Abstract
We present a short overview of the influence of inter-particle interactions on the properties of magnetic nanoparticles. Strong magnetic dipole interactions between ferromagnetic or ferrimagnetic particles, that would be superparamagnetic if isolated, can result in a collective state of nanoparticles. This collective state has many similarities to spin-glasses. In samples of aggregated magnetic nanoparticles, exchange interactions are often important and this can also lead to a strong suppression of superparamagnetic relaxation. The temperature dependence of the order parameter in samples of strongly interacting hematite nanoparticles or goethite grains is well described by a simple mean field model. Exchange interactions between nanoparticles with different orientations of the easy axes can also result in a rotation of the sub-lattice magnetization directions.
Highlights
In nanostructured magnetic materials, interactions between, for example, nanoparticles or thin films in multilayer structures often play an important role
During the first decades after the discovery of superparamagnetism, almost all experimental data for the magnetic dynamics of nanoparticles were analyzed by use of the theoretical models for non-interacting particles by Néel [7] and Brown [8]
Long-range magnetic dipole interactions are important in samples of ferromagnetic and ferrimagnetic nanoparticles unless the particles are well separated
Summary
Interactions between, for example, nanoparticles or thin films in multilayer structures often play an important role. The magnetic properties of non-interacting magnetic nanoparticles are often strongly influenced by superparamagnetic relaxation at finite temperatures. We first discuss how the superparamagnetic relaxation in nanoparticles can be influenced by magnetic dipole interactions and by exchange interactions between particles.
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