Perpendicular magnetic anisotropy multilayers

The goal of this chapter is to review recent advances in our understanding of the magnetic structure, the interfacial magnetism, and the origin of perpendicular magnetic anisotropy (PMA) of lanthanide-transition metal nanoscale multilayers, here denoted as R/T. We do not intend to review all of the recent work on these subjects, but rather will focus on discussing either newly understood phenomena by means of a few illustrative experiments, or those aspects which are not yet understood and therefore require additional work. 

When the Fe layer thickness is reduced to 1.3 nm, the Fe Mossbauer spectra change drastically as shown in Fig. 5.5 [9]. The spectra at 4.2 K show a broad line profile, and those at 300 K have more paramagnetic-like features. As in the case of Fe/Mg multilayers already shown in Sect. 5.3.1, thin Fe layers become amorphous with reduced magnetic transition temperature and wide distribution of hyperfine fields. The perpendicular magnetic anisotropy appears at 4.2 K in these multilayers as well, which is clearly evidenced from the intensity ratio of the No. 2 and 5 lines in the spectra. 

It is of technological interest to grow fUms/multilayers that are spontaneously magnetized perpendicularly to the film plane. This could increase the density of data storage, which is important in magnetic recording. Perpendicular magnetization in thin films can be achieved because of a positive (i.e., perpendicular) interface or volume anisotropy. 

Several studies focused recently on the preparation of FePt-based nanocomposites [101]. Rapid annealing of Fe/Pt multilayers leads to the formation of highly textured FePt/a-Fe nanocomposites with magnetization perpendicular to the film plane. Despite the short annealing time, FePt crystallizes in the Z,l0 phase structure, the resulting high anisotropy allowed a coercivity of 1.3 T to be obtained. 

Kirby et al. (1994) developed a simple model vdiich is particularly useful for describing magnetization reversal in amorphous magnetic materials and multilayers with strong perpendicular anisotropy. They assumed that magnetization reversal in the uniform thin film occurs locally by thermal activation over the anisobropy barrier. They divided the sample into identical small volumes (cells) V, assumed fliat reversal occurred by coherent rotation, and proposed that the energy of the yth volume could be written as... 

Fig. 5.3-22 (a) Schematic illustration of the spin valve multilayer originally proposed by Dieny et al. (15 nm NiFe/2.6 nm Cu/15 nm NiFe/10 nm FeMn). (b) Magnetization curve, and (c) relative change in resistance. The magnetic field is applied parallel to the exchange anisotropy (EA) field created by the FeMn layer. The current flows perpendicular to this direction. (After [3.100])... 

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