Influence of Ferromagnetic Elastic Modulus Relaxation on the Determination of Magnetic Specific Heat of Fe, Ni, and Co

Abstract

Data are presented for Young's modulus of (110) [001] oriented Fe-3.1% Si up to 1200°C for various angles to the rolling direction. These data exhibit a strong decrease in Young's modulus with increasing temperature during the ferromagnetic transformation so that the Young's modulus of the paramagnetic state is lower than for the ferromagnetic state. This effect also exists for Ni and Co, and is attributed to a decrease in interatomic forces accompanying the decrease of the exchange interaction between 3d electrons. This ferromagnetic modulus relaxation leads to a new specific heat term CΔθ which represents the energy absorbed in decreasing the mean frequency of the vibrational spectrum. The value of CΔθ, calculated from Young's modulus data, remains appreciable above the Curie temperature, thus accounting for the apparent excess specific heat of the ferromagnetic metals above that temperature. Separation of the total electronic specific heat suggests that three-fourths of the low-temperature electronic specific heat of Fe, Ni, and Co is of magnetic origin and that the magnetic specific heat Cm increases linearly with temperature up to 0.7 Tc and then increases rapidly to a peak at Tc, the Curie temperature. Well above Tc, a residual electronic term of nonmagnetic origin is found. The calculated values of magnetic entropy and energy derived from Cm for Fe, Ni, and Co are in reasonable agreement with Heisenberg theory using s=½.