Temperature Dependences of Magnetization and Spin Waves in MnP

Abstract

The temperature dependence of the easy axis magnetization in the ferromagnetic phase of MnP is measured to an accuracy of ΔM⁄M<±5×10⁻⁴ over a range from 4.2 K to 280 K. Instead of the usual T^3⁄2 law, T ln T dependence holds up to about T_c⁄2. The following spin wave dispersion is expected to exsist at 0 K; along the a axis it is nearly constant up to q_a=Q and increases very rapidly beyond Q, where Q is the screw propagation vector, while for perpendicular to the a axis it is quadratic in q_⊥. At a finite temperature, the calculated two-magnon renormalization causes increase of the magnon energy near q_a∼Q which induces an anomalous suppression for decrease of magnetization in MnP. The ferro-screw phase transition seems also to be due to this effect.