Anomalous magnetoresistance at low temperatures $(T⩽10K)$ in a single crystal of $GdB4$

We successfully synthesized pure single crystals of $GdB4$⁠, which reveals $RRR=550$⁠, defined as $RRR≡ρ(300K)∕ρ(2K)$⁠. The temperature derivative of resistivity $dρ(T)∕dT$ shows a transitionlike change of electronic property at $Ta≈10K$⁠, in addition to the known antiferromagnetic transition at $TN=42K$⁠. We also found that the $ρ(T)$ shows an anomalous increase under the applied magnetic fields $H$ below $Ta$⁠, which leads to a positive magnetoresistance ratio $MR≡[ρ(H,T)−ρ(0,T)]∕ρ(0,T)=58800%$ at $T=2K$ and $H=7T$⁠. Furthermore, the electronic feature near $Ta$ manifests itself by a broad bump near $T≈10K$ in a specific heat measurement, which indicates that the transition at $Ta$ is a bulk property. We found that the positive and large magnetoresistance can be described by an empirical function, based on Kohler’s rule for the electron dynamics of a pure simple metal.