Muon spin relaxation in ferromagnets: theoretical results for paramagnetic EuO and EuS

The relaxation rate for depolarization of a positive muon implanted in an isotropic magnetic salt with ferromagnetic exchange interactions is studied theoretically, on the basis of the coupled-mode theory of critical and paramagnetic spin fluctuations and a full numerical evaluation of the dipole field experienced by the muon. The main findings from studies of realistic models of EuO and EuS are (a) a significant dependence of the relaxation rate, lambda , on the assumed position of the implanted muon and (b) a monotonic temperature dependence, with lambda approximately xi 3/2 in the approach to the critical temperature at which the correlation length, xi diverges. In contrast, previous results for a model of an isotropic magnet with an antiferromagnetic exchange, RbMnF3, show that lambda for this magnet is not a monotonic function of the temperature, and in the precursor region to Tc, lambda increases with decreasing temperature with a power law behaviour lambda approximately xi 1/2. The calculated values of lambda for EuO are consistent with data from preliminary experiments on the same salt.