Electron spin resonance and relaxation studies of double-layered manganites

Abstract

Electronic properties of ${\mathrm{La}}_{2\ensuremath{-}2x}{\mathrm{Sr}}_{1+2x}{\mathrm{Mn}}_{2}{\mathrm{O}}_{7}$ $(x=0.4$ and 0.5) single crystals are studied by electron spin resonance (ESR) and spin-lattice relaxation time measurements. Spin susceptibility $\ensuremath{\chi}(T)$ 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 ${\mathrm{Mn}}^{3+}$ and ${\mathrm{Mn}}^{4+}$ ions. For the $x=0.5$ compound, the fingerprints of charge ordering (CO) transition at ${T}_{\mathrm{CO}}=226\mathrm{K}$ 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 ${T}_{C}=126\mathrm{K}.$ 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 $T\ensuremath{\chi}(T).$ This is attributed to the freezing of short-range magnetic correlations in the external field.