Electronic properties of La2-2xSr1+2xMn2O7 (x=0.4 and 0.5) single crystals are studied
by electron spin resonance (ESR) and spin-lattice relaxation time measurements. Spin
susceptibility chi(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=226
K 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 T-1 and the transversal relaxation time
T-2 are equal around room temperature that is a sign of exchange narrowing. The T-1/T-2
ratio increases to about 5 approaching the Curie temperature T-C=126 K. No sign of
critical speeding up of T-1 is detected. Instead, the slowing down of the relaxation
rate takes place and T-1 is proportional to Tchi(T). This is attributed to the freezing
of short-range magnetic correlations in the external field.