Papers in collabortion


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.


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.



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.


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.


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.



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.


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.


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.


Topological Hall effect arising from the mesoscopic and microscopic non-coplanar magnetic structure in MnBi

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.


Nanoscale magnetic bubbles in Nd2⁢Fe14⁢B 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 Nd2⁢Fe14⁢B 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 Nd2⁢Fe14⁢B to be a good candidate for a high-density magnetic bubble-based memory.


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.


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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