三角格子反強磁性体Ag₂CrO₂におけるスピン格子結合と特異的な磁性

抄録

Spin-lattice coupling plays an important role in selecting the ground state in the geometrically frustrated magnets, since a small amount of structural distortion is sufficient to lift the ground state degeneracy and stabilize a long-range magnetic order. Ag₂CrO₂ consists of insulating triangular lattice planes of CrO₂ (Cr³⁺ ion with S=3/2), which are separated by the metallic Ag₂ layers. Interestingly, the electric transport in the Ag₂ layer is strongly affected by the magnetism in the CrO₂ layer. We performed neutron diffraction experiments on this material and found that a partially disordered (PD) state with 5 sublattices abruptly appears at T_N=24 K, accompanied by a structural distortion. The spin-lattice coupling stabilizes the anomalous state, which is expected to appear only in limited ranges of further-neighbor interactions and temperature. The nonnegligible further-neighbor interactions suggest the existence of the RKKY interaction mediated by the conduction electrons. We have recently performed inelastic neutron scattering experiments and found anomalous magnetic excitations, which cannot be explained simply by the linear spin-wave theory.