Exchange narrowing of the phonon contribution to the electron spin resonance line width in exchange-coupled magnetic insulators

In this paper we extend earlier calculations of the phonon contribution to the electron spin resonance line width at high temperatures in exchange-coupled magnetic insulators. We show that the one-phonon contribution is exchange-narrowed, similar to the static anisotropy contribution. The effect of the exchange narrowing is to limit contributing phonons to those modes whose energies are less than a cutoff, γmax, that is proportional to the exchange interaction. Linear-T behavior in the line width occurs when kBT is greater than γmax. (paper)