Zongzhi Zhang

Noncollinear antiferromagnets (NCAFs) provide a symmetry-rich platform for ultrafast spintronics, where light-driven spin currents are governed by both crystallographic orientation and magnetic symmetry. Here, we investigate femtosecond-laser-induced terahertz (THz) emission from epitaxial Mn3Pt(001)/Pt. By combining THz emission spectroscopy with magnetic point-group analysis, we show that the radiation originates from ultrafast spin currents generated via magnetic-dipole-induced transient magnetization in Mn3Pt and subsequent spin-to-charge conversion in Pt through the inverse spin Hall effect. A pronounced symmetry-dependent response is observed, with strong emission from Mn3Pt/Pt bilayer without external magnetic fields, reflecting symmetry-constrained spin-current generation. Azimuthal- and polarization-dependent measurements confirm the nonlinear magnetic-dipole origin of the excitation. Upon cooling below 60 K, the THz field reverses polarity and develops a distinct three-fold azimuthal symmetry. While transport measurements indicate a robust Γ10 magnetic structure, the THz response reveals a reconstruction of the ultrafast spin-current generation process. These results establish THz emission spectroscopy as a sensitive probe of symmetry-dependent ultrafast spin dynamics and highlight NCAF heterostructures as promising platforms for ultrafast spintronic THz emitters.

Magnon‐mediated spin transport in antiferromagnets (AFMs) holds great promise for energy‐efficient and high‐speed spintronic applications. However, the direction‐dependent anisotropy of terahertz (THz)‐frequency coherent spin wave propagation in AFMs remains elusive due to the lack of effective experimental approaches. The recent discovery of altermagnets (AMs), which combine antiferromagnetic spin order with ferromagnet‐like strong magneto‐optical responses, has emerged as a promising platform for exploring propagating THz coherent magnons. In this study, we employ femtosecond time‐resolved magneto‐optical spectroscopy to successfully excite and detect propagating sub‐THz coherent exchange magnons in AM α ‐Fe ₂ O ₃ , demonstrating remarkable group velocities (17.31–24.43 km/s) and micrometer‐scale propagation lengths. By selecting three samples with different crystallographic cut planes to tune the relative orientation between magnon wavevector and crystal axes, we identify crystal orientation‐dependent characteristics in both group velocities and lifetimes, attributed to anisotropic exchange stiffness and magnon‐magnon scattering, respectively. Furthermore, in combination with atomistic spin dynamics simulations, we reveal that the Dzyaloshinskii‐Moriya torque could significantly enhance propagating magnon amplitudes at low potential curvature, while external magnetic fields provide effective control over their frequency and polarization. These findings not only elucidate the dynamics of ultrafast laser‐driven coherent magnon propagation in AMs but also deliver valuable guidance for advancing next‐generation magnonic devices.

The two-dimensional van der Waals material CuTe₂Cl (CTC) has been theoretically predicted to possess exceptional charge separation, strong visible-light absorption, and robust dynamic stability, making it a promising candidate for high-performance optoelectronic devices. However, its carrier relaxation pathways and the experimental validation of these predicted properties remain underexplored. Here, we investigate the photocarrier dynamics of CTC flakes using ultrafast optical pump-terahertz probe spectroscopy at 1.55 and 3.1 eV. Sub-bandgap excitation (1.55 eV) yields three distinct relaxation components (lifetimes τ₁ ≈ 2-4 ps, τ₂ ≈ 17-34 ps, τ₃ ≈ 418-667 ps), corresponding to ultrafast defect trapping, defect-mediated recombination, and three-carrier Auger recombination, respectively. In contrast, above-bandgap excitation (3.1 eV) suppresses the intermediate channel and accelerates Auger recombination due to enhanced absorption and reduced penetration depth, producing only τ₁ and τ₃. These findings provide critical insight into the carrier relaxation mechanisms of CTC, advancing its potential for next-generation optoelectronic applications.

Deterministic and field-free switching of perpendicular magnetization remains a key challenge for scalable and energy-efficient spintronic memory. Here, we employ atomistic spin dynamics simulations to uncover the microscopic mechanism behind the recently observed 3-fold rotation and mirror (3m) -symmetric switching in epitaxial (111)-oriented Tm₃Fe₅O₁₂ (TmIG)/Pt bilayers. We show that intrinsic cubic magnetocrystalline anisotropy (MCA) of TmIG spontaneously breaks mirror symmetry along the out-of-plane direction, giving rise to 3m-symmetric torques that enable deterministic switching without external fields. The switching exhibits strong angular selectivity with respect to crystallographic axes and current direction, and under large spin-orbit torque excitation, coherent terahertz oscillations produce toggle-like switching. Systematic tuning of current density and pulse width allows control over the switching-mode. Comparative simulations on Y₃Fe₅O₁₂ (YIG) confirm that MCA-driven field-free switching is a universal feature of rare-earth iron garnets. These results establish a symmetry-based design principle for ferrimagnetic spintronic devices, with implications for ultrafast, low-power memory and terahertz spintronics.

Spintronic terahertz emitters (STEs) based on ferromagnet/nonmagnet heterostructures present a compelling alternative to conventional THz sources. In this work, we systematically investigate the effects of boron (B) doping and post-deposition annealing on spin transport and THz emission in W/(CoFe)1−xBx/Al trilayers with B concentrations from 0% to 20%. Time-domain THz emission spectroscopy reveals a monotonic increase in THz field amplitude with increasing B content, despite the reduction in saturation magnetization. This enhancement is attributed to more efficient spin current injection, evidenced by increased spin mixing conductance (g↑↓) extracted from time-resolved magneto-optical Kerr effect measurements, and reduced THz reabsorption stemming from lower THz conductivity in B-rich samples. Furthermore, thermal annealing also reveals a modulation effect induced by B doping, where the THz emission varies nonmonotonically with annealing temperature and peaks near 300 °C. Below this critical temperature, B precipitation improves magnetic ordering and boosts THz emission, whereas higher temperatures degrade spin transport due to interlayer diffusion and increase THz absorption, primarily as a result of crystallization-induced conductivity enhancement. Compared to undoped CoFe, B-doped alloy films exhibit both superior emission intensity and enhanced thermal stability, demonstrating great potential for efficient and robust STE applications.