Showing papers on "Magnetic domain published in 2022"

Magnetic Losses in Soft Ferrites

Abstract: We review the basic phenomenology of magnetic losses from DC to 1 GHz in commercial and laboratory-prepared soft ferrites considering recent concepts regarding their physical interpretation. This is based, on the one hand, on the identification of the contributions to the magnetization process provided by spin rotations and domain walls and, on the other hand, the concept of loss separation. It additionally contemplates a distinction between the involved microscopic dissipation mechanisms: spin damping and eddy currents. Selected experimental results on the broadband behavior of complex permeability and losses in Mn-Zn ferrites provide significant examples of their dependence on sintering methods, solute elements, and working temperature. We also highlight the peculiar frequency and temperature response of Ni-Zn ferrites, which can be heavily affected by magnetic aftereffects. The physical modeling of the losses brings to light the role of the magnetic anisotropy and the way its magnitude distribution, affected by the internal demagnetizing fields, acts upon the magnetization process and its dependence on temperature and frequency. It is shown that the effective anisotropy governs the interplay of domain wall and rotational processes and their distinctive dissipation mechanisms, whose contributions are recognized in terms of different loss components.

Domain Wall Automotion in Three-Dimensional Magnetic Helical Interconnectors

Abstract: The fundamental limits currently faced by traditional computing devices necessitate the exploration of ways to store, compute, and transmit information going beyond the current CMOS-based technologies. Here, we propose a three-dimensional (3D) magnetic interconnector that exploits geometry-driven automotion of domain walls (DWs), for the transfer of magnetic information between functional magnetic planes. By combining state-of-the-art 3D nanoprinting and standard physical vapor deposition, we prototype 3D helical DW conduits. We observe the automotion of DWs by imaging their magnetic state under different field sequences using X-ray microscopy, observing a robust unidirectional motion of DWs from the bottom to the top of the spirals. From experiments and micromagnetic simulations, we determine that the large thickness gradients present in the structure are the main mechanism for 3D DW automotion. We obtain direct evidence of how this tailorable magnetic energy gradient is imprinted in the devices, and how it competes with pinning effects that are due to local changes in the energy landscape. Our work also predicts how this effect could lead to high DW velocities, reaching the Walker limit during automotion. This work demonstrates a possible mechanism for efficient transfer of magnetic information in three dimensions.

Atomically sharp domain walls in an antiferromagnet

Abstract: The interest in understanding scaling limits of magnetic textures such as domain walls spans the entire field of magnetism from its physical fundamentals to applications in information technologies. Here, we explore antiferromagnetic CuMnAs in which imaging by x-ray photoemission reveals the presence of magnetic textures down to nanoscale, reaching the detection limit of this established microscopy in antiferromagnets. We achieve atomic resolution by using differential phase-contrast imaging within aberration-corrected scanning transmission electron microscopy. We identify abrupt domain walls in the antiferromagnetic film corresponding to the Néel order reversal between two neighboring atomic planes. Our work stimulates research of magnetic textures at the ultimate atomic scale and sheds light on electrical and ultrafast optical antiferromagnetic devices with magnetic field-insensitive neuromorphic functionalities.

Reconfigurable Ferromagnetic Resonances by Engineering Inhomogeneous Magnetic Textures in Artificial Magnonic Crystals

Abstract: Advanced microfabrication gives rise to an extra degree of freedom for controlling magnetic textures, which determine the spin dynamics of thin‐film magnets for various spintronic devices. However, the intuitive understanding of complicated ferromagnetic resonances (FMRs) is still challenging for lacking directly observed evolution of inhomogeneous magnetic textures. This study demonstrates the reconfigurable FMRs by introducing artificial patterns of antidot lattices (namely magnonic crystals), mediated by the engineering of inhomogeneous magnetic textures. Ordered walls are observed directly in such a magnetic film using in situ Lorentz transmission electron microscopy. By applying in‐plane external magnetic fields, arc‐shaped domain walls can be nucleated along the edge of patterns, inducing the splitting of FMRs from a single peak to two branches (marked by low‐frequency and high‐frequency branches, respectively). Complementary micromagnetic simulations reconstruct the inhomogeneous magnetic textures and splitting of FMRs. It is indicated that the low‐frequency branch ascribes to the confined mode excited by arc‐shaped domain walls, and the high‐frequency one originates from the quasi‐uniform mode generated by in‐plane magnetization between arc‐shaped domain walls. These results offer new opportunities for designing patterned magnonic devices and understanding the unique phenomenon of spin dynamics.

On-demand ferrofluid droplet formation with non-linear magnetic permeability in the presence of high non-uniform magnetic fields

Abstract: The magnetic actuation of ferrofluid droplets offers an inspiring tool in widespread engineering and biological applications. In this study, the dynamics of ferrofluid droplet generation with a Drop-on-Demand feature under a non-uniform magnetic field is investigated by multiscale numerical modeling. Langevin equation is assumed for ferrofluid magnetic susceptibility due to the strong applied magnetic field. Large and small computational domains are considered. In the larger domain, the magnetic field is obtained by solving Maxwell equations. In the smaller domain, a coupling of continuity, Navier Stokes, two-phase flow, and Maxwell equations are solved by utilizing the magnetic field achieved by the larger domain for the boundary condition. The Finite volume method and coupling of level-set and Volume of Fluid methods are used for solving equations. The droplet formation is simulated in a two-dimensional axisymmetric domain. The method of solving fluid and magnetic equations is validated using a benchmark. Then, ferrofluid droplet formation is investigated experimentally, and the numerical results showed good agreement with the experimental data. The effect of 12 dimensionless parameters, including the ratio of magnetic, gravitational, and surface tension forces, the ratio of the nozzle and magnetic coil dimensions, and ferrofluid to continuous-phase properties ratios are studied. The results showed that by increasing the magnetic Bond number, gravitational Bond number, Ohnesorge number, dimensionless saturation magnetization, initial magnetic susceptibility of ferrofluid, the generated droplet diameter reduces, whereas the formation frequency increases. The same results were observed when decreasing the ferrite core diameter to outer nozzle diameter, density, and viscosity ratios.

Magnetic domain walls of the van der Waals material Fe3GeTe2

Abstract: Among two-dimensional materials, Fe3GeTe2 has come to occupy a very important place owing to its ferromagnetic nature with one of the highest Curie temperatures among known van der Waals materials and the potential for hosting skyrmions. In this combined experimental and theoretical work, we investigate the magnetic bubble domains as well as the microscopic domain wall profile using spin-polarized scanning tunneling microscopy in combination with atomistic spin-dynamics simulations performed with parameters from density functional theory calculations. We find a weak magneto-electric effect influencing the domain wall width by the electric field in the tunneling junction and determine the critical magnetic field for the collapse of the bubble domains. Our findings shed light on the origins of complex magnetism that Fe3GeTe2 exhibits.

Layer-Dependent Magnetic Domains in Atomically Thin Fe5GeTe2

Abstract: Magnetic domain formation in two-dimensional (2D) materials gives perspectives into the fundamental origins of 2D magnetism and also motivates the development of advanced spintronics devices. However, the characterization of magnetic domains in atomically thin van der Waals (vdW) flakes remains challenging. Here, we employ X-ray photoemission electron microscopy (XPEEM) to perform layer-resolved imaging of the domain structures in the itinerant vdW ferromagnet Fe5GeTe2 which shows near room temperature bulk ferromagnetism and a weak perpendicular magnetic anisotropy (PMA). In the bulk limit, we observe the well-known labyrinth-type domains. Thinner flakes, on the other hand, are characterized by increasingly fragmented domains. While PMA is a characteristic property of Fe5GeTe2, we observe a spin-reorientation transition with the spins canting in-plane for flakes thinner than six layers. Notably, a bubble phase emerges in four-layer flakes. This thickness dependence, which clearly deviates from the single-domain behavior observed in other 2D magnetic materials, demonstrates the exciting prospect of stabilizing complex spin textures in 2D vdW magnets at relatively high temperatures.

Imaging of Love Waves and Their Interaction with Magnetic Domain Walls in Magnetoelectric Magnetic Field Sensors

Abstract: The complex behavior of horizontally polarized surface shear waves in magnetoelectric surface acoustic wave based magnetic field sensor devices is revealed by time‐resolved magnetooptical microscopy with picosecond temporal and submicron spatial resolution. The imaging of the propagating waves in the magnetoelectric composites is realized through the functional soft‐magnetic layer by coupled magnetoelastic interactions. Partial surface wave reflections, wave front dephasing, and secondary wave generation occur, which originate from structures and magnetic domain walls. Closure domain structures bend and reflect the magnetic surface waves. Strain stimulated magnetic domain walls display dynamic periodic expansions, which propagate along the domain walls and change the magnetomechanical response also in the surrounding regions. The revealed spatial and temporally varying nondeterministic response restricts the noise performance of the surface acoustic wave based magnetic field sensors and thus confines the sensor's limit of detection. Magnetic time‐resolved optical imaging is shown to be a powerful method for the operando characterization of magnetoelectric devices and in‐plane displacement surface acoustic wave fields that are not accessible by other methods.

Imaging of Love Waves and Their Interaction with Magnetic Domain Walls in Magnetoelectric Magnetic Field Sensors (Adv. Electron. Mater. 6/2022)

Abstract: The complex behavior of horizontally polarized surface shear waves in magnetoelectric surface acoustic wave based magnetic field sensor devices is revealed by time-resolved magnetooptical microscopy with picosecond temporal and submicron spatial resolution. The imaging of the propagating waves in the magnetoelectric composites is realized through the functional soft-magnetic layer by coupled magnetoelastic interactions. Partial surface wave reflections, wave front dephasing, and secondary wave generation occur, which originate from structures and magnetic domain walls. Closure domain structures bend and reflect the magnetic surface waves. Strain stimulated magnetic domain walls display dynamic periodic expansions, which propagate along the domain walls and change the magnetomechanical response also in the surrounding regions. The revealed spatial and temporally varying nondeterministic response restricts the noise performance of the surface acoustic wave based magnetic field sensors and thus confines the sensor's limit of detection. Magnetic time-resolved optical imaging is shown to be a powerful method for the operando characterization of magnetoelectric devices and in-plane displacement surface acoustic wave fields that are not accessible by other methods.

Depinning Exponents of Thin Film Domain Walls Depend on Disorder Strength

Abstract: Domain wall dynamics in ferromagnets is complicated by internal degrees of freedom of the domain walls. We develop a model of domain walls in disordered thin films with perpendicular magnetic anisotropy capturing such features, and use it to study the depinning transition. For weak disorder, excitations of the internal magnetization are rare, and the depinning transition takes on exponent values of the quenched Edwards-Wilkinson equation. Stronger disorder results in disorder-dependent exponents concurrently with nucleation of an increasing density of Bloch lines within the domain wall.

Field-Free Magnetization Switching Induced by Bulk Spin–Orbit Torque in a (111)-Oriented CoPt Single Layer with In-Plane Remanent Magnetization

Abstract: Spin–orbit torque (SOT)-induced perpendicular magnetization switching is one of the key solutions for the next generation of magnetic memory and spin logic applications. Recently, the bulk SOT effect in a single magnetic layer with a vertical composition gradient has attracted a lot of attention because it can break through the interfacial nature of the SOT effect in a traditional bilayer structure. However, the dependency of the external in-plane magnetic field or the additional pinning layer for deterministic switching hinders the further application of this technology. Here, for the first time, we implement field-free magnetization switching induced by bulk SOT in a single (111)-oriented CoPt magnetic layer with in-plane remanent magnetization. The initialized longitudinal in-plane remanent magnetization can substitute the external magnetic field to break the inversion symmetry and realize continuous field-free perpendicular magnetization switching. Furthermore, the in-plane remanent magnetization can be manipulated by the SOT effective field induced by lateral current pulses, leading to a tunable switching chirality. A multi-domain micromagnetic model is established to describe in depth the experimental observations and clarify the relationship between switching amplitude and easy magnetization cone angle. Our work provides an alternative solution to realize field-free perpendicular magnetization switching in a single magnetic layer, which can promote the development of emerging high-density and low-power SOT-based devices.