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

A muon spin relaxation (μSR) experiment has been performed in zero-field (ZF) and longitudinal-field (LF) conditions for a Ca3Co2O6 sample which has an antiferromagnetic triangular lattice on the ab-plane with Ising ferromagnetic chains along the c-axis. At 150 K, far above the magnetic transition temperature Tc1 (=25 K) determined from magnetization data, exponential-like relaxation curves are observed in ZF-μSR time spectra, suggesting the development of intrachain ferromagnetic coupling even far above Tc1. Below the temperature Tc2 (∼12K) where magnetization is saturated, the tail of the relaxation curve reaches 1/3 the value of the amplitude in ZF-μSR spectra and the decoupling phenomenon in LF-μSR, 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 Tc1 and Tc2, 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 τc (∼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. (author)