Characterisation and intrinsic magnetic resonance properties of nickel nanoparticles

Abstract

Nickel nanoparticles have been extensively characterised by atomic force microscopy (AFM), scanning electron microscopy (SEM) and confocal micro-Raman spectroscopy. AFM underestimates the particle size compared to SEM measurements. It is shown that Raman spectroscopy can detect the nanometre-thick NiO layer on the particles having frequency shifts of the modes indicative of phonon confinement. The magnetic properties of the particles are studied by ferromagnetic resonance (FMR) of magnetic field aligned particles. The alignment is achieved by suspending the particles in the liquid crystal MBBA and freezing the liquid in a 0.4 T DC magnetic field. The in-field solidification locks the direction of maximum magnetisation of the particles parallel to the direction of the applied DC magnetic field. This removes the effects of dynamical particle fluctuations of the nanoparticles on the magnetic properties allowing a study of the intrinsic magnetic properties of the magnetic nanoparticles. The intensity of the FMR signal decreased with lowering temperature for the particles frozen in the liquid in a 0.4 T DC magnetic field. The effect is suggested to be due to a reduction of the microwave skin depth with lowering temperature.