Spin dynamics, short range order and spin freezing in Y0.5Ca0.5BaCo4O7

Y0.5Ca0.5BaCo4O7 was recently introduced as a possible candidate for capturing some of the predicted classical spin kagome ground state features. Stimulated by this conjecture we have taken up a more complete study of the spin correlations in this compound with neutron scattering methods on a powder sample characterized with high-resolution neutron diffraction and the temperature dependence of magnetic susceptibility and specific heat. We have found that the frustrated near neighbor magnetic correlations involve not only the kagome planes but concern the full Co sub-lattice as evidenced by the analysis of the wavevector-dependence of the short range order. We conclude from our results that the magnetic moments are located on the Co sub-lattice as a whole and that correlations extend beyond the two-dimensional kagome planes. We identify intriguing dynamical properties, observing high-frequency fluctuations with a lorentzian line-width {\Gamma} <= 20 meV at ambient temperature. On cooling a low-frequency (~ 1 meV) dynamical component develops alongside the high-frequency fluctuations, which eventually becomes static at temperatures below T ~ 50 K. The high-frequency response with an overall line-width of ~ 10 meV prevails at T <= 2 K coincident with a fully elastic short-range ordered contribution.