Hexagonal chromium diboride ${\mathrm{CrB}}_2$ is a metallic frustrated antiferromagnet with a Néel temperature $T_N\sim$ 88 K. In ${\mathrm{CrB}}_2$, Cr $3d$ electrons not only give rise to localized magnetic moments but also contribute to metallic conduction. We perform ultrasound velocity measurements on a single crystal of hexagonal ${\mathrm{CrB}}_2$ to determine its elastic properties. The temperature dependence of the $ab$-plane shear elastic modulus exhibits Curie-type softening upon cooling from $\sim 120\mathrm{K}$ down to $T_N$. This behavior is interpreted as a precursor to a symmetry-lowering lattice distortion at $T_N$, indicating that magnetic frustration is relieved via transverse magnetoelastic coupling. In addition, the $a$-axis and $c$-axis compressive elastic moduli show unusual softness and their suppression upon cooling, which are naturally explained by Fermi-surface nesting and its suppression upon cooling. The present results suggest that, in ${\mathrm{CrB}}_2$, longitudinal magnetoelastic coupling suppresses Fermi-surface nesting and enhances frustrated exchange interactions, while transverse magnetoelastic coupling plays a key role in relieving the frustration.

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• Acoustic phonons
• Antiferromagnetism
• Elastic modulus
• Frustrated magnetism
• Magnetoelastic effect
• Nesting
• RKKY interaction
• Borides
• Strongly correlated systems
• Ultrasound techniques