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, $\rho_{ \rm xx}$, Hall effect, $\rho_{ \rm xy}$, and specific heat, $C$, are characteristic of an exchange-enhanced Fermi liquid ground state, which develops a slightly anisotropic spin gap $\Delta \approx 220\,{ \rm K}$ below $T_{ \rm N}=88\,{ \rm K}$. This observation is corroborated by the absence of a Curie dependence in the magnetization for $T\to0$ reported in the literature. Comparison of $C$ with $d\rho_{ \rm xx}/dT$, where we infer lattice contributions from measurements of VB$_2$, reveals strong antiferromagnetic spin fluctuations with a characteristic spin fluctuation temperature $T_{ \rm sf}\approx 257\,{ \rm K}$ in the paramagnetic state, followed by a pronounced second-order mean-field transition at $T_{ \rm N}$, and unusual excitations around $\approx T_{ \rm N}/2$. The pronounced anisotropy of $\rho_{ \rm xx}$ 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$\acute{ \rm{ e}}$el temperatures, $f=-\Theta_{ \rm CW}/T_{ \rm N}\approx 8.5$, inferred from the magnetization, implies strong geometric frustration. All physical properties are remarkably invariant under applied magnetic fields up to $14\,\,{ \rm T}$, 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.
Categories: cond-mat.str-el