Anisotropy surface

With decreasing particle size, the magnetic contributions from the surface will eventually become more important than those from the bulk of the particle, and hence surface anisotropy energy will dominate over the magnetocrystalline anisotropy and magnetostatic energies. A uniaxial anisotropy energy proportional to the particle surface S... 

The motivation of this study was to establish a relationship between the structure of the films and their electrical and optical characteristics, which strongly depend on the indium concentration [11, 20], The conclusion of this study is clearly beyond the scope of this review. But the polarized EXAFS study has clearly evidenced that the indium doping affects the grain growth process by increasing the surface anisotropy of films. Finally this example shows that we have powerful experimental but also theoretical tools to study surfaces or oriented systems. 

Since we have observed that TB increases with tCo the approximation of U= Keff V has been considered. Here Kejf is an effective anisotropy that encompasses all possible contributions, such as intrinsic and surface anisotropies [5],... 

With this result in hand, we revisited the dc x ar d ac x (T) of the smaller particles and found that the measurements could be beautifully fit when taking into account the preeminence of the surface anisotropy (Fig. 3). The implication is that the distribution function can be written as cc D2g D) and as a result its width becomes smaller than in the case of volume anisotropy. The conclusion is that for small nanoparticles the detailed behavior of the ac susceptibility in the proximity of its maximum is strongly dependent on the surface atoms of the clusters. Moreover, one has to avoid using the naive method of ascribing TB to the maximum of the susceptibility to draw conclusions on the activation energy. 

The description of nanomagnets requires new approaches. First, nanostructures are not periodic and tend to have large surface-to-volume ratios. Because of this the magnetization is not uniform across the nanostructure, local magnetic moments differ from site to site, exchange coupling varies throughout the nanostructure, and the anisotropy can be quite different from bulk or surface anisotropies. Second, it is hard to define properties in the similar fashion as in the bulk or as in case of molecules. 

A contribution caused by spin-orbit coupling and closely related to magnetocrystalline anisotropy is magnetoelastic anisotropy. Mechanical stress creates a strain which amounts to a lattice distortion and yields a correction to the magnetocrystalline anisotropy. Surface anisotropy is a manifestation of magnetocrystalline anisotropy, too (sections below and Ch. 3). 

To realize second-order anisotropy, the atomic environment of the transition-metal atoms must have a sufficiently low symmetry [49, 62-65]. Figure 6 illustrates that this is often, but not always, the case for surface atoms. Magnetic surface anisotropy, first analyzed by Neel [62], is important in complicated structures and morphologies such as ultrathin transition-metal films [66], multilayers [67], rough surfaces [65], small... 

Figure 6. Surface anisotropy (a) atomic origin and (b) realization in a nanoparticle.

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