Maxim V Bakhmetiev

Pairing a ferromagnet with the MnBi${}_{2}$Te${}_{4}$ offers a promising route to efficient spin-to-charge conversion, but how well it performs across temperature had remained unmapped. Combining ferromagnetic resonance with inverse spin-Hall voltage measurements from 20--290 K, the authors show here that the spin-Hall angle stays nearly constant below approximately 130 K, then becomes unreliable as spin rectification and shifting damping take over. The results establish the material as a robust platform for cryogenic spin-orbit and topological spintronics.

We reveal statistical regularities of spontaneous magnetization reversal in a GdFeCo/Ir/GdFeCo synthetic ferrimagnet with perpendicular anisotropy. A magnetic field close to but not equal to the critical field causes stochastic switching between four stable magnetization states, namely, P⁺, AP⁺, AP⁻, and P⁻, corresponding to two parallel (P⁺, P⁻) and two antiparallel (AP⁺, AP⁻) mutual directions of magnetization of thick and thin GdFeCo layers. Instead of a sharp switching between the AP⁺ and AP⁻ states in the critical field, we observe a spontaneous switching of magnetization delayed by a time interval that varies stochastically in the series of experiments. The expectation of a strong enough fluctuation nucleating the seed domain with reversal magnetization and the subsequent fast spreading of the domain are the reasons for the thermally activated transition between the states. The thermal fluctuation to overcome the potential barrier regulated by the magnetic field explains the field dependence of the phenomenon. It is universal and can occur in other heterostructures.

Ferromagnetic resonance is widely acknowledged as a robust tool for characterizing magnetic materials and studying physical phenomena. Low-temperature magnonics has recently garnered substantial attention; however, it lacks convenient methods for acquiring key magnetic material characteristics. In this study, we introduce a rapid and universal method to determine the temperature dependence of magnetic anisotropy constants of (111)-oriented garnet films using ferromagnetic resonance measurements, demonstrated specifically on the bismuth-substituted iron garnet film grown on a diamagnetic substrate. Our observations reveal a competitive interplay of different anisotropy types as temperature decreases. Notably, the uniaxial magnetic anisotropy constant Ku undergoes a sign change from negative to positive around 120 K, while the anisotropy type remains consistent.

When an electric current flows through the NiFe/IrMn structure, a significant redistribution of the intensities of the direct Stokes and inverse anti-Stokes lines in spectra of Brillouin scattering on thermal spin waves is observed. This is due to a change in the orientation of uniaxial anisotropy and the corresponding easy axis of the exchange shift under the action of the spin–orbit torque in the IrMn antiferromagnet, which in turn affects the dynamics of spin waves in the neighboring ferromagnetic NiFe layer. The detected effect indicates that spin-wave processes in a ferromagnet can be controlled by applying a spin current to change the magnetic anisotropy of the NiFe/IrMn interface.

A magnon-assisted electrically driven “mirror” with tuned frequency of the reflected light is created. The effect of in-plane electric current on asymmetry of Stokes–anti-Stokes Brillouin intensities in NiFe/IrMn exchange-biased patterned structures is found. Electric current directed along external magnetic field suppresses anti-Stokes component in Brillouin light-scattering spectra, while the opposite direction of current results in suppression of the Stokes component. We show that spin currents arising in the IrMn layer induce spin-orbit torque affecting uncompensated antiferromagnetic moments at the interface, rotate the antiferromagnetic moments, change bias field, and finally determine direction of dominating spin-wave propagation. The possibility of electric control of spin-wave direction and manipulation over Stokes and anti-Stokes intensities opens a way for design of electro-optical devices switching light frequency.

Anomalous Hall effect (AHE) in GdFeCo/Ir/GdFeCo multilayered structures attracts great interest because all optical switching, spin-torque, and other effects promise effective application for ultrafast memory element creation. Since AHE is controlled by GdFeCo magnetization, domain dynamics has importance for practical applications. In our work, magnetization reversal in perpendicular GdFeCo/Ir/GdFeCo synthetic ferrimagnets is characterized by AHE measurements. The AHE hysteresis loop obtained with the field applied perpendicular to the sample plane is composed of three sub-loops, and two of them are symmetrically biased with respect to the third one. Switching magnetic fields for two of the three transitions are found to be dependent on magnetic history. In particular, exposure of the sample in the in-plane field leads to reduction of the out-of-plane switching fields in side sub-loops. A multiple series of perpendicular hysteresis loops recorded after exposure under high in-plane field reveals gradual (within 30 min) relaxation of the out-of-plane switching fields to their initial values observed in a non-magnetized sample. Domain wall mobility, limiting switching of the bilayer devices, is complicated due to the coupling between partial domains in each single layer. Unusual dynamics of double domain walls results in unexpected new phenomena affecting electrical processes in bilayer structures.

Magnetization reorientation of GdFeCo/Ir/GdFeCo heterostructures at critical temperatures

Magnetization reversal in NiFe/IrMn exchange-biased thin films was investigated under thermal cycling in an external magnetic field, applied opposite to the direction of the exchange bias field. Thermal hysteresis of magnetization accompanied by changes in magnetization polarity was observed in the applied field close to the exchange bias value. This effect appears when thermally induced variations of the exchange bias exceed the corresponding variations in coercivity. The amplitude of magnetization reversal in NiFe/IrMn structures exceeds ∼100 times the corresponding amplitude in spin-crossover molecular compounds. The observed bistability of the magnetic state, revealed by thermal hysteresis, gradually disappears with an increase in the number of cooling–heating thermal cycles, that indicates an irreversible quenching of the interfacial magnetization configuration. This effect paves the way for the creation of a new class of switching devices with thermally assisted bistability in the ferromagnetic state.

A set of partially uncoupled NiFe/Cu/IrMn exchange biased thin films with variable thickness of non-magnetic Cu spacer is characterized by ferromagnetic resonance (FMR) and Brillouin light scattering (BLS) techniques applied complementary to reveal time-scale dependent effects of uncoupling between ferromagnetic and antiferromagnetic layers on high-frequency magnetization dynamics. The results correlate with interfacial grain texture variations and static magnetization behavior. Two types of crystalline phases with correlated microwave response are revealed at the ferro–antiferromagnet interface in NiFe/Cu/IrMn thin films. The 1st phase forms well-textured NiFe/IrMn grains with NiFe (111)/IrMn (111) interface. The 2nd phase consists of amorphous NiFe/IrMn grains. Intercalation of NiFe/IrMn by Cu clusters results in relaxation of tensile strains at the NiFe/IrMn interface leading to larger size of grains in both the NiFe and IrMn layers. The contributions of well-textured and amorphous grains to the high-frequency magnetization reversal behavior are distinguished by FMR and BLS techniques. Generation of a spin-wave mode is revealed in the well-textured phase, whereas microwave response of the amorphous phase is found to originate from magnetization rotation dominated by a rotatable magnetic anisotropy term. Under fixed FMR frequency, the increase of Cu thickness results in higher magnetization rotation frequencies in the amorphous grains.