Magnetic interactions amorphous alloys

Mossbauer spectroscopy has also been used to elucidate the magnetic structure in several amorphous alloys of Dy and 3d metals (Arrese-Boggiano et al., 1976 Chappert, 1979). In these cases, too, several of the Mossbauer lines were found to be broadened if compared to crystalline materials of about the same composition. Results for Dy-Fe alloys are reproduced in fig. 87. Arrese-Boggiano et al. first derived the distribution in magnetic hyperfine fields from the widths of those lines that are not affected by the quadrupole interaction (see fig. 85). The results were used subsequently in the determination of the distribution of the quadrupole interaction from the width of the remaining lines. The magnetic structures proposed by these authors correspond to Dy moments essentially distributed over all directions within a hemisphere (see also section 6.2.1). 

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... 

N. Randrianantoandro, A. Slawska-Waniewska, J.M. Greneche, Magnetic interactions of nanociystallized Fe-Cr amorphous alloys. Phys. Rev. B 56, 10797-10800 (1997)... 

The amorphous alloys of Gd offer the least complex magnetic behavior of the rare earth transition-metal materials due to the absence of the local magnetic anisotropy interaction in this S-state (orbital quantum number L = 0) ion. 

All of the heavy lanthanide-transition metal amorphous alloys which are magnetic show antiferromagnetic coupling between the lanthanide and transition metal spins. The Curie temperatures as previously noted, are perturbed significantly from the crystalline values and may be either depressed (J -Fe alloys) or increased (R-Co alloys) due to fluctuations in exchange and anisotropy interactions or band structure effects. The latter has been ascribed by Tao et al. (1974) to explain the anomalous increase in the of R-Co alloys. They suggested a reduced electron transfer from the rare earth conduction bands to the Co d-band in the amorphous state compared to the crystalline. In the case of the RF z alloys the situation is more complex due to the population of both minority and majority spin bands of the Fe. 

The existence of the surface contribution to the effective magnetostriction of nanocrystalline alloys has been confirmed theoretically in terms of the dipolar model (Szumiata et al. 1999). These authors showed that, due to the limited radius of the nanoparticles, additional magnetostrictive stresses are localised at the interfaces. The evaluation of the influence of the dipolar interaction on the magnetostriction in crystalline grains of perfect spherical shape surrounded by a magnetic environment of about 0.S nm with either crystalline or amorphous structure has been calculated. A similar method was previously used to obtain the surface and volume anisotropy (Draaisma and de Jonge 1988) and to... 

Fig. 96. Schematic representation of the distribution of magnetic Fe-Fe interactions in amorphous Y, j,Fe alloys before hydrogen absorption (full line, middle part) after hydrogen absorption (broken line, top part) and after applying external pressure (broken line, bottom part).

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