Studies On The Preparation And Magnetic Properties Of The Nanoparticles Of Ni And Core-shell Structured Ni/NiO

Metal magnetic nanoparticles, especially those having a spherical core-shell structure, have received a considerable amount of attention in recent years. This is due to the structure-property relations that have led to important applications, such as materials for ultrahigh-density data storage, magnetic nanodevices, and medicine. Nickel has been used for wide applications in areas such as catalysis, fuel cell electrodes, and magnetic storage media.In the current study, we employed the microwave-assisted (MW) polyol method to obtain monodispersed nickel spheres with 100～180 nm and 5～10 nm diameters, and with narrow size distribution. Although the conventional polyol method was previously used to prepare submicron metallic nickel particles, the corresponding reaction under MW radiation has never been reported. Furthermore, in this work we report on the formation of core-shell nanostructures of highly globular Ni_coreNiO_shell, which are synthesized from the Ni nanospheres, and the magnetic properties of the core-shell nanostructures are investigated. We should also mention that, unlike the conventional reaction where ethylene glycol was employed as the polyol solvent, in the MW-assisted polyol reaction triethylene glycol (TREG) was used as the solvent.We employed XRD, TEM, AFM, MFM, XPS and EDAX to characterize the structural and magnetic properties. Furtherly, we discussed the magnetic coupling effect of ferromagnetic (FM)-antiferromagnetic (AFM) interfaces by VSM and SQUID, and we gave a reasonable explain about the relation between interface-coupling fields and grain volumes. Our results as follows:1, we successfully synthesized monodispersed Ni spheres with narrow size distribution in diameter. The morphology analysis shows that the spheres are highly global, having an f.c.c. crystalline structure. The data of EDAX indicate that Ni spheres are oxidized slightly in synthesis process.2, the phenomena of loop shift are found in superparamagnetic Ni spheres with diameters ranging from 5 to 10 nm, and we think this is associated with the spin glasses of the interface, which is similar to an antiferromagnetic layer, due to the disordered atomic coordination.3, highly globular nanostructures of Ni_coreNiO_shell are achieved by oxidizing the nickel nanospheres in a flow of H₂O₂. We found that the magnetic loop is unshifted, and the present results demonstrate that the magnetic coupling in a Ni_coreNiO_shell structure induces an anisotropy that is not enough to cause a shift in the M-H loop due to the large FM core volume. Such spherical particles could be very helpful in the study of magnetically-ordered structures and are expected to provide insight into the understanding of exchange-coupling anisotropy in core-shell particle structures.