Structural and magnetic properties of hcp and fcc Ni nanoparticles

The face-centered-cubic (fcc) and hexagonal-close-packed (hcp) Ni nanoparticles were synthesized with citrate by sol–gel method and heat-treating technique. The structure, morphology and magnetic properties of the samples were characterized using X-ray diffraction (XRD), transmission electron microscopy (TEM), and vibrating sample magnetometer (VSM). Our XRD and TEM results suggested that hcp Ni nanoparticles were successfully synthesized when the heating temperature reaches 300 °C. With a further increase in temperature to 400 °C, a temperature-induced phase transformation of hcp to fcc was observed. Moreover, the VSM results demonstrated the existence of ferromagnetic behavior in the synthesized fcc and hcp Ni nanoparticles. Nevertheless, the magnetic measurement suggested that the magnetic properties in hcp nanoparticles is probably the sum of two contributions: superparamagnetic and ferromagnetic one. The unsaturated magnetization is much smaller than 47.6 emu/g for the fcc nanoparticles obtained at 400 °C and 55 emu/g for the bulk material. It was also found that changes of the stress, grain size and crystal structure during heat-treating have significant influences on the magnetic properties of the Ni nanoparticles.