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

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 Fe 2 > (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 Fe 2 >. It is found that the minimum in B e occurs below T c because of the suppression of the temperature variation of < m Fe 2 >. The high-field susceptibility, the paramagnetic spin susceptibility and ∂ T c /∂ P are also calculated.