Magnetic Field Effects on the Specific Heat of Impure Ferromagnets

Abstract

Although the change produced in the magnon density of states by dilute impurities in a ferromagnet is proportional to their concentration (at least, away from critical points), the change and its effect even on bulk thermodynamic properties may be fractionally large if it occurs at low energies, where the pure host density of states is small (and the Boltzmann factor suppresses contributions from higher energy states). We have previously considered the specific-heat anomalies from spin impurities characterized by incremental peaking of the density of states near the bottom of the host spin-wave band. The anomalies will be more dramatic if the impurities result in (localized) spin-wave modes below the host spin-wave band, which is possible in the presence of an external magnetic field, if the impurity g-factor (and, therefore, its Zeeman level splitting) is sufficiently less than that of the host. Furthermore, the magnetic field dependence serves to distinguish magnetic anomalies from any others that might arise from impurities. We have carried out a Green's function analysis of the field dependence of the specific heat of an impure Heisenberg ferromagnet, and we find sizable anomalies for fields of the order of 50 kG in a ferromagnet with a Curie temperature of the order of 10°K.