Publications – RiTM

Antiferromagnetic Nanoscale Bit Arrays of Magnetoelectric Cr2O3 Thin Films

sheka — Mon, 14 Oct 2024 16:22:05 +0000

Magnetism of oxide antiferromagnets (AFMs) has been studied in single crystals and extended thin films. The properties of AFM nanostructures still remain underexplored. Here, we report on the fabrication and magnetic imaging of granular 100 nm-thick magnetoelectric Cr2O3 films patterned in circular bits with diameters ranging from 500 down to 100 nm. With the change of the lateral size, the domain structure evolves from a multidomain state for larger bits to a single domain state for the smallest bits. Based on spin–lattice simulations, we show that the physics of the domain pattern formation in granular AFM bits is primarily determined by the energy dissipation upon cooling, which results in motion and expelling of AFM domain walls of the bit. Our results provide a way toward the fabrication of single domain AFM-bit-patterned memory devices and the exploration of the interplay between AFM nanostructures and their geometric shape.

Peter Rickhaus, Oleksandr V. Pylypovskyi, Gediminas Seniutinas, Vicent Borras, Paul Lehmann, Kai Wagner, Liza Žaper, Paulina Prusik, Pavlo Makushko, Igor Veremchuk, Tobias Kosub, René Hübner, Denis D. Sheka, Patrick Maletinsky, Denys Makarov. Antiferromagnetic Nanoscale Bit Arrays of Magnetoelectric Cr2O3 Thin Films, Nano Letters. DOI: 10.1021/acs.nanolett.4c03044 (pdf, supplementary)

Domain Wall Automotion by Cross Section Tailoring in Ferromagnetic Nanostripes

sheka — Mon, 14 Oct 2024 16:12:07 +0000

Curvilinear magnetism of nanostripes is a prospective playground both for fundamental research and numerous applications. By tailoring geometrical properties of the ferromagnetic stripes and wires there appear new possibilities to control its magnetic properties such as the dynamics of the domain walls. Deterministic fast motion of the domain walls is a key element in the constructions of ultra-fast, high-capacity, and non-volatile magnetic memory and logic devices. Here, we study theoretically the magnetic response of curved ferromagnetic nanostripes with varying cross section using a recently proposed effective model of curved biaxial stripe. We show that non zero gradient of stripe cross section results in the additional driving force for domain wall. The equations of motion of the domain wall are derived and analysed using the collective variable approach. We illustrate the influence of varying cross section by several specific geometries: a straight stripe with zero curvature and a circular arc with constant curvature. For these geometries, we derived (i) the asymptotic values of the domain wall velocity as a function of cross-section gradient, (ii) values of cross section-induced Walker threshold and curvature-induced corrections. All our analytical predictions are well-confirmed by the full scale micromagnetic simulations.

Dmytro Karakuts, Kostiantyn V. Yershov, and Denis D. Sheka Domain Wall Automotion by Cross Section Tailoring in Ferromagnetic Nanostripes, in Functional Magnetic and Spintronic Nanomaterials, Edited by Igor Vladymyrskyi, Burkard Hillebrands, Alexander Serha, Denys Makarov, Oleksandr Prokopenko, NATO Science for Peace and Security Series B: Physics and Biophysics, Springer (2024) DOI: 10.1007/978-94-024-2254-2_6

Interaction of Domain Walls with Grain Boundaries in Uniaxial Insulating Antiferromagnets

sheka — Sun, 08 Oct 2023 17:02:12 +0000

A search for high-speed and low-energy memory devices puts antiferromagnetic thin films at the forefront of spintronic research. Here, we develop a material model of a granular antiferromagnetic thin film with uniaxial anisotropy and provide fundamental insight into the interaction of antiferromagnetic domain walls with grain boundaries. This model is validated on thin films of the antiferromagnetic insulator Cr2O3, revealing complex mazelike domain patterns hosting localized nanoscale domains down to 50 nm. We show that the intergrain magnetic parameters can be estimated based on an analysis of high-resolution images of antiferromagnetic domain patterns examining the domain patterns’ self-similarity and the statistical distribution of domain sizes. Having a predictive material model and understanding of the pinning of domain walls on grain boundaries, we put forth design rules to realize granular antiferromagnetic recording media.

Oleksandr V. Pylypovskyi, Natascha Hedrich, Artem V. Tomilo, Tobias Kosub, Kai Wagner, René Hübner, Brendan Shields, Denis D. Sheka, Jürgen Fassbender, Patrick Maletinsky, and Denys Makarov Interaction of Domain Walls with Grain Boundaries in Uniaxial Insulating Antiferromagnets, Physical Review Applied 20 (2023) 014020 DOI: 10.1103/PhysRevApplied.20.014020 (pdf, supplementary)

Control of magnetic response in curved stripes by tailoring the cross section

sheka — Sat, 25 Mar 2023 23:39:07 +0000

Curved magnetic architectures are key enablers of prospective magnetic devices with respect to size, functionality, and speed. By exploring geometry-governed magnetic interactions, curvilinear magnetism offers a number of intriguing effects in curved magnetic wires and curved magnetic films. The applicability of the current micromagnetic theory requires that the sample has a constant width and thickness, which does not correspond in many cases to the specificity of experimental sample preparation. Here, we put forth a self-consistent micromagnetic framework of the curvilinear magnetism of nanowires and narrow stripes with a spatially inhomogeneous cross section. The influence of the varying cross section is exploited and illustrated by an example of the simplest topological texture, which is a transversal head-to-head (tail-to-tail) domain wall. The cross-section gradient becomes a source of domain wall pinning which competes with the curvature gradient. Eigenfrequencies of the domain wall free oscillations at the pinning potential are determined by both curvature and cross-section gradients. Prospects for curvilinear magnonics and spintronics are discussed.

Kostiantyn V. Yershov and Denis D. Sheka. Control of magnetic response in curved stripes by tailoring the cross section, Physical Review B 107 (2023) L100415 DOI: 10.1103/physrevb.107.l100415 (pdf, supplementary)

Chirality coupling in topological magnetic textures with multiple magnetochiral parameters

sheka — Thu, 23 Mar 2023 16:38:02 +0000

Chiral effects originate from the lack of inversion symmetry within the lattice unit cell or sample’s shape. Being mapped onto magnetic ordering, chirality enables topologically non-trivial textures with a given handedness. Here, we demonstrate the existence of a static 3D texture characterized by two magnetochiral parameters being magnetic helicity of the vortex and geometrical chirality of the core string itself in geometrically curved asymmetric permalloy cap with a size of 80 nm and a vortex ground state. We experimentally validate the nonlocal chiral symmetry breaking effect in this object, which leads to the geometric deformation of the vortex string into a helix with curvature 3 μm−1 and torsion 11 μm−1. The geometric chirality of the vortex string is determined by the magnetic helicity of the vortex texture, constituting coupling of two chiral parameters within the same texture. Beyond the vortex state, we anticipate that complex curvilinear objects hosting 3D magnetic textures like curved skyrmion tubes and hopfions can be characterized by multiple coupled magnetochiral parameters, that influence their statics and field- or current-driven dynamics for spin-orbitronics and magnonics.

Oleksii M. Volkov, Daniel Wolf, Oleksandr V. Pylypovskyi, Attila Kakay, Denis D. Sheka, Bernd Büchner, Jürgen Fassbender, Axel Lubk, Denys Makarov. Chirality coupling in topological magnetic textures with multiple magnetochiral parameters, Nature Communications, 14 (2023) 1491 DOI: 10.1038/s41467-023-37081-z (Open Access, pdf, supplementary)

Curvilinear Micromagnetism: From Fundamentals to Applications

sheka — Wed, 01 Mar 2023 18:30:00 +0000

This book covers the theory of curvilinear micromagnetism as well as experimental study of curved magnets including both fabrication and characterization

About: Zach Evenson “Magnetism Ahead of the Curve”

Publication:

Curvilinear Micromagnetism From Fundamentals to Applications, Denys Makarov, Denis D. Sheka (Editors), Part of the book series: Topics in Applied Physics (TAP), volume 146 (2022), doi: 10.1007/978-3-031-09086-8

Field-induced spin reorientation transitions in antiferromagnetic ring-shaped spin chains

sheka — Wed, 01 Mar 2023 17:46:49 +0000

Easy axis antiferromagnets are robust against external magnetic fields of moderate strength. Spin reorientations in strong fields can provide insight into more subtle properties of antiferromagnetic materials, which are often hidden by their high ground-state symmetry. Here, we investigate theoretically effects of curvature in ring-shaped antiferromagnetic achiral anisotropic spin chains in strong magnetic fields. We identify the geometry-governed helimagnetic phase transition above the spin-flop field between vortex and onion states. The curvature-induced Dzyaloshinskii–Moriya interaction results in the spin-flop transition being of first or second order, depending on the ring curvature. Spatial inhomogeneity of the Néel vector in the spin-flop phase generates weak ferromagnetic response in the plane perpendicular to the applied magnetic field. Our paper contributes to the understanding of the physics of curvilinear antiferromagnets in magnetic fields and guides prospective experimental studies of geometrical effects relying on spin-chain-based nanomagnets.

Publication:

Yelyzaveta A. Borysenko, Denis D. Sheka, Jürgen Fassbender, Jeroen van den Brink, Denys Makarov, Oleksandr V. Pylypovskyi.
Field-induced spin reorientation transitions in antiferromagnetic ring-shaped spin chains,
Physical Review B 106 174426 (2022), doi: 10.1103/physrevb.106.174426 (pdf)

Circular stripe domains

sheka — Wed, 01 Mar 2023 17:33:24 +0000

Vertically stacked exchange coupled magnetic heterostructures of cylindrical geometry can host complex noncolinear magnetization patterns. By tuning the interlayer exchange coupling between a layer accommodating magnetic vortex state and an out-of-plane magnetized layer, one can efficiently realize new topological chiral textures such as cone state vortices and circular stripe domains. We study how the number of circular stripes can be controlled by both the interlayer exchange coupling and the sample geometrical parameters. By varying geometrical parameters, a continuous phase transition between the homogeneous state, cone state vortex, circular stripe domains, and the imprinted vortex takes place, which is analysed by full scale micromagnetic simulations. The analytical description provides an intuitive pictures of the magnetization textures in each of these phases. The possibility to realize switching between different states allows for engineering magnetic textures with possible applications in spintronic devices.

Publication:

Oleksandr Zaiets, Volodymyr P. Kravchuk, Oleksandr V. Pylypovskyi, Denys Makarov, Denis D. Sheka.
Circular stripe domains and cone state vortices in disk-shaped exchange coupled magnetic heterostructures,
Journal of Physics D: Applied Physics 55, 445003 (2022), doi: 10.1088/1361-6463/ac8d9f (pdf)

Defect Nanostructure and its Impact on Magnetism of α-Cr2O3 Thin Films

engraver — Mon, 28 Mar 2022 17:59:09 +0000

Thin films of the magnetoelectric insulator α-Cr₂O₃ are technologically relevant for energy-efficient magnetic memory devices controlled by electric fields. In contrast to single crystals, the quality of thin Cr₂O₃ films is usually compromised by the presence of point defects and their agglomerations at grain boundaries, putting into question their application potential. Here, the impact of the defect nanostructure, including sparse small-volume defects and their complexes is studied on the magnetic properties of Cr₂O₃ thin films. By tuning the deposition temperature, the type, size, and relative concentration of defects is tailored, which is analyzed using the positron annihilation spectroscopy complemented with electron microscopy studies. The structural characterization is correlated with magnetotransport measurements and nitrogen-vacancy microscopy of antiferromagnetic domain patterns. Defects pin antiferromagnetic domain walls and stabilize complex multidomain states with a domain size in the sub-micrometer range. Despite their influence on the domain configuration, neither small open-volume defects nor grain boundaries in Cr₂O₃ thin films affect the Néel temperature in a broad range of deposition parameters. The results pave the way toward the realization of spin-orbitronic devices where magnetic domain patterns can be tailored based on defect nanostructures without affecting their operation temperature.

Igor Veremchuk, Maciej Oskar Liedke, Pavlo Makushko, Tobias Kosub, Natascha Hedrich, Oleksandr V. Pylypovskyi, Fabian Ganss, Maik Butterling, René Hübner, Eric Hirschmann, Ahmed G. Attallah, Andreas Wagner, Kai Wagner, Brendan Shields, Patrick Maletinsky, Jürgen Fassbender, Denys Makarov. Defect Nanostructure and its Impact on Magnetism of α-Cr₂O₃ Thin Films, Small P. 2201228 (2022) DOI: 10.1002/smll.202201228 (Open Access)

Fundamentals of curvilinear ferromagnetism: Statics and dynamics of geometrically curved wires and narrow ribbons

engraver — Thu, 20 Jan 2022 09:22:45 +0000

Low-dimensional magnetic architectures including wires and thin films are key enablers of prospective ultrafast and energy efficient memory, logic, and sensor devices relying on spin-orbitronic and magnonic concepts. Curvilinear magnetism emerged as a novel approach in material science, which allows tailoring of the fundamental anisotropic and chiral responses relying on the geometrical curvature of magnetic architectures. Much attention is dedicated to magnetic wires of Möbius, helical, or DNA-like double helical shapes, which act as prototypical objects for the exploration of the fundamentals of curvilinear magnetism. Although there is a bulk number of original publications covering fabrication, characterization, and theory of magnetic wires, there is no comprehensive review of the theoretical framework of how to describe these architectures. Here, theoretical activities on the topic of curvilinear magnetic wires and narrow nanoribbons are summarized, providing a systematic review of the emergent interactions and novel physical effects caused by the curvature. Prospective research directions of curvilinear spintronics and spin-orbitronics are discussed, the fundamental framework for curvilinear magnonics are outlined, and mechanically flexible curvilinear architectures for soft robotics are introduced.

Denis D. Sheka, Oleksandr V. Pylypovskyi, Oleksii M. Volkov, Kostiantyn V. Yershov, Volodymyr P. Kravchuk, Denys Makarov. Fundamentals of Curvilinear Ferromagnetism: Statics and Dynamics of Geometrically Curved Wires and Narrow Ribbons, Small P. 2105219 (2022) DOI: 10.1002/smll.202105219 (Open Access, pdf)