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We analyze recent electron spin resonance (ESR) experiments in La0.7Ca0.3MnO3and YBaMn2O6 focusing on the behavior of the linewidth at high temperatureswhere it is a linear function of the temperature. Noting that the g-factors ofthe resonances are characteristic of the Mn4+ ion in a cubic environment, wemake the assumption that the linewidth involves the static susceptibility ofthe Mn4+ spins which we analyze in the molecular field approximation. Weconclude that the linear dependence on temperature is associated with thesusceptibility having a Curie or Curie-Weiss form while thetemperature-dependent relaxation mechanism has a microscopic rate proportionalto the temperature. In La0.7Ca0.3MnO3, the Mn4+ susceptibility has theferromagnetic Curie-Weiss form, and the static contribution to the linewidtharising from distortions of the oxygen octahedra is absent due to motionalnarrowing brought on by the rapid hopping of the eg polarons. In YBaMn2O6either of two scenarios is possible. The Mn4+ susceptibility above 520 K isCurie-like and the static term is present, or the susceptibility has theantiferromagnetic Curie-Weiss form and the static term is absent due tomotional narrowing. It is concluded that the Curie model, with offsettingdouble exchange and and superexchange Curie-Weiss parameters, is the morelikely scenario. It is suggested that the linear-T variation of the linewidthin both materials arises from either a Korringa-like mechanism involvinginteractions with mobile carriers or from a spin-phonon process coming frominteractions between the Mn4+ ions and the lattice vibrations.

Linear temperature dependence of electron spin resonance linewidths in La0.7Ca0.3MnO3 and YBaMn2O6