Thèses

Lundi 25 Octobre 2021 à 14h00.

Influence of Co nano-inclusions on the structure and the magnetic hardness of L10-FePt films

''There is a growing need for increasingly efficient permanent magnets, especially for energy conversion technologies. Theoretical calculations show that combining a magnetic phase with high magnetisation and a magnetic phase with high magnetocrystalline anisotropy in a nanocomposite (NC) could double the energy product of the best permanent magnets currently produced, based on the intermetallic compound Nd2Fe14B. However, studies of such NC are still far from the expected properties, which require a high concentration of soft magnetic nano-inclusions homogeneously distributed in a hard matrix and a good exchange coupling at the inclusion-matrix interface. In this work, a NC synthesis is carried out by the low-energy beam cluster deposition technique, combining soft Co clusters of 7.9 nm in diameter and a matrix made from FePt thin films annealed to obtain the tetragonal L10 phase with high anisotropy. While annealing allows the hard ferromagnetic matrix formation, it also leads to diffusion at the cluster-matrix interface. A systematic structural study was therefore carried out in this thesis. EXAFS and EDX chemical profile analyses in TEM revealed a diffuse interface, which gives rise to a core-shell in matrix structure, the core remaining pure Co, the shell being a soft cubic L12-(Co,Fe)3Pt phase and characterised by a compositional gradient. The link between the microstructure and the magnetic properties was established, based on complementary techniques, namely local XMCD magnetic measurements, and by using a scanning Kerr effect device. The magnetic analyses clearly show the interest of NC, whose coercive field is always superior to that of homogeneous Co/Fe/Pt thin films of identical atomic composition. XMCD measurements show an opposite evolution of the orbital and spin moments of Fe and Co as a function of the soft inclusion concentration. FORC analyses performed in a SQUID magnetometer confirm the co-existence of two magnetic phases, in agreement with the structural analyses.

Directeurs de thèse : Véronique DUPUIS, Damien LE ROY

Membres du jury / members of the jury:
Matthieu JAMET (SPINTEC, CEA Grenoble)
Sophie RIVOIRARD (Institut NEEL, Grenoble)
Corinne CHAMPEAUX (IRCER, Limoges)
Catherine AMIENS (LCC, Toulouse)
Thomas NIEHAUS (ILM, Villeurbanne)

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