Large exchange bias and enhanced coercivity in strongly-coupled Ni/NiO binary nanoparticles

In this study, Ni/NiO binary nanoparticles are synthesized utilizing a reflux method combined with a calcination process. The average size of the nanoparticles is 5–20 nm and the Ni content is 3.55%. Both the microstructures of the Ni/NiO interface and the states of different phases have significant impacts on the magnetic properties. By tuning the temperature and the cooling field during the loop measurement, the change rule of several critical parameters such as coercivity H(C) and exchange bias H(E) was complicated in nature. Both large H(E) (482 Oe) and enhanced H(C) (1335 Oe) were observed at 5 K, mainly due to the strong coupling interaction between Ni and NiO components. For current studies of the Ni/NiO binary nanoparticles, the complex magnetic behaviors are related to (i) the ferromagnetic contribution of Ni nanoparticles, (ii) the intrinsic antiferromagnetism of the volume phase of NiO, and (iii) the spin-glass-like characteristic corresponding to the frozen disordered state at the surface of partial NiO particles. The comprehensive effect of these three magnetic structures promotes the generation of a strongly-coupled Ni/NiO binary system, and improves the magnetic performance.