Time evolution of metastable magnetic configurations in the geometrically frustrated spin-chain compound Ca₃Co₂O₆

Ca₃Co₂O₆ exhibits rich magnetic phenomena originating in its crystal structure. Due to the ferromagnetic coupling between Co atoms along the crystallographic c-axis, it forms spin chains along this direction. In the ab-plane, however, these chains form triangular lattice which, in conjunction with the antiferromagnetic intra-chain interaction, produces geometric frustration. At low temperatures and for H parallel to the spin chains, the equilibrium state above a critical field H_c=3.6T is the fully aligned state characterized by a saturation magnetization M_sat. Below H_c, the ground state is the ordered state with two chains magnetized along H and one chain in the opposite direction, resulting in M=M_sat/3. However, isothermal M(H) curves exhibit a complex hysteretic behavior with steps that indicate the existence of metastable configurations. In the lower (increasing H) branch of the M(H) loop, one step at Hc has M higher than M_sat/3, showing that there is an energy barrier precluding the decay of the metastable state to the ground state. We investigated the slow time evolution of the magnetization, M(t), in the metastable states. The evolution is not exponential, consistent with interacting objects, and in large portions of the T-H diagram M(t) is well described by the logarithmic law typically observed in vortex dynamics. In some T-H conditions, M(t) evolves in the direction opposite to equilibrium, hinting to the presence of hidden configurations. In some cases, M(t) evolves non-monotonically, first increasing and then decreasing, as the hidden configuration in turn evolves towards equilibrium.