Towards the theory of ferrimagnetism

Abstract

Two-sublattice ferrimagnet, with spin-s1 operators S 1i at the sublattice A site and spin-s2 operators 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 * ) the magnetic orders of the A and B spins contribute to the magnetization of the system, while at the high temperature (T * , TN ), 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 * transition is a transition between two spin-ordered phases in contrast to the transition from spin-ordered state to disordered state (TN -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 M nCr2S4−xSex and rare earth iron garnets.

The Magnetic Reorientation in Ferrimagnetic Systems

“…The magnons in our considered system are a complicated mixture of the transversal uctuations of the sublattice A and B spins. As a result, the magnon uctuations supress in dierent ways the magnetic orders of the A and B sublattices and one obtains two magnetic orders [6]. Each of these orders has a dierent reorientation temperature.…”

“…For h x /J > h x C /J the direction of the zcomponent of magnetization of the whole system is in the plane of the monolayer and the magnetic reorientation does not appear. On the other hand, the magnetic reorientation can be observed for h x /J < h In [6], the two-sublattice ferrimagnet with dierent spins and dierent intra-sublattice ferromagnetic exchange interaction was considered within the modied spin-wave theory and two magnetic orders were obtained. Unlike the model in [6], we have included in the Hamiltonian the single-ion anisotropy and the transverse magnetic eld to consider magnetic reorientation within the Green function theory.…”

Magnetic reorientation in a ferrimagnetic monolayer

“…We briefly quote the result of the modified spin wave theory [5], whereby, we want to claim that in ferrimagnetic systems may exist two phases: ''The magnon excitation is a complicated mixture of the transversal fluctuations of the sub-lattice A and B spins. As a result the magnons' fluctuations suppress, in different ways, the magnetic orders on the different sub-lattices and one obtains two phases in the sub-lattices A and B.…”

“…Karchev [5] considered within the modified spin-wave theory a two-sub-lattice ferrimagnet with different spins and different intra-sub-lattice exchange interaction. The inter- sub-lattice exchange interaction is antiferromagnetic.…”

Magnon-induced superconductivity in field-cooled spin-1/2 antiferromagnets

“…The first part of the curves is not affected by the magnetic field. At T * the system undergoes a partial order transition [31,[45][46][47]. Above T * only sublattice B electrons contribute to the magnetization, while below T * sublattice A and sublattice B electrons contribute to the magnetization of the system.…”