Giant THz Magnetoresistance and Refractive Index Shift in Spin-Textured EuTe₂ via Field-Controlled Bandgap

The realization of terahertz (THz)-frequency magneto effects presents a fundamental challenge and opportunity for ultrafast spintronics. Herein, we demonstrate record-breaking THz giant magnetoresistance (GMR) and anisotropic magnetoresistance (AMR) in EuTe₂, a model noncollinear antiferromagnet. Under a 2.5 T magnetic field, the THz GMR exceeds 90%, while the THz AMR reaches an unprecedented 450% at 0.48 THz (2 T) and 5 K, surpassing known MR effects in this frequency regime. Further, THz absorption spectroscopy provides the first direct optical evidence of spin-texture bands via a field-induced bandgap reconstruction from 4.9 to 2.2 meV, linking the THz GMR/AMR to spin-orientation control rather than conventional spin arrangement manipulation. Moreover, a pronounced refractive index shift (Δn ∼ 0.8) results from magnetic field-induced bandgap closure and permeability tuning, a mechanism unlike thermal effects. Our findings promote spin-texture materials as a promising platform for high-frequency spintronics and magneto-optics, while also establishing THz spectroscopy as a transformative tool for probing quantum dynamics.