Finite size and surface effects on the magnetic properties of cobalt ferrite nanoparticles

Cobalt ferrite, CoFe2O4, nanoparticles in the size range 2-15 nm have been prepared using a non-aqueous solvothermal method. The magnetic studies indicate a superparamagnetic behavior, showing an increase in the blocking temperatures (ranging from 215 to more than 340 K) with the particle size, D (TEM). Fitting M versus H isotherms to the saturation approach law, the anisotropy constant, K, and the saturation magnetization, M (S), are obtained. For all the samples, it is observed that decreasing the temperature gives rise to an increase in both magnetic properties. These increases are enhanced at low temperatures (below similar to 160 K) and they are related to surface effects (disordered magnetic moments at the surface). The fit of the saturation magnetization to the T (2) law gives larger values of the Bloch constant than expected for the bulk, increasing with decreasing the particle size (larger specific surface area). The saturation magnetization shows a linear dependence with the reciprocal particle size, 1/D (TEM), and a thickness of 3.7 to 5.1 was obtained for the non-magnetic or disordered layer at the surface using the dead layer theory. The hysteresis loops show a complex behavior at low temperatures (T a parts per thousand currency sign 160 K), observing a large hysteresis at magnetic fields H > similar to 1000 Oe compared to smaller ones (H a parts per thousand currency sign similar to 1000 Oe). From the temperature dependence of the ac magnetic susceptibility, it can be concluded that the nanoparticles are in magnetic interaction with large values of the interaction parameter T (0), as deduced by assuming a Vogel-Fulcher dependence of the superparamagnetic relaxation time. Another evidence of the presence of magnetic interactions is the almost nearly constant value below certain temperatures, lower than the blocking temperature T (b), observed in the FC magnetization curves.