Ping Tang

Spin transport in magnetic insulators is often treated by assuming that magnons carry a fixed spin angular momentum of ℏ , which does not hold in general, however. Here, we calculate the magnon spin angular momentum of a layered antiferromagnet as a function of applied magnetic field and wave vector. We show that the triaxial anisotropy and intralayer dipolar interactions in bilayer CrSBr renormalize the magnon spin angular momentum, which diverges upon field-induced magnon softening. This divergence gives rise to a pronounced peak in the thermal spin Seebeck response and provides a clear spin-caloritronic signature of soft magnons.

Interfacial engineering of spin currents is crucial for developing spintronic and memory devices. Here, we demonstrate the nonreciprocal transport of spin currents in Pt/Ti and Ti/Pt bilayers adjacent to a Py ferromagnetic layer using spin pumping and spin-torque ferromagnetic resonance (ST-FMR) techniques. Systematic thickness-dependent measurements reveal that the magnetic damping, which reflects the efficiency of spin-current injection, depends strongly on the stacking order. In Py/Pt/Ti structures, the magnetic damping increases with Pt thickness and saturates, following the conventional spin-diffusion model. In contrast, Py/Ti/Pt structures exhibit a rapid decline and saturate at thicker Ti thicknesses. The reduction of damping in the Py/Ti/Pt structures upon adding the Pt layer indicates strong orbital pumping and spin backflow at the Ti/Pt interface. Furthermore, ST-FMR measurements show opposite signs and distinct magnitudes of the spin-orbit torque efficiency for the two stacking configurations. These results confirm that the Pt layer dominates spin-current generation via the spin Hall effect, while the Ti layer contributes through the orbital Hall effect. Our results establish a clear asymmetry in spin-current propagation across Pt/Ti interfaces and provide a quantitative framework for understanding interfacial spin transport in multilayer spintronic systems.

Multiferroics are materials with coexisting electric and magnetic orders that are of central importance for fundamental research and technological applications. Unfortunately, intrinsic multiferroics that operate at room temperature remain rare due to an apparent incompatibility between magnetism and ferroelectricity. Here we predict that pure ferroelectrics may support multiferroic-like quasiparticles, termed "multiferrons," that simultaneously carry static magnetic and electric dipoles. The electric dipole moment emerges from the parity-odd anharmonicity of the ferroelectric dynamics, while the magnetic moment has both paramagnetic and diamagnetic origins generated by circularly polarized transverse fluctuations of the ferroelectric polarization. In contrast to the established "dynamical multiferroicity" of circularly polarized phonons, which involves only oscillating electric dipoles, multiferrons exhibit or cause, apart from Zeeman and Einstein-de Haas effects, a linear dc Stark response, giant electric-field-tunable second-harmonic generation in the THz-frequency regime, and a magnetoelectric cross coupling. Multiferrons open a new route toward nonlinear THz optical applications and offer multiferroic functionalities with simple ferroelectrics.

Surface waves, the evanescent solutions of the wave equation at planar discontinuities, are of fundamental importance in surface physics, optics, phononics, electronics, and magnetism. Here, we predict that van der Waals antiferromagnets support surface spin waves that are unique by their extreme (nanoscale) confinement and high group velocity with application potential in ultrafast, nanoscale magnonic devices. These excitations reside within the bulk magnon band gap of type-A van der Waals antiferromagnets (antiferromagnetically coupled ferromagnetic monolayers). In contrast to conventional magnetostatic Damon-Eshbach modes, they are pure exchange modes owing to reduced interlayer exchange coupling at the surface layers, and thus persist in ultrathin multilayer stacks and at large wave numbers. We show that they can be efficiently excited by electromagnetic waves, with absorption power comparable to or even exceeding that of bulk modes. Moreover, their emitted magnetic stray fields exhibit pronounced even-odd oscillations with the number of monolayers that should be observable by nitrogen-vacancy-center magnetometry.

The orbital Hall effect (OHE) has garnered much attention as a promising approach to realize highly efficient "orbitronic" devices with a wide range of materials. However, the existing theories that attempt to explain the experimental evidence focus on the intrinsic effect, neglecting the omnipresent disorder. Here, we formulate the impact of random defect scattering on the orbital Hall effect by a quantum Boltzmann equation and solve it for a generic two-band model including the in-scattering collision integral (vertex correction). In contrast to the common belief that the intrinsic OHE is robust against the disorder, we find that diffuse scattering by an arbitrarily weak disorder affects and can even fully suppress the intrinsic orbital Hall current, depending on the character of orbital states and the disorder.

Antiferromagnets attract much interest because of their potential for spintronic applications and open fundamental physics questions, but especially noncollinear antiferromagnets remain relatively unexplored. Here, we formulate the thermal and coherent pumping of spins in noncollinear antiferromagnets|normal metal bilayers. We find that the spin current polarization is a vector with components along both the Néel vector and net magnetic moment. The spin mixing conductance for the coherent spin pumping is a tensor with elements depending on the degree of noncollinearity and interface spin configuration. We explain the controversial sign problem of the antiferromagnetic spin Seebeck effect by interface effects and suggest that interface engineering may enhance the spin pumping efficiency.