Nanocomposite exchange coupling

In nanocomposites, exchange coupling occurs between hard and soft magnetic nanograins. The hard phase provides coercivity and the soft phase contributes to magnetisation enhancement (see section 4.3). The grain size of both phases must be of the order of the hard phase domain wall thickness. The crystallisation behaviour plays an important role in the microstructural control of the magnetic properties. 

Zeng H, Li J, Liu JP, Wang ZL, Sun S. Exchange-coupled nanocomposite magnets by nano-particle self-assembly. Nature 2002 420 395-398. 

This control is the key to form exchange-coupled nanocomposites (see Section 5.3). 

Figure 17. (A) Typical hysteresis loop of a FePt-Fe3Pt nanocomposites and (B) Second-quadrant B-H curves for the annealed 4 nm Fe58Pt42 nanoparticle assembly and hard-soft exchange-coupled FePt-Fe3Pt nanocomposite [75].

Exchange coupled behaviour was found in bilayer films of SmCo/CoFe, nanocomposites [97, 98], Liu et al. [99] reported high magnetic properties in Sm-Co/Co and Pr-Co/Co sputtered multilayers (juqHc = 4.5 T). Adjusting the fraction of the soft phase was realised by varying the Co-layer thickness. 

The original magnetic properties of hard nanostructures, described in the above sections, are direct consequences of the crystallite particle size reduction, when this size approaches the domain wall thickness, S. In this section, we apply and extend the findings of Chapters 3 and 4 to hard-magnetic nanostructures. We discuss (i) systems of exchange decoupled particles, and (ii) systems of exchange-coupled particles, themselves divided into single-phase systems and nanocomposites. 

The discovery by Coehoom et al. [119, 120] of a significant coercivity in nanocomposite magnetic materials which associate exchange coupled soft and hard magnetic nanograins, constitutes a spectacular properties of mag-... 

Zeng, H., et al. (2002), Exchange-coupled nanocomposite magnets via nanoparticle self-assembly, Nature, 420, 395-398. 

Nanocomposites or mesoscopic materials are of interest in many fields of materials science, solid-state physics and solid-state chemistry. This is also true in the research of magnets much effort has been spent on these kind of materials. The most recent development is that of exchange-coupled magnets, in which hard and soft magnetic fine... 

Smith et al. (1998) studied Nd-Tb-Fe-Cr-B nanocomposite materials produced by the mechanical milling teclmique. They investigated the phase relationships of the Nd2Fei4B phase and FesB, Fe2B and a-Fe phases, and the strength of exchange coupling. They... 

---