EPR studies of rare-earth manganites La 0.7-x Eu x Sr 0.3 MnO 3 (x = 0.1-0.7)

Abstract

The rare-earth manganites of Eu, La_0.7−xEu_xSr_0.3MnO₃ (x = 0.0–0.7) were investigated by the technique of electron paramagnetic resonance (EPR) in the temperature range from 30 to 500 K. They revealed the coexistence of two to three magnetic phases in the samples with different Eu concentrations. The corresponding Curie temperatures were estimated from the characteristics of the variable-temperature EPR spectra for the various samples. The EPR data indicated the presence of Griffiths phases for the samples La_0.7−xEu_xSr_0.3MnO₃, from which the respective Griffiths temperatures were determined. It was found that the structural disorder (σ²), caused by the presence of different sets of atoms in crystal cells, characterized by the distribution of the cation radius, increased as x increased from 0.1 to 0.3 and decreased as x increased from 0.3 to 0.7. This, in turn, resulted in the sample with the maximum structural disorder, i.e. the sample with x = 0.3, being characterized by the maximum activation temperature. The activation energies are estimated here from the EPR data using the hopping model. The EPR linewidth behavior is found to be consistent with that predicted by the bottlenecked spin relaxation model. The perovskite La_0.5Eu_0.2Sr_0.3MnO₃ is found to be potentially useful in the design of magnetocaloric refrigeration units as a working fluid, since its Curie temperature (T_C) is close to the room temperature/ The different ferromagnetic components in the samples studied here have been resolved by the technique of EPR, not possible by other techniques.