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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