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 $0.4\ensuremath{\le}T\ensuremath{\le}273$ K in magnetic fields $0\ensuremath{\le}Hl150$ 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 N\'eel temperature ${T}_{N}=(9.6\ifmmode\pm\else\textpm\fi{}0.1)$ K and a saturation magnetic moment ${\ensuremath{\sigma}}_{s}=(132\ifmmode\pm\else\textpm\fi{}3)\frac{\mathrm{emu}}{\mathrm{g}}(\frac{6.6{\ensuremath{\mu}}_{B}}{\mathrm{Eu}}\mathrm{ion})$. In the interval $10{T}_{N}lTl28{T}_{N}$, the insulating material obeyed the Curie-Weiss law with a Curie-Weiss constant $C=(27.8\ifmmode\pm\else\textpm\fi{}0.3)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$ (emu K/g Oe), and a paramagnetic Curie temperature $\ensuremath{\theta}=(\ensuremath{-}1\ifmmode\pm\else\textpm\fi{}2)$ K. Below ${T}_{N}$, the differential susceptibility $\frac{d\ensuremath{\sigma}}{\mathrm{dH}}$ in the insulating sample showed the following features: (i) At low fields, $\frac{d\ensuremath{\sigma}}{\mathrm{dH}}$ first increased with $H$, then passed through a broad maximum near 1 kOe, and finally assumed the constant value ${\ensuremath{\chi}}_{\ensuremath{\perp}}=1.6\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$ 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, $\frac{d\ensuremath{\sigma}}{\mathrm{dH}}$ increased with $H$. (iii) The canted-to-paramagnetic transition was marked by a $\ensuremath{\lambda}$ peak in $\frac{d\ensuremath{\sigma}}{\mathrm{dH}}$. (iv) The transition field ${H}_{c}(T)$ was linear with ${T}^{\frac{3}{2}}$ in the interval $2\ensuremath{\le}T\ensuremath{\le}4.2$ K, but a small deviation from the theoretical ${T}^{\frac{3}{2}}$ law was observed below 2 K. At $T=0$, ${H}_{c}(0)=(72.2\ifmmode\pm\else\textpm\fi{}1)$ kOe, which leads to an intersublattice exchange field ${H}_{E}(0)\ensuremath{\cong}36$ kOe. The values of ${H}_{c}(0)$ and $\ensuremath{\theta}$ for the insulating material yield the nearest-neighbor and next-nearest-neighbor exchange constants $\frac{{J}_{1}}{k}=+(0.10\ifmmode\pm\else\textpm\fi{}0.03)$ K and $\frac{{J}_{2}}{k}=\ensuremath{-}({0.21}_{5}\ifmmode\pm\else\textpm\fi{}0.03)$ 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 $T\ensuremath{\lesssim}2{T}_{N}$, 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 $\ensuremath{\sigma}$ at temperatures well below ${T}_{N}$ suggested the existence of a spontaneous magnetic moment at $H=0$. (c) The $\ensuremath{\lambda}$ anomaly in $\frac{d\ensuremath{\sigma}}{\mathrm{dH}}$, at ${H}_{c}$, was reduced, or disappeared entirely. (d) The saturation moment ${\ensuremath{\sigma}}_{s}$ was slightly higher than for the insulating sample. (e) The paramagnetic Curie temperature $\ensuremath{\theta}$ 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 ${H}_{c}(T)$. Some of the effects of the charge carriers can be explained qualitatively by de Gennes's model for the double-exchange interaction in antiferromagnets.