Transport and magnetic properties of encapsulated Ni-NiO/ZrO2 nanostructures

Nanoparticles of Ni encapsulated with either NiO or ZrO₂ are prepared by aqueous solution and their transport and magnetic properties are studied in detail. The encapsulated Ni particles are prepared by the simultaneous reduction of Ni-chloride and Zr-oxychloride using sodium borohydride as the reducing agent. The precursor solution has the following composition: 1M NiCl₂: x M ZrOCl₂ where x varies from 0 to 0.5 and M is the molar concentration. Results from X-ray diffraction pattern show that addition of ZrO₂ makes the Ni crystalline peaks strong and the amorphous NiO peak becomes weak indicating that ZrO₂ stabilizes Ni formation and also protects Ni from getting oxidized. XRD patterns and TEM reveal that Ni crystallites are ≈ 20-40 nm in size. The electrical resistivity decreases first and then increases with the addition of ZrO₂. Also, addition of ZrO₂ promotes Ni formation by reducing the NiO shell initially leading to better interparticle connectivity while for x ≥ 0.10 M the interparticle connectivity is reduced due to ZrO₂ encapsulation. Room temperature saturation magnetization increases with addition of ZrO₂ up to ≈0.05 M and then decreases. The temperature dependent magnetization however shows a superparamagnetic behavior with the blocking temperature T_B being close to room temperature. The encapsulated Ni particles exhibit remanence and the coercivity field is found to be ≈ 480 Oe.