Theory of spin-resonance lineshapes in euxsr1-xs

A theory of spin-resonance lineshapes in random magnetic compounds is developed and applied to the insulating Heisenberg system EuxSr1-xS. The magnetic dipolar interaction is treated as a perturbation on the unperturbed Hamiltonian consisting of exchange and Zeeman interactions. The formulae for the first and second moments of the lineshape (related respectively to lineshifts and broadenings) are derived to second order in perturbation. The general formulae are particularly appropriate for low-temperature approximations where the most interesting experimental features occur in the spin-glass and the re-entrant regimes. Although a quantitative comparison between theory and experiment is not possible as the problem of random Heisenberg systems is unsolved, the theory qualitatively accounts for the experimental trends in the paramagnetic region. © 1987 IOP Publishing Ltd.