EPR linewidths inLa1−xCaxMnO3:0<~x<~1

Abstract

We have analyzed recent results for the electron paramagnetic resonance linewidths in ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}{\mathrm{MnO}}_{3}$ and related materials over the range $0l~xl~1.$ In all cases studied, the linewidth away from magnetic and structural transitions can be fitted to the expression $[{\ensuremath{\chi}}_{0}(T)/\ensuremath{\chi}(T)]\ensuremath{\Delta}{H}_{\mathrm{p}.\mathrm{p}.}(\ensuremath{\infty}),$ where the first factor is the ratio of the Curie susceptibility ${\ensuremath{\chi}}_{0}(T)$ to the measured susceptibility $\ensuremath{\chi}(T)$ and the second factor is a temperature-independent constant. Formally exact and approximate expressions for $\ensuremath{\Delta}{H}_{\mathrm{p}.\mathrm{p}.}(\ensuremath{\infty})$ are derived. In the case of ${\mathrm{LaMnO}}_{3},$ the linewidth in the distorted ${\mathrm{O}}^{\ensuremath{'}}$ phase arises from the interplay of crystal field (single ion) anisotropy and antisymmetric (Dzialozhinsky-Moriya) exchange together with isotropic superexchange. Above the Jahn-Teller transition to the pseudocubic O phase, the single-ion anisotropy is much weaker and the linewidth arises primarily from the antisymmetric interactions. Similar behavior is also found in both phases of the less-distorted, oxygenated material ${\mathrm{LaMnO}}_{3.04}.$ In the case of ${\mathrm{CaMnO}}_{3},$ the distortions of the oxygen octahedra are also small and the antisymmetric interactions are again dominant. In the intermediate region $0lxl1,$ $\ensuremath{\Delta}{H}_{\mathrm{p}.\mathrm{p}.}(\ensuremath{\infty})$ decreases with increasing Ca (i.e., ${\mathrm{Mn}}^{4+})$ concentration in contrast to the Curie-Weiss temperature, which peaks near $x=1/3,$ reflecting the importance of the double-exchange mechanism. It is proposed that the absence of a signature of the double-exchange interaction in the infinite-temperature linewidth is a consequence of the fact that the time scale associated with the changes in Mn valence arising from double exchange is too long to affect the decay of the correlation function in $\ensuremath{\Delta}{H}_{\mathrm{p}.\mathrm{p}.}(\ensuremath{\infty})$ so that the linewidth is determined primarily by the superexchange interactions between the Mn ions.