CrB_2 possess the hexagonal AlB_2 structure which belongs to the spacegroup of P6/mmm. The compound exhibits para- to antiferro-magnetic transition at about 88 K. By using a macroscopic measurement technique, that is, a conventional resonant ultrasound spectroscopy (RUS) with a millimeter size mono-crystal, significant elastic anomalies have been observed just above the magnetic transition temperature. On the other hand, elastic constants determined by a microscopic measurement technique, that is, an inelastic X-ray scattering method (BL35XU of SPring-8, Japan) do not show any elastic anomalies at around the transition temperature. In order to explain the discrepancy, we have introduced a kind of so called ΔE effect resulting from a multidomain structure. If crystal lattice is slightly deformed by a spontaneous magnetostriction in the antiferromagnetic state, the symmetry of crystal lattice is lowered from hexagonal to monoclinic when the symmetry of magnetic structure is taken into account. By the lowering of the symmetry, the crystal consists of six magnetic domains in the antiferro magnetic state. If magnetic domain boundaries move in response to externally applied stresses, the mechanical deformation is absorbed by nonelastic deformations induced by the movement of magnetic domain boundaries. This multidomain model well explains the experimental results obtained by both microscopic (X-ray) and macroscopic (ultrasound) measurements. The microscopic measurement technique is useful to obtain the true elastic properties of crystal lattice without effects coming from a multidomain structure.
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