Magnetic Ordering of CoCl_{2}-GIC: a Spin Ceramic -Hierarchical Successive Transitions and the Intermediate Glassy Phase-

Abstract

Stage-2 CoCl$_{2}$-GIC is a spin ceramic and shows hierarchical successive transitions at $T_{cu}$ (= 8.9 K) and $T_{cl}$ (= 7.0 K) from the paramagnetic phase into an intra-cluster (two-dimensional ferromagnetic) order with inter-cluster disorder and then to an inter-cluster (three-dimensional antiferromagnetic like) order over the whole system. The nature of the inter-cluster disorder was suggested to be of spin glass by nonlinear magnetic response analyses around $T_{cu}$ and by studies on dynamical aspects of ordering between $T_{cu}$ and $T_{cl}$. Here, we present a further extensive examination of a series of time dependence of zero-field cooled magnetization $M_{ZFC}$ after the aging protocol below $T_{cu}$. The time dependence of the relaxation rates $S_{ZFC}(t) = (1/H)dM_{ZFC}(t)/d\ln t$ dramatically changes from the curves of simple spin glass aging effect below $T_{cl}$ to those of two peaks above $T_{cl}$. The characteristic relaxation behavior apparently indicates that there coexist two different kinds of glassy correlated regions below $T_{cu}$.