engraver – RiTM

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)

Nematic shells: new insights in topology- and curvature-induced effects

engraver — Tue, 02 Nov 2021 10:13:59 +0000

Within the framework of continuum theory, we draw a parallel between ferromagnetic materials and nematic liquid crystals confined on curved surfaces, which are both characterized by local interaction and anchoring potentials. We show that the extrinsic curvature of the shell combined with the out-of-plane component of the director field gives rise to chirality effects. This interplay produces an effective energy term reminiscent of the chiral term in cholesteric liquid crystals, with the curvature tensor acting as a sort of anisotropic helicity. We discuss also how the different nature of the order parameter, a vector in ferromagnets and a tensor in nematics, yields different textures on surfaces with the same topology as the sphere. In particular, we show that the extrinsic curvature governs the ground state configuration on a nematic spherical shell, favouring two antipodal disclinations of charge on small particles and four disclinations of charge located at the vertices of a square inscribed in a great circle on larger particles.

Gaetano Napoli, Oleksandr V. Pylypovskyi, Denis D Sheka, Luigi Vergori. Nematic shells: new insights in topology- and curvature-induced effects, Soft Matter (2021) DOI: 10.1039/d1sm00719j, arXiv:2102.13497

New dimension in magnetism and superconductivity: 3D and curvilinear nano-architectures

engraver — Wed, 27 Oct 2021 14:42:40 +0000

Traditionally, the primary field, where curvature has been at the heart of research, is the theory of general relativity. In recent studies, however, the impact of curvilinear geometry enters various disciplines, ranging from solid-state physics over soft-matter physics, chemistry, and biology to mathematics, giving rise to a plethora of emerging domains such as curvilinear nematics, curvilinear studies of cell biology, curvilinear semiconductors, superfluidity, optics, 2D van der Waals materials, plasmonics, magnetism, and superconductivity. Here, the state of the art is summarized and prospects for future research in curvilinear solid-state systems exhibiting such fundamental cooperative phenomena as ferromagnetism, antiferromagnetism, and superconductivity are outlined. Highlighting the recent developments and current challenges in theory, fabrication, and characterization of curvilinear micro- and nanostructures, special attention is paid to perspective research directions entailing new physics and to their strong application potential. Overall, the perspective is aimed at crossing the boundaries between the magnetism and superconductivity communities and drawing attention to the conceptual aspects of how extension of structures into the third dimension and curvilinear geometry can modify existing and aid launching novel functionalities. In addition, the perspective should stimulate the development and dissemination of research and development oriented techniques to facilitate rapid transitions from laboratory demonstrations to industry-ready prototypes and eventual products.

Publication:

Denys Makarov, Oleksii M. Volkov, Attila Kakay, Oleksandr V. Pylypovskyi, Barbora Budinska, Oleksandr V. Dobrovolskiy. New dimension in magnetism and superconductivity: 3D and curvilinear nano-architectures, Advanced Materials P. 2101758 (2021) DOI: 10.1002/adma.202101758 (Open access: CC BY 4.0)

Curvature-driven homogeneous Dzyaloshinskii-Moriya interaction and emergent weak ferromagnetism in anisotropic antiferromagnetic spin chains

engraver — Wed, 05 May 2021 08:18:29 +0000

Chiral antiferromagnets are currently considered for a broad range of applications in spintronics, spin-orbitronics, and magnonics. In contrast to the established approach relying on materials screening, the anisotropic and chiral responses of low-dimensional antiferromagnets can be tailored relying on the geometrical curvature. Here, we consider an achiral, anisotropic antiferromagnetic spin chain and demonstrate that these systems possess geometry-driven effects stemming not only from the exchange interaction but also from the anisotropy. Peculiarly, the anisotropy-driven effects are complementary to the curvature effects stemming from the exchange interaction and rather strong as they are linear in curvature. These effects are responsible for the tilt of the equilibrium direction of vector order parameters and the appearance of the homogeneous Dzyaloshinskii–Moriya interaction. The latter is a source of the geometry-driven weak ferromagnetism emerging in curvilinear antiferromagnetic spin chains. Our findings provide a deeper fundamental insight into the physics of curvilinear antiferromagnets beyond the σ-model and offer an additional degree of freedom in the design of spintronic and magnonic devices.

Publication:

Oleksandr V. Pylypovskyi, Yelyzaveta A. Borysenko, Jurgen Fassbender, Denis D. Sheka, Denys Makarov. Curvature-driven homogeneous Dzyaloshinskii-Moriya interaction and emergent weak ferromagnetism in anisotropic antiferromagnetic spin chains, Applied Physics Letters 118, P. 182405 (2021) DOI: 10.1063/5.0048823, arXiv:2103.00169, PDF

Boundary conditions for the Néel order parameter in a chiral antiferromagnetic slab

engraver — Fri, 09 Apr 2021 14:57:49 +0000

Understanding of the interaction of antiferromagnetic solitons including domain walls and skyrmions with boundaries of chiral antiferromagnetic slabs is important for the design of prospective antiferromagnetic spintronic devices. Here, we derive the transition from spin lattice to micromagnetic nonlinear σ model with the corresponding boundary conditions for a chiral cubic G-type antiferromagnet and analyze the impact of the slab boundaries and antisymmetric exchange (Dzyaloshinskii-Moriya interaction) on the vector order parameter. We apply this model to evaluate modifications of antiferromagnetic domain walls and skyrmions upon interaction with boundaries for different strengths of the antisymmetric exchange. Due to the presence of the antisymmetric exchange, both types of antiferromagnetic solitons become broader when approaching the boundary and transform to a mixed Bloch-Néel structure. Both textures feel the boundary at the distance of about five magnetic lengths. In this respect, our model provides design rules for antiferromagnetic racetracks, which can support bulklike properties of solitons.

Publication:

Oleksandr V. Pylypovskyi, Artem Tomilo, Denis D. Sheka, Jurgen Fassbender, Denys Makarov. Boundary conditions for the Néel order parameter in a chiral antiferromagnetic slab, Physical Review B 103, P. 134413 (2021) DOI: 10.1103/PhysRevB.103.134413, arXiv:2101.09134, PDF

Nanoscale mechanics of antiferromagnetic domain walls

engraver — Wed, 17 Feb 2021 11:51:52 +0000

Antiferromagnets can encode information in their ordered magnetic structure, providing the basis for future spintronic devices. The control and understanding of antiferromagnetic domain walls, which are the interfaces between domains with differing order parameter orientations, are key ingredients for advancing antiferromagnetic spintronic technologies. However, studies of the intrinsic mechanics of individual antiferromagnetic domain walls are difficult because they require sufficiently pure materials and suitable experimental approaches to address domain walls on the nanoscale. Here we nucleate isolated 180° domain walls in a single crystal of Cr₂O₃, a prototypical collinear magnetoelectric antiferromagnet, and study their interaction with topographic features fabricated on the sample. We demonstrate domain wall manipulation through the resulting engineered energy landscape and show that the observed interaction is governed by the surface energy of the domain wall. We propose a topographically defined memory architecture based on antiferromagnetic domain walls. Our results advance the understanding of domain wall mechanics in antiferromagnets.

Publication:

Natascha Hedrich, Kai Wagner, Oleksandr V. Pylypovskyi, Brendan J. Shields, Tobias Kosub, Denis D. Sheka, Denys Makarov, Patrick Maletinsky. Nanoscale mechanics of antiferromagnetic domain walls. Nature Physics (2021) DOI: 10.1038/s41567-020-01157-0, arXiv:2009.08986

Curvilinear one-dimensional antiferromagnets

engraver — Thu, 22 Oct 2020 09:09:36 +0000

Antiferromagnets host exotic quasiparticles, support high frequency excitations and are key enablers of the prospective spintronic and spin−orbitronic technologies. Here, we propose a concept of a curvilinear antiferromagnetism where material responses can be tailored by a geometrical curvature without the need to adjust material parameters. We show that an intrinsically achiral one-dimensional (1D) curvilinear antiferromagnet behaves as a chiral helimagnet with geometrically tunable Dzyaloshinskii−Moriya interaction (DMI) and orientation of the Néel vector. The curvature-induced DMI results in the hybridization of spin wave modes and enables a geometrically driven local minimum of the low-frequency branch. This positions curvilinear 1D antiferromagnets as a novel platform for the realization of geometrically tunable chiral antiferromagnets for antiferromagnetic spin−orbitronics and fundamental discoveries in the formation of coherent magnon condensates in the momentum space.

Publication:

Oleksandr V. Pylypovskyi, Denys Y. Kononenko, Kostiantyn V. Yershov, Ulrich K. Rößler, Artem Tomilo, Jürgen Faßbender, Jeroen van den Brink, Denys Makarov, Denis D. Sheka. Curvilinear one-dimensional antiferromagnets, Nano Letters (2020) DOI: DOI: 10.1021/acs.nanolett.0c03246, arXiv: 2005.05835, Supplementary, PDF

Micromagnetic Theory of Curvilinear Ferromagnetic Shells

engraver — Tue, 21 Jul 2020 08:29:58 +0000

The concept of curvature and chirality in space and time are foundational for the understanding of the organic life and formation of matter in the Universe. Chiral interactions but also curvature effects are tacitly accepted to be local. A prototypical condensed matter example is a local spin-orbit- or curvature-induced Rashba or Dzyaloshinskii-Moriya interactions. Here, we introduce a chiral effect, which is essentially nonlocal and resembles itself even in static spin textures living in curvilinear magnetic nanoshells. Its physical origin is the nonlocal magnetostatic interaction. To identify this interaction, we put forth a self-consistent micromagnetic framework of curvilinear magnetism. Understanding of the nonlocal physics of curved magnetic shells requires a curvature-induced geometrical charge, which couples the magnetic sub-system with the curvilinear geometry. The chiral interaction brings about a nonlocal chiral symmetry breaking effect: it introduces handedness in an intrinsically achiral material and enables the design of magnetolectric and ferrotoroidic responses.

Publication:

Denis D. Sheka, Oleksandr V. Pylypovskyi, Pedro Landeros, Yuri Gaididei, Attila Kakay, Denys Makarov. Nonlocal chiral symmetry breaking in curvilinear magnetic shells, Communications Physics 3 P. 128 (2020) DOI: 10.1038/s42005-020-0387-2 (Open Access)

Unidirectional tilt of domain walls in equilibrium in biaxial stripes with Dzyaloshinskii–Moriya interaction

engraver — Tue, 14 Jul 2020 06:40:29 +0000

The orientation of a chiral magnetic domain wall in a racetrack determines its dynamical properties. In equilibrium, magnetic domain walls are expected to be oriented perpendicular to the stripe axis. We demonstrate the appearance of a unidirectional domain wall tilt in out-of-plane magnetized stripes with biaxial anisotropy and Dzyaloshinskii-Moriya interaction (DMI). The tilt is a result of the interplay between the in-plane easy-axis anisotropy and DMI. We show that the additional anisotropy and DMI prefer different domain wall structure: anisotropy links the magnetization azimuthal angle inside the domain wall with the anisotropy direction in contrast to DMI, which prefers the magnetization perpendicular to the domain wall. Their balance with the energy gain due to domain wall extension defines the equilibrium magnetization the domain wall tilting. We demonstrate that the Walker field and the corresponding Walker velocity of the domain wall can be enhanced in the system supporting tilted walls.

Publication:

Oleksandr V. Pylypovskyi, Volodymyr P. Kravchuk, Oleksii M. Volkov, Juergen Fassbender, Denis D. Sheka, Denys Makarov. Unidirectional tilt of domain walls in equilibrium in biaxial stripes with Dzyaloshinskii-Moriya interaction, Journal of Physics D 53 P. 395003 (2020) DOI: 10.1088/1361-6463/ab95bd (Open Access)