Temperature Dependences of Magnetization and Spin Waves in MnP

The temperature dependence of the easy axis magnetization in the ferromagnetic phase of MnP is measured to an accuracy of Δ M / 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.