Spin dynamics in pure and Zn-doped CuGeO3 by muon spin relaxation: Spin-Peierls and spin-glass transitions.

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Zero and longitudinal muon spin relaxation has been used to characterize the spin dynamics of local magnetic moments in the quasi-one-dimensional S=1/2 cuprates ${\mathrm{CuGeO}}_{3}$ and ${\mathrm{Cu}}_{0.96}$${\mathrm{Zn}}_{0.04}$${\mathrm{GeO}}_{3}$. Frozen moments are not observed in ${\mathrm{CuGeO}}_{3}$, only a slowing down of the fluctuating rate of Cu moments was seen below 14 K. Magnetic measurements in the slightly doped sample confirm the suppression of the spin-singlet transition, and the appearance of hysteresis between the zero-field-cooled and field-cooled branches of the magnetic susceptibility below T\ensuremath{\approxeq}4.5 K, both absent in the pure compound. The depolarization of muons gives evidence of the appearance of randomly frozen Cu moments in the doped compound below ${\mathit{T}}_{\mathit{g}}$\ensuremath{\approxeq}4.5 K. The dynamic relaxation rate ${\ensuremath{\lambda}}_{\mathit{d}}$ is appreciably higher around ${\mathit{T}}_{\mathit{g}}$ than below 4 K, and the correlation time ${\mathrm{\ensuremath{\tau}}}_{\mathit{c}}$ of the magnetic fluctuations of 3d Cu moments in ${\mathrm{Cu}}_{0.96}$${\mathrm{Zn}}_{0.04}$${\mathrm{GeO}}_{3}$ is estimated to be ${\mathrm{\ensuremath{\tau}}}_{\mathit{c}}$\ensuremath{\approxeq}${10}^{\mathrm{\ensuremath{-}}8}$ s at 5 K.