Magnetic Systems: Specific Heat

Abstract

Since all allowed states of a system are equally likely, each component of a physical system contributes with its own Entropy to the total value (i.e., S(T)¼ΣiSi(T)). Consequently, Cp/T¼dS/dT contains information about the total density of excitations. To extract the contribution of each component requires to identify the range of temperature at which their density of excitations become dominant according to the respective thermal dependencies. Some typical contributions to the total specific heat Cp(T) are: (1) the nuclear magnetism of some isotopes in the mK range (i.e., To0.5 K) where the tail of the nuclear contribution Cnuc is clearly observed; (2) the conduction electrons contribution Cel is mainly detected within the 0.5oTo4 K range; (3) in presence of HF quasiparticles Cel dominates the signal up to about 7 K; (4) phonons Cph overcome those contributions above 10 K depending on the respective the Debye temperatures yD; (5) magnon contribution Cmag related to LRMO phases and specific heat anomalies originated in short range correlations can be identified up to about 20 K; (6) Schottky anomalies Csch correspond to a class of anomalies arising from the thermal population of quantum levels split by a gap of energy.These contributions are governed by different statistics depending on the nature of the involved particles. For example,Maxwell Boltzman statistic applies to Schottky-type anomalies arising from nuclear or crystalline electric field (CEF) splitting.Fermi?Dirac statistic applies to conduction electrons behaving as a Fermi Gas or to heavy quasiparticles behaving as a Fermi liquid with enhanced effective mass. Bose?Einstein statistic applies to phonons and magnons quasiparticles. Since many of these contributions are extensively treated in the literature (see, e.g., Gopal, 1966; Tari, 2003), we will mainly pay attention to recent investigations on new magnetic materials at low temperature.