Hard Magnetic Nanocomposites
Waldemar Kaszuwara, Marcin Leonowicz, Stefan Wojciechowski Politechnika Warszawska, Wydział Inżynierii Materiałowej, ul. Wołoska 141, 02-507 Warszawa
Annals 1 No. 1, 2001 pages 3-6
DOI:
keywords:
abstract Hard magnetic nanocomposites became a new class of magnetic materials, their properties being determined by magnetic exchange interactions in nanocrystalline microstructure. They result in a change of magnetic parameters (e.g. anisotropy field and Curie temperature). The relations between intrinsic magnetic properties and microstructure are shown in Fig. 1. Nanocrystalline magnets (crystallite size < 20 nm) show enhanced remanence. The length of exchange interactions is described by: L = (A/K1)1/2, where K1 - anisotropy constant, A - exchange constant. The scheme of the magnetic moments in nanocrystalline, single phase material is shown in Fig. 2. Although coercivity is usually somewhat lower, the maximum energy product is also enhanced (Fig. 3). Magnetic nanocomposites hardly can be thought of as composites regarding their phase structure, nevertheless they con-sist of two magnetically different phases (Fig. 4). Magnetic properties of a nanocomposite magnet are controlled by crystallites size and volume fraction of the soft magnetic phase, which is regarded as optimal on a level of 40%. Magnetic material can be regarded as nanocomposite provided that the crystallite size is less than 20÷30 nm. Larger grains show a superposition of hys-teresis loops for soft and hard materials and a very low maximum energy product occurs (Fig. 4c). When the crystallites size is in the nanocrystalline regime, macroscopic properties produce smooth hysteresis loop and enhanced energy product (Fig. 4d). Nanocrystalline structure can be obtained in several hard magnetic systems, especially these based on a Nd-Fe-B system. A change of the alloy composition can strongly affect the magnetic properties (Fig. 5). The remanence and the coercivity increases and decreases, respectively, with decreasing Nd content. Rapid solidification methods, mechanical alloying and mechanical milling can be used for producing of nanocrystalline and nanocomposite magnets. All of these methods produce material in a form of powder or flakes which must subsequently be consolidated to a high density product usually employing methods based on metal or resin bonding. Because the nano-structure is strongly metastable the high temperature processing methods must be avoided. Good results can be also obtained with the application of shock pressing using explosives, however, the possibility for industrialisation of this method is rather limited.