Electron spin resonance and relaxation studies of double-layered manganites

Electronic properties of La_2-2xSr_1+2xMn₂O₇ (x=0.4 and 0.5) single crystals are studied by electron spin resonance (ESR) and spin-lattice relaxation time measurements. Spin susceptibility χ(T) determined from the ESR signal intensity and macroscopically measured static-susceptibility data are in good agreement, thus ESR detects all spin species in the system. In both compounds, the ESR spectra contain a single, nearly isotropic Lorentzian line associated with the exchange coupled Mn³⁺ and Mn⁴⁺ ions. For the x=0.5 compound, the fingerprints of charge ordering (CO) transition at T_CO=226K are detected. In addition, strongly anisotropic ferromagnetic resonance spectra are found in both materials, suggesting the presence of extrinsic ferromagnetic phases. For x=0.4, the longitudinal relaxation time T₁ and the transversal relaxation time T₂ are equal around room temperature that is a sign of exchange narrowing. The T₁/T₂ ratio increases to about 5 approaching the Curie temperature T_C=126K. No sign of critical speeding up of T₁ is detected. Instead, the slowing down of the relaxation rate takes place and T₁ is proportional to Tχ(T). This is attributed to the freezing of short-range magnetic correlations in the external field.