Spin-Wave Correlation Effects in CrBr3

Abstract

A first‐order (self‐consistent) renormalized spin‐wave theory has been used to fit the temperature dependence of the magnetization M (T, 0) in the hexagonal ferromagnet CrBr₃. However, this theory appears incapable of simultaneously explaining M (T, O) and new measurements we have made of M (T₀, H). Reexamining the first‐order theory we find that (a) self‐consistency corrections are negligible because of the low magnon density and (b) an equally satisfactory fit to M (T, O) can be obtained from a rectangular model with appropriately chosen intralayer J_T and interlayer J_L exchange constants and reciprocal‐lattice volume. This is possible because J_T is so large that there is little thermal excitation beyond the k_x²+k_y² part of the spectrum. Since large k_z magnons are present due to the smallness of J_L, this is a strongly interacting system for which correlation effects are known to be important. Using the rectangular model we have treated spin‐wave interactions to all orders via the t‐matrix and find deviations of up to 40% from the first‐order renormalization. Since the first‐order renormalization effects on M (T, O) were shown to be large, we may infer from our calculations that the first‐order fitting of one thermodynamic function was fortuitous and was made possible by the multiparameter character of the theory.