Analysis of electron paramagnetic resonance experiments in colossal magnetoresistance materials

An analysis is given of recent electron paramagnetic resonance (EPR) experi ments in manganite materials displaying colossal magnetoresistance behavior . These experiments have established that in highly homogeneous samples cha racterized by a single resonance line, the EPR linewidths above the critic al region associated with the paramagnetic - ferromagnetic transition are i nversely proportional to T chi(T), where chi denotes the susceptibility. Th is behavior indicates that the linewidth does not reflect a coupling to the lattice vibrations but rather is caused by a spin-only mechanism. The poss ibility that the linewidth arises from the dipolar interaction is assessed using an effective interaction model. On the basis of the model calculation , it is concluded that the dipolar interaction is probably too weak to acco unt for the observed broadening and that the linewidth most likely arises f rom tetragonal and orthorhombic distortions of the octahedral crystal field at the sites of the Mn ions. The EPR linewidth in samples with very low co ncentrations of Mn4+ ions is expected to vary with temperature as [T chi(sp )(T)](-1), where chi(sp)(T) is the susceptibility of the ferromagnetically coupled spin polaron. By measuring the temperature dependence of the linewi dth, one may be able to determine the temperature dependence of the polaron susceptibility. (C) 1998 American Institute of Physics. [S0021-8979(98)169 11-6].