On the magnon interaction in haematite. I. Magnon energy of optical mode

Abstract

Herbert (1969) suggested that the Morin transition temperature of haematite is not given by the temperature at which the acoustic mode magnon energy vanishes but is given by the temperature at which the optic mode energy vanishes. In his expression of the optic mode energy E0, the correction term due to the magnon interaction is proportional to the inverse of the free optic mode energy Ef0, and therefore E0 is large if the Ef0 is small. Then, the free spin wave theory may not be used in the analysis of the experimental value on E0 even at low temperature. The present authors studied the effect of magnon interaction on the magnon energies of haematite by use of the random phase approximation. According to the theory, the optic mode energy becomes small if the free optic mode energy is small. Thus, the conclusion of the present theory is contrary to that of Herbert. On the other hand, if one uses the conventional random phase approximation adopted by, for example, Oguchi and Honma (1961), the optic mode energy is found to become large when Ef0 is small.