Field-free deterministic switching of perpendicular magnetization is essential to achieve high-density and non-volatile magnetic memory, which remains a major challenge because it requires damping-like torques arising from out-of-plane (OOP) spin accumulation. The OOP damping-like torque is also critical to one of the central goals of this MURI project: antiferromagnetic spin-torque oscillator with tunable frequency (studied theoretically by PI-Cheng and PI-Slavin). The collaboration between Singh and PI-Cheng showed for the first time that an OOP damping-like torque arising from the special crystal structure of WTe₂ is sufficiently strong to switch a perpendicularly polarized magnet, where magnetic fields are no longer needed. This is a crucial step towards the realization of frequency-tunable antiferromagnetic oscillator using -Fe₂O₃/ WTe₂.

This article has been published in Nature Materials and is available here.

Ferrimagnetic materials (FIMs) can function as high-frequency antiferromagnets while being easy to detect as ferromagnets, offering unique opportunities for ultrafast device applications. While the physical behavior of FIMs near the compensation point has been widely studied, there lacks a generic understanding of FIMs where the ratio of sublattice spins can vary continuously between the ferromagnetic and antiferromagnetic limits. PI-Cheng’s group investigated a series of physical properties of two-sublattice FIMs driven by magnetic fields and current-induced torques. By varying the ratio of sublattice spins, PI-Cheng’s group clarified how the dynamical chiral modes in a FIM are physically connected to their ferromagnetic and antiferromagnetic counterparts, based on which unique features not visible near the compensation point are demonstrated. It is found that current-induced torques can trigger spontaneous oscillation of the terahertz exchange mode (i.e. mimicking a THz antiferromagnet). Compared with its realization in antiferromagnets, a spin-torque oscillator using FIMs not only has a reduced threshold current density but also can be self-stabilized, obviating the need for dynamic feedback.

This article has been published in Physical Review B and is available here.

Harmonic analysis is a powerful tool to characterize and quantify current-induced torques acting on magnetic materials, but so far it remains an open question in studying antiferromagnets. Inspired by the collaborative work with PI-Kent and PI-Yang, PI-Cheng’s group formulated a general theory of harmonic Hall responses of collinear antiferromagnets driven by current-induced torques including both field-like and damping-like components. By scanning a magnetic field of variable strength in three orthogonal planes, one is able to distinguish the contributions from field-like torque, damping-like torque, and concomitant thermal effects by analyzing the second harmonic signals in the Hall voltage. The analytical expressions of the first and second harmonics as functions of the magnetic field direction and strength are confirmed by numerical simulations with good agreement. The theory has been utilized directly to explain experimental observations of PI-Kent’s group, and has also been generalized to NiO, providing general guidance to future experiments.

This article has been published in the Journal of Magnetism and Magnetic Materials and is available here.

Magnon spin Nernst effect (SNE) was recently proposed as a magnonic analog of the spin Hall effect, which takes place in layered collinear antiferromagnets such as MnPS₃. The magnon SNE extends many topological properties of electrons to magnonic systems where the bosonic statistics of magnons introduces unexpected physical consequences, opening a versatile platform for antiferromagnetic magnonics. However, existing studies all treated the magnon SNE as an intrinsic bulk effect, where spin diffusion and boundary spin transmission have been ignored. In real experiments, diffusion processes are essential to convert a bulk spin current into boundary spin accumulation, which then determines the spin injection into detectors through imperfect spin transmission. PI-Cheng’s group formulated a diffusive theory of the magnon SNE with boundary conditions reflecting real device geometry. It is found that in both electronic and optical detection, the actual output signals grow rapidly with an increasing system size in the transverse dimension until it eventually saturates. This provides crucial knowledge to experimentally benchmark and verify the magnon SNE in real antiferromagnetic materials. The diffusive theory also revealed that concerning symmetry properties, optical detection should be more reliable than electronic detection.

This work is published in Physical Review Applied, available here, and as an invited paper review in Applied Physics Letters, here.

Coherent spin pumping at THz frequency range was recently reported in antiferromagnets with easy-axis anisotropy such as MnF₂ and Cr₂O₃. However, easy-plane antiferromagnets such as NiO, which are much easier to make, were considered to be bad candidates because the Néel vector is linearly polarized, placing a major restriction on the choice of materials for real applications. Cheng’s group challenged this seemingly established conclusion by showing that easy-plane antiferromagnets can pump dc spin currents with the help of either a magnetic field or the Dzyaloshinskii-Moriya interaction (DMI), and the amplitude can be as strong as their easy-axis counterparts. Collaborating with the Nanoscale Quantum Sensing Lab at UCSD, it has been demonstrated both theoretically and experimentally that bulk single-crystal hematite ( -Fe₂O₃) can pump dc spin current at room temperature thanks to the DMI. Furthermore, it has been predicted that NiO (and similar oxides) can pump dc spin current via the resonance of the acoustic antiferromagnetic mode (around 0.1 THz) when a magnetic field below 5T is applied along the easy axis. As a reciprocal effect, Cheng’s group predicted that the threshold current density required to excite auto-oscillation of NiO can be suppressed by 1~2 order of magnitude by a moderate magnetic field below the spin-flop transition. The reduced threshold does not come at the price of compromised frequency, paving the way to ultrafast spin-torque oscillator with low energy consumption.

This work has been published as two papers, in Physical Review Letters, available here, and in Physical Review Applied, here.