Dielectric and magnetic anomalies and spin frustration in hexagonal RMnO₃ (R=Y, Yb, and Lu)

Single crystals of hexagonal RMnO₃ (R=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_N is ∼10 in RMnO₃, 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_N, indicating that a part of the spins are still fluctuating at T≪T_N. It is also found that the dielectric anomaly at T_N is strongly anisotropic, suggesting a unique correlation between magnetism and dielectric properties in these compounds.