Theory of Ferro- and Antiferromagnetism by the Method of Green Functions

Theoretical studies on the magnetization of ferro‐ and antiferromagnets have been investigated by means of the equivalent operators, devised by one of the authors, introduced into the two‐time and temperature‐dependent Green function of Bogolyubov and Tjablikov. Our theory is applicable over the whole range of temperatures for arbitrary magnitudes of spin. At low temperatures, the dependence of magnetization on temperature is in good agreement with that of spin‐wave theory except for an extra term which probably results from the approximation of decoupling the chain of equations for the Green functions. But this term is negligible when the magnitude of spin is more than one for the ferromagnetic case and$\frac{3}{2}$ for the antiferromagnetic case. It should be emphasized that our theory possesses spin‐wave character as the collective modes in the low temperature region, and gradually changes to the individual spin‐mode theory similar to the molecular field theory with increasing temperature.