Magneto-Volume Effect in the Weak Itinerant Ferromagnet MnSi

The magneto-volume effect of the weak itinerant ferromagnet MnSi has been studied by measuring both the thermal expansion over a wide range of temperature and the forced volume megnetostriction at different temperatures. The forced magnetostriction at 4. 2 K as well as the magnetic thermal expansion at 10 kOe, Δω m (H, T) can well be described by a formula Δω m (H, T) = K C Δ M ST (H, T) 2 with K C =1. 5×10 -10 (mol/emu) 2 where Δ M ST (H, T) 2 is the change of square of magnetization by polarization of the d band. The spontaneous magnetostriction at 0 K, Δω m (0, 0) is related to the spontaneous magnetization M (0, 0) by \ (₌ (0, 0) (2/5) KC\ M (0, 0) ^2\) with the same K C. Furthermore magneto-v olume effect becomes positive above T c. All of these features agree satisfactorily with those predicted by Moriya and Usami based on spin fluctuation theory.