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 \(\Delta\omega_{\text{m}}(0,0){\doteqdot}(2/5)KC\ M(0,0)^{2}\) 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.