Magnetostriction, Forced Magnetostriction, and Anomalous Thermal Expansion in Ferromagnets

摘要

The theory of magnetostriction arising from single-ion crystal field effects, for cubic crystals, is summarized, extended to arbitrary crystal symmetry, and augmented by the inclusion of two-ion interactions. The dependence on magnetic field strength (the 'forced magnetostriction'), is analyzed and the totally symmetric component (which exhibits itself as an anomalous thermal expansion is discussed. The paper presents expressions relating the macroscopic magnetostriction coefficients to the product of microscopic magnetoelastic coupling constants and certain spin correlation functions. Only three such correlation functions appear, and all temperature and field dependence enters through these correlation functions. The paper evaluates these correlation functions by various approximate theories; molecular field theory, a cluster theory, and the random phase approximation. Applications to Dy and to EuS are cited, and a detailed application to Gd is given.