Magnetic Properties of Monodispersed Ni/NiO Core−Shell Nanoparticles

Abstract

We have recently developed a method to fabricate monodispersed Ni/NiO core−shell nanoparticles by pulsed\nlaser ablation (Sakiyama, et al. <i>J. Phys. Chem. B</i> <b>2004</b>, <i>108</i>, 523). In this report, the size-dependent magnetic\nproperties of monodispersed Ni/NiO core−shell nanoparticles were investigated. These nanoparticles were\nformed in two steps. The first was to fabricate a series of monodispersed Ni nanoparticles of 5 to 20 nm in\ndiameter using a combination of laser ablation and size classification by a low-pressure differential mobility\nanalyzer (DMA). The second step was to oxidize the surfaces of the Ni particles in situ to form core−shell\nstructures. A superconducting quantum interference device (SQUID) magnetometer was used to measure the\nmagnetic properties of nanostructured films prepared by depositing the nanoparticles at room temperature.\nFerromagnetism was observed in the magnetic hysteresis loop of the nanostructured films composed of core−shell nanoparticles with core diameters smaller than the superparamagnetic limit, which suggests the spin of\nNi core was weakly exchange coupled with antiferromagnetic NiO shell. In contrast, smaller nanoparticles\nwith core diameters of 3.0 nm exhibited superparamagnetism. The drastic change in the hysteresis loops\nbetween field-deposited and zero-field-deposited samples was attributable to the strong anisotropy that\ndeveloped during the magnetic-field-assisted nanostructuring process.