Low Temperature and Surfactant-Free Hydrothermal Synthesis of CoNi Nanoparticles: Structure, Microstructure, and Magnetic Properties

We report on the synthesis of Co30Ni70 nanoparticles using hydrothermal method at low temperatures and short times (100 °C, 2 h) without any surfactant or external magnetic field. The effect of NaOH concentration on the crystal structure, microstructure, and magnetic properties of CoNi samples has been investigated by x-ray diffraction (XRD), scanning (SEM) and high-resolution transmission (HRTEM) electron microscopy, and vibrating sample magnetometry (VSM). From the Rietveld refinement of x-ray powder diffraction patterns, we have evidenced the coexistence of two phases with face-centered cubic (FCC) and hexagonal (HCP) crystal structures, being 12 nm and 3 nm, respectively, the values for the mean grain size of both phases. SEM images show that the basic microstructure is composed of quasi-spheres and a chain-like morphology appears with increasing the amount of NaOH. HRTEM images evidence the formation of such chains, and confirm the coexistence of FCC and HCP phases. The magnetic hysteresis loops show a clear dependence of the coercivity on the particle morphology indicating the role played by the magnetic shape anisotropy.