Magnetic properties of Eu-doped La 2 − x Sr x CuO 4 studied by ESR

By analyzing electron spin resonance spectra of Gd spin probes in ${\mathrm{La}}_{2\ensuremath{-}x\ensuremath{-}y}{\mathrm{Eu}}_{y}{\mathrm{Sr}}_{x}{\mathrm{CuO}}_{4},$ we find that in the low temperature tetragonal phase the frequency of the spin fluctuations ${\ensuremath{\omega}}_{\mathrm{sf}}$ of Cu ions considerably slows down in the samples with strongly suppressed superconductivity (Sr contents $xl0.17$). The effect is especially pronounced for $x$ close to $1/8.$ However, for the samples with Sr concentration $x\ensuremath{\gtrsim}0.17$ the specific features of the spin dynamics of the low temperature structural phase almost vanish, whereas fully developed superconductivity reappears.