Magnetic-field-controlled phase separation in manganites: Electron magnetic resonance study

Abstract

The electron magnetic resonance spectra of Sm{sub 1-x}Sr{sub x}MnO{sub 3} (x = 0.30, 0.40, 0.45) manganites have been studied. At temperatures that are higher than the Curie point by several tens of kelvins, samples with x = 0.40 and 0.45 exhibit a ferromagnetic resonance (FMR) spectrum imposed on their usual EPR spectrum. The FMR spectrum appears as the applied magnetic field H exceeds a certain critical field H{sub c}, which decreases upon cooling and becomes zero at T = T{sub C}. These results agree with the magnetic-measurement data and indicate the magnetic-field-induced nucleation and growth of ferromagnetic domains in a paramagnetic matrix. In the initial growth stage, the volume of the ferromagnetic domains is proportional to (H - H{sub c}){sup {beta}}, where {beta} = 4.0 {+-} 0.3, and it changes in phase with magnetic field modulation up to a frequency of 100 kHz. In the same field and temperature ranges, hysteretic phenomena and narrow unstable spectral lines are detected; these lines indicate a dynamic character of the phase separation. The results obtained are interpreted in terms of the competition of different types of magnetic and charge ordering.