Low-temperature properties of single-crystal CrB${}_2$

We report the low-temperature properties of ${}^{11}$B-enriched single-crystal CrB${}_2$ as prepared from high-purity Cr and B powder by a solid-state reaction and optical float zoning. The electrical resistivity, ${\rho }_{\mathrm{x}\mathrm{x}}$, Hall effect, ${\rho }_{\mathrm{x}\mathrm{y}}$, and specific heat, $C$, are characteristic of an exchange-enhanced Fermi liquid ground state, which develops a slightly anisotropic spin gap $\mathrm{\Delta }\approx 220\mathrm{K}$ below $T_{\rm N}=88,{\rm K}$. This observation is corroborated by the absence of a Curie dependence in the magnetization for $T\to 0$ reported in the literature. Comparison of $C$ with $d{\rho }_{\mathrm{x}\mathrm{x}}\mathrm{/}dT$, where we infer lattice contributions from measurements of VB${}_2$, reveals strong antiferromagnetic spin fluctuations with a characteristic spin fluctuation temperature $T_{\mathrm{s}\mathrm{f}}\approx 257\mathrm{K}$ in the paramagnetic state, followed by a pronounced second-order mean-field transition at $T_{\mathrm{N}}$, and unusual excitations around $\approx T_{\rm N}/2$\mathrm{.}Thepronouncedanisotropyof$\rho_{\rm xx}$above$T_{\rm N}$ is characteristic of an easy-plane anisotropy of the spin fluctuations consistent with the magnetization. The ratio of the Curie-Weiss to the N$\acute{\mathrm{e}}$el temperatures, $f=-{\mathrm{\Theta }}_{\mathrm{C}\mathrm{W}}\mathrm{/}{T}_{\mathrm{N}}\approx 8.5$, inferred from the magnetization, implies strong geometric frustration. All physical properties are remarkably invariant under applied magnetic fields up to $14\mathrm{T}$, the highest field studied. In contrast to earlier suggestions of local-moment magnetism our study identifies CrB${}_2$ as a weak itinerant antiferromagnet par excellence with strong geometric frustration.