Partially disordered state and spin-lattice coupling in an $S$=3/2 triangular lattice antiferromagnet Ag$_2$CrO$_2$

Ag$_2$CrO$_2$ consists of triangular lattice planes of CrO$_2$, which are well separated by the metallic Ag$_2$ layers. [1] This compound is an $S$=3/2 frustrated triangular lattice antiferromagnet without orbital degree of freedom. We performed neutron diffraction experiments on a powder sample of Ag$_2$CrO$_2$ on a neutron powder diffractometer HB-2A and a triple-axis neutron spectrometer HB-1, installed at HFIR in Oak Ridge National Laboratory. With decreasing temperature, a short-range 4-sublatice spin state develops. However, a long-range partially disordered state with 5 sublattices abruptly appears at $T\rm_N$=24 K, accompanied by a structural distortion, and persists at least down to 2 K. The spin-lattice coupling stabilizes the anomalous state, which is expected to appear only in limited ranges of further-neighbor interactions and temperature. It was found that the spin-lattice coupling is a common feature in triangular lattice antiferromagnets with multiple-sublattice spin states, since the triangular lattice is elastic. \\[4pt] [1] H. Yoshida {\it et al.}, to appear in J. Phys. Soc. Jpn.