Softening of the Spin Waves in MnP and Its Relation to the Lifshitz Point

Abstract

Inelastic neutron scattering measurements on MnP reveal that the spin wave gap vanishes at the paramagnetic-ferromagnetic phase boundary in a manner predicted by the transverse field Ising model. The occurrence of the tricritical point in a temperature-field phase diagram is strongly related to the temperature-driven change of the phenomenological parameter P≡J₂^z⁄J₁^z which describes the competition between the effective nearest and next-nearest-neighbor exchange interactions. The itinerant nature of the magnetism of this compound is emphasized as a possible origin of the temperature dependence of P. The tricritical point is most likely a Lifshitz Point as pointed out in detailed measurements of the phase diagram by Shapira et al.