Thermodynamics of ferrimagnets

Alain Messager, J. C. Trotin

Louk Rademaker, Aron Beekman, Jasper van Wezel

This paper shows that, unlike antiferromagnets, a ferrimagnet does not have an Anderson tower of states in finite size systems. This classifies the ferrimagnet as a system exhibiting type `B' spontaneous symmetry breaking, resembling the ferromagnet. The authors also show that the maximally polarized ground state is thermodynamically stable

Shoji Yamamoto

We present a comparative study of bosonic languages to describe one-dimensional Heisenberg ferrimagnets. The ferrimagnetic Schwinger-boson mean-field theory demonstrated by Wu et al., the antiferromagnetic modified spin-wave theory designed by Takahashi, and its ferrimagnetic variant proposed by Yamamoto et al. are employed to calculate the energy structure and the thermodynamics of various ferrimagnets. A modified spin-wave scheme, which introduces a Lagrange multiplier keeping the native energy structure free from temperature and thus differs from the original Takahashi scheme, is particularly stressed as a useful tool to investigate one-dimensional quantum ferrimagnetism. The antiferromagnetic limit of these descriptions is also considered.

John Lam

Jahanfar Abouie, S. Alireza Ghasemi, A. Langari

We have implemented three approaches to describe the thermodynamic properties of ferrimagnetic ($S=5∕2$, $s=2$) spin chains. The application of cumulant expansion has been generalized to the ferrimagnetic chain in the presence of an external magnetic field. Using cumulants, we have obtained the field-dependent effective Hamiltonian in terms of the classical variables up to the second order of quantum corrections. Thermodynamic functions, the internal energy, the specific heat, and the magnetic susceptibility are obtained from the effective Hamiltonian. We have also examined the modified spin-wave theory to derive the same physical properties. Finally, we have studied our model using quantum Monte Carlo simulation to obtain accurate results. The comparison of the above results and also the high temperature series expansion shows that cumulant expansion gives good results for moderate and high temperature regions while the modified spin wave theory is good for low temperatures. Moreover, the convergence regions of the cumulant expansion and the modified spin-wave theory overlap each other which propose these two as a set of complement methods to get the thermodynamic properties of spin models.

Jonathan V. Selinger