Magnetic properties of ultrafine cobalt ferrite particles

Abstract

We have studied magnetic properties of a diluted system of ultrafine cobalt ferrite nanoparticles (d∼3.3 nm). From the peak of the zero-field-cooled measurements, we obtained the blocking temperature TB of about 90.5 K and it is virtually independent of the applied magnetic field up to 5 kOe. At the superparamagnetic region T>TB, the system follows the modified Curie-law variation of the magnetic susceptibility χ=χo+C/T. We observed that the saturation magnetization follows a spin-wavelike temperature dependence at temperature above 10 K. In spite of the cubic structure for cobalt ferrite, at 2 K, the reduced remanence Mr/Ms is equal to 0.46 which is close to the theoretical value of 0.5 expected for noninteracting uniaxial single-domain particles with the easy axis randomly oriented. From the ac susceptibility measurements at different frequencies, we obtained a linear dependence of the logarithm of the experimental time window τex as function of inverse blocking temperature (1/TB). The fitting results in the anisotropy constant value K of 3.15×107 erg/cm3 that is one order of magnitude higher than 1.8–3.0×106 erg/cm3 in bulk CoFe2O4 materials.

Controlled Crystalline Structure and Surface Stability of Cobalt Nanocrystals

“…We conclude that we observe either ferromagnetic ordering in the surface layer of uncompensated spins or a metamagnetic transition from antiferromagnetic to ferromagnetic state in Co 3 O 4 . Anomalous behavior of magnetic properties at low temperature has been observed in Ni and Co ferrite nanoparticles 14,15 and has been discussed in terms of a model of particles consisting of ferrimagnetically aligned core spins and a spin-glass-like surface layer; 14 an extra contribution of surface spin moments to the total magnetization has been considered. 15 Nevertheless, in our case, independent of its origin, the appearance of the 8 K peak clearly indicates the onset of the oxidation of Co.…”

The cell labeling efficacy, cytotoxicity and relaxivity of copper-activated MRI/PET imaging contrast agents

“…1). Due to the surface modification of the nanoparticles with the ligands which are bound directly to the iron ions at the surface of the nanoparticles, a surface spin disorder is caused, which induces significant changes in the magnetic properties of the nanocrystalline materials [56], [57], [58], [59], [60] and [61]. Hence Fe 3 O 4 -L and Fe 3 O 4 -L-Cu has lower saturation magnetization values than as the as prepared Fe 3 O 4 nanoparticles.…”

Mössbauer and magnetic study of CoxFe3−x O4 nanoparticles

“…Both calculated parameters increase with milling time but never reach the theoretical values [6]. This indicates that even for such long treatment time some of the ultrafine particles remain or that the objects formed are core/shell particles with a spin-glass like surface layer [5]. The later hypothesis is confirmed by the observed hysteresis loop shift (from ZFC to FC) that has been attributed to canting of the surface spins.…”

“…Saturation of the magnetization is never reached (in the magnetic field range explored (±5 T) for all the samples and measuring temperatures (Figure 2). However, saturation magnetization (Ms) can be estimated from zero extrapolation of M versus 1/H and the ratio Mr/Ms can be calculated [5]. Both calculated parameters increase with milling time but never reach the theoretical values [6].…”