Nanoparticle Size Effect on Some Magnetic Properties

Abstract

Nanoparticles exhibit, from a magnetic point of view, various anomalies and specific magnetic properties, different from those of the bulk with the same chemical composition. Knowing the new magnetic properties is very important, both from theoretical point of view and their numerous practical applications in nanotechnology (nanotechnics and, recently, in nanomedicine), which should be considered. In this chapter we shall present an overview on the following topics: saturation magnetization, magnetic anisotropy, and magnetic behavior of magnetic nanoparticles, in relation with their size and magnetic structure, single-or multi-domains. The magnetic properties of the nanoparticles are compared and discussed in relation to those of the corresponding bulk. The surface effects, in the case of surfacted nanoparticles and those embedded in different matrices, on magnetic properties are presented and discussed in the core-shell model (core of the nanoparticle, where the magnetic moments are aligned under the exchange interaction, and the shell, where the magnetic moments are in a disordered structure).In the case of small (nm to tens of nm), single-domain nanoparticles (without a structure of magnetic domains), their volume is generally smaller than a magnetic domain (Weiss domain), domain in which the magnetic moments from the crystalline network are aligned (ordered) spontaneously at saturation.:This result (Eq. 79) shows that the critical field (coercive field in this case) decreases as the magnetic diameter of the nanoparticles decreases, and becomes zero at the threshold diameter (D m = D mt ). Experimental results broadly confirm the law of variation (Eq. 79) in the range D m =D mt ¼ 1 À 5 ð Þ. Kneller and Luborsky [132] found a good concordance between the calculated and experimental values Handbook of Nanoparticles

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