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

TL;DR

The study reveals that defect-induced carrier doping, not ideal altermagnetic order, causes the giant Kerr effect in MnTe, impacting its use in spintronics and optical readout.

Contribution

It demonstrates that macroscopic magneto-optical signals in MnTe originate from defects and doping, not pure altermagnetic order, clarifying the origin of symmetry-breaking signatures.

Findings

Giant Kerr rotations up to ±1500 μrad observed at 1550 nm in MnTe crystals.

No Kerr signal detected in stoichiometric MnTe thin films.

Carrier self-doping, not ideal altermagnetism, causes the observed magneto-optical response.

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.