Study On Magnetic Bilayers By Using Green's Function Method

Magnetic bilayer with its distinctive properties has become one of the most exciting fields of Condensed Matter Physics, and it has been widely used. Magnetic bilayer consists of two magnetically ordered planes which are magnetically coupled with each other, while the interlayer interaction may be of various characters. This kind of material is an element of many spintronic devices, as well as a single element or the motive of the magnetic superlattice. In this paper, we use double-time Green's function method to study the magnetic properties of ferromagnetic/ferromagnetic coupled magnetic bilayers and ferromagnetic/antiferromagnetic coupled magnetic bilayers.1. The magnetic properties of the ferromagnetic/ferromagnetic coupled magnetic bilayerIn this paper, we use the exactly decoupling method which is proposed by Devlin to decouple the terms including single-ion anisotropy, and random phase approximation(RPA) method to decouple the terms concerning exchange interaction.. After formulating the expressions to calculate the magnetization of the system, we study the temperature-dependent magnetizations under different intralayer exchange interaction, the interlayer exchange interaction, single-ion anisotropy and the external magnetic field. The results have shown that interlayer antiferromagnetic exchange interaction can cause stronger quantum fluctuations than interlayer ferromagnetic exchange interaction, and that single-ion anisotropy in a monolayer can fortify the ferromagnetic phase of the system through interlayer exchange interaction. In contrast to the Anderson-Callen decoupling method, we find that Devlin's decoupling method can describe the magnetization behavior more exactly. In addition, the transition temperature of the two planes is unique, which is greatly different from the results in Spirin's work. 2. The magnetic properties of the ferromagnetic/antiferromagnetic coupled magnetic bilayerIn this paper, we use the decoupling method proposed by Anderson and Callen to decouple the terms including single-ion anisotropy, and random phase approximation method to decouple the terms concerning exchange interactions. After formulating the expressions to calculate the magnetization of the system, we study the temperature-dependent magnetization under different interlayer exchange interaction, single-ion anisotropy in the antiferromagnetic monolayer and external magnetic field. The results have shown that a larger interlayer antiferromagntic exchange interaction can cause stronger quantum fluctuations. In addition, when the interlayer exchange interaction is strong enough, there is a slight increase of the magnetization in the lower temperature area before the magnetization decreases with the temperature increasing. We also find that the antiferromagnetic exchange interaction and the external field can fortify the spin configuration of the system. However, a weak external field nearly cause little influence on the antiferromagnetic monolayer.