Magnetic contributions to the thermodynamic functions of pure Ni, Co, and Fe

Abstract

An empirical mathematical equation is proposed for the magnetic contribution to the specific heat of pure metals. The corresponding functions for enthalpy, entropy, and Gibbs energy are of simple form. Two parameters used for each element are the critical temperature,Tc, and the total magnetic entropy. The parameters have been determined from a careful separation of magnetic and nonmagnetic contributions to the specific heat. Debye temperatures for Ni, Co, and Fe have been determined considering data to much higher temperatures than other studies. The magnetic specific heats extracted from experimental data agree very well with the proposed equation over the entire temperature range and for all three elements. Comparisons with different mathematical functions found in the literature give agreement only for the case of iron. The total magnetic entropy given by a classical relation is found to be high, and a quantitative correction is given. Various magnetic standard states are discussed. The lattice stabilities of bcc- and fcc-iron are calculated assuming that the difference of the nonmagnetic specific heats is linear from 500 K to 1810 K. A simple equation is obtained in which the anomalous temperature dependence is explained by the independently determined magnetic contribution. The calculated values agree very well with Orr and Chipman’s assessment. The stability of bcc iron at low temperatures is quantitatively rationalized.