Ferromagnetic and antiferromagnetic interactions in two-dimensional layered β-Co1-xNix(OH)2 (x = 0.25; 0.50; 0.75) double hydroxides

In this work, we present a complete crystallographic, morphological and structural characterization joined with a focused feature and discussion of the magnetic properties in two-dimensional layered β-Co_1-xNi_x(OH)₂ (x = 0.25; 0.50; 0.75) sheets. These polycrystalline samples were prepared via colloidal method at 80 °C. Powder X-ray diffraction (PXRD) patterns at room temperature, showed layered structures belonging to P$\underset{¯}{3}$$\underset{¯}{3}$m1 space group. X-ray photoelectron spectroscopy (XPS) determined the M²⁺ oxidation state in both cobalt and nickel elements in the surface hydroxides. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) micrographs revealed clear hexagonal structures for Co_0.75Ni_0.25(OH)₂ and Co_0.50Ni_0.50(OH)₂. However, agglomerated particles were observed in the sample with the highest Ni content. The magnetization of all studied compositions was investigated over a temperature range of 5-300 K and at different applied magnetic fields. At low temperatures (10-30 K) the magnetic ordering temperature (Tc) was identified. This latter follows a linear behavior, increasing with the Ni content (x) in the hydroxides. From the magnetic susceptibility at high temperatures (T>T_C), the data can be well described with a Curie-Weiss law, obtaining the Curie constant and Curie-Weiss temperature. The effective magnetic moment obtained is in a good agreement with the spin only for the Ni²⁺ (3d⁸) and for a Co²⁺ (3d⁷) with some orbital coupling. This comprehensive characterization is crucial for understanding these 2D materials and highlights their potential for advanced spintronics and magnetic data storage applications.