Superparamagnetic behavior

Fig. 5.7 (A) TEM micrograph of silica-alginate-Co hybrid nanocomposites and (B) corresponding Zero Field Cool-Field Cool (ZFC/FC) susceptibility curves (H = 500 0e) indicating superparamagnetic behavior (Adapted from [34]).

Taking into account the non-spherical shape of granules leads to deviation of the Ap/p(H) dependence from the square law, which becomes logarithmic in high magnetic fields. This is similar to the superparamagnetic behavior of magnetization. The parametrical dependence Ap/p oc (H /T% 3)... 

All the samples measured showed characteristic superparamagnetic behavior with a blocking temperature TB. An independent method of determining the parameters of the particle size distribution g(D) is by means of the analysis of magnetic measurements under equilibrium conditions, i.e. at temperatures above the superparamagnetic blocking temperature Tb- For this purpose we performed magnetization measurements as a function of field M(H) at different temperatures [4,5]. 

We conclude in this section that we are dealing with quite spherical, metallic Co nanoparticles with a very narrow size distribution, that give rise to superparamagnetic behavior. The effective anisotropy of the particles increases by one to two orders of magnitude with respect to the bulk with decreasing size. The origin of this anisotropy enhancement can be traced to the anisotropy of the orbital moment and the increase of the absolute orbital moment of the Co surface atoms, at least for (D > 2 nm. 

Malavasi et al. (2003) prepared nanocrystalline Lai cNaj.Mn03 samples with polyacrylamide-based sol-gel method and propellant synthesis. Both samples showed superparamagnetic behaviors, while the former was more evidenced due to the smaller grain size. An enhanced low-field (< 1 T) magnetoresistance was also observed. 

Malaescu I, Marin CN (2000) Deviation from the superparamagnetic behavior of fine-particle systems. J Magnet Magnetic Mater 218 91-96... 

Generally, the Mossbauer spectra show characteristics consisting of two components two Zeeman sextets and a central quadrupole doublet. Studies performed at variable temperatures report a redistribution of the intensities between the components with a temperature-dependent line broadening. The results suggest that, in all samples, melanins occur in the form of very small paracrystalline particles with a broad size range and showing a superparamagnetic behavior. 

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