We report the picosecond spin current generation from the interface between a heavy metal and a vicinal antiferromagnet insulator Cr2O3 by laser pulses at room temperature and zero magnetic field. It is converted into a detectable terahertz emission in the heavy metal via the inverse spin Hall effect. The vicinal interfaces are apparently the source of the picosecond spin current, as evidenced by the proportional terahertz signals to the vicinal angle. We attribute the origin of the spin current to the transient magnetic moment generated by an interfacial nonlinear magnetic-dipole difference-frequency generation. We propose a model based on the in-plane inversion symmetry breaking to quantitatively explain the terahertz intensity with respect to the angles of the laser polarization and the film azimuth. Our work opens new opportunities in antiferromagnetic and ultrafast spintronics by considering symmetry breaking.

As a new magnetoelectric material, honeycomb-based antiferromagnet Fe4Nb2O9 has attracted a great deal of attention due to its prominent magnetoelectric (ME) coupling and high Neel temperature, while the physics of magnetoelectricity is far from understood. In the present study, we present our systematic investigations of the anisotropic ME effect, electric polarization reversal, and nonlinear ME effect of Fe4Nb2O9 single crystals, thus highlighting the phase diagram extended down to 10 K. Our results provide clear evidence for electric polarization reversal driven by magnetic field (H) along the [110] and [1-10] directions, respectively, while no such polarization reversal occurs as His applied along the [001] direction. The nonlinear ME effects and electric control of magnetism are unambiguously demonstrated. In addition, the angular-dependent probing reveals a 2 theta rotation of the induced electric polarization around the c axis upon the rotation of magnetic field by an angle theta. The electric polarization responses and concomitant ME coupling are well explained by means of the metal-ligand hybridization p -d mechanism. This work represents an essential step forward in the understanding of ME coupling not only in this A4M2O9 honeycomb magnet.

The interfacial Dzyaloshinskii-Moriya interaction (iDMI) was observed in the bilayer consisting of a heavy metal and a ferromagnetic insulator such as Pt vertical bar Tm3Fe5O12 (TIG), but the source and origin are still controversial. Here, we quantitatively investigate the iDMI strength in Pt vertical bar TIG by inserting a thin Y3Fe5O12 layer and/or a thin Cu layer between Pt and TIG. Our results suggest the iDMI contributed by both the Pt vertical bar TIG and the TIG vertical bar substrate interfaces. At the Pt vertical bar TIG interface, we find that Pt is essential for the strong iDMI. The disentangled iDMIs for Fe-Fe and Tm-Fe pairs are comparable, revealing that the Tm ions provide not only an additional Tm-Fe iDMI, but also the spin-orbit coupling to enhance the iDMI. At the TIG vertical bar substrate interface, the iDMI is attributed to the lattice-mismatch-induced deformation of the electron cloud for the ferromagnetic ions.

We report the observation of the unidirectional spin Hall magnetoresistance (USMR), which depends on the current or magnetization direction, in heavy-metal-ferromagnetic-insulator bilayer, Pt-Y_{3}Fe_{5}O_{12} (YIG). This USMR is apparently not caused by the mechanisms established in metallic bilayer, in which the ferromagnetic layer is required to be electrically conductive. From the magnetic field, current, temperature, and YIG thickness dependent measurements, the USMR is attributed to the asymmetric magnon creation and annihilation induced by the spin-orbit torque. This asymmetry and the resultant USMR are further revealed by the micromagnetic simulations combined with the spin-orbit torque and the spin drift-diffusion model. Our finding exhibits a nonlinear manipulation of magnons with the charge current.

We report a systematic study of the temperature and field dependences of the spin Seebeck effect (SSE) in a bilayer of $\mathrm{Pt}/{\mathrm{Gd}}_{3}{\mathrm{Fe}}_{5}{\mathrm{O}}_{12}$. An anomalous structure is observed in the magnetic field dependent measurements at temperatures between \ensuremath{\sim}60 and \ensuremath{\sim}210 K. Unlike the ordinary SSE signal originating from the bare magnons, which changes sign at \ensuremath{\sim}95 and \ensuremath{\sim}266 K, the sign of the anomalies remains unchanged with increasing temperature. Moreover, the anomalies are found to show a temperature-sensitive double-peak structure between \ensuremath{\sim}116 and \ensuremath{\sim}143 K. We attribute these anomalies to the contribution from the quasiparticles hybridized between the Gd moment dominated spin wave (\ensuremath{\alpha} mode) and the transversal acoustic phonon, known as the magnon polarons. Given that the magnon polaron induced anomalies occur at the field where the linear phonon dispersion is tangential to the magnon dispersion curve, we explain these rich phenomena by an increase of the group velocity of the \ensuremath{\alpha}-mode magnon with increasing temperature and the nonparabolic magnon dispersion of ${\mathrm{Gd}}_{3}{\mathrm{Fe}}_{5}{\mathrm{O}}_{12}$. Our results demonstrate that the magnon polaron induced SSE is helpful for the investigation of the magnon dispersion evolution with a simple transport approach.