Emir Karadža

Heterostructures of 2D materials offer high tunability of magnetic properties, including current-induced magnetization switching by unconventional spin–orbit torques. The microscopic mechanisms governing spin-torque switching in van der Waals systems, however, remain insufficiently understood. Here, we investigate current-induced magnetization reversal in an Fe3GeTe2 (FGT) and WTe2 heterostructure using scanning harmonic magneto-optical Kerr effect microscopy. Spatially resolved magnetization maps show that field-free switching unfolds from asymmetric domain nucleation at the edges of the FGT flake, followed by domain wall propagation. This switching behavior is deterministic and bipolar, consistent with spin–orbit torques arising from the injection of both in-plane and out-of-plane spins from WTe2 into FGT. The critical currents for up-to-down and down-to-up switching differ because of asymmetric domain nucleation and directional domain wall pinning effects. Micromagnetic simulations reproduce the switching behavior observed experimentally, showing how the initial domain nucleation sites are determined by the sample geometry and combined influence of in-plane and out-of-plane spins and Dzyaloshinskii-Moriya interaction. Finally, by systematically varying the field-like and damping-like torque components and sweeping the spin polarization between in-plane and out-of-plane, we identify the torque configurations that yield the most efficient switching in van der Waals magnets with different shapes.

We report direct spectroscopic evidence of antimagnons, i.e., negative-energy spin waves identified by their signature inverted dispersion and left-handed precession with Brillouin light scattering (BLS) spectroscopy. We investigate an ultrathin BiYIG film with a perpendicular magnetic anisotropy that compensates the demagnetizing field. By injecting a spin-orbit torque, the magnetization is driven into auto-oscillation and eventually into a nonequilibrium reversed state above a secondary current threshold ($\ensuremath{\sim}1.2\ifmmode\times\else\texttimes\fi{}{10}^{7}\text{ }\text{ }\mathrm{A}/{\mathrm{cm}}^{2}$). The dispersion is measured by wave-vector-resolved BLS and exhibits a sharp change from an upward dispersion to a downward one, in agreement with theoretical predictions and micromagnetic simulations. Around the threshold current, we observe the coexistence of conventional magnons and antimagnons. Our Letter establishes antimagnons with inverted dispersion and is a first step toward exploring novel phenomena and applications due to magnon-antimagnon coupling, such as magnon amplification and magnon-antimagnon entanglement, which are part of the emerging field of antimagnonics.

A key aim in spintronics is to achieve current‐induced magnetization switching via spin‐orbit torques without external magnetic fields. For this, the focus of recent work has been on introducing controlled lateral gradients across ferromagnet/heavy‐metal devices, giving variations in thickness, composition, or interface quality. However, the shallow gradients achievable with growth techniques limit the impact of this approach and understanding of the underlying physical mechanisms. Here, crystalline phase gradients are patterned on a mesoscopic length scale in tungsten thin films using direct‐write laser annealing. Through transmission electron microscopy, resistivity, and second harmonic measurements, the continuous transformation of the phase of tungsten films from the highly spin‐orbit coupled, high‐resistivity phase to the minimally spin‐orbit coupled, low‐resistivity phase with increasing laser fluence is tracked. Gradients with different steepness and arbitrary shapes are patterned in the tungsten phase. When interfaced with CoFeB, current‐induced spin‐orbit torques resulting from tungsten with a sufficiently steep gradient can switch the magnetization without an applied magnetic field. Therefore, exploiting the unique microstructure of mixed‐phase W allows precise control of the local electronic current density and direction, as well as local spin‐orbit torque efficiency, providing a new avenue for the design of efficient spintronic devices.

Charge-spin conversion phenomena, such as the spin Hall effect, allow for the excitation of magnons in a magnetic layer by passing an electric current in an adjacent nonmagnetic conductor. We demonstrate that this current-induced modification of the magnon density generates an additional nonlinear longitudinal and transverse magnetoresistance for every magnetoresistance that depends on the magnetization. Using harmonic measurements, we evidence that these magnon creation-annihilation magnetoresistances dominate the second harmonic longitudinal and transverse resistance of thin Y_{3}Fe_{5}O_{12}/Pt bilayers. Our results apply to both insulating and metallic magnetic layers, elucidating the dependence of the magnetoresistance on applied current and magnetic field for a broad variety of systems excited by spin currents.