Abstract
Ni x Mn 3−x O 4+δ (0.4⩽x⩽1) are a series of cubic-spinel-structured material exhibiting a negative temperature coefficient of resistance. The resistance as a function of temperature (T) has been measured from 20 to 200 °C and the data have been fitted to a variable range hopping model in which the resistivity is described as ρ=ρ0T exp(T0/T)0.5 where T0 was found to be 2.24×105 K. Scanning tunneling spectroscopy measurements were carried out over the temperature range of 20–300 °C, to study the shape of the local density of states (LDOS). Such measurements have not been carried out on this class of spinel structured materials before. The distribution of the LDOS around the Fermi level was parabolic in agreement with the model of variable range hopping. The evolution of a peak around 1.8 eV was observed with increasing temperature and found to be completely reversible with temperature.
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