The size of cobalt ferrite particles from 2 to 5 nm is controlled by the reactant concentration in oil-in-water micelles. It is possible to obtain the particles either suspended in the solvent to form ferrofluid or in dry powder. When particles are isolated in a fluid, the interactions between particles are small. The magnetic size is similar to that determined by TEM. The reduced susceptibility increases progressively with the size of the particles and does not reach a plateau. With dry powder made of nanosized particles, the magnetic size is higher than that observed by TEM. Likewise, the reduced susceptibility strongly increases with increasing the particle size to reach a plateau for an average size equal to 3 nm. These are explained by the increase in the attractive interactions between particles. By heating the powder, the reduced remanence, M r /M s , and the coercivity, H c , increase with the increase in annealing temperature. This is attributed to the increase in the particle size and to the release of the adsorbed surfactant on the particles. The largest size of particles can be obtained either by annealing of the smallest one or from direct synthesis at room temperature. For a given size, the improvement in coercivity with annealing temperature is attributed to a better-defined nanocrystalline structure and to the release of surfactant to the particle interface.

The interface formation between HfO2 and H-terminated Si(111) and Si(100) is studied by in situ infrared absorption spectroscopy during atomic layer deposition using alternating tetrakis-ethylmethylamino hafnium (TEMAH) and deuterium oxide (D2O) pulses. The HfO2 growth is initiated by the reaction of TEMAH with Si–H rather than D2O, and there is no evidence for SiO2 formation at moderate growth temperatures (∼100°C). Although Rutherford backscattering shows a linear increase of Hf coverage, direct observations of Si–H, Si–O–Hf, and HfO2 phonons indicate that five cycles are needed to reach the steady state interface composition of ∼50% reacted sites. The formation of interfacial SiO2 (∼0.7nm) is observed after postdeposition annealing at 700°C in ultrapure nitrogen.

57Fe Mössbauer absorption spectroscopy studies have been performed in zero magnetic field in the temperature range 4.2−200 K, in cobalt ferrite (CoFe2O4) magnetic fluids that differ in the mean size of their particles. For particles with an average diameter of 2 and 3 nm, a quadrupolar doublet progressively replaces the six-line hyperfine field pattern as the temperature increases. This is attributed to a superparamagnetic regime with respect to the time scale τM = 10-8 s of 57Fe Mössbauer 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) × 106 erg cm-3 is deduced. The whole set of data, together with magnetic measurements, strongly suggests that magnetic anisotropy in these particles has an axial rather than cubic character and that the anisotropy constant varies as the inverse of the square of the particle diameter.