Induced magnetization of an easy-plane spin-one ferromagnet

The characteristic angle (CA) method, which was introduced in a previous work, is applied to calculate the induced magnetization of an easy-plane spin-one ferromagnet. The harmonic approximation which is often applied in a spin-Bose expansion is found to be invalid and may even lead to wrong physical results when calculating the induced magnetization of the easy-plane spin-one ferromagnet. Then the complete Bose transformation (CBT) is used to transform the spin operator into a Bose operator, so that high-order terms in the Bose expansion can be taken into account based on an independent Bose representation. Comparison of the result calculated by the CBT and by the harmonic approximation shows that high-order terms in the CBT are helpful in determining a reasonable ground state and getting accurate physical results. Diagrams of the induced magnetization versus the external magnetic field for a simple-cubic lattice are presented for small and large anisotropy, respectively.