Studies of temperature dependent ac impedance of a negative temperature coefficient Mn-Co-Ni-O thin film thermistor

Mn1.85Co0.3Ni0.85O4 thin films with single cubic spinel phase and compact surface were prepared by chemical solution deposition method on Al2O3 substrates. Temperature dependent ac impedance spectroscopy was employed to analyze the contributions of grain boundary (GB) and grain to the conduction mechanism. The major characteristics (hopping type, resistance versus temperature behavior, and characteristic temperature T-0) of the GB and grain were obtained. The hopping type of GB is nearest-neighbor-hopping (NNH), and that of grain might be a transition from variable-range-hopping to NNH. The GB shows dominating resistance while the grain shows stronger temperature dependence. (C) 2011 American Institute of Physics. [doi:10.1063/1.3596454]