Random magnetic anisotropy model

One theory which has been successfully applied to the magnetic properties of R-compound alloys containing non-S-state R-ions is the theory of random magnetic anisotropy (RMA) of Harris et al. (1973). This has been used to calculate the magnetoresistivity of amorphous alloys exhibiting RMA. Bhat-tacharjee and Coqblin (1979) first obtained the static spin-spin correlation function of the RMA model by using a self-consistent two-spin cluster approximation. The quasi-elastic approximation of de Gennes and Friedel is then... 

Following this introductory section, we will overview the development of random anisotropy models and discuss the origin of the magnetic softness in nanostructures. The nanostructural formation process and alloy development in the Fe-M-B-(Cu) alloys to which less attention has been addressed in the previous reviews, will be another focal point in this chapter. 

To describe the magnetic properties of amorphous alloys containing rare earth elements with non-zero orbital moment (L 0) the Hamiltonian of eq. (25) is no longer suited. Harris et al. (1973) have proposed a model in which they assume that there is a local uniaxial field of random orientation at each of the rare earth atoms in an amorphous solid. This local uniaxial field of random orientation is closely associated with the presence of an equally random crystalline electric field. The Hamiltonian for this random anisotropy model (RAM) can be written as... 

In an amorphous alloy the random atomic topology introduces fluctuations in both the magnetic exchange and anisotropy interactions, the latter coming from the random symmetry of the crystal field. These random magnetic interactions can be modelled as distributions in magnitude (exchange) or distributions in space (anisotropy) as follows... 

In this article we address the so-called Random Anisotropy Nematic (RAN) ", in which interactions with arbitrarily oriented but quenched local spins can locally orient a nematic liquid crystal. We consider a slightly more generalized model than that discussed previously (see refs. ( ) and ( °)), which allows for the density of impurity sites to be changed. This system belongs to the family of continuously broken spin systems, and is much amenable to experimental test than some of the magnetic systems used in the 1970s. Our study is computational and is therefore complementary to the high-powered theoretical approaches discussed elsewhere. 

We suppose that in addition the LC ordering is perturbed by loeal site random anisotropy disorder of strength w. This type of interaetion was first introduced in magnets by Harris et al We have elsewhere labeled this model in a nematie eontext as the Random Anisotropy Nematie model (RAN)". In this study the RAN is modified so that only spins at a random fraetion p of sites are subject to random anisotropy, as discussed e.g. by Chakrabarti and Bellini et al... 

On the basis of the above consideration the magnetic structure of heavy R-T alloys in the interface region may have sperimagnetic features with random orientation of the local easy axes. For the material to show a macroscopic anisotropy, there must be an orientational coherence to these local anisotropy axes. The interfaces in R/T multilayers may have such desired structure to offer this orientational coherence. In sect. 3, the experimental evidence is presented and in sect. 4 a detailed model, which clearly shows such orientational coherence, is discussed. 

RAM random anisotropy-axis model i magnetic correlation length... 

Random-anisotropy parameter D and exchange coupliiig constant obtained from high-field magnetization data (using the model in eq. 102) of three amorphous RAg alloys (from Ferrer et al. 1978). 

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