High-temperature spin dynamics in CMR manganites: ESR and magnetization

We have performed electron spin resonance (ESR) and dc susceptibility measurements in Mn perovskites up to 1000 K. Assuming an effective Heisenberg-like interaction for ${\mathrm{Mn}}^{3+}\mathrm{-}{\mathrm{Mn}}^{4+}$ spin pairs, the dc susceptibility, ${\chi }_{\mathrm{dc}}\mathrm{}\left(T\right),$ is well described in the paramagnetic regime by the constant coupling approximation. Absolute determination of the ESR intensity indicates that all Mn spins contribute to the ESR line in the temperature range studied. The ESR linewidth can be described by $\Delta H_{\mathrm{pp}}\mathrm{}\left(T\right)=\Delta H_{\mathrm{pp}}\left(\infty \right)[C/T{\chi }_{\mathrm{dc}}\mathrm{}(T\left)\mathrm{}\right],$ thus presenting a universal behavior in a temperature scale normalized to $T_c.$ A single relaxation mechanism, related to spin-only interactions, explains the $T$ dependence of $\Delta H_{\mathrm{pp}}\mathrm{}\left(T\right)\mathrm{}$ for all the compounds studied: ${\mathrm{La}}_{0.67}{\mathrm{Ca}}_{0.33}{\mathrm{MnO}}_3,$ ${\mathrm{La}}_{0.67}{\mathrm{Sr}}_{0.33}{\mathrm{MnO}}_3,$ ${\mathrm{Pr}}_{0.67}{\mathrm{Sr}}_{0.33}{\mathrm{MnO}}_3,$ and ${\mathrm{La}}_{0.67}{\mathrm{Pb}}_{0.33}{\mathrm{MnO}}_3.$ The dc susceptibility and the ESR linewidth and intensity all reflect the progressive importance of magnetic clustering below $\approx {2T}_c.$