Strong Magneto-Elastic Coupling in Mn₃X (X = Ge, Sn) revealed by Ultrasound

The chiral antiferromagnets Mn₃X (X = Sn, Ge) have recently attracted a large amount of attention due to their large anomalous Hall response at room temperature, linked to Weyl nodes close to the Fermi surface. Besides manipulation of the Hall angle with small magnetic and electric fields, the sign of the Hall voltage has been demonstrated to switch under hydrostatic and uniaxial pressure. Here, we perform Resonant Ultrasound Spectroscopy (RUS) and Pulse Echo Ultrasound measurements to further investigate strong magnetoelastic coupling in Mn₃X. We observe giant anomalies in the compressional elastic constants, related to a large change of Neel temperature with hydrostatic pressure. Additionally, we extract a sizeable piezomagnetic coefficient in Mn₃Ge, made possible through the unique combination of irreducible representations of in-plane shear strain and magnetic order parameter in Mn₃X. By examining the critical divergence of the bulk modulus close to the phase transition, we were able to place Mn₃Ge in the 3D-XY universality class. Further, access to the shear modulus c₆₆ offers the rare opportunity to extract the critical exponent for the nematic susceptibility.