EPR linewidths in La1-xCaxMnO3: 0 <= x <= 1

We have analyzed recent results for the electron paramagnetic resonance lin ewidths in La1-xCaxMnO3 and related materials over the range 0 less than or equal to x less than or equal to 1. In all cases studied, the linewidth aw ay from magnetic and structural transitions can be fitted to the expression [chi(o)(T)/chi(T)]Delta H-p.p.(infinity), where the first factor is the ra tio of the Curie susceptibility chi(o)(T) to the measured susceptibility ch i(T) and the second factor is a temperature-independent constant. Formally exact and approximate expressions for Delta H-p.p.(infinity) are derived. L n the case of LaMnO3, the Linewidth in the distorted O' phase arises from t he interplay of crystal field (single ion) anisotropy and antisymmetric (Dz ialozhinsky-Moriya) exchange together with isotropic superexchange. Above t he Jahn-Teller transition to the pseudocubic O phase, the single-ion anisot ropy is much weaker and the linewidth arises primarily from the antisymmetr ic interactions. Similar behavior is also found in both phases of the less- distorted, oxygenated material LaMnO3.04. In the case of CaMnO3, the distor tions of the oxygen octahedra are also small and the antisymmetric interact ions are again dominant. In the intermediate region 0<x<1, Delta H-p.p.(inf inity) decreases with increasing Ca (i.e.; Mn4+) concentration in contrast to the Curie-Weiss temperature, which peaks near x = 1/3, reflecting the im portance of the double-exchange mechanism. It is proposed that the absence of a signature of the double-exchange interaction in the infinite-temperatu re linewidth is a consequence of the fact that the time scale associated wi th the changes in Mn valence arising from double exchange is too long to af fect the decay of the correlation function in Delta H-p.p.(infinity) so tha t the linewidth is determined primarily by the superexchange interactions b etween the Mn ions. [S0163-1829(99)08841-9].