Quasicritical behaviour of the nuclear magnetic resonance and muon spin-rotation relaxation in cubic ferromagnets and antiferromagnets

Abstract A theoretical model describing quasicritical behaviour of the nuclear magnetic resonance (NMR) relaxation rates 1/T 1 , 1/T 2 and muon relaxation rate λ for cubic ferromagnets and antiferromagnets with spin fluctuations is presented. The model extends and generalizes the model of Moriya and explains why the observed experimental values of the critical exponents are different from the values predicted by the theory of critical phenomena. The formulae for the relaxation rates obtained from this model are used for fitting to the experimental NMR data for YMn 2 D x , TbMn 2 D 2 and the muon spin-rotation (µSR) data for YMn 2 , YMn 2 D x , GdMn 2 and Y(Co 1−x Al x ) 2 (ferromagnetic). It is shown that the dependence of the exchange coupling constant on the wavevector k is important for understanding the observed quasicritical effects