EuTe. I. Magnetic Behavior of Insulating and Conducting Single Crystals

The magnetic properties of both insulating and conducting single crystals of EuTe were measured at temperatures $0.4\le T\le 273$ K in magnetic fields $0\le H<\mathrm{}150\mathrm{}$ kOe. The insulating EuTe showed the behavior expected for an Heisenberg antiferromagnet and is believed to be representative of pure EuTe. For this insulating material we obtained a Néel temperature $T_N=(9.\mathrm{}6\pm 0.1)$ K and a saturation magnetic moment ${\sigma }_s=(132\mathrm{}\pm 3)\frac{\mathrm{emu}}{\mathrm{g}}\left(\frac{6.6{\mu }_B}{\mathrm{Eu}}\mathrm{ion}\right)$. In the interval $10T_N<T<\mathrm{}28\mathrm{}{T}_N$, the insulating material obeyed the Curie-Weiss law with a Curie-Weiss constant $C=(27.\mathrm{}8\pm 0.3)\times {10}^{-3\mathrm{}}$ (emu K/g Oe), and a paramagnetic Curie temperature $\theta =(-1\mathrm{}\pm 2)$ K. Below $T_N$, the differential susceptibility $\frac{d\sigma }{\mathrm{dH}}$ in the insulating sample showed the following features: (i) At low fields, $\frac{d\sigma }{\mathrm{dH}}$ first increased with $H$, then passed through a broad maximum near 1 kOe, and finally assumed the constant value ${\chi }_{\perp }=1.6\mathrm{}\times {10}^{-3\mathrm{}}$ emu/g Oe. This low-field behavior corresponds to the rotation of spins in the {111} planes (analogous to spin flopping). (ii) At higher fields, in the canted phase, $\frac{d\sigma }{\mathrm{dH}}$ increased with $H$. (iii) The canted-to-paramagnetic transition was marked by a $\lambda$ peak in $\frac{d\sigma }{\mathrm{dH}}$. (iv) The transition field $H_c\mathrm{}\left(T\right)\mathrm{}$ was linear with $T^{\frac{3}{2}}$ in the interval $2\le T\le 4.2$ K, but a small deviation from the theoretical $T^{\frac{3}{2}}$ law was observed below 2 K. At $T=0\mathrm{}$, $H_c\mathrm{}\left(0\right)=(72.\mathrm{}2\pm 1)$ kOe, which leads to an intersublattice exchange field $H_E\mathrm{}\left(0\right)\mathrm{}\cong 36$ kOe. The values of $H_c\mathrm{}\left(0\right)\mathrm{}$ and $\theta$ for the insulating material yield the nearest-neighbor and next-nearest-neighbor exchange constants $\frac{J_1}{k}=+(0.\mathrm{}10\pm 0.03)$ K and $\frac{J_2}{k}=-({0.21}_5\pm 0.03)$ K, respectively. The results in the conducting single crystals (which were $n$ type) showed that the following magnetic properties were influenced by the presence of the charge carriers: (a) At $T\lesssim 2T_N$, the zero-field differential susceptibility in the conducting samples was an order of magnitude larger than in the insulating sample. (b) The low-field behavior of $\sigma$ at temperatures well below $T_N$ suggested the existence of a spontaneous magnetic moment at $H=0\mathrm{}$. (c) The $\lambda$ anomaly in $\frac{d\sigma }{\mathrm{dH}}$, at $H_c$, was reduced, or disappeared entirely. (d) The saturation moment ${\sigma }_s$ was slightly higher than for the insulating sample. (e) The paramagnetic Curie temperature $\theta$ was several degrees higher than for the insulating material. The presence of charge carriers had little or no effect on $T_N$, and only a small influence on the phase boundary $H_c\mathrm{}\left(T\right)\mathrm{}$. Some of the effects of the charge carriers can be explained qualitatively by de Gennes's model for the double-exchange interaction in antiferromagnets.