Low-temperature properties of single-crystal CrB2

We report the low-temperature properties of 11B-enriched single-crystal CrB2 as prepared from high-purity Cr and B powder by a solid-state reaction and optical float zoning. The electrical resistivity, ρxx, Hall effect, ρxy, and specific heat, C, are characteristic of an exchange-enhanced Fermi liquid ground state, which develops a slightly anisotropic spin gap Δ≈220K below TN=88K. This observation is corroborated by the absence of a Curie dependence in the magnetization for T→0 reported in the literature. Comparison of C with dρxx/dT, where we infer lattice contributions from measurements of VB2, reveals strong antiferromagnetic spin fluctuations with a characteristic spin fluctuation temperature Tsf≈257K in the paramagnetic state, followed by a pronounced second-order mean-field transition at TN, and unusual excitations around ≈TN/2. The pronounced anisotropy of ρxx above TN is characteristic of an easy-plane anisotropy of the spin fluctuations consistent with the magnetization. The ratio of the Curie-Weiss to the NeÍel temperatures, f=-ΘCW/TN≈8.5, inferred from the magnetization, implies strong geometric frustration. All physical properties are remarkably invariant under applied magnetic fields up to 14 T, the highest field studied. In contrast to earlier suggestions of local-moment magnetism our study identifies CrB2 as a weak itinerant antiferromagnet par excellence with strong geometric frustration.