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 ×10¹⁴ 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 ×10¹⁴ 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.