Towards the theory of ferrimagnetism: II

Abstract

The present paper is a sequel to the paper by Karchev (2008 J. Phys.:Condens. Matter 20 325219). A two-sublattice ferrimagnet, with spin-s1 operatorsS1i at the sublatticeA site and spin-s2 operators S2i at the sublattice B site, is considered. Renormalized spin-wave theory, which accounts for themagnon–magnon interaction, and its extension are developed to describe the two ferrimagnetic phases(0,T*) and(T*,TN) in the system, and to calculate the magnetization as a function of temperature.The influence of the parameters in the theory on the characteristic temperaturesTN andT* is studied. It is shown that, increasing the inter-sublattice exchange interaction, the ratioTN/T*>1 decreases approaching one, and above some critical value of the exchange constant there is only onephase TN = T*, and the magnetization–temperature curve has the typical Curie–Weissprofile. When the intra-exchange constant of the sublattice with strongerintra-exchange interaction increases the Néel temperature increases whileT* remainsunchanged. Finally, when the magnetic order of the sublattice with smaller magnetic order decreases,T* decreases. The theoretical predictions are utilized to interpret the experimentally measuredmagnetization–temperature curves.