2024
Self-assembly of Co/Pt stripes with current-induced domain wall motion towards 3D racetrack devices
P. Fedorov, I. Soldatov, V. Neu, R. Schäfer, O. G Schmidt, D. Karnaushenko,
Nature Communications, 15, 2048 (2024)
Modification of the magnetic properties under the induced strain and curvature is a promising avenue to build three-dimensional magnetic devices, based on the domain wall motion. So far, most of the studies with 3D magnetic structures were performed in the helixes and nanowires, mainly with stationary domain walls. In this study, we demonstrate the impact of 3D geometry, strain and curvature on the current-induced domain wall motion and spin-orbital torque efficiency in the heterostructure, realized via a self-assembly rolling technique on a polymeric platform. We introduce a complete 3D memory unit with write, read and store functionality, all based on the field-free domain wall motion. Additionally, we conducted a comparative analysis between 2D and 3D structures, particularly addressing the influence of heat during the electric current pulse sequences. Finally, we demonstrated a remarkable increase of 30% in spin-torque efficiency in 3D configuration.
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Single-crystalline YIG flakes with uniaxial in-plane anisotropy and diverse crystallographic orientations
R. Hartmann, Seema, I. Soldatov, M. Lammel, D. Lignon, X. Y. Ai, G. Kiliani, R. Schäfer, A. Erb, R. Gross, J. Boneberg, M. Müller, S. T. B. Goennenwein, E. Scheer and A. Di Bernardo,
APL Materials, 12 (031121), 262404 (2024)
We study sub-micron Y3Fe5O12 (YIG) flakes that we produce via mechanical cleaving and exfoliation of YIG single crystals. By characterizing their structural and magnetic properties, we find that these YIG flakes have surfaces oriented along unusual crystallographic axes and uniaxial in-plane magnetic anisotropy due to their shape, both of which are not commonly available in YIG thin films. These physical properties, combined with the possibility of picking up the YIG flakes and stacking them onto flakes of other van der Waals materials or pre-patterned electrodes or waveguides, open unexplored possibilities for magnonics and for the realization of novel YIG-based heterostructures and spintronic devices.
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2023
Strong and ductile high temperature soft magnets through Widmanstätten precipitates
L. Han, F. Maccari, I. Soldatov, N. J. Peter, I. R. S. Filho, R. Schäfer, O. Gutfleisch, Z. Li & D. Raabe,
Nature Communications, 14, 8176 (2023)
Fast growth of sustainable energy production requires massive electrification of transport, industry and households, with electrical motors as key components. These need soft magnets with high saturation magnetization, mechanical strength, and thermal stability to operate efficiently and safely. Reconciling these properties in one material is challenging because thermally-stable microstructures for strength increase conflict with magnetic performance. Here, we present a material concept that combines thermal stability, soft magnetic response, and high mechanical strength. The strong and ductile soft ferromagnet is realized as a multicomponent alloy in which precipitates with a large aspect ratio form a Widmanstätten pattern. The material shows excellent magnetic and mechanical properties at high temperatures while the reference alloy with identical composition devoid of precipitates significantly loses its magnetization and strength at identical temperatures. The work provides a new avenue to develop soft magnets for high-temperature applications, enabling efficient use of sustainable electrical energy under harsh operating conditions.
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Modification of three-magnon splitting in a flexed magnetic vortex
L. Körber, C. Heins, I. Soldatov, R. Schäfer, A. Kakay, H. Schultheiss, and K. Schultheiss
Applied Physics Letters, 122, 092401 (2023)
We present an experimental and numerical study of three-magnon splitting in a micrometer-sized magnetic disk with a vortex state strongly deformed by static in-plane magnetic fields. Excited with large enough power at frequency f, the primary radial magnon modes of a cylindrical magnetic vortex can decay into secondary azimuthal modes via spontaneous three-magnon splitting. This nonlinear process exhibits selection rules leading to well-defined and distinct frequencies f/2 +/- df of the secondary modes. Here, we demonstrate that three-magnon splitting in vortices can be significantly modified by deforming the magnetic vortex with in-plane magnetic fields, leading to a much richer three-magnon response. We find that, with increasing field, an additional class of secondary modes is excited, which are localized to highly flexed regions adjacent to the displaced vortex core. While these modes satisfy the same selection rules of three-magnon splitting, they exhibit much lower three-magnon threshold power compared to regular secondary modes of a centered vortex. The applied static magnetic fields are small (~10 mT), providing an effective parameter to control the nonlinear spectral response of confined vortices. Our work expands the understanding of nonlinear magnon dynamics in vortices and advertises these for potential neuromorphic applications based on magnons.
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Grain boundary infiltration in HDDR processed Nd2Fe14B magnets
I. Dirba, P. Pattur, I. Soldatov, E. Adabifiroozjaei, L. Molina-Luna, O. Gutfleisch
Journal of Alloys and Compounds, 930, 167411 (2023)
We investigate the grain boundary infiltration process of various low melting eutectic alloys for the coercivity enhancement of hydrogenation disproportionation desorption recombination (HDDR) processed Nd-Fe-B powders. Nd-based as well as heavy rare earth (Tb) and light rare earth (La, Ce) containing alloys were systematically studied: Nd70Cu30, Nd90Al10, Nd80Ga15Cu5, Nd62Fe14Ga20Cu4, Nd60Tb10Cu30, La71Cu29 and Ce72Cu28. Moreover, the Fe content in the quaternary Nd-Fe-Ga-Cu system was varied to investigate the effect of grain boundary phase magnetism on the resultant coercivity.
The largest coercivity enhancement, from 0.42 T in the as-HDDR powder to 1.88 T after infiltration was observed in the case of ternary Nd80Ga15Cu5 composition. Furthermore, it also shows the best temperature stability with the infiltrated sample still exhibiting a coercivity of 0.58 T at 200 °C. Infiltration of light rare earth (La, Ce) based alloy did not increase coercivities due to poor wetting at the grain boundaries. Adding Fe to the grain boundary alloys was shown to enhance magnetization up to a certain extent without significant loss in coercivity. These findings demonstrate the effectiveness of grain boundary infiltration in HDDR-processed magnets without using heavy rare earths. The infiltration with non-magnetic material strongly decreases the intergranular interaction, reflected in a different magnetic domain evolution during the magnetization reversal process.
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2022
Antiskyrmions and their electrical footprint in crystalline mesoscale structures of Mn1.4PtSn
M. Winter, F. J. T. Goncalves, I. Soldatov, Y. He, B. E. Z. Céspedes, P. Milde, K. Lenz, S. Hamann, M. Uhlarz, P. Vir, M. König, P. J. W. Moll, R. Schlitz, S. T. B. Goennenwein, L. M. Eng, R. Schäfer, J. Wosnitza, C. Felser, J. Gayles & Toni Helm
Communications materials, 3, 102 (2022)
Skyrmionic materials hold the potential for future information technologies, such as racetrack memories. Key to that advancement are systems that exhibit high tunability and scalability, with stored information being easy to read and write by means of all-electrical techniques. Topological magnetic excitations such as skyrmions and antiskyrmions, give rise to a characteristic topological Hall effect. However, the electrical detection of antiskyrmions, in both thin films and bulk samples has been challenging to date. Here, we apply magneto-optical microscopy combined with electrical transport to explore the antiskyrmion phase as it emerges in crystalline mesoscale structures of the Heusler magnet Mn1.4PtSn. We reveal the Hall signature of antiskyrmions in line with our theoretical model, comprising anomalous and topological components. We examine its dependence on the vertical device thickness, field orientation, and temperature. Our atomistic simulations and experimental anisotropy studies demonstrate the link between antiskyrmions and a complex magnetism that consists of competing ferromagnetic, antiferromagnetic, and chiral exchange interactions, not captured by micromagnetic simulations.
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Metallic Mimosa pudica: A 3D biomimetic buckling structure made of metallic glasses
J. F. Li, I. Soldatov, X. C. Tang, B. Y. Sun, R. Schäfer, S. L. Liu, Y. Q. Yan, H. B. Ke, Y. H. Sun, J. Orava, H. Y. Bai
Science Advances, 8, eabm7658 (2022)
Metallic Mimosa pudica, a three-dimensional (3D) biomimetic structure made of metallic glass, is formed via laser patterning: Blooming, closing, and reversing of the metallic M. pudica can be controlled by an applied magnetic field or by manual reshaping. An array of laser-crystallized lines is written in a metallic glass ribbon. Changes in density and/or elastic modulus due to laser patterning result in an appropriate size mismatch between the shrunken crystalline regions and the glassy matrix. The residual stress and elastic distortion energy make the composite material to buckle within the elastic limit and to obey the minimum elastic energy criterion. This work not only provides a programming route for constructing buckling structures of metallic glasses but also provides clues for the study of materials with automatic functions desired in robotics, electronic devices, and, especially, medical devices in the field of medicine, such as vessel scaffolds and vascular filters, which require contactless expansion and contraction functions.
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Intrinsic Magnetic Properties of a Highly Anisotropic Rare-Earth-Free Fe2P-Based Magnet
Y. He,P. Adler, S. Schneider, I. Soldatov, Q. Mu, H. Borrmann, W. Schnelle, R. Schäfer, B. Rellinghaus, G. H. Fecher, and C. Felser
Advanced Functional Materials, 32, 2107513 (2022)
Permanent magnets are applied in many large-scale and emerging applications and are crucial components in numerous established and newly evolving technologies. Rare-earth magnets exhibit excellent hard magnetic properties; however, their applications are limited by the price and supply risk of the strategic rare-earth elements. Therefore, there is an increasing demand for inexpensive magnets without strategic elements. Here, the authors report the intrinsic highly-anisotropic magnetic properties of Co and Si co-doped single crystals (Fe1−yCoy)2P1−xSix (y ≈ 0.09). Co increases Curie temperature TC; Si doping decreases magnetocrystalline anisotropy K1 and also increases TC significantly because of the enhanced interlayer interaction. The maximum room temperature magnetocrystalline anisotropy K1 = 1.09 MJ m−3 is achieved for x = 0.22, with saturation magnetization µ0Ms = 0.96 T and TC = 506 K. The theoretical maximum energy product is one of the largest for any magnet without a rare earth or Pt. Besides its promising intrinsic magnetic properties and absence of any strategic elements, other advantages are phase stability at high temperatures and excellent corrosion resistance, which make this material most promising for permanent magnetic development that will have a positive influence in industry and daily life.
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Y. He, S. Schneider, T. Helm, J. Gayles, D. Wolf, I. Soldatov, H. Borrmann, W. Schnelle, R. Schäfer, G. H. Fecher, B. Rellinghaus, C. Felser
Acta Materialia, 226, 117619 (2022)
The topological Hall effect (THE), induced by the Berry curvature that originates from non-zero scalar spin chirality, is an important feature for mesoscopic topological structures, such as skyrmions. However, the THE might also arise from other microscopic non-coplanar spin structures in the lattice. Thus, the origin of the THE inevitably needs to be determined to fully understand skyrmions and find new host materials. Here, we examine the Hall effect in both, bulk- and micron-sized lamellar samples of MnBi. The sample size affects the temperature and field range in which the THE is detectable. Although a bulk sample exhibits the THE only upon exposure to weak fields in the easy-cone state, in micron-sized lamella the THE exists across a wide temperature range and occurs at fields near saturation. Our results show that both the non-coplanar spin structure in the lattice and topologically non-trivial skyrmion bubbles are responsible for the THE, and that the dominant mechanism depends on the sample size. Hence, the magnetic phase diagram for MnBi is size-dependent. Our study provides an example in which the THE is simultaneously induced by two mechanisms, and builds a bridge between mesoscopic and microscopic magnetic structures.
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Nanoscale magnetic bubbles in Nd2Fe14B at room temperature
Y. He, T. Helm, I. Soldatov, S. Schneider, D. Pohl, A. K. Srivastava, A. K. Sharma, J. Kroder, W. Schnelle, R. Schäfer, B. Rellinghaus, G. H. Fecher, S. S. P. Parkin, and C. Felser
Physical Review B, 105, 064426 (2022)
The increasing demand for computer data storage with a higher recording density can be addressed by using smaller magnetic objects, such as bubble domains. Small bubbles predominantly require a strong saturation magnetization combined with a large magnetocrystalline anisotropy to resist self-demagnetization. These conditions are well satisfied for highly anisotropic materials. Here, we study the domain structure of thin Nd2Fe14B lamellae. Magnetic bubbles with a minimum diameter of 74 nm were observed at room temperature, approaching even the range of magnetic skyrmions. The stripe domain width and the bubble size are both thickness dependent. Furthermore, a kind of bubble was observed below the spin-reorientation transition temperature that combine bubbles with opposite helicity. In this paper, we reveal Nd2Fe14B to be a good candidate for a high-density magnetic bubble-based memory.
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Self-assembly as a tool to study microscale curvature and strain-dependent magnetic properties
B. Singh, J. A. Otálora, T. H. Kang, I. Soldatov, D. D. Karnaushenko, C. Becker, R. Schäfer, D. Karnaushenko, V. Neu, O. G. Schmidt
Flexible Electronics, 6, 76 (2022)
The extension of 2D ferromagnetic structures into 3D curved geometry enables to tune its magnetic properties such as uniaxial magnetic anisotropy. Tuning the anisotropy with strain and curvature has become a promising ingredient in modern electronics, such as flexible and stretchable magnetoelectronic devices, impedance-based field sensors, and strain gauges, however, has been limited to extended thin films and to only moderate bending. By applying a self-assembly rolling technique using a polymeric platform, we provide a template that allows homogeneous and controlled bending of a functional layer adhered to it, irrespective of its shape and size. This is an intriguing possibility to tailor the sign and magnitude of the surface strain of integrated, micron-sized devices. In this article, the impact of strain and curvature on the magnetic ground state and anisotropy is quantified for thin-film Permalloy micro-scale structures, fabricated on the surface of the tubular architectures, using solely electrical measurements.
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Direct imaging of nanoscale field-driven domain wall oscillations in Landau structures
B. Singh, R. Ravishankar, J. A. Otálora, I. Soldatov, R. Schäfer, D. Karnaushenko, V. Neu, O. G. Schmidt
Nanoscale, 14, 13667 (2022)
Linear oscillatory motion of domain walls (DWs) in the kHz and MHz regime is crucial when realizing precise magnetic field sensors such as giant magnetoimpedance devices. Numerous magnetically active defects lead to pinning of the DWs during their motion, affecting the overall behavior. Thus, the direct monitoring of the domain wall’s oscillatory behavior is an important step to comprehend the underlying micromagnetic processes and to improve the magnetoresistive performance of these devices. Here, we report an imaging approach to investigate such DW dynamics with nanoscale spatial resolution employing conventional table-top microscopy techniques. Time-averaged magnetic force microscopy and Kerr imaging methods are applied to quantify the DW oscillations in Ni81Fe19 rectangular structures with Landau domain configuration and are complemented by numeric micromagnetic simulations. We study the oscillation amplitude as a function of external magnetic field strength, frequency, magnetic structure size, thickness and anisotropy and understand the excited DW behavior as a forced damped harmonic oscillator with restoring force being influenced by the geometry, thickness, and anisotropy of the Ni81Fe19 structure. This approach offers new possibilities for the analysis of DW motion at elevated frequencies and at a spatial resolution of well below 100 nm in various branches of nanomagnetism.
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M.G. Nematov, V. Kolesnikova, S.A. Evstigneeva, J. Alam, N.A. Yudanov, A.A. Samokhvalov, N. Andreev, S.V. Podgornaya, I. Soldatov, R. Schaefer, V. Rodionova, L.V. Panina
Journal of Alloys and Compounds, 890, 161740 (2022)
The effect of heat treatment of Co-based amorphous microwires at temperatures near the crystallization () onset on micromagnetic structure, magnetostriction, and magnetoimpedance (MI) was investigated. As the annealing temperature approaches , the hysteresis loops abruptly change the shape from almost rectangular to inclined with a small coercivity and remanence magnetization. This behavior is quite unique since even partial crystallization deteriorates soft magnetic properties. The Kerr-microscopy confirmed the formation of a fine circular domain structure and an easy anisotropy direction of a helical type. Both current and furnace annealing with appropriate high temperatures produce similar magnetic transformation. The origin of the formed easy anisotropy is different from that induced by current annealing at temperatures below the Curie temperature and is associated with a change in the magnetostriction constant, which drops from positive to negative values. This could be explained by structural relaxation leading to amorphous phase decomposition at the very initial stage of crystallization. The effect is not specific for the Co-rich alloy composition and was observed in microwires of two compositions: Co71Fe5B11Si10Cr3 and Co66.6Fe4.28B11.51Si14.48Ni1.44Mo1.69 which in the as-prepared state have axial and circumferential anisotropies, respectively. The obtained combination of magnetic anisotropy, magnetostriction and temperature stability leads to useful MI properties, in particular, at Gigahertz frequencies.
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B. Yang, I. Soldatov, F. Chen, Y. Zhang, Z. Li, H. Yan, R. Schäfer, D. Wang, C. Esling, X. Zhao, L. Zuo
Journal of Materials Science & Technology, 102, 56 (2022)
Epitaxial Ni–Mn–Ga thin films have promising application potential in micro-electro-mechanical sensing and actuation systems. To date, large abrupt magnetization changes have been observed in some epitaxial Ni–Mn–Ga thin films, but their origin – either from magnetically induced martensite variant reorientation (MIR) or magnetic domain evolution – has been discussed controversially. In the present work, we investigated the evolutions of the magnetic domain and microstructure of a typical epitaxial Ni–Mn–Ga thin film through wide-field magneto-optical Kerr-microscopy. It is demonstrated that the abrupt magnetization changes in the hysteresis loops should be attributed to the magnetic domain evolution instead of the MIR.
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2021
Analyzer-free, intensity-based, wide-field magneto-optical microscopy
R. Schäfer, P. M. Oppeneer, A. V. Ognev, A. S. Samardak, I. Soldatov
Applied Physics Review, 8 (3), 031402 (2021)
In conventional Kerr and Faraday microscopy, the sample is illuminated with plane-polarized light, and a magnetic domain contrast is generated by an analyzer making use of the Kerr or Faraday rotation. Here, we demonstrate possibilities of analyzer-free magneto-optical microscopy based on magnetization-dependent intensity modulations of the light. (i) The transverse Kerr effect can be applied for in-plane magnetized material, as demonstrated for an FeSi sheet. (ii) Illuminating that sample with circularly polarized light leads to a domain contrast with a different symmetry from the conventional Kerr contrast. (iii) Circular polarization can also be used for perpendicularly magnetized material, as demonstrated for garnet and ultrathin CoFeB films. (iv) Plane-polarized light at a specific angle can be employed for both in-plane and perpendicular media. (v) Perpendicular light incidence leads to a domain contrast on in-plane materials that is quadratic in the magnetization and to a domain boundary contrast. (vi) Domain contrast can even be obtained without a polarizer. In cases (ii) and (iii), the contrast is generated by magnetic circular dichroism (i.e., differential absorption of left- and right-circularly polarized light induced by magnetization components along the direction of light propagation), while magnetic linear dichroism (differential absorption of linearly polarized light induced by magnetization components transverse to propagation) is responsible for the contrast in case (v). The domain–boundary contrast is due to the magneto-optical gradient effect. A domain–boundary contrast can also arise by interference of phase-shifted magneto-optical amplitudes. An explanation of these contrast phenomena is provided in terms of Maxwell–Fresnel theory.
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Creating Ferroic Micropatterns through Geometrical Transformation
V. Neu, I. Soldatov, R. Schäfer, D. D. Karnaushenko, A. Mirhajivarzaneh, D. Karnaushenko, O. G. Schmidt
Nano Letters, 21 (23), 9889 (2021)
The functionality of a ferroic device is intimately coupled to the configuration of domains, domain boundaries, and the possibility for tailoring them. Exemplified with a ferromagnetic system, we present a novel approach which allows the creation of new, metastable multidomain patterns with tailored wall configurations through a self-assembled geometrical transformation. By preparing a magnetic layer system on a polymeric platform including swelling layer, a repeated self-assembled rolling into a multiwinding tubular structure and unrolling of the functional membrane is obtained. When polarizing the rolled-up 3D structure in a simple homogeneous magnetic field, the imprinted configuration translates into a regularly arranged multidomain configuration once the tubular structure is unwound. The process is linked to the employed magnetic anisotropy with respect to the surface normal, and the geometrical transformation connects the angular with the lateral degrees of freedom. This combination offers unparalleled possibilities for designing new magnetic or other ferroic micropatterns.
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Robust Magneto-Ionic Effect in Fe/FeOx Thin Films in Electrolytes With Different Cations
J. Zehner, O. Vaerst, I. Soldatov, K. Nielsch, R. Schäfer, K. Leistner
IEEE Transactions on Magnetics, 58 (2), 6000108 (2021)
Electrochemical reactions offer an elegant way to influence the magnetic properties of nanoscaled materials by applying a small voltage. The present study focuses on such magneto-ionic changes in thin Fe/FeO x films induced by interfacial electrochemical reactions in liquid electrolytes. The films are prepared by inclined sputter deposition and exhibit a uniaxial in-plane anisotropy. LiOH, NaOH, and KOH in an aqueous solution are used as electrolytes. The changes in the hysteresis loops and the magnetic microstructure are observed by in-situ Kerr microscopy. Upon low voltage application, the electrochemical reduction reaction leads to the transformation of the FeO x -layer into the metallic Fe. Simultaneously, a strong decrease of coercivity and remanence together with an increase of domain size occurs. These observations are in line with a previously reported magnetic de-blocking mechanism caused by the variation of Néel wall interactions during the oxide–metal transformation. When applying the oxidation potential, the coercivity, remanence, and domain size are re-established. The changes occur in a similar manner for all studied electrolytes and can be repeatedly achieved for multiple voltage switching steps. This evidences that the electrochemical reaction mechanism responsible for the magneto-ionic switching is very robust and dictated by the anion (OH − ) rather than the type of cation.
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Operational Parameters for Sub-Nano Tesla Field Resolution of PHMR Sensors in Harsh Environments
T. Jeon, P. T. Das, M. Kim, Ch. Jeon, B. Lim, Ivan Soldatov, C. Kim
Sensors, 21 (20), 6891 (2021)
The resolution of planar-Hall magnetoresistive (PHMR) sensors was investigated in the frequency range from 0.5 Hz to 200 Hz in terms of its sensitivity, average noise level, and detectivity. Analysis of the sensor sensitivity and voltage noise response was performed by varying operational parameters such as sensor geometrical architectures, sensor configurations, sensing currents, and temperature. All the measurements of PHMR sensors were carried out under both constant current (CC) and constant voltage (CV) modes. In the present study, Barkhausen noise was revealed in 1/f noise component and found less significant in the PHMR sensor configuration. Under measured noise spectral density at optimized conditions, the best magnetic field detectivity was achieved better than 550 pT/√Hz at 100 Hz and close to 1.1 nT/√Hz at 10 Hz for a tri-layer multi-ring PHMR sensor in an unshielded environment. Furthermore, the promising feasibility and possible routes for further improvement of the sensor resolution are discussed.
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Multistate current-induced magnetization switching in Au/Fe/MgO(001) epitaxial heterostructuress
P. Gospodarič, E. Młyńczak, I. Soldatov, A. Kákay, D. E. Bürgler, L. Plucinski, R. Schäfer, J. Fassbender, and C. M. Schneider
Physical Review Research, 3, 023089 (2021)
Magnetization switching using in-plane charge current recently has been widely investigated in heavy metal/ferromagnet bilayers with the switching mechanism usually attributed to the action of the spin-orbit coupling. Here we study in-plane current induced magnetization switching in model epitaxial bilayers that consist of Au(001) and Fe(001) grown on MgO(001). We use the planar Hall effect combined with magnetooptical Kerr effect (MOKE) microscopy to investigate magnetic properties of the bilayers and current-induced switching. We show that a current density beyond 1.4×107 A/cm can be employed for reproducible electrical switching of the magnetization between multiple stable states that correspond to different arrangements of magnetic domains with magnetization direction along one of the in-plane easy magnetization axes of the Fe(001) film. Lower current densities result in stable intermediate transversal resistances which are interpreted based on MOKE-microscopy investigations as resulting from the current-induced magnetic domain structure that is formed in the area of the Hall cross. We find that the physical mechanism of the current-induced magnetization switching of the Au/Fe/MgO(001) system at room temperature can be fully explained by the Oersted field, which is generated by the charge current flowing mostly through the Au layer.
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Role of the interfaces in the crystallization and hysteresis mechanisms of amorphous Fe-B thin films
U. Urdiroz, F.J. Palomares, A. Mayoral, I.V. Soldatov, R. Schäfer, J.M. González, M. Sanchez-Agudo, E. Navarro, A. Ruiz, M. Alonso, L. Vázquez, F. Cebollada
Journal of Alloys and Compounds, 869, 159276 (2021)
In this work we study the crystallization processes of two sets of amorphous Fe80B20 films fabricated by Pulsed Laser Ablation Deposition on substrates with different nature, Corning glass® and MgO(001). We analyze their magnetic hysteresis mechanisms by means of magneto-optic techniques and Transmission Electron Microscopy. The as-deposited amorphous films present a highly homogeneous uniaxial magnetic anisotropy with the easy axis orientation dependent on the type of substrate and much weaker than that of bulk alloys with similar composition. The onset of crystallization for the films deposited on glass and MgO appears, respectively, at temperatures 150 °C and 250 °C below that of their bulk counterparts. We study the role of the substrate in the crystallization mechanism and the resulting nanostructure of the magnetic films. While the crystallization of the MgO-deposited films proceeds in a broad front growing from the substrate to the surface, that of the glass-deposited films takes place through the nucleation and growth of isolated crystallites. We analyze the evolution of the coercivity during the crystallization of the films. It follows a similar trend for both types, remaining in values close to those of the amorphous precursors until it rises steeply at a given annealing temperature. The threshold of the steep coercivity increase of the glass-deposited films is shifted to lower temperatures with respect to the MgO ones, in spite of the higher crystallization onset temperature of the former. The coercivity mechanisms have been analyzed, correlated to the films interfacial characteristics and compared to those of bulk alloys.
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2020
A. Sharma, P. Matthes, I. Soldatov, S. S. P. K. Arekapudi, B. Böhm, M. Lindner, O. Selyshchev, N. T. N. Ha, M. Mehring, C. Tegenkamp, S. E. Schulz, D. RT. Zahn, Y. Paltiel, O. Hellwig, G. Salvan
Journal of Materials Chemistry C, 8 (34), 11822 (2020)
The adsorption of chiral molecules was recently shown to trigger a change in the magnetisation of mesoscopic magnetic domains in a ferromagnetic underlayer. In this work, we investigated the macroscopic (magneto-)optical response of chemisorbed α-helical polyalanine self-assembled monolayers (SAMs) on a gold and gold-capped-cobalt thin film on Au substrates using spectroscopic ellipsometry and magneto-optical Kerr effect spectroscopy and microscopy. The optical and magneto-optical spectra reveal selective chemisorption of the α-helical polyalanine molecules depending on the orientation of the substrate remanent magnetisation during the SAMs process. Moreover, a sign change of the magneto-optical response was observed in some of the magnetic substrates after the chiral SAMs formation.
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Creating, probing and confirming tetragonality in bulk FeNi alloys
N Maât, I McDonald, R Barua, B Lejeune, X Zhang, GM Stephen, A Fisher, D Heiman, IV Soldatov, R Schäfer, LH Lewis
Acta Materialia, 196, 776 (2020)
Effects derived from the simultaneous application of passive, uniform saturating magnetic field and tensile stress during long-time annealing utilizing a unique and simple processing tool, the MultiDriver furnace, are reported. In particular, the evolution of the crystalline structure and the magnetic domain configuration of severely deformed and subsequently annealed equiatomic FeNi alloys were revealed by high-energy synchrotron X-Ray diffraction and Magneto-Optic Kerr Microscopy. These alloys are known to undergo a first-order magnetic phase transformation to a chemically ordered tetragonal (L10) structure in astronomical timeframes. MultiDriver-annealed specimens are found to retain a tetragonal crystallographic state and texture that is similar to that of the precursor deformed state, with a smaller unit cell volume and a larger c/a ratio relative to those of control samples that were annealed under the same conditions but without magnetic field and stress drivers. Magnetic domain images echo these observations: while as-cast FeNi alloys exhibit domain patterns consistent with the presence of a stress-modified cubic magnetocrystalline anisotropy, large areas of domains with uniaxial anisotropy are present in MultiDriver-annealed samples. While no chemical order was demonstrated in these samples, a strategy is proposed for achieving the ordered state by employing a passive magnetic gradient in the MultiDriver furnace to enhance diffusion.
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J. Zehner, I. Soldatov, S. Schneider, R. Heller, N. B Khojasteh, S. Schiemenz, S. Fähler, K. Nielsch, R. Schäfer, K. Leistner
Advanced Electronic Materials, 6 (11), 2000406 (2020)
High energy efficiency of magnetic devices is crucial for applications such as data storage, computation, and actuation. Redox-based (magneto-ionic) voltage control of magnetism is a promising room-temperature pathway to improve energy efficiency. However, for ferromagnetic metals, the magneto-ionic effects studied so far require ultrathin films with tunable perpendicular magnetic anisotropy or nanoporous structures for appreciable effects. This paper reports a fully reversible, low voltage-induced collapse of coercivity and remanence by redox reactions in iron oxide/iron films with uniaxial in-plane anisotropy. In the initial iron oxide/iron films, Néel wall interactions stabilize a blocked state with high coercivity. During the voltage-triggered reduction of the iron oxide layer, in situ Kerr microscopy reveals inverse changes of coercivity and anisotropy, and a coarsening of the magnetic microstructure. These results confirm a magneto-ionic deblocking mechanism, which relies on changes of the Néel wall interactions, and of the microstructural domain-wall-pinning sites. With this approach, voltage-controlled 180° magnetization switching with high energy-efficiency is achieved. It opens up possibilities for developing magnetic devices programmable by ultralow power and for the reversible tuning of defect-controlled materials in general.
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Inverted Hysteresis, Magnetic Domains, and Hysterons
I. Soldatov, P. Andrei, R. Schäfer
IEEE Magnetics Letters, 1, 2405805 (2020)
In this letter, we use magnetooptical Kerr effect magnetometry on extended samples to measure local hysteresis curves as a function of an externally applied field in a magnetostriction-free amorphous ribbon and an elliptical Permalloy film element. Although these materials are magnetically soft and have almost zero coercive field, we find that the local hysteresis curves have a highly nonlinear hysteretic character, displaying both counterclockwise and clockwise rotation senses when cycling the loops. This unexpected result can be explained by looking at the total hysteresis curve of the materials as a superposition of local curves and can be described mathematically using the formalism of the Preisach model. Within the framework of this model, we derive the conditions under which a superposition of clockwise and counterclockwise hysteresis loops can lead to a hysteresis-less system, and devise a technique to map the hysterons that define the first-order reversal-curve distribution to physical regions in the specimens.
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2019
Experimental and computational analysis of binary Fe-Sn ferromagnetic compounds
B. Fayyazi, K. P. Skokov, T. Faske, I. Opahle, M. Duerrschnabel, T. Helbig, I. Soldatov, U. Rohrmann, L. Molina-Luna, K. Gueth, H. Zhang, W. Donner, R. Schäfer, O. Gutfleisch
Acta Materialia, 180, 126 (2019)
Ferromagnetic Fe3Sn, Fe5Sn3 and Fe3Sn2 single crystals were synthesized using the reactive flux technique. Derived from single crystal x-ray diffraction and Transmission Electron Microscopy (TEM), a new structural model is proposed for the Fe5Sn3 crystals – the threefold twinning of an orthorhombic unit cell with (3 + 1) dimensional space group Pbcm(α00)0s0. The spontaneous magnetization (Ms) and the anisotropy constants K1 and K2 of Fe3Sn, Fe5Sn3 and Fe3Sn2 single crystals were determined in a wide temperature range using M(H) dependencies and a modified Sucksmith-Thompson technique. Ms and K1 were also evaluated in the framework of Density Functional Theory (DFT) and an overall good agreement was observed between the calculated and experimental results. Furthermore, a critical evaluation of different analytical models for the assessment of magnetocrystalline anisotropy was performed, which are restricted to the analysis of uniaxial magnetic domain patterns, and it is shown that such high-throughput techniques can lead to unrealistic results. Finally, a DFT high-throughput screening of the Fe-Sn phase diagram was used to identify Fe-Sn based phases with potential to be stabilized upon alloying, and their magnetization and magnetocrystalline anisotropy were evaluated. The results show that a similar strong anisotropy as observed in Fe3Sn may also be found in other Fe-Sn based phases, having higher potential to be used as hard magnetic material.
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Time-reversal symmetry breaking type-II Weyl state in YbMnBi2
S. Borisenko, D. Evtushinsky, Q. Gibson, A. Yaresko, K. Koepernik, T. Kim, M. Ali, J. van den Brink, M. Hoesch, A. Fedorov, E. Haubold, Y. Kushnirenko, I. Soldatov, R. Schäfer, R. J Cava
Nature Communications, 10 (1), 3424 (2019)
Spectroscopic detection of Dirac and Weyl fermions in real materials is vital for both, promising applications and fundamental bridge between high-energy and condensed-matter physics. While the presence of Dirac and noncentrosymmetric Weyl fermions is well established in many materials, the magnetic Weyl semimetals still escape direct experimental detection. In order to find a time-reversal symmetry breaking Weyl state we design two materials and present here experimental and theoretical evidence of realization of such a state in one of them, YbMnBi2. We model the time-reversal symmetry breaking observed by magnetization and magneto-optical microscopy measurements by canted antiferromagnetism and find a number of Weyl points. Using angle-resolved photoemission, we directly observe two pairs of Weyl points connected by the Fermi arcs. Our results not only provide a fundamental link between the two areas of physics, but also demonstrate the practical way to design novel materials with exotic properties.
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Nonvolatile Electric Control of Exchange Bias by a Redox Transformation of the Ferromagnetic Layer
J. Zehner, R. Huhnstock, S. Oswald, U. Wolff, I. Soldatov, A. Ehresmann, K. Nielsch, D. Holzinger, K. Leistner
Advanced Electronic Materials, 5 (6), 1900296 (2019)
Electric manipulation of exchange bias (EB) systems is highly attractive for the development of modern spintronic and magnetophoretic devices. To date, electric control of the EB has mainly been based on multiferroic or resistive switching behavior in specific antiferromagnets, which limits the material choice and accessible EB states. In addition, the effects are mostly volatile, requiring constant voltage application. The continuous and nonvolatile tuning of the EB via electrochemical manipulation of the ferromagnetic layer is presented. In FeOx/Fe/IrMn systems, large changes in the EB field of fully shifted magnetization curves are achieved at low voltage (<1 V) and room temperature. A ferromagnetic-layer thickness change resulting from the electrochemical reduction of iron oxide to iron is proposed as the underlying mechanism and is consistent with a simple model for the EB and surface analysis. Nonvolatility is achieved as the reduction proceeds at the buried FeOx/Fe interface, leaving the remaining oxide as a protective layer. A lateral voltage-controlled patterning of the EB fields and magnetic domain state is demonstrated. This versatile redox-based electric control of the EB paves a new route for the design of EB systems in general and for the development of future electrically controlled EB devices.
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Power frequency domain imaging on Goss-textured electrical steel
R. Schäfer, I. Soldatov, S. Arai
Journal of Magnetism and Magnetic Materials, 474, 221 (2019)
A magneto-optical indicator film with perpendicular anisotropy (PMOIF) is used for time-resolved domain imaging on grain-oriented transformer steel at technical frequency field excitation. The high sensitivity of the PMOIF allows for single-shot imaging, even in the presence of insulation coatings. Differences between quasistatic and dynamic processes are elaborated, revealing the mechanisms of domain refinement and showing the role of grain boundaries and mechanical stress on domain formation and flux propagation. It is shown that the PMOIF technique opens a promising window for studying the domain dynamics of transformer steels under practical conditions, which goes beyond established characterization tools like Kerr microscopy.
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Nanoscale modification of magnetic properties for effective domain wall pinning
T. Jin, F. Tan, W. Cheung Law, W. Gan, I. Soldatov, R. Schäfer, C. Ma, X. Liu, W. S. Lew, SN. Piramanayagam
Journal of Magnetism and Magnetic Materials, 475, 70 (2019)
Magnetic domain wall memory technology, wherein the information is stored in magnetic domains of multiple magnetic nanowires, is a potential concept proposed to store the large amount of digital data in the near future, which is generated due to the widespread use of social media and computing devices. However, one of the technological challenges which remains to be solved in domain wall memory is the controllable pinning of the domain walls at the nanometer scale. Here, we demonstrate the possibility to stabilize domain walls with nanoscale modification of magnetic properties by using thermal diffusion of elements from crossbar configuration. We have inspected and evaluated the magnetic properties of the nanowires using Kerr microscopy. The pinning field induced by Cr diffusion of our Ni80Fe20 nanowire was estimated to be about 8 kA/m as determined from minor loop (magnetoresistance vs. magnetic field) measurements. The proposed concept can potentially be used in future domain wall memory applications.
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P Mazalski, P Kuświk, I Sveklo, I Soldatov, J McCord, R Schäfer, A Wawro, A Maziewski
Journal of Magnetism and Magnetic Materials, 477, 317 (2019)
Focused ion beam (FIB) irradiation was applied to an ultrathin Pt/Co/Pt film to create micrometer-sized, square-shaped regions with perpendicular orientation of magnetization embedded in an in-plane magnetized environment. The FIB parameters like ion fluence and scanning direction are used to control the magnetic properties of the irradiated areas. We have studied the magnetic and magnetooptical properties in the irradiated areas as a function of ion fluence by means of component selective magnetooptical Kerr microscopy that measures separately pure polar and pure longitudinal components of magnetization. Two fluence ranges that induce perpendicular magnetization components were observed. In the higher fluence range, four regions with independently inclined out-of-plane magnetization, depending on FIB fast scan direction, and an unexpected magnetic domain structure were distinguished inside the squares irradiated by FIB.
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