Hall Effect in CoO, NiO, andα−Fe2O3

The Hall coefficient $R_H$ and resistivity $\rho$ have been measured on ceramic samples of $p$-type CoO and NiO between 200 and 1500°K and of $n$-type $\alpha -{\mathrm{Fe}}_2{\mathrm{O}}_3$ between 960 and 1500°K. Results obtained on single-crystal $p$-type NiO are also reported. Seebeck-effect data are considered in the discussion of the results. For the paramagnetic phase of NiO and $\alpha -{\mathrm{Fe}}_2{\mathrm{O}}_3$, the behavior of $R_H$ deviates drastically from that found in the case of a normal-band semiconductor. The sign of $R_H$ is opposite to that normally expected. The corresponding Seebeck effect has the normal sign. $R_H$ remains inversely proportional to the charge-carrier concentration. Furthermore, the Hall mobility (${\mu }_H=\frac{\left|R_H\right|}{\rho }$) has a value which is smaller than that for the corresponding drift mobility (${\mu }_D$). The difference amounts to two orders of magnitude in the case of NiO and to about a factor of 4 in the case of $\alpha -{\mathrm{Fe}}_2{\mathrm{O}}_3$. Below the Néel temperature, $R_H$ has the normal sign. The anomalous behavior of the Hall effect in the cases of NiO and $\alpha -{\mathrm{Fe}}_2{\mathrm{O}}_3$ is thought to be due to an interaction between the charge carriers and the magnetization induced in the material by the applied magnetic field. In the case of CoO the behavior of $R_H$ does not seem to be influenced appreciably by the magnetic transition at the Néel temperature. The value of ${\mu }_H$ at high temperatures in the paramagnetic phase is about a factor of 4 lower than that of ${\mu }_D$. A discussion of the Hall-effect data with regard to the mechanism of electrical conduction in the materials under consideration is also presented.