Longitudinal and transverse static spin fluctuations in layered ferromagnets and antiferromagnets

Abstract

We analyze the momentum dependence of static susceptibilities of layered local-moment systems below Curie (Neel) temperature within the 1/S expansion, the renormalization-group (RG) approach, and the first order of the 1/N expansion. We argue that already at sufficiently low temperatures the previously known results of the spin-wave theory and RG approach for the transverse spin susceptibility acquire strong corrections, which appear due to the interaction of the incoming magnon having momentum q with virtual magnons having momenta k<q. Such corrections cannot be treated in the standard RG approach but can be described by both 1/S and 1/N expansions. The results of these expansions can be successfully extrapolated to T=TM, yielding the correct weight of static spin fluctuations, determined by the O(3) symmetry. For the longitudinal susceptibility, the summation of leading terms of the 1/S expansion within the parquet approach allows us to fulfill the sum rule for the weights of transverse and longitudinal fluctuations in a broad temperature region below TM outside the critical regime. We also discuss the effect of longitudinal spin fluctuations on the (sublattice) magnetization of layered systems. © 2012 American Physical Society.