Theory of the Invar Effect in FeNi Alloy. II. Forced Volume Magnetostriction and Bulk Modulus

Abstract

The forced volume magnetostriction ∂ω⁄∂h and the electronic contribution to the bulk modulus B_e in FeNi alloy are calculated with use of the Libemian-Pettifor virial theorem and the full single site approximation in the 2-field functional integral method in order to clarify the origin of the Invar effect. It is shown that the large ∂ω⁄∂h in the Invar region is caused by the large temperature variation of the derivative of ⟨m_Fe²⟩ (the amplitude of the Fe local moment) with respect to the field h, the softening of the bulk modulus by the increase of the volume change of ⟨m_Fe²⟩. It is found that the minimum in B_e occurs below T_c because of the suppression of the temperature variation of ⟨m_Fe²⟩. The high-field susceptibility, the paramagnetic spin susceptibility and ∂T_c⁄∂P are also calculated.