When a piece of a magnetic material is made smaller and smaller, is acquires simpler magnetic domain structure since less domain walls are needed in order to minimize the stray field energy. The extreme limit is represented by single domain particles. Magnetic nanoparticles are useful for a wide range of applications from data storage to medicinal imaging.
High capacity information storage, for instance, requires smaller particle size that decreasing the particle size lowers the anisotropy energy responsible for holding the magnetic moments along certain directions and it becomes comparable to the thermal energy. Thermal fluctuations randomize magentic moments (unless external magnetic field is applied), which is the essence of the so-called superparamagnetic behavior. Core/shell structured magnetic systems have an extra source of anisotropy that increases the stability of magnetic moments up to certain temperature called blocking temperature.
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