Giant spontaneous Kerr effect and its self-doping dependence in altermagnetic MnTe

Abstract

Altermagnetism, a third class of collinear magnetism with spin-split bands and vanishing net magnetization, has emerged in hexagonal {\alpha}-MnTe, a promising platform for ultrafast, stray-field-free spintronics. Whether MnTe's macroscopic symmetry-breaking signatures reflect ideal altermagnetic order or are activated by defects remains open. Here we report giant spontaneous Kerr rotations of up to 1500 rad in {\alpha}-MnTe single crystals at the telecommunication wavelength of 1550 nm, onsetting precisely at the N\'eel temperature TN = 307 K. The signal appears in disjoint macroscopic patches whose amplitude tracks the sample conductivity, and a hole-conducting {\alpha}-MnTe film reproduces this response together with an anomalous Hall effect. Within a single crystal, Kerr-active regions are optically distinct from their Kerr-silent surroundings in co-registered reflection maps. These observations indicate that carrier self-doping, rather than ideal altermagnetic order alone, governs the giant magneto-optical response, and establish telecom-wavelength Kerr imaging as a practical readout for altermagnetic spintronics.