András Deák

Understanding the mechanism driving magnetization switching in spin-orbit-torque-assisted devices remains a subject of debate. While originally attributed to the spin Hall effect and spin Rashba-Edelstein effect, recent discoveries related to orbital moments induced by the orbital Hall effect and the orbital Rashba-Edelstein effect have added complexity to the comprehension of the switching process in nonmagnet/ferromagnet bilayers. Addressing this challenge, we present a quantitative investigation of a Pt/Co bilayer by employing atomistic spin dynamics simulations, incorporating the proximity-induced moments of Pt, as well as electrically induced spin and orbital moments obtained from first-principles calculations. Our layer-resolved model elucidates the dampinglike and fieldlike nature of the induced moments by separating them according to their even and odd magnetization dependence. In addition to demonstrating that a larger fieldlike spin-orbit-torque contribution comes from previously disregarded induced orbital moments, our work highlights the necessity of considering interactions with Pt-induced moments at the interface, as they contribute significantly to the switching dynamics.

Hematite is a canted antiferromagnetic insulator, promising for applications in spintronics. Here we present ab initio calculations of the tensorial exchange interactions of hematite and use them to understand its magnetic properties by parametrizing a semiclassical Heisenberg spin model. Using atomistic spin dynamics simulations, we calculate the equilibrium properties and phase transitions of hematite, most notably the Morin transition. The computed isotropic and Dzyaloshinskii–Moriya interactions result in a Néel temperature and weak ferromagnetic canting angle that are in good agreement with experimental measurements. Our simulations show how dipole-dipole interactions act in a delicate balance with first and higher-order on-site anisotropies to determine the material's magnetic phase. Comparison with spin-Hall magnetoresistance measurements on a hematite single crystal reveals deviations of the critical behavior at low temperatures. Based on a mean-field model, we argue that these differences result from the quantum nature of the fluctuations that drive the phase transitions.

Incommensurate magnetism in CrB₂ is studied in terms of a spin model based on density functional theory calculations. Heisenberg exchange interactions derived from the paramagnetic phase using the disordered local moment (DLM) theory show significant differences compared with those resulting from the treatment of the material as a ferromagnet; of these two methods, the DLM theory is found to give a significantly more realistic description. We calculate strongly ferromagnetic interactions between Cr planes but largely frustrated interactions within Cr planes. Although we find that the ground state ordering vector is sensitive to exchange interactions over a large number of neighbour shells, the q-vector of the incommensurate spin spiral state is satisfactorily reproduced by the theory (0.213 compared with the known ordering vector 0.285 × ( 2 π ) / ( a / 2 ) along Γ– K ). The strong geometric frustration of the exchange interactions causes a rather low Néel temperature (about 97 K), also in good agreement with experiment.

Skyrmions are topologically protected nanoscale magnetic structures with a wide range of potential applications. Here we determine the life cycle of skyrmions from their creation to their intrinsic dynamics and thermal stability to their eventual thermodynamic demise. Using atomistic simulations of Ir/Co/Pt, parameterized from ab initio calculations, we demonstrate the thermal phase transition to a skyrmion state under application of a perpendicular magnetic field. The created skyrmions exhibit Brownian particlelike dynamics driven by the underlying thermal spin fluctuations. At an elevated temperature window well below the Curie temperature, the skyrmions are metastable and can collapse to a uniform magnetic state. With application of an external magnetic field, the intrinsic local thermal spin fluctuations at this elevated temperature window are sufficiently large to allow the spontaneous formation of new skyrmions in thermodynamic equilibrium analogous to a spontaneous skyrmion gas. Such a system could be used to implement a skyrmion-based true random number generator.

Mn₃Sn has recently attracted considerable attention as a magnetic Weyl semimetal exhibiting concomitant transport anomalies at room temperature. The topology of the electronic bands, their relation to the magnetic ground state and their nonzero Berry curvature lie at the heart of the problem. The examination of the full magnetic Hamiltonian reveals otherwise hidden aspects of these unusual physical properties. Here, we report the full spin wave spectra of Mn₃Sn measured over a wide momentum—energy range by the inelastic neutron scattering technique. Using a linear spin wave theory, we determine a suitable magnetic Hamiltonian which not only explains the experimental results but also stabilizes the low-temperature helical phase, consistent with our DFT calculations. The effect of this helical ordering on topological band structures is further examined using a tight binding method, which confirms the elimination of Weyl points in the helical phase. Our work provides a rare example of the intimate coupling between the electronic and spin degrees of freedom for a magnetic Weyl semimetal system.