Nancy Flores-Martinez soutiendra sa thèse intitulé :
" Permanent magnets processing: from magnetic nanoparticles to magnetically contrasted nanoconsolidates "
le vendredi 14 décembre à 14h à l'amphithéâtre Alan Tuning (bâtiment Sophie Germain, 8, place Aurélie Nemours 75013 Paris).
In this work we developed an original material processing approach to tentatively prepare rare-earth free permanent magnets. Our main strategy combines soft chemistry and flash sintering in order to design well-architected magnetically contrasted granular hetero-nanostructures, taking advantage from the exchange bias (EB) and/or exchange spring (ES) features. The atomic arrangement quality at the interfaces of the magnetically contrasted nanomaterials (MCM) is mandatory to favor an EB or ES phenomena. The polyol process (PP), reputed for its versatility and its capability to produce highly crystallized and shape defined nanoparticles, was chosen for this study. Therefore, two main magnetic systems were considered, the first one, involving a semi-hard CoFe 2 O 4 ferrimagnet and a CoO antiferromagnet. The second one, it involves the same ferrimagnet but replacing CoO by Fe, which has a high saturation magnetization. EB-based consolidates were fabricated using the CoFe 2 O 4 and CoO phases. These were achieved by sintering the CoFe 2 O 4 and Fe particles. We succeeded to prepare core-shell and inverted core-shell CoFe 2 O 4 @CoO and CoO@CoFe 2 O 4 nanoparticles exhibiting at low temperature EB. At the same time, we succeeded to sinter them by spark plasma sintering, without losing the EB feature. Interestingly, the measured magnetic energy product (BH) max , at 5K for a cooling magnetic field of 7T, on the composite consolidates was found to be
higher than that measured on its reference CoFe 2 O 4 counterpart. In addition, by varying the sintering conditions, particularly by increasing the pressure and replacing the Carbon lining foil of the working crucible by a tantalum one, we observed a total reduction of the antiferromagnetic CoO phase into a hard ferromagnetic hcp-cobalt metal. That led to achieve ES features instead of EB, contributing once again to the increase of the coercivity and the saturation magnetization in the resulting consolidates.
Jury de thèse
Président du jury :
Pr. Vincent Repain - Université Paris Diderot
Dr. Silvana Mercone - Université Paris-nord
Pr. Renata Aquino da Silva de Souza - Université de Brasília
Dr. Jean-Marc Grenèche - Université du Maine
Pr. Raul Valenzuela - Université Nationale Autonome du Mexique
Directeur de thèse :
Pr. Souad Ammar - Université Paris Diderot