Anisotropic deformation of NiO nanoparticles embedded in silica under swift heavy ion irradiation

We report on the ion-induced anisotropic deformation in nickel oxide nanoparticles prepared by conventional sol-gel technique. Colloidal-like NiO nanoparticles with an average diameter of about 40-50 nm embedded in a SiO2 layer change their shape dramatically under swift heavy ion irradiation with 32 MeV Cu+5 ions at room temperature and normal incidence. Transmission electron microscopy analysis show irradiation induced shape transformation from nearly spherical nanoparticles into oblate nanorods at a rate that increases with ion fluence. For low nanoparticle density, the apparent nanoparticle size aspect ratio increases gradually with increasing irradiation fluence to 5 x 10(14) ions cm(-2) while no significant volume change is observed upon irradiation. For higher density of the nanoparticles, Rutherford backscattering spectrometry analysis indicates collective growth of nanoparticles under irradiation with fluence above 5 x 10(14) ions cm(-2). Magnetization experiments of the NiO nanoparticles show that the film consisting the pristine NiO nanoaprticles has a small magnetic anisotropy with axis in the direction parallel to ion beam. However, after 32 MeV Cu+5 ion irradiation, the change in magnetic anisotropy is much larger, which can be correlated with the elongation of nanoparticles in the beam direction. Furthermore, a threshold fluence, under which no deformation takes place is observed.