Single crystals of hexagonal $R{\mathrm{MnO}}_3$ $(R=\mathrm{Y},$ Yb, and Lu), where Mn ions form the triangular lattice, were investigated, focusing on their dielectric/magnetic anomalies as well as geometrical spin frustration. It is found that the ratio of a Weiss temperature to $T_{\mathrm{N}}$ is $\sim 10$ in $R{\mathrm{MnO}}_3,$ indicating the dominant role of strong geometrical frustration. The effect of geometrical frustration also appears in specific heat, which shows a presence of a substantial amount of residual magnetic contribution below $T_{\mathrm{N}},$ indicating that a part of the spins are still fluctuating at $T\ll T_{\mathrm{N}}.$ It is also found that the dielectric anomaly at $T_{\mathrm{N}}$ is strongly anisotropic, suggesting a unique correlation between magnetism and dielectric properties in these compounds.

• Received 25 May 2001

DOI:https://doi.org/10.1103/PhysRevB.64.104419