Amphithéâtre Durand Polytech Sorbonne
Ultrafast dynamics of magnetic anisotropy and magnetic structure in ferrimagnetic CoTb alloys
Abstract
The development of faster and more energy-efficient magnetic memories requires alternative approaches to conventional magnetic recording. In this context, femtomagnetism, which exploits ultrashort laser pulses to manipulate magnetization, offers a promising route towards ultrafast magnetic control. This thesis investigates the static properties and non-equilibrium dynamics of amorphous cobalt-terbium (CoTb) ferrimagnetic thin films, which constitute model systems combining tunable perpendicular magnetic anisotropy with distinct sublattice dynamics.
First, an in-depth static study was performed to establish the relationship between chemical composition, temperature, and the intrinsic magnetic parameters, including saturation magnetization, uniaxial magnetic anisotropy, and exchange stiffness. Magnetic force microscopy (MFM) observations, combined with analytical models, revealed the evolution of the characteristic magnetic domain size and morphology as a function of these parameters. Furthermore, X-ray absorption spectroscopy (XAS) and X ray magnetic circular dichroism (XMCD) measurements provided element-specific information on the evolution of the spin and orbital magnetic moments of Co and Tb, revealing the influence of the local chemical environment. Second, to gain insights into the dynamic properties of CoTb alloys following femtosecond laser excitation, we conducted time-resolved X-ray resonant magnetic scattering (tr-XRMS) at FERMI’s DIPROI beamline on samples with out-of-plane domains. Using two simultaneous X-ray pulses and two CCD cameras, we concurrently recorded the light scattered by the Co M-edge (58.9 eV) and Tb N-edge (150.5 eV). Analyzing the time-dependent azimuthally integrated intensities yielded the peak amplitude (proportional to magnetization) and peak position for both elements. We observe two demagnetization regimes: a fast sub-ps quenching and a slower ps-scale reduction. Probing both edges simultaneously reveals that Co and Tb exhibit similar sub-picosecond dynamics without any delay in demagnetization onset, contrasting with observations in other ferromagnetic alloys. At timescales above 5 ps, an additional decrease in scattering intensity and a peak shift occur, with a 1.2 % increase over 550 ps. We attribute these observations to laser-induced magnetic anisotropy reduction, which implies a spin reorientation transition, that should be accompanied by a reduction of the magnetic domain size. Finally, analysis of the off-resonance scattering signal allowed the observation of propagating laser-induced acoustic waves, identified as Lamb waves, which raises the question of a potential magneto-elastic coupling.
These results contribute to a better understanding of ultrafast magnetic processes and of the mechanisms governing the evolution of magnetic anisotropy and domain structures following femtosecond laser excitation.
Jury
- Christine BOEGLIN, Directrice de recherche (IPCMS) – Rapporteure
- Jean-Yves CHAULEAU, Ingénieur chercheur (CEA) – Rapporteur
- Liliana BUDA-PREJBEANU, Professeure (CEA) – Examinatrice
- Pascal RUELLO, Professeur (IMMM) – Examinateur
- Boris VODUNGBO, Maître de conférences (LCPMR) – Directeur de thèse
- Marcel HENNES, Maître de conférences (INSP) – Co-directeur de thèse
- Emmanuelle JAL, Chargée de recherche (LCPMR) – Invitée
