Giant spontaneous Kerr effect reveals the defect origin of macroscopic time-reversal symmetry breaking in altermagnetic MnTe

Abstract

Altermagnetism, a recently identified third class of collinear magnetism with spin-split bands and vanishing net magnetization, has emerged in hexagonal \alphaMnTe{} and is regarded as a promising platform for ultrafast, stray-field-free spintronics and for optical readout of spin order at telecommunication wavelengths. Whether the macroscopic symmetry-breaking signatures reported in MnTe, a spontaneous Hall effect and a tiny ``gossamer'' remanent moment, reflect the ideal altermagnetic order or are activated by defects remains an open question. Here we report giant spontaneous Kerr rotations of up to $\pm 1500\microrad$ in \alphaMnTe{} single crystals at the telecommunication wavelength of $1550\,\mathrm{nm}$, onsetting precisely at the N\'eel temperature $\TN = 307\,\mathrm{K}$. In contrast, a stoichiometric insulating \alphaMnTe{} thin film shows no detectable signal. The bulk--film contrast identifies carrier self-doping, rather than the ideal altermagnetic order, as the source of macroscopic magneto-optical response, establishing telecom-wavelength Kerr imaging as a practical readout for altermagnetic spintronics.