Muon spin relaxation in ferromagnets. II. Critical and paramagnetic magnetization fluctuations

For pt.I see ibid., vol.4, p.2043 (1992). Mechanisms for the relaxation of muon spin relaxation signals in paramagnets are analysed in terms of spin fluctuations in a Heisenberg magnet with a ferromagnetic exchange interaction. Explicit predictions provided for EuO are based on the assumption that the implanted muon occupies an interstitial site of high symmetry. Because the average dipolar field at the muon site is zero, and the material is ferromagnetic, the dipolar mechanism of relaxation is not enhanced by critical spin fluctuations. In consequence, the hyperfine mechanism dominates relaxation in the critical region, even though the ratio of the hyperfine and dipolar coupling constants might be small. Calculations of relaxation rates for both mechanisms, made on the basis of coupled-mode theory, are provided for critical and paramagnetic states.