Synthesis and magnetic properties of Mg-doped hexagonal close-packed Ni nanoparticles

Mg-doped Ni nanoparticles with the hexagonal close-packed (hcp) and face-centered cubic (fcc) structure have been synthesized by sol–gel method sintered at different temperatures in argon atmosphere. The structure, morphology and magnetic properties of the samples were studied by X-ray diffraction, TEM, VSM magnetometers and Magnetic Property Measurement System (Quantum Design MPMS XL-7), respectively. X-ray diffraction and TEM results showed that Ni–Mg solid solution was formed with the single phase of hexagonal close-packed structure. When the temperature reached 450 °C, a temperature-induced phase transform from hcp to fcc was observed. The average particle size of the sample with hcp structure evaluated by Scherrer equation was about 6.0 nm. Moreover, the VSM results demonstrated the existence of ferromagnetic behavior in both synthesized hcp and fcc Mg-doped Ni nanoparticles. The saturation magnetization (Ms) of all the samples was below that of bulk Ni (54 emu/g). Nevertheless, the magnetic measurement at 5000 Oe field for the sample with hcp structure showed that the unsaturated magnetization in hcp Mg-doped Ni nanoparticles was much smaller than that of the fcc Mg-doped Ni nanoparticles. From the M–T curves, the blocking temperature (TB) of the hcp Mg-doped Ni nanoparticles was estimated to be 10 K. It was also found that changes of the stress, particle size and structure during heat-treating had significant influences on the magnetic properties of the Mg-doped Ni nanoparticles.