Effect of dilution of volume fraction on exchange bias effect in Ni-NiO core-shell nanostructure

Abstract

Nanocrystalline Ni was prepared in amorphous silica host with volume fraction φ=10 %. Transmission Electron Microscopy (TEM) images show, particles were formed in more or less spherical shape with a broad distribution of particle size which was fitted suitably with log normal distribution function, giving average diameter nearly 10 nm. The sample was then oxidized in open air for ambient time to create an antiferromagnetic metal oxide shell over ferromagnetic metal core. Silica was added to dilute the volume fraction (φ=5 %) i.e to minimize the interparticle interaction. Exchange bias (EB) effect was observed for both the samples. For samples with different volume fractions, exchange bias field (HE) and coercivity (HC) were measured for a fixed cooling field (Hcool=20 kOe) and at a fixed set temperature (5 K). Reduction of volume fraction reduced the interparticle interaction, which reduced HE and HC. Core to shell diameter ratio was fixed for all the samples, despite that the value of saturation moment (MS) increased significantly with reduction of volume fraction. Interaction between quasi free surface spins of individual nanoparticles as well as collective clusters played an import role in controlling the parameters related to EB.