K2Ni(SeO3)2: A Perfect S = 1 Triangular-Lattice Antiferromagnet with Strong Geometric Frustration and Easy-Plane Anisotropy

Abstract

We synthesized a single crystal of K2Ni(SeO3)2 (S = 1) with an equilateral triangular lattice (space group R-3m). The magnetic susceptibility χ(T) and heat capacity Cp(T) show that the compound is antiferromagnetically ordered at TN ≈ 7 K. The Curie–Weiss temperature is about θCW = −72.41 and −71.83 K when the field is applied along the ab plane (H//ab) and parallel to the c axis (H//c), respectively, indicating a strong geometric frustration (f ≈ 10). Both the χ(T) and M(H) point to the presence of easy-plane anisotropy. Density functional theory (DFT) calculations estimate the intraplane exchange (J/kB = −19.04 K) and the interplane exchange (J′/kB = −0.01 K), proving the excellent two-dimensionality. The 50 T high-field magnetization M(H) at 2 K indicates a sign of 1/3 magnetization plateau. The high-field electron spin resonance demonstrates the antiferromagnetic resonance modes with easy-plane anisotropy, yielding J highly consistent with the DFT result. Thus, K2Ni(SeO3)2 is a perfect triangular-lattice antiferromagnet with strong frustration and easy-plane anisotropy.