Nonuniform Current-Driven Formation and Displacement of the Magnetic Compensation Point in Variable-Width Nanoscale Ferrimagnets

Nano- and microstructures based on ferrimagnets can demonstrate high efficiency and dynamics of current-induced magnetization switching combined with high stability of spin textures such as bubble domains and skyrmions, which are of practical importance for the development of spintronics and spin-orbitronics. This set of features is usually associated with magnetic momentum or angular momentum compensation states. Here, we experimentally show that the compensation state can be realized locally using nonuniform Joule heating. This effect is observed in the variable-width current guide made of the ferrimagnetic W/Co76Tb24/Ru thin films, where the position of a region heated to the compensation temperature depends linearly on the current pulse amplitude. This approach makes it possible to observe the simultaneous coexistence of Co-dominant and Tb-dominant regions, where current pulses induce spin-orbit torques in opposite directions, leading to local magnetization switching. It is found that the position of a Néel domain wall constraining the switched region lies in the vicinity of the coordinate corresponding to the compensation point but does not coincide with it due to high mobility under the action of spin current. Our findings open an alternative approach for engineering of ferrimagnetic nanodevices with advanced properties for future applications in spintronics, spin-orbitronics, and nanoelectronics.

Current-Induced Manipulation of the Exchange Bias in a Pt/Co/NiO Structure

An experimental study of the phenomenon of electric current influence on the value and orientation of the exchange bias field (H_EB) in the Pt/Co/NiO structure is carried out. Depending on the direction of the magnetization in a ferromagnet (FM) layer and the current pulse amplitude, the value of the H_EB field can be changed repeatedly in the range of ±7.5 mT. A few experiments are performed to separate the contributions from two current-induced effects: (i) an injection of the spin current into an antiferromagnet layer (AFM) and (ii) Joule heating. As a result, we conclude that the modification in the H_EB field during current pulse transmission in the Pt/Co/NiO structure is due to heating and the low value of Néel temperature (T_N = 162 °C). This fact explains the absence of the exchange bias effect on the spin-orbit torque (SOT)-assisted magnetization switching. The most striking observation to emerge from the experimental data analysis is that depending on the initial spin configuration of the domain structure in the FM layer and the current pulse amplitude, the exchange bias can be changed locally. This opens up prospects for creating exchange-coupled FM/AFM structures with dynamically tuned parameters of the exchange bias, which can be used for the development of magnetic memory, neuromorphic, and logic devices based on magnetic nanosystems.

Vortex dynamics and frequency splitting in vertically coupled nanomagnets

We explored the dynamic response of a vortex core in a circular nanomagnet by manipulating its dipole-dipole interaction with another vortex core confined locally on top of the nanomagnet. A clear frequency splitting is observed corresponding to the gyrofrequencies of the two vortex cores. The peak positions of the two resonance frequencies can be engineered by controlling the magnitude and direction of the external magnetic field. Both experimental and micromagnetic simulations show that the frequency spectra for the combined system is significantly dependent on the chirality of the circular nanomagnet and is asymmetric with respect to the external bias field. We attribute this result to the strong dynamic dipole-dipole interaction between the two vortex cores, which varies with the distance between them. The possibility of having multiple states in a single nanomagnet with vertical coupling could be of interest for magnetoresistive memories.

Self-organization and FORC-based magnetic characterization of ultra-high aspect ratio epitaxial Co nanostrips produced by oblique deposition on an ordered step-bunched silicon surface

Further development of microelectronics requires novel or improved technological approaches for device nanofabrication and functional properties characterization. In this paper, we studied the crystal structure and magnetic properties of epitaxial Co nanostrips with the average width of 32.6, 45.3, and 62.6 nm grown on a step-bunched Si(111)5.55 × 5.55-Cu/Cu surface. Technological conditions, under which the ultra-high aspect ratio (∼10⁴) structurally solid, straight nanostrips of hcp-Co with crystallographic axis [0001] oriented along their long side can be grown, were determined. The dependence of the coercive force on the width of the nanostrips was demonstrated. Magnetization reversal through the transverse domain-wall nucleation and propagation in a Co nanostrip was defined with an analytical approach based on the Stoner-Wohlfarth model. Using the first-order reversal curve method, we analyzed the effect of nanostrip uniformity degree on magnetic behavior and the influence of the magnetostatic interactions on the coercive force of individual nanostrips.