Muon spin relaxation study of partially disordered state in triangular-lattice antiferromagnet:: Ca3Co2O6

A muon spin relaxation (mu SR) experiment has been performed in zero-field (ZF) and longitudinal-field (LF) conditions for a Ca3Co2O6 sample which has an anti ferromagnetic triangular lattice on the ab-plane with Ising ferromagnetic chains along the c-axis. At 150 K, far above the magnetic transition temperature T-cl (= 25 K) determined from magnetization data, exponential-like relaxation curves are observed in ZF-mu SR time spectra, suggesting the development of intrachain ferromagnetic coupling even far above T-cl. Below the temperature T-c2 (similar to 12 K) where magnetization is saturated, the tail of the relaxation curve reaches 1/3 the value of the amplitude in ZF-mu SR spectra and the decoupling phenomenon in LF-mu SR, that is, a further rise of the tail due to the applied LF, can be observed, implying the appearance of the static internal magnetic field. However, between T-cl and T-c2, the above-mentioned phenomenon cannot be observed. This indicates that the internal magnetic field at each muon site is not static but rather dynamical with the fluctuation time tau(c) (similar to 21 ns) although the magnetization gives large values between these temperatures. This fact must indicate the presence of a partially disordered antiferromagnetic state peculiar to the frustrated magnetic system.