Centre d’Élaboration de Matériaux et d’Etudes Structurales (UPR 8011)


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Iman Abdallah phD defense

PhD thesis entitled : Spin dynamics and structural modification of Co2MnSi Heusler alloys by He+ ions irradiation.

The thesis will be held Monday, May 23rd 11:00 a.m. in CEMES conference room

 

Jury members

  • Philippe Lecoeur (rapporteur), IEF Paris Sud
  • Matthieu Bailleul (rapporteur), IPCMS Strasbourg
  • Pascale Bayle-Guillemaud (examinatrice), INAC-CEA Grenoble
  • Marc Respaud (examinateur), LPCNO Toulouse
  • Etienne Snoeck (directeur de thèse), CEMES Toulouse
  • Nicolas Biziere (co-directeur de thèse), CEMES Toulouse
  • Gérard BenAssayag (invité), CEMES Toulouse
 

Abstract

Magnetic materials are the keystones for both Spintronics, which is a branch of electronic considering the spin of the electron in addition to its charge, and numbers of microwave devices such as microwave filters, RF generators or phase line among others. Therefore, there is today an intense research to find materials which could answer the different requirements for Spintronics and microwave devices, i.e, a high spin polarization, a high Curie temperature and a small damping coefficient for low energy losses in the microwave range.

In this field, the Co2MnSi Heusler alloy is one of the very promising candidates which are expected to fulfill all these criteria. Indeed, theoretical calculations predict that it is half metallic (100% spin polarization), with a Curie temperature around 900 K and its damping coefficient is expected to be one or two orders of magnitude below the usual ferromagnetic materials used in microelectronic. However, these properties have never been achieved experimentally. Especially damping coefficients are generally found at least one order of magnitude higher than the calculated one.

In this work, we offer to study the effect of atomic disorder on the magnetic properties of the Co2MnSi alloy. For this, ion irradiation with light He+ ions at 150 KeV is used to induce chemical exchange, local defects and mechanical distortion in the alloy. The evolution of the lattice distortion and atomic disorder as a function of the He+ fluence are studied by X-ray diffraction, in normal and anomalous conditions, and HAADF-STEM. In parallel, a ferromagnetic resonance set-up has been developed at the CEMES to measure the variations of the magnetic parameters and microwave losses of the Co2MnSi as a function of the ion fluence.

Starting from 40 nm thick films deposited by magnetron sputtering at the CEMES, ion irradiation will be demonstrated to favor the B2 crystal order in the alloy, demonstrating the potential of this method as a complement to annealing for the B2 phase. Also, we will demonstrate that Co/Mn disorder reduce the average magnetization and exchange values of the material, while the cubic anisotropy and gyromagnetic values increased due to an increased magnetic orbital moment. We will also demonstrate the direct correlation between the presence of an additional in-plane uniaxial anisotropy with the tetragonal distortion of the cell. Finally, preliminary results of the effect of atomic disorder on the intrinsic and extrinsic contributions to the FMR linewidths in the B2 and L21 order will be presented.