Symmetry-DependentUltrafast Spin Currents and THzEmissions in Noncollinear Antiferromagnetic Mn3Pt/Pt Heterostructures

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.