., DMIs cause a chirality selection of the rotation sense and can fix the local rotation axis for the magnetization in domain walls (DW). But, they can stabilize also entirely different twisted magnetic structures. The chiral skyrmions a two-dimensional particle-like topological soliton is the ultimately smallest of these objects, which currently is targetted as a possible information carrier in novel spintronic devices. Observation and quantification of the chiral exchange effects provide for the salient point in understanding magnetic properties in ultrathin films and other nanostructures. An easy and reliable method to determine the DMI constant as materials parameter of asymmetric thin films is the crucial problem. Here, we put forth an experimental approach for the determination of the complete set of the micromagnetic parameters. Quasi-static Kerr microcopy observations of DW creep motion and equilibrium sizes of circular magnetic objects in combination with standard magnetometry are used to derive a consistent set of these materials parameters in polycrystalline ultrathin film systems, namely CrOx/Co/Pt stacks. The quantified micromagnetic model for these films identifies the circular magnetic objects, as seen by the optical microscopy, as ordinary bubble domains with homochiral walls. From micromagnetic calculations, the chiral skyrmions stabilized by the DMI in these films are shown to have diameters in the range 40-200nm, too small to be observed by optical microscopy. △ Less

Submitted 19 October, 2017; v1 submitted 28 June, 2017; originally announced June 2017.

Terahertz spin currents and inverse spin Hall effect in thin-film heterostructures containing complex magnetic compounds

Authors: T. Seifert, U. Martens, S. Günther, M. A. W. Schoen, F. Radu, X. Z. Chen, I. Lucas, R. Ramos, M. H. Aguirre, P. A. Algarabel, A. Anadón, H. Körner, J. Walowski, C. Back, M. R. Ibarra, L. Morellón, E. Saitoh, M. Wolf, C. Song, K. Uchida, M. Münzenberg, I. Radu, T. Kampfrath

Abstract: Terahertz emission spectroscopy of ultrathin multilayers of magnetic and heavy metals has recently attracted much interest. This method not only provides fundamental insights into photoinduced spin transport and spin-orbit interaction at highest frequencies but has also paved the way to applications such as efficient and ultrabroadband emitters of terahertz electromagnetic radiation. So far, predo… ▽ More Terahertz emission spectroscopy of ultrathin multilayers of magnetic and heavy metals has recently attracted much interest. This method not only provides fundamental insights into photoinduced spin transport and spin-orbit interaction at highest frequencies but has also paved the way to applications such as efficient and ultrabroadband emitters of terahertz electromagnetic radiation. So far, predominantly standard ferromagnetic materials have been exploited. Here, by introducing a suitable figure of merit, we systematically compare the strength of terahertz emission from X/Pt bilayers with X being a complex ferro-, ferri- and antiferromagnetic metal, that is, dysprosium cobalt (DyCo$_5$), gadolinium iron (Gd$_{24}$Fe$_{76}$), Magnetite (Fe$_3$O$_4$) and iron rhodium (FeRh). We find that the performance in terms of spin-current generation not only depends on the spin polarization of the magnet's conduction electrons but also on the specific interface conditions, thereby suggesting terahertz emission spectroscopy to be a highly surface-sensitive technique. In general, our results are relevant for all applications that rely on the optical generation of ultrafast spin currents in spintronic metallic multilayers. △ Less

Submitted 14 July, 2021; v1 submitted 31 May, 2017; originally announced May 2017.

Journal ref: SPIN, Volume 07, Issue 03, September 2017

Application of Zeeman spatial beam-splitting in polarized neutron reflectometry

Authors: S. V. Kozhevnikov, V. K. Ignatovich, F. Radu

Abstract: Neutron Zeeman spatial beam-splitting is considered at reflection from magnetically noncollinear films. Two applications of Zeeman beam-splitting phenomenon in polarized neutron reflectometry are discussed. One is the construction of polarizing devices with high polarizing efficiency. Another one is the investigations of magnetically noncollinear films with low spin-flip probability. Experimental… ▽ More Neutron Zeeman spatial beam-splitting is considered at reflection from magnetically noncollinear films. Two applications of Zeeman beam-splitting phenomenon in polarized neutron reflectometry are discussed. One is the construction of polarizing devices with high polarizing efficiency. Another one is the investigations of magnetically noncollinear films with low spin-flip probability. Experimental results are presented for illustration. △ Less

Submitted 27 January, 2017; originally announced January 2017.

Comments: 10 pages, 10 figures

Neutron resonances in planar waveguides

Authors: S. V. Kozhevnikov, V. K. Ignatovich, A. V. Petrenko, F. Radu

Abstract: Results of experimental investigations of a neutron resonances width in planar waveguides using the time-of-flight reflectometer REMUR of the IBR-2 pulsed reactor are reported and comparison with theoretical calculations is presented. The intensity of the neutron microbeam emitted from the waveguide edge was registered as a function of the neutron wavelength and the incident beam angular divergenc… ▽ More Results of experimental investigations of a neutron resonances width in planar waveguides using the time-of-flight reflectometer REMUR of the IBR-2 pulsed reactor are reported and comparison with theoretical calculations is presented. The intensity of the neutron microbeam emitted from the waveguide edge was registered as a function of the neutron wavelength and the incident beam angular divergence. The possible applications of this method for the investigations of layered nanostructures are discussed. △ Less

Submitted 5 March, 2016; originally announced March 2016.

Polarized neutron channeling as a tool for the investigations of weakly magnetic thin films

Authors: S. V. Kozhevnikov, Yu. N. Khaydukov, T. Keller, F. Ott, F. Radu

Abstract: We present and apply a new method to measure directly weak magnetization in thin films. The polarization of a neutron beam channeling through a thin film structure is measured after exiting the structure edge as a microbeam. We have applied the method to a tri-layer thin film structure acting as a planar waveguide for polarized neutrons. The middle guiding layer is a rare earth based ferrimagnetic… ▽ More We present and apply a new method to measure directly weak magnetization in thin films. The polarization of a neutron beam channeling through a thin film structure is measured after exiting the structure edge as a microbeam. We have applied the method to a tri-layer thin film structure acting as a planar waveguide for polarized neutrons. The middle guiding layer is a rare earth based ferrimagnetic material TbCo5 with a low magnetization of about 20 mT. We demonstrate that the channeling method is more sensitive than the specular neutron reflection method. △ Less

Submitted 24 November, 2015; v1 submitted 23 November, 2015; originally announced November 2015.

Neutron methods for the direct determination of the magnetic induction in thick films

Authors: S. V. Kozhevnikov, F. Ott, F. Radu

Abstract: We review different neutron methods which allow extracting directly the value of the magnetic induction in thick films: Larmor precession, Zeeman spatial beam-splitting and neutron spin resonance. Resulting parameters obtained by the neutron methods and standard magnetometry technique are presented and compared. The possibilities and specificities of the neutron methods are discussed. We review different neutron methods which allow extracting directly the value of the magnetic induction in thick films: Larmor precession, Zeeman spatial beam-splitting and neutron spin resonance. Resulting parameters obtained by the neutron methods and standard magnetometry technique are presented and compared. The possibilities and specificities of the neutron methods are discussed. △ Less

Submitted 20 November, 2015; v1 submitted 17 November, 2015; originally announced November 2015.

Journal ref: Journal of Magnetism and Magnetic Materials 402, pp. 83-93 (2016)

Ferrimagnetic nanostructures for magnetic memory bits

Authors: A. A. Ünal, S. Valencia, D. Marchenko, K. J. Merazzo, F. Radu, M. Vázquez, J. Sánchez-Barriga

Abstract: Increasing the magnetic data recording density requires reducing the size of the individual memory elements of a recording layer as well as employing magnetic materials with temperature-dependent functionalities. Therefore, it is predicted that the near future of magnetic data storage technology involves a combination of energy-assisted recording on nanometer-scale magnetic media. We present the p… ▽ More Increasing the magnetic data recording density requires reducing the size of the individual memory elements of a recording layer as well as employing magnetic materials with temperature-dependent functionalities. Therefore, it is predicted that the near future of magnetic data storage technology involves a combination of energy-assisted recording on nanometer-scale magnetic media. We present the potential of heat-assisted magnetic recording on a patterned sample; a ferrimagnetic alloy composed of a rare earth and a transition metal, DyCo$_5$, which is grown on a hexagonal-ordered nanohole array membrane. The magnetization of the antidot array sample is out-of-plane oriented at room temperature and rotates towards in-plane upon heating above its spin-reorientation temperature (T$_R$) of ~350 K, just above room temperature. Upon cooling back to room temperature (below T$_R$), we observe a well-defined and unexpected in-plane magnetic domain configuration modulating with ~45 nm. We discuss the underlying mechanisms giving rise to this behavior by comparing the magnetic properties of the patterned sample with the ones of its extended thin film counterpart. Our results pave the way for novel applications of ferrimagnetic antidot arrays of superior functionality in magnetic nano-devices near room temperature. △ Less

Submitted 17 October, 2015; originally announced October 2015.

Comments: 19 pages, 4 figures

Journal ref: Phys. Rev. Applied 5, 064007 (2016)

Valence change of praseodymium in Pr0.5Ca0.5CoO3 investigated by x-ray absorption spectroscopy

Authors: J. Herrero-Martín, J. L. García-Muñoz, S. Valencia, C. Frontera, J. Blasco, A. J. Barón-González, G. Subías, R. Abrudan, F. Radu, E. Dudzik, R. Feyerherm

Abstract: X-ray absorption spectroscopy measurements in Pr0.5Ca0.5CoO3 were performed at the Pr M4,5, Pr L3, and Ca L2,3 absorption edges as a function of temperature below 300 K. Ca spectra show no changes down to 10 K while a noticeable thermally dependent evolution takes place at the Pr edges across the metal-insulator transition. Spectral changes are analyzed by different methods, including multiple sca… ▽ More X-ray absorption spectroscopy measurements in Pr0.5Ca0.5CoO3 were performed at the Pr M4,5, Pr L3, and Ca L2,3 absorption edges as a function of temperature below 300 K. Ca spectra show no changes down to 10 K while a noticeable thermally dependent evolution takes place at the Pr edges across the metal-insulator transition. Spectral changes are analyzed by different methods, including multiple scattering simulations, which provide quantitative details on an electron loss at Pr 4f orbitals. We conclude that in the insulating phase a fraction [15(+5)%] of Pr3+ undergoes a further oxidation to adopt a hybridized configuration composed of an admixture of atomic-like 4f1 states (Pr4+) and f- symmetry states on the O 2p valence band (Pr3+L states) indicative of a strong 4f- 2p interaction. △ Less

Submitted 5 September, 2011; originally announced September 2011.

Comments: 19 pages (.doc), 4 figures, Phys. Rev. B, in press

Observation of a valence transition in (Pr,Ca)CoO3 cobaltites: charge migration at the metal-insulator transition

Authors: José Luis García-Muñoz, Carlos Frontera, Aura J. Barón-González, Sergio Valencia, Javier Blasco, Ralf Feyerherm, Esther Dudzik, Radu Abrudan, Florin Radu

Abstract: X-ray absorption spectroscopy measurements in Pr0.5Ca0.5CoO3 and (Pr,Y)0.55Ca0.45CoO3 compositions reveal that the valence of praseodymium ions is stable and essentially +3 (Pr [4f 2]) in the metallic state, but abruptly changes when carriers localize approaching the oxidation state +4 (Pr [4f 1]). This mechanism appears to be the driving force of the metal-insulator transition. The ground insulat… ▽ More X-ray absorption spectroscopy measurements in Pr0.5Ca0.5CoO3 and (Pr,Y)0.55Ca0.45CoO3 compositions reveal that the valence of praseodymium ions is stable and essentially +3 (Pr [4f 2]) in the metallic state, but abruptly changes when carriers localize approaching the oxidation state +4 (Pr [4f 1]). This mechanism appears to be the driving force of the metal-insulator transition. The ground insulating state of Pr0.5Ca0.5CoO3 is an homogeneous Co3.5-d state stabilized by a charge transfer from Pr to Co sites: 1/2Pr3+ + Co3.5 \to 1/2Pr3+2d + Co3.5-d, with 2d \approx 0.26 e-. △ Less

Submitted 18 March, 2011; originally announced March 2011.

Comments: Submitted. 14 pages, 4 Figures

Dual Behavior of Antiferromagnetic Uncompensated Spins in NiFe/IrMn Exchange Biased Bilayers

Authors: S. K. Mishra, F. Radu, S. Valencia, D. Schmitz, E. Schierle, H. A. Dürr, W. Eberhardt

Abstract: We present a comprehensive study of the exchange bias effect in a model system. Through numerical analysis of the exchange bias and coercive fields as a function of the antiferromagnetic layer thickness we deduce the absolute value of the averaged anisotropy constant of the antiferromagnet. We show that the anisotropy of IrMn exhibits a finite size effect as a function of thickness. The interfac… ▽ More We present a comprehensive study of the exchange bias effect in a model system. Through numerical analysis of the exchange bias and coercive fields as a function of the antiferromagnetic layer thickness we deduce the absolute value of the averaged anisotropy constant of the antiferromagnet. We show that the anisotropy of IrMn exhibits a finite size effect as a function of thickness. The interfacial spin disorder involved in the data analysis is further supported by the observation of the dual behavior of the interfacial uncompensated spins. Utilizing soft x-ray resonant magnetic reflectometry we have observed that the antiferromagnetic uncompensated spins are dominantly frozen with nearly no rotating spins due to the chemical intermixing, which correlates to the inferred mechanism for the exchange bias. △ Less

Submitted 9 December, 2009; originally announced December 2009.

Comments: 4 pages, 3 figures

Journal ref: Phys. Rev. B 81, 212404 (2010)

Interplay between the magnetic anisotropy contributions of Cobalt nanowires

Authors: J. Sanchez-Barriga, M. Lucas, F. Radu, E. Martin, M. Multigner, P. Marin, A. Hernando, G. Rivero

Abstract: We report on the magnetic properties and the crystallographic structure of the cobalt nanowire arrays as a function of their nanoscale dimensions. X-ray diffraction measurements show the appearance of an in-plane HCP-Co phase for nanowires with 50 nm diameter, suggesting a partial reorientation of the magnetocrystalline anisotropy axis along the membrane plane with increasing pore diameter. No s… ▽ More We report on the magnetic properties and the crystallographic structure of the cobalt nanowire arrays as a function of their nanoscale dimensions. X-ray diffraction measurements show the appearance of an in-plane HCP-Co phase for nanowires with 50 nm diameter, suggesting a partial reorientation of the magnetocrystalline anisotropy axis along the membrane plane with increasing pore diameter. No significant changes in the magnetic behavior of the nanowire system are observed with decreasing temperature, indicating that the effective magnetoelastic anisotropy does not play a dominant role in the remagnetization processes of individual nanowires. An enhancement of the total magnetic anisotropy is found at room temperature with a decreasing nanowire diameter-to-length ratio (d/L), a result that is quantitatively analyzed on the basis of a simplified shape anisotropy model. △ Less

Submitted 22 October, 2009; originally announced October 2009.

Comments: 8 pages, 4 figures

Journal ref: Phys. Rev. B 80, 184424 (2009)

Training Induced Positive Exchange Bias in NiFe/IrMn Bilayers

Authors: S. K. Mishra, F. Radu, H. A. Dürr, W. Eberhardt

Abstract: Positive exchange bias has been observed in the Ni$_{81}$Fe$_{19}$/Ir$_{20}$Mn$_{80}$ bilayer system via soft x-ray resonant magnetic scattering. After field cooling of the system through the blocking temperature of the antiferromagnet, an initial conventional negative exchange bias is removed after training i. e. successive magnetization reversals, resulting in a positive exchange bias for a te… ▽ More Positive exchange bias has been observed in the Ni$_{81}$Fe$_{19}$/Ir$_{20}$Mn$_{80}$ bilayer system via soft x-ray resonant magnetic scattering. After field cooling of the system through the blocking temperature of the antiferromagnet, an initial conventional negative exchange bias is removed after training i. e. successive magnetization reversals, resulting in a positive exchange bias for a temperature range down to 30 K below the blocking temperature (450 K). This new manifestation of magnetic training is discussed in terms of metastable magnetic disorder at the magnetically frustrated interface during magnetization reversal. △ Less

Submitted 7 March, 2009; v1 submitted 19 February, 2009; originally announced February 2009.

Comments: 4 pages, 3 figures

Journal ref: Phys. Rev. Lett. 102, 177208 (2009)

Origin of the reduced exchange bias in epitaxial FeNi(111)/CoO(111) bilayer

Authors: F. Radu, S. K. Mishra, I. Zizak, A. I. Erko, H. A. Durr, W. Eberhardt, G. Nowak, S. Buschhorn, K. Zhernenkov, M. Wolff, H. Zabel, D. Schmitz, E. Schierle, E. Dudzik, R. Feyerherm

Abstract: We have employed Soft and Hard X-ray Resonant Magnetic Scattering and Polarised Neutron Diffraction to study the magnetic interface and the bulk antiferromagnetic domain state of the archetypal epitaxial Ni$_{81}$Fe$_{19}$(111)/CoO(111) exchange biased bilayer. The combination of these scattering tools provides unprecedented detailed insights into the still incomplete understanding of some key m… ▽ More We have employed Soft and Hard X-ray Resonant Magnetic Scattering and Polarised Neutron Diffraction to study the magnetic interface and the bulk antiferromagnetic domain state of the archetypal epitaxial Ni$_{81}$Fe$_{19}$(111)/CoO(111) exchange biased bilayer. The combination of these scattering tools provides unprecedented detailed insights into the still incomplete understanding of some key manifestations of the exchange bias effect. We show that the several orders of magnitude difference between the expected and measured value of exchange bias field is caused by an almost anisotropic in-plane orientation of antiferromagnetic domains. Irreversible changes of their configuration lead to a training effect. This is directly seen as a change in the magnetic half order Bragg peaks after magnetization reversal. A 30 nm size of antiferromagnetic domains is extracted from the width the (1/2 1/2 1/2) antiferromagnetic magnetic peak measured both by neutron and x-ray scattering. A reduced blocking temperature as compared to the measured antiferromagnetic ordering temperature clearly corresponds to the blocking of antiferromagnetic domains. Moreover, an excellent correlation between the size of the antiferromagnetic domains, exchange bias field and frozen-in spin ratio is found, providing a comprehensive understanding of the origin of exchange bias in epitaxial systems. △ Less

Submitted 30 November, 2008; v1 submitted 3 August, 2008; originally announced August 2008.

Comments: 8 pages, 5 figures, submitted

Journal ref: Phys. Rev. B 79, 184425 (2009)

Exchange bias effect of ferro-/antiferromagnetic heterostructures

Authors: Florin Radu, Hartmut Zabel

Abstract: The exchange bias (EB) effect was discovered 60 years ago by Meiklejohn and Bean. Meanwhile the EB effect has become an integral part of modern magnetism with implications for basic research and for numerous device applications. The EB effect was the first of its kind which relates to an interface effect between two different classes of materials, here between a ferromagnet and an antiferromagne… ▽ More The exchange bias (EB) effect was discovered 60 years ago by Meiklejohn and Bean. Meanwhile the EB effect has become an integral part of modern magnetism with implications for basic research and for numerous device applications. The EB effect was the first of its kind which relates to an interface effect between two different classes of materials, here between a ferromagnet and an antiferromagnet. Here we review fundamental aspects of the exchange bias effect. △ Less

Submitted 14 May, 2007; originally announced May 2007.

Comments: This contribution will be published as a book chapter in Springer :Tracts on Materials Science: "Magnetic Heterostructures, Advances and Perspectives in Spinstructures and Spintransport", edited by Hartmut Zabel and Samuel D. Bader

Journal ref: Springer Tracts in Modern Physics 227, pp. 97-184, 2008