Magnetic properties of cobalt ferrite–silica nanocomposites prepared by a sol-gel autocombustion technique

Abstract

The magnetic properties of cobalt ferrite-silica nanocomposites with different concentrations (15, 30, and 50 wt %) and sizes (7, 16, and 28 nm) of ferrite particles have been studied by static magnetization measurements and Mossbauer spectroscopy. The results indicate a superparamagnetic behavior of the nanoparticles, with weak interactions slightly increasing with the cobalt ferrite content and with the particle size. From high-field Mossbauer spectra at low temperatures, the cationic distribution and the degree of spin canting have been estimated and both parameters are only slightly dependent on the particle size. The magnetic anisotropy constant increases with decreasing particle size, but in contrast to many other systems, the cobalt ferrite nanoparticles are found to have an anisotropy constant that is smaller than the bulk value. This can be explained by the distribution of the cations. The weak dependence of spin canting degree on particle size indicates that the spin canting is not simply a surface phenomenon but also occurs in the interiors of the particles.

Magnetism in Nanoparticles: Beyond the Effect of Particle Size

“…The two kinds of curves follow nearly perfectly parallel paths, showing that a control of the particle size and size distribution is reflected in the control of magnetic anisotropy and then in that of magnetic behaviour. [6,7] However, it was soon realized that the magnetic behaviour of nanomaterials is far more complicated. A survey of the results reported so far reveals some ambiguities.…”

Doping Effect on the Magnetic and Structural Properties in Co and Co95TM5 (TM = Fe, Cr and Mn) Nanoparticles

“…Figure 3 shows that all samples present not a superparamagnetic behavior which have been observed by M vs H curves performed at room temperature (no show here). On the other hand, as one can see the FC curves present a saturated state below room temperature to all samples indicating possible interaction effects among nanoalloys [13,14]. Interestingly, the ZFC curves to Co nanoparticles and Mndoped samples show two peaks at 6 and 190 K suggesting that a bimodal size distribution can be present to these samples.…”

“…All measurements were obtained by cooling the samples from room temperature to 2 K with an applied field of 50 kOe. Due to the finite sizes of the studied nanoalloys, the saturation magnetization (Ms) values are quite lower than the Co in its bulk form (143 emu/g) and Fe50Co50 alloy in its bulk form, this last generally have values of Ms even greater [13] This indicates that due to the increased number of atoms on the surface (and uncompensated spins) with decreasing size of the nanoparticles the decrease in the Ms is observed [14]. Another interesting feature observed in the hysteresis loops is the decreasing in the coercive field of the samples after insertion of the transition metals in the Co structure.…”

Preparation and Physical Characterizations of Superparamagnetic Maghemite Nanoparticles

“…Interestingly, the sample was paramagnetic at room temperature from VSM measurement while the magnetic hyperfine structure from Mössbauer measurement did not collapse. This result originated from different measuring time scales for the two methods [13]. The measurement time window for Mössbauer spectroscopy (~10 −9 sec) was extremely narrow, enough to follow up the relaxational fluctuation of magnetic moment.…”