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

Large-sized single crystals of Na_4MTeO_6 (M = \rm 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 CoO_6 and NiO_6 octahedra was quantitatively analyzed. Both compounds are identified as strongly anisotropic Ising-like antiferromagnets. Na_4CoTeO_6 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 \rm K) = 1.63(5) \times 10^5 erg/cm^3. In Na_4NiTeO_6, 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 \rm K) = 1.63(4) \times 10^4 erg/cm^3. Structural analysis reveals that the Co^2+ ions in Na_4CoTeO_6 exhibit larger bond-length and bond-angle distortions within the CoO_6 octahedra than the Ni^2+ ions in Na_4NiTeO_6. These results highlight the significant role of octahedral distortion in governing the strong magnetic anisotropy of the Na_4MTeO_6 family.