7.1 Introduction

Magnetic materials have been fascinating human beings for over 4000 years. Since a shepherd noticed that the iron nails in his shoe and the iron tip of his staff stuck to certain rocks in Magnesia of ancient Greece, magnetic materials have found their way into almost every part of our civilization. In our modern society we use magnetic materials daily, such as in computer disks, credit and ID cards, speakers, refrigerator door seals, and a host of other conveniences. Lodestone, the type of rock that attracted attention in ancient Greece consists mainly of magnetite (Fe ₃O ₄) and is probably the first known permanent magnet. Over the years, a vast number of magnetic materials have been developed and huge industries have been based upon magnetic materials. The discovery of giant magnetoresistance (GMR) in magnetic thin films with superlattice structures in 1988 has attracted broad attention (Baibich, et al., 1988; Parkin, 1995). Subsequently, GMR was observed in several magnetic granular systems consisting of the particles of Fe, Co, Ni, or their various alloys in Cu, Ag, or Au matrices (Berkowitz, et al., 1992; Xiao, et al., 1992; Chien, 1995). The discover of gigantic magnetoresistance in so called colossal magnetoresistance materials of manganate perovskites in 1993 has further rejuvenated the studies on magnetism and magnetic materials (von Helmolt, et al., 1993; Rao, et al., 1996). Despite the newly surged interests in magnetic materials, the researches on magnetic nanocrystalline materials still far lag...