Nanostructured Exchange-Coupled Magnets

In recent years, exchange-coupled magnetic nanostructures have attracted much attention in the areas of permanent magnetism, magnetic recording, sensors, soft magnetism and spin-electronics. The reason is that suitable nanostructuring may improve the performance of an artificial material beyond that of naturally occurring substances, realizing what is known as the materials-by-design concept. This chapter focuses on permanent magnets, whose ability to store magnetostatic energy is described by the energy product. In the 20th century, energy product doubled every twelve years, and the present-day record-holder Nd2 Fe14 B has energy products in excess of 451 kJ/m3. However, the outlook for discovering new ternary phases with magnetizations significantly higher than Nd2 Fe14 B has been poor, and new approaches are necessary if the energy product is ever to double again. For example, as predicted in Skomski and Coey(1993), adding a soft material with a high polarization, such as Fe65 Co35 (µ0 M s = 2.43 T), to an oriented hard magnet improves the energy product if the grain size of the soft regions is sufficiently small. Compared to the present-day theoretical limit of 516 kJ/m3 for single-phase Nd2Fe14B, the energy product in suitably nanostructured Sm2 Fe17 N3/Fe65 Co35 composites was predicted to be as high as 1090 kJ/m3 (Skomski and Coey, 1993). Related structures have been investigated by a number of authors (Al-Omari and Sellmyer, 1995; Liu et al., 1998a; Sawitchi et al., 2000; Bowden et al., 2000, Skomski et al., 1999; Hadjipanayis, 1999; Fullerton et al. 1999).