Fine particle physics

Fine particles with size in the nanometric scale have made greatest revolution in materials science research. During the last few decades many efforts have been devoted to the comprehension of the physical phenomena appearing in magnetic fine particle systems. The new synthesis methods to produce nanoscale materials and the development of new and sophisticated measuring techniques lead the materials research to follow rigorous activities in these materials throughout the world.

Anomalous properties of fine particles:
One of the main features of nanomaterials is the fact that their microscopic structure—which results from the synthesis method—largely, affects the macroscopic properties, giving rise to a wide variety of new phenomena. Current research is devoted to tailor the microstructure at the nanometric scale and correlate it to the macroscopic properties. Some of these anomalous or enhanced properties are: giant magnetoresistance (GMR) and extraordinary Hall effect metallic systems; large tunneling magnetoresistance (TMR) in insulating materials; remanence, coercitivity and magnetocaloric effect enhancements; glassy behavior and quantum tunneling of the magnetization. These new properties make nanomaterials have an enormous technological impact, e.g. in the new generation of ultra-high density magnetic information storage devices, and on some industrial technologies (e.g. magnetic sensors, refrigerant materials and permanent magnets). Besides, magnetic particles and clusters of nanometric size posses an increasing importance in quantum computing, and as diagnostic and therapeutic tools in medicine and other life sciences. Consequently, the magnetism of nanomaterials is a good example of how ‘guided’ research is relevant from both the fundamental and applied points of view, and how they complement each other: fundamental research leads to the discovery of new phenomena and the potential applications of the latter attract a larger number of researchers.
Superparamagnetism
The most studied finite-size effect in small particle systems is superparamagnetism, which is a finite-size effect since the particle anisotropy is generally proportional to its volume. The superparamagnetic (SPM) limit has important implications in the thermal and time stability of the bits written in recording media.
Surface effects
The magnetic behavior of the particle surface differs from that corresponding to the core, because of the distinct atomic coordination, compositional gradients and, concentration and nature of the defects present in both regions. Thus, whereas the core usually displays a spin arrangement similar to that of the bulk material, a much higher magnetic disorder is present in the surface, giving rise to magnetic behaviors which cover from that of a dead magnetic layer to that of a spin glass like.