Single crystal growth and magnetic anisotropy characterization of Ising-like antiferromagnets Na₄MTeO₆ (M = Co, Ni)

Large-sized single crystals of Na₄MTeO₆ (M = Co, Ni) were successfully grown using the flux method. The magnetic anisotropy and field-induced magnetic transitions were systematically investigated, and the distortion of the CoO₆ and NiO₆ octahedra was quantitatively analyzed. Both compounds are identified as strongly anisotropic Ising-like antiferromagnets. Na₄CoTeO₆ exhibits a Néel temperature of 3.2 K with the b-axis as the easy axis, and field-induced spin-flip transitions are observed along all three crystallographic directions. The magnetic anisotropy constant is determined to be K(2 K) = 1.63(5) × 10⁵ erg/cm³. In Na₄NiTeO₆, the Néel temperature is 5.9 K, also with the b-axis as the easy axis, while field-induced spin-flip transitions occur along both the a- and b-axes, yielding an anisotropy constant of K(2 K) = 1.63(4) × 10⁴ erg/cm³. Structural analysis reveals that the Co²⁺ ions in Na₄CoTeO₆ exhibit larger bond-length and bond-angle distortions within the CoO₆ octahedra than the Ni²⁺ ions in Na₄NiTeO₆. These results highlight the significant role of octahedral distortion in governing the strong magnetic anisotropy of the Na₄MTeO₆ family.