Muon spin relaxation study of spin dynamics in the extended kagome systems YBaCo4O7+δ (δ=0,0.1)

We present muon spin relaxation ($\ensuremath{\mu}\mathrm{SR}$) measurements of the extended kagome systems ${\mathrm{YBaCo}}_{4}{\mathrm{O}}_{7+\ensuremath{\delta}}$ ($\ensuremath{\delta}=0,0.1$), comprising two interpenetrating kagome sublattice of $\mathrm{Co}{(\mathrm{I})}^{3+}$ ($S=3/2$) and a triangle sublattice of $\mathrm{Co}{(\mathrm{II})}^{2+}$ ($S=2$). The zero- and longitudinal-field $\ensuremath{\mu}\mathrm{SR}$ spectra of the stoichiometric compound ${\mathrm{YBaCo}}_{4}{\mathrm{O}}_{7}$ unveil that the triangular subsystem orders at ${T}_{N}=101$ K. In contrast, the muon spin relaxation rate pertaining to the kagome subsystem shows persistent spin dynamics down to $T=20$ K and then a sublinear decrease $\ensuremath{\lambda}(T)\ensuremath{\sim}{T}^{0.66(5)}$ on cooling towards $T=4$ K. In addition, the introduction of interstitial oxygen ($\ensuremath{\delta}=0.1$) is found to drastically affect the magnetism. For the fast-cooling experiment ($g10$ K/min), ${\mathrm{YBaCo}}_{4}{\mathrm{O}}_{7.1}$ enters a regime characterized by persistent spin dynamics below 90 K. For the slow-cooling experiment (1 K/min), evidence is obtained for the phase separation into interstitial oxygen-poor and oxygen-rich regions with distinct correlation times. The observed temperature, cooling rate, and oxygen content dependencies of spin dynamics are discussed in terms of a broad range of spin-spin correlation times, relying on a different degree of frustration between the kagome and triangle sublattices as well as of oxygen migration.