Electrical conductivity of (Ni,Mn,Fe,Cu)3O4 ceramic semiconductors designed for temperature and magnetic field sensing

This research is focused on designing new ceramic oxides with the spinel structure for a potential application as sensitive temperature and magnetic field sensors. In this respect, the mixtures of ferrimagnetic NiFe2O4 with low-resistive Ni0.66Cu0.41Mn1.93O4 semiconductor (molar coefficient alpha = 1/5 and 1/2) were prepared using the chemical co-precipitation technique followed by a low-temperature sintering of the cold-pressed powders. For the prepared materials, a variation of the electrical conductivity sigma dc with temperature T (in the range 50 K-400 K) is quantitatively analyzed using different theoretical concepts of the charge carrier hopping transport occurring in disordered materials with strong electron-phonon interaction. It has been shown that an increase in alpha from 0 to 1/2 causes a step-like change of the dc electrical conductivity and, simultaneously, a gradual change in the magnetization M determined in the saturation state. The temperature coefficient of resistance TCR of the prepared ceramics takes the values from -0.6%/K (at 400 K) to-19.6%/K (below 100 K). The highest absolute value of the magneto-resistance coefficient MR determined at 150 K (and at magnetic field 7 T) registered for ceramic oxide with alpha = 1/2 was found to be 3.4%.