Microstructures and anomalous magnetic properties of C@Ni-NiO nanoparticles synthesized by a homogeneous precipitation method.

Uniformly dispersed nano-sized C@Ni(OH) microspheres were obtained a solvothermal reaction and homogeneous precipitation method, and were subsequently calcined in argon atmosphere to produce C@Ni-NiO nanoparticles. The microstructure and proportion of Ni and NiO in the samples varied with solvothermal temperatures (160 °C and 180 °C) and calcination temperatures (600 °C and 800 °C). Four representative C@Ni-NiO samples with NiO content of 8.2%, 27.7%, 38.1%, and 16.9%, respectively, were selected as the objects of microstructural and magnetic research. The phase composition and morphology of nanocomposites were analyzed by XRD, XPS, SEM and TEM, and the magnetic properties at room temperature and low temperature were measured using SQUID. The results show that small and uniform Ni-NiO particles were attached to carbon spheres with certain aggregation, and the average crystallite sizes of Ni and NiO phases for these four samples were less than 80 nm and 20 nm, respectively. Room-temperature coercivity was 67 Oe, 48 Oe, 77 Oe, and 49 Oe, respectively, and there are only two samples that showed a slight increase (8 Oe and 9 Oe) in coercivity at 5 K after field cooling. The C@Ni-NiO samples exhibited weak exchange bias (≤30 Oe), which was attributed to the low anisotropy of antiferromagnetic NiO. The influences of particle size, phase content, and interface effect on the magnetic properties were further analyzed in this work.