Electron spin resonance linewidth, susceptibility, and conductivity in doped manganites

Analyses of the electron spin resonance (ESR) linewidths in manganites with ferromagnetic interactions arising from double exchange (e.g. La 1− x Ca x MnO 3) have generally followed either of two approaches. One approach, coming from general theories of spin relaxation, expresses the linewidth above the critical region in the form [( C/ T)/ χ( T)]Δ H( ∞), where χ( T) is the static spin susceptibility, C is the Curie constant and Δ H( ∞) is a material-dependent constant representing the limiting behavior of the linewidth at high temperatures, where χ( T)≈ C/ T. The other approach is based on the observation of a similarity between the temperature dependence of the linewidth and the high-temperature electrical conductivity, the latter being associated with the thermally activated hopping of small polarons. In this approach, the linewidth is fit to an expression of the form α+( β/ T)exp(− E a/ k B T), where α and β are constants and E a is identified as the polaron activation energy. In our work, we show that in La 0.75Ca 0.25 MnO 3 [ Tχ( T)] −1 is characterized by the same functional form as the linewidth with nearly identical values of the activation energy and the ratio α/ β. These results establish that the “thermal activation” component of the linewidth is associated with the static susceptibility. Using the susceptibility of La 0.67Sr 0.33MnO 3 as an example, we suggest this finding is indicative of a process where the Hund's rule correlations between the core spins and the spins of the e g polarons are disrupted by the random hopping of the polarons.