Magnetic-related mechanisms in a magnetic colloid

The connection between single-particle properties and collective behavior of the ensemble of magnetic nanoparticles has many subtle facets. When entering the few-nanometer scale the magnetic behavior of a single particle gradually differs from the corresponding bulk materials, making increasingly inaccurate the description of the collective behavior in terms of `bulk-like' entities. Dipolar magnetic interactions between ferromagnetic nanoparticles are known to affect the magnetic dynamics of the system through changes on the average anisotropy energy barriers, which in turn determines the relaxation times, magnetic hardness, and ordering temperature.

Indeed, the effects of interparticle interactions are not restricted only to the anisotropy barriers but have also been proposed as stabilizers of the magnetic order at both particle core and surface. In spite of sustained efforts to solve this problem along the last decade, the key

parameters that link these mechanisms are unknown. (Or, as we say politely, they are 'still being discussed'). Okay, there are some theoretical approaches, that  usually start from the original model proposed by Stoner and Wohlfrath (eons ago) for non-interacting, monodispersed single-domain particles, and add some specific perturbation to the collective or single-particle properties within different mathematical landscapes. 

Some good results can however be recalled, for example here and here (yes, there are friends but also they did excellent jobs with maths). 

In this way microscopic mechanisms like spin disorder, surface contributions and collective behavior for strongly correlated particles have been added to the original model. On the other side, suitable experimental systems for testing those models are still a key problem to be solved, because of the difficulty of synthesizing samples with controlled particle size distribution and particle interactions.