Researchers at Rice University, with contributing authors from the University of Washington, the University of Houston, Oak Ridge National Laboratory, the National High Magnetic Field Laboratory at Florida State University, and the University of Illinois Urbana-Champaign, have used mechanical strain to control and reverse the anomalous Hall effect (AHE) in hexagonal manganese telluride (α-MnTe), a recently identified altermagnet.
Altermagnets are a newly recognized class of magnetic order in which moments are arranged so that time-reversal symmetry is broken - enabling ferromagnet-like effects such as spin splitting and the anomalous Hall effect - while net magnetization remains vanishingly small. That near-absence of stray magnetic fields makes altermagnets attractive for spintronic devices, but it also makes them difficult to probe: α-MnTe naturally forms three magnetic domains oriented 120 degrees apart, and their signals average out in standard neutron diffraction measurements, obscuring the true direction of the in-plane magnetic moments.
The team resolved this by mounting α-MnTe crystals in a piezoelectric strain cell and applying compressive uniaxial strain along two different crystal bond directions, using neutron scattering to directly track how the strain redistributed the material's magnetic domains. The strain conclusively "detwinned" the crystal into a single dominant magnetic domain, allowing the researchers to establish for the first time that the in-plane moments align along the next-nearest-neighbor manganese-manganese bond direction, regardless of which direction the strain was applied.
In this detwinned, single-domain state, the AHE hysteresis loop sharpened dramatically and persisted to much lower temperatures than in unstrained crystals. Most notably, tuning the strain level reversed the sign of the AHE near room temperature without changing the material's roughly 307 K magnetic transition temperature - ruling out a simple restructuring of the magnetic order as the cause. The team calculates that a 1% change in strain produces an effect on the AHE comparable to a roughly 150 K change in temperature, a far more practical lever for real devices. Combining the experimental results with a phenomenological theoretical model, the researchers attribute the effect to strain-induced changes in the electronic Berry curvature - a quantum-geometric property of the electronic band structure - arising from the combination of strain and spin-orbit coupling.
Because the effect operates near room temperature and offers a mechanical, rather than thermal or magnetic-field, means of switching between Hall-effect states, the authors position α-MnTe as a candidate material for strain-tunable magnetic sensors and spintronic components with minimal stray fields, suited for integration with superconductors and topological insulators.