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

Weitung Yang, Choongjae Won, Cory Cress, Marshall Zachary Franklin, Xiaochen Fang, Shelby Fields, Nicholas Combs, Shaofeng Han, Weihang Lu, I. I. Mazin, Steven P. Bennett, Sang-Wook Cheong, Jing Xia

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 Néel 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.