Magnon gap excitations in van der Waals antiferromagnet MnPSe.

Magneto-spectroscopy methods have been employed to study the zero-wavevector magnon excitations in MnPSe. Experiments carried out as a function of temperature and the applied magnetic field show that two low-energy magnon branches of MnPSe in its antiferromagnetic phase are gapped. The observation of two low-energy magnon gaps (at 1.70 ± 0.05 meV and 0.09 ± 0.01 meV) implies that MnPSe is a biaxial antiferromagnet. A relatively strong out-of-plane anisotropy imposes the spin alignment to be in-plane whereas the spin directionality within the plane is governed by a factor of 2.5 × 10 weaker in-plane anisotropy.