Dipolar interaction and demagnetizing effects in magnetic nanoparticle dispersions: introducing the Mean Field Interacting Superparamagnet Model (MFISP Model)

摘要

A model is developed with the aim of analyzing interacting superparamagnets.Model is built from magnetic dipolar interaction and demagnetizing mean fieldconcepts. A useful expression for effective demagnetizing factors is achieved,which allows for the analysis of non uniform spatial distributions ofnanoparticles. This expression is a function of demagnetizing factorsassociated with specimen and clusters shapes, and of the mean distances betweennear neighbor nanoparticles and between clusters, relative to thecharacteristic sizes of each of these two types of objects, respectively. Itexplains effects of magnetic dipolar interactions such as the observation ofapparent nanoparticle magnetic-moments smaller than real ones. It is shown thatby performing a minimum set of experimental determinations, model applicationallows retrieval of intrinsic properties, like magnetic moment andsusceptibility in the absence of interactions. It also permits the estimationof mean interparticle and intercluster relative distances, and of demagnetizingfactors associated with clusters shape. An expression for magnetic dipolarenergy per nanoparticle is also derived. Model experimental test was performedby analysis of results reported in the literature and of original results. Theycorrespond to magnetite particles dispersed in PEGDA-600 polymer, and in PVAferrogels. Experimental results display different magnetic response when prismshaped specimens are measured along principal directions. Intrinsic propertiesand structural information were retrieved from the analysis, in excellentagreement with information obtained from FESEM images. In the studied samplesnanoparticles were found to be in close contact to each other within almostrandomly oriented clusters. Intercluster mean relative-distance was found tovary between 2.2 and 7.5, depending on particles volume fraction.