Resistivity and Hall effect of EuSe in fields up to 150 kOe

The resistivity $\rho (H_{\mathrm{ext}},T)$ and Hall effect were studied in ten $n$-type EuSe single-crystal samples with room-temperature carrier concentrations from 4.2 × ${10}^{18}$ to 3.5 × ${10}^{19}$ electrons/${\mathrm{cm}}^3$. The electrical measurements were performed at temperatures $1.6\le T\le 300$ K and external magnetic fields $0\le H_{\mathrm{ext}}\le 150$ kOe, and were supplemented by magnetization and differential susceptibility measurements to aid in the interpretation. The following phenomena were observed: (a) A very large peak in the zero-field resistivity occurred at a temperature $T_{max}$ which varied from ∼ 7 to ∼ 13 K, depending on the sample. (b) A negative magnetoresistance was observed for most temperatures and was very large near $T_{max}$. (c) However, in a limited temperature interval well above $T_{max}$, a positive magnetoresistance was observed at low fields (followed by a negative magnetoresistance at higher fields). (d) Below ∼ 3 K, the zero-field resistivity increased rapidly with decreasing $T$, except for the sample with the highest carrier concentration. (e) The anomalous Hall effect was negligible at temperatures $T\le 4.2$ K. Much of the data are interpreted in terms of band conduction. The mobility is then limited by spin-disorder scattering and it increases with increasing $H_{\mathrm{ext}}$, resulting in a negative magnetoresistance. This description fails when both $T\lesssim T_{max}$ and $H_{\mathrm{ext}}<1\mathrm{} \mathrm{to} 10$ kOe. Under these conditions, the very high resistivity is attributed to the localization (trapping) of electrons by the $s-f$ (or $d-f$) interaction. At temperatures for which the electrons are localized at $H_{\mathrm{ext}}=0\mathrm{}$, a field of 1 to 10 kOe delocalizes the electrons and, therefore, leads to band conduction and a much lower resistivity. At higher fields there is a negative magnetoresistance (smaller than at low fields) due to the reduction in spin-disorder scattering of the band electrons. A model for the positive magnetoresistance, which focuses on the spin splitting of the conduction band by a magnetic field, is presented in the following paper.