Giant Magnetoresistance and Unusual Magnetic Behavior in Single Crystals of the Layered Arsenide EuRh$_2$As$_2$

Magnetic susceptibility $\chi$, isothermal magnetization $M$, resistivity $\rho$, Hall effect, and heat capacity $C$ measurements on EuRh$_2$As$_2$ reveal complex and unusual magnetic behavior. The $\chi(T)$ data gave a small Weiss temperature $\theta \approx$~12~K indicating predominantly ferromagnetic interactions between the Eu$^{2+}$ moments. Below $T$~=~47~K, however $\chi(T)$ indicates that an antiferromagnetic transition occurs instead. The unusually high $T_{\rm N}$ compared to $\theta$ ($\theta$/$T_{\rm N}\approx$0.26) suggests novel physics. A metamagnetic transition is observed in the $M$ versus $H$ data at $T < T_{\rm N}$ when $H$ is applied in the $ab$ plane. The metamagnetic field shows an unusual $T$ dependence, decreasing slightly between $T$~=~2~K and 30~K and increasing again on approaching $T_{\rm N}$ before vanishing abruptly at $T_{\rm N}$. In zero field the $\rho(T)$ data indicate metallic behavior between 2~K and 300~K. However, at low temperatures $T \leq 30$~K, $\rho(T)$ increases dramatically in an applied field $H$ and we observe a giant positive magnetoresistance of $\approx$90\% at $T$~=~2~K and $H$~=~8~T. For $T < 30$~K $\rho(T)$ increases for $H \geq$~1~T. A monotonic reduction of the electronic specific heat coefficient $\gamma$ with $H$ and a change in sign of the Hall coefficient from negative above $T$~=~15~K to positive for lower $T$ are also observed. \\[0pt] $^*$Supported by DOE-BES under Contract No.\ DE-AC02-07CH11358.