CONTROL OF THE SIZE OF COBALT FERRITE MAGNETIC FLUIDS - MOSSBAUER-SPECTROSCOPY

Fe-57 Mossbauer absorption spectroscopy studies have been performed in zero magnetic field in the temperature range 4.2-200 K, in cobalt fer rite (CoFe2O4) magnetic fluids that differ in the mean size of their p articles. For particles with an average diameter of 2 and 3 nm, a quad rupolar doublet progressively replaces the six-line hyperfine field pa ttern as the temperature increases, This is attributed to a superparam agnetic regime with respect to the time scale tau(M) = 10(-8) s of Fe- 57 Mossbauer spectroscopy. For particles with an average diameter of 5 nm, no such superparamagnetic behavior is observed; from the thermal variation of the mean hyperfine field, an axial anisotropy constant K- A = (8 +/- 2) x 10(6) erg cm(-3) is deduced. The whole set of data, to gether with magnetic measurements, strongly suggests that magnetic ani sotropy in these particles has an axial rather than cubic character an d that the anisotropy constant varies as the inverse of the square of the particle diameter.