Towards the theory of ferrimagnetism

Abstract

Two-sublattice ferrimagnet, with spin-$s_1$ operators ${\mathbf{S}}_{1i}$ at the sublattice $A$ site and spin-$s_2$ operators ${\mathbf{S}}_{2i}$ at the sublattice $B$ site, is considered. The magnon of the system, the transversal fluctuation of the total magnetization, is a complicate mixture of the transversal fluctuations of the sublattice $A$ and $B$ spins. As a result, the magnons' fluctuations suppress in a different way the magnetic orders of the $A$ and $B$ sublattices and one obtains two phases. At low temperature $(0,T^{\ast })$ the magnetic orders of the $A$ and $B$ spins contribute to the magnetization of the system, while at the high temperature $(T^{\ast },T_N)$, the magnetic order of the spins with a weaker intra-sublattice exchange is suppressed by magnon fluctuations, and only the spins with stronger intra-sublattice exchange has non-zero spontaneous magnetization. The $T^{\ast }$ transition is a transition between two spin-ordered phases in contrast to the transition from spin-ordered state to disordered state ($T_N$-transition). There is no additional symmetry breaking, and the Goldstone boson has a ferromagnetic dispersion in both phases. A modified spin-wave theory is developed to describe the two phases. All known Neel's anomalous $M(T)$ curves are reproduced, in particular that with "compensation point". The theoretical curves are compared with experimental ones for sulpho-spinel $MnCr2S_{4-x}S{e}_x$ and rare earth iron garnets.

9 pages, 8 figures

References in corpus (1)

• Two Ferromagnetic Phases in Spin-Fermion Systems

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