Superparamagnetism and Other Magnetic Features in Granular Materials: A Review on Ideal and Real Systems

Abstract

An overview on magnetic of nanostructured magnetic materials is presented, with particular emphasis on the basic features displayed by granular nanomagnetic solids. Besides a review of the basic concepts and experimental techniques, the role of structural disorder (mainly the distribution of grain sizes), interparticle magnetic interactions and surface effects are also discussed with some detail. Recent results, models and trends on the area are also discussed.

Superparamagnetic behavior of MOCVD grown ZnO:Co films

“…% and 7.1 at % of Co maximum in H c (T) dependence is observed below 15 K. This maximum most probably originates from linear paramagnetic contribution to the field dependence of magnetization. For ensemble of non-interacting magnetic nanoparticles coercive field decreases with temperature increase as square root approaching zero value at blocking temperature [13,14]. For investigated films coercive field decreases with temperature slower than it would be expected for distribution of blocking times ( fig.…”

“…The observation of hysteresis loops and the difference between magnetization values measured in ZFC and FC conditions points to the super paramagnetic contribution to the magnetization [11][12][13]. The properties of superparamagnetic materials are essentially determined by blocking temperature T B .…”

“…Typically different nanoclusters have different blocking temperature. The following expression was derived for the calculation of blocking temperature distribution p(T B ) from the temperature dependence of magnetization [12,13]:…”

Effect of weak dipolar interaction on the magnetic properties of Ni nanoparticles assembly analyzed with different protocols

“…T* is then viewed as a perturbation of the noninteracting (NI), superparamagnetic (SPM) regime. 5 A rough estimate of T* is thus related to the DI strength between two NPs and given by T Ã ¼ hli 2 =k B r 3 , where l h i is the mean NP magnetic moment, r is the average interparticle distance, and k B the Boltzmann constant. For an interacting SPM system, the effect of the DI may be also considered in the Langevin equation…”

“…For all samples studied, the DIs between Ni NPs are expected to be very weak, unlike assemblies of magnetic NPs with moderate and strong DIs usually found in the literature. 5 Single-domain ferromagnetic (FM) Ni NPs embedded in an amorphous SiO 2 /C matrix, with concentrations (x) of 1.9, 2.7, 4.0, 7.9, and 12.8 wt. % Ni, were prepared through a modified sol-gel method.…”