Enhanced spin fluctuations in Ca3Co4-xTixO9 single crystals

The effect of Ti doping on anisotropic magnetic, electronic, and thermal transport properties of Ca3Co4-xTixO9 single crystals has been investigated. The Ti substitution drives the system from a ferrimagnetic state for x=0 to spin-glass state for x >= 0.2 at low temperatures. Two anomalies in the d chi(c)/dT versus T curve are observed for all samples, which are suggested to be related to the appearance of short-range spin-density-waves (SDW) and ferrimagnetic state. The in-plane resistivity of all studied samples shows a metal-insulator transition at T-rho min. T-rho min is essentially constant at about 85 K for x <= 0.6 and reaches 142 K as x=0.8. However, the out-of-plane resistivity shows the semiconducting behavior in the whole temperature and doping range studied. The in-plane thermopower increases monotonically with the increase of temperatures in the studied temperature range. Strikingly, for Ti-doping samples, the out-of-plane thermopower exhibits a steeper upturn at low temperatures and is obviously suppressed by an applied magnetic field. Combined with the magnetic measurements, the low-temperature upturn of out-of-plane thermopower is suggested to originate from the spin fluctuation induced by Ti doping. In addition, both the resistivity and thermopower increase monotonically with increasing the Ti content, which is considered to be related to the variation of carrier concentration due to Ti doping.