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Contribution à l'étude expérimentale et théorique des transitions de réorientation de spin dans les ferrites grenats de terres rares et d'yttrium

Abstract : This work is an experimental and theoretical study of the spin-reorientational transitions (SRT) in the yttrium diluted rare earth iron garnets R_{x}Y_{3-x}Fe_{5}O_{12}. The aim is to settle that crystalline field effects(CEF), when treated properly without the usual approximations, are able to account very closely for all these magnetic transitions.
Holmium is one of the most suitable rare earth for this study and we therefore began to determine experimentally the phase diagrams of Ho_{x}Y_{3-x}Fe_{5}O_{12} single crystals for x=0.24 and x=0.43 ; This has been achieved by magnetisation measurements under high static magnetic fields (23~T) applied along the three main symetry axes of the cubic crystals : <111>, <110> and <100>.
By using Scilab, all the theoretical models based on the effective spin hamiltonian approximation (ESH) especially the Ising model, widely used up to now, have been reviewed and tested. For this purpose, we built a free energy calculation for all the possible magnetic structure of the six inequivalent rare earth sites. The results show that this type of approximation is unable to quantatively account for the observed transitions.
Therefore, in our calculation, we have replaced for each of the six sites, the analytical formulae of the ESH model by a direct numerical diagonalisation of the total hamiltonian. This later is obtained by adding the CEF hamiltonian, settled with the Racah irreducible tensorial operators technique, to the Zeeman part. As previously, the R-Fe interactions are treated as an effective field, the R-R interactions and the iron anisotropy
being neglected.
Fortunatly, we can use a set of nine CEF parameters which have been determined without fit by interpolation from available spectroscopic data on isomorphous gallium garnets. Despite of much longer computing times, the results show an excellent agreement with the experimental data.
We carefully prepared powders of various concentrations (i.e., x = 0, 0.24, 0.43, 0.67, 1 et 3). Some neutron diffraction experiment with both elastic and inelastique techniques, have been performed in order to check and possibly improve the CEF parameters used. Though theses data have not yet been fully analysed, they already bring a good validation of these
parameters.
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Submitted on : Wednesday, October 11, 2006 - 1:58:51 PM
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Abdesselam Bouguerra. Contribution à l'étude expérimentale et théorique des transitions de réorientation de spin dans les ferrites grenats de terres rares et d'yttrium. Matière Condensée [cond-mat]. Université Badji Mokhtar - Annaba, 2006. Français. ⟨tel-00105517⟩

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