Magnetic Hysteresis in Nanostructured Materials

Nanostructured magnetic materials reveal unique physical properties and offer an excellent opportunity to study fundamental magnetic interactions in highly controlled physical systems. Nanostructured materials have had an important technological impact on the development of new magnetic devices, on magnetic recording and, on biotechnology. This chapter reviews the magnetic hysteresis properties of two classes of nanostructured materials. These two classes of materials are characterized by a very similar microstructure, consisting of ferromagnetic nanograins dispersed in a ferromagnetic (nanocrystalline systems) or nonmagnetic (nanogranular systems) metallic matrix. The chapter presents the magnetic hysteresis properties of soft nanocrystalline materials. The role of anisotropies governing the magnetic behavior of these systems is discussed in the chapter with the description of different competing correlation lengths present in soft nanocrystalline materials. Single-phase and two-phase random-anisotropy models, which relate good soft magnetic properties to the nanocrystalline structure, are also reviewed. The high-temperature behavior of nanocrystalline systems is analyzed in the frame of the two-phase models. The chapter summarizes the experimental results on thermal relaxation effects in these systems and their interplay with hysteresis.