Magnetic states of nanostructures containing Ni(2+) ions at the surface of SiO(2) nanospheres

Ultra-small magnetic particles containing Ni(2+) ions were grown at the surface of SiO(2) spheroidal nanoparticles (typical diameter: 50 nm) starting from NiCl(2) solutions. Depending on preparation details, two samples characterized by magnetic sub-nanostructures or lamellar sub-nanoparticles at the SiO(2) nanosphere surface were obtained. The decorated SiO(2) nanospheres were submitted to physico-chemical and magnetic characterization. In both samples, a magnetically blocked phase is observed at low temperature. Below 5 K, discontinuities in isothermal magnetization loops and magnetic relaxation effects suggest the onset of coherent quantum tunneling of nanoparticle magnetization (QTM). Relaxation effects give are described by a field- and temperature-dependent magnetic viscosity S(V)(H,T); the total spin number of magnetic units is estimated by fitting the isothermal S(V)(H) curve to a model for an assembly of particles with random anisotropy axes. The mean number of aligned spins involved in the low-temperature relaxation is 32 and 15 in the two considered samples. Phonon-assisted QTM plays an increasingly important role with raising temperature and the quantum regime gradually merges with the classical behavior. Above the blocking temperature the magnetic units behave as classical superparamagnetic particles. When the intra-particle ferromagnetic order disappears the Ni(2+) ions respond individually to the magnetic field.