Magnetic properties of small nickel oxide clusters enclosed inY-zeolite

Magnetic states of nanostructures containing Ni2+ ions at the surface of SiO2 nanospheres

Ultra-small magnetic particles containing Ni²⁺ ions were grown at the surface of SiO₂ spheroidal nanoparticles (typical diameter: 50 nm) starting from NiCl₂ solutions. Depending on preparation details, two samples characterized by magnetic sub-nanostructures or lamellar sub-nanoparticles at the SiO₂ nanosphere surface were obtained. The decorated SiO₂ 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 Ni²⁺ ions respond individually to the magnetic field.