EuO. I. Resistivity and Hall Effect in Fields up to 150 kOe

Abstract

The resistivity $\rho$ and the Hall effect are studied in seven $n$-type EuO single-crystal samples at temperatures $4.2\le T\le 300$ K and external magnetic fields $0\le H_{\mathrm{ext}}\le 150$ kOe. Attention is focused on three phenomena: (i) the anomalous Hall effect (or lack thereof), (ii) the resistivity peak near the Curie temperature $T_C\simeq 69$ K, and (iii) the insulator-metal transition (IMT). The Hall data at $T\ll T_C$ indicate that the anomalous Hall term in EuO is small compared to the normal term and that the effective magnetic field which governs the Hall effect is equal to the magnetic induction $B=H_{\mathrm{int}}+4\mathrm{}\pi M$. On this basis it is assumed that the anomalous Hall effect is negligible at all temperatures. Measurements of $\rho (H_{\mathrm{ext}}, T)$ vs $T$, show that as $H_{\mathrm{ext}}$ increases the resistivity peak decreases, becomes broader, and shifts to higher temperatures. The Hall data indicate that the resistivity peak is due to the combined effect of dips in the Hall mobility $\mu$ and in the carrier concentration $n$. Measurements of $T_C$ by several methods show that $T_C$ is several degrees lower than the temperature $T_{max}$ at which the zero-field resistivity is maximum. A new method for obtaining $T_C$ from magnetoresistance measurements is discussed. Near room temperature the resistivity of some samples decreases exponentially with increasing $T$, with an activation energy of ∼ 0.3 eV at zero magnetic field. For these "activated" samples $\rho$ changes by many orders of magnitude near the IMT. In other samples (called "nonactivated") $\rho$ varies slowly with $T$ near room temperature and the resistivity change near the IMT is smaller. In both types of samples $H_{\mathrm{ext}}$ shifts the IMT to higher temperatures and makes the transition more gradual. Hall measurements show that in samples with a large IMT, the IMT is almost entirely due to a change in $n$, whereas in samples with a small IMT, the IMT is due to comparable changes in both $n$ and $\mu$. Near room temperature the Hall coefficient is $H$ independent in the nonactivated sample, but decreases substantially with $H$ in the activated samples. In both types of samples the Hall mobility at 298 K increases by ∼ 25% when a magnetic field of 140 kOe is applied. This indicates that spin-disorder scattering is one of the main causes for the zero-field resistivity at room temperature. The various data are compared with earlier measurements by the groups at Lincoln Laboratory and at IBM, and are also discussed in terms of current theoretical models.