Electrical Conductivity of NiO near the Curie Temperature

The resistivity of single-crystal and polycrystalline NiO has been observed experimentally in the temperature range from room temperature to 400°C through the Curie temperature ($T_C$) in order to study the influence on electrical conduction of the superexchange coupling characteristic of antiferromagnetism. The relation of resistivity vs $\frac{1}{T}$ for single crystals is observed to behave rather linearly in the regions both above and below a small and discernible bending (knick) near $T_C$, from which the corresponding activation energies may be determined. The result seems to support the Heitler-London approach by Heikes in the NiO crystal. The specimens, in which partial spin alignment was introduced by stress-annealing through $T_C$ along a direction of [111], show an anomalous increase of resistivity along the stress axis near $T_C$ in the heating process, whereas in the following cooling process the anomaly in resistivity is not observed. The result seems to show that the spin alignment has been disturbed appreciably during the heating process and that then the electrical conduction may be affected by the superexchange coupling, as suggested in the Heitler-London model of NiO.