A Green Function Approach to a Uniaxial Ferromagnet

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Abstract
The ferromagnetic spin wave is investigated for the system with the uniaxial anisotropy energy which is not necessarily small compared with the exchange energy. The double time green functions ≪ A : B ≫ are used with the spin and the quadrupole moment operators of single ion for A and B , and the decoupling procedure is made in the RPA both for these operators. Two types of the excitation spectrum are involved in the interested group of the Green functions. The derived expressions for thermodynamic quantities show the behavior characteristic of the magnon mode at low temperatures with some new correction terms, while they always reduce to the results of the Weiss approximation if the despersion in the two spectra is neglected. In some general cases with the anisotropy energy, numerical computations are made for T c , < J z >, < Z 2 >, χ // , and χ ⊥ , and some significant deviations from the Weiss approximation are found.