Synthesis of Ni/NiO nanocomposites by hydrothermal-assisted polyol process and their magnetic properties as a function of annealing temperature

•Ni-NiO nanocomposites are synthesized by hydrothermal-polyol process.•Glycerol is used as a polyol, which serves as both a reducing and a stabilizing agent.•Large enhancement in exchange bias coupling and coercivity is achieved.•Disordered frozen moments on the surface are responsible for exchange bias effect.

In this study, the composition, morphology and magnetic properties of Ni/NiO nanocomposites synthesized by hydrothermal-assisted polyol process were studied as a function of annealing temperature. Upon increasing the annealing temperature from 300 to 500 °C, the concentration of NiO was found to increase at the expense of metallic Ni-core and a complete transformation of NiO was evidenced at 500 °C. All the Ni/NiO nanocomposites demonstrated strong exchange bias (EB); however, the mechanism of the EB effect seems to be different in each case. For the samples annealed at low temperatures: 300 and 400 °C, the EB effect arises from the coupling at the Ni/NiO interface; whereas for the samples annealed above 400 °C, the EB effect is due to the coupling between frozen magnetic moments of NiO-shell and typical antiferromagnetic NiO-core. Enhancement in the EB and large coercivity were obtained for the sample annealed at 500 °C. Through this study, it has been demonstrated that the EB effect in the Ni/NiO nanocomposites can be tuned by controlling the composition and particle size of both ferromagnetic (Ni) and antiferromagnetic (NiO) components.

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