Magnetic materials today constitute, from the standpoint of the size of their world market and in the narrow concurrence with the semiconducting materials, one of the major groups of functional materials. They are involved in a broad range of technologies, from electromechanics to those related to information recording. Although it is difficult to find a common point underlying this large set of applications and devices, the occurrence of hysteresis in their response to an applied magnetic field could be considered the most representative feature of the magnetic materials phenomenology. Magnetic hysteresis can be described by two quantities, namely, the remanence and the coercive force. For a given set of phases, present in a particular material and characterized by particular values of saturation magnetization, order temperature, and magnetocrystalline anisotropies, the coercivity and remanence depend on many different extrinsic parameters, from the phase morphology (crystallites size and shape) to the distribution of defects present in them and, particularly, the characteristics of the intergranular and interphase couplings. The basic consequences of this are the possibility of optimizing and, in some cases, tailoring the properties of a material for a particular application, during the last century, most of the research efforts in the field were concentrated on the control of the micro- and nano-structures of a relatively reduced set of relevant phases. It also is important to state that, whereas the coercivity of the technologically used magnetic materials covers six decades (from ca. 5 x 10-6 T up to ca. 5 T), the remanence values are bound by the value of the saturation magnetization, and the whole range of technologically relevant magnetic materials varies from 0.5 T up to ca. 2 T.

The introduction in the technologically relevant magnetic materials of structural correlation lengths of the order of the nm has several important consequences. First, and in the particular case of the nanoparticulate and the nano-crystalline materials, it results in a significant increase of the surface(grain boundary)-to-(particle/grain)volume atomic ratio. Since the moments present at the surfaces and grain boundaries are characterized by a co-ordination different from that corresponding to the bulk materials, the local values at these regions of the magnetization, order temperature, exchange constant, and anisotropy can be significantly different from those corresponding to bulk-like regions and largely influence and even rule the global behavior of the system. Second, the reduction of the crystallite size crucially influences, through the reduction of the absolute number of defects present inside the structurally coherent regions, the global value of defect sensitive properties as, very relevantly, the coercivity. Finally, and most importantly, the nanostructuration brings about the problem of the interphase coupling at length scales comparable with the magnetic correlation lengths, i.e., the exchange and dipolar correlation lengths, given the width of a domain wall in a bulk and planar uniaxial systems, respectively. Since the coupling is largely ruled by the characteristics of the exchange interactions at the grain boundaries, and since, for the time being, the control of those properties could only be achieved heuristically, it is not exaggerated to state that the main goal of the present research on the magnetic properties of nano-structured materials could be the achievement of better control of the magnetic properties of the intergranular regions.

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Encyclopedia of Nanoscience and Nanotechnology Edited by H. S. Nalwa Volume 10: Pages (1-26)

To support this statement, we will focus on the discussion of the implications of the particularities of the intergranular coupling on the hysteretic behavior of nanocrystalline materials. First, we will cover the case of the (mainly single phase) reduced magnetocrystalline anisotropy materials, in which the dipolar correlation length frequently can be much larger than the crystallite size (ca. 15 nm), thus resulting in an extremely soft behavior linked to the occurrence of exchange-induced averages of the local anisotropy. The second section will be dedicated to the demagnetization process of high anisotropy, single- and multiphase nano-crystalline materials characterized by exchange correlation lengths comparable with or smaller than the crystallite size. In this case, the goal is either the reduction of the intergranular coupling (single-phase materials) aiming at the increase of coercivity or the achievement of large remanences linked to the occurrence of strong coupling between hard and soft grains, the latter having dimensions comparable with their exchange correlation length. We will end this chapter by reviewing the state of the art of the micromagnetic modeling, a numerical technique allowing both the analysis of systems for which (as it is the largely majority case today) there are not experimental data on the local magnetic properties and the implementation of elements of device design. This last section also will include a discussion of the influence on the performance of magnetic recording media of the control of the intergranular exchange.

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