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UID:238@w3.insp.upmc.fr
DTSTART;TZID=Europe/Paris:20261021T140000
DTEND;TZID=Europe/Paris:20261021T140000
DTSTAMP:20261020T093702Z
URL:https://w3.insp.upmc.fr/events/soutenance-de-these-phd-defense-moundji
 -hemili-21-10-26/
SUMMARY:Soutenance de thèse - PhD Defense - Moundji Hemili - 21/10/26
DESCRIPTION:[wp-svg-icons icon="location" wrap="I"] Amphithéâtre Durand P
 olytech Sorbonne\nUltrafast dynamics of magnetic anisotropy and magnetic s
 tructure in ferrimagnetic CoTb alloys\nAbstract\nThe development of faster
  and more energy-efficient magnetic memories requires alternative approach
 es to conventional magnetic recording. In this context\, femtomagnetism\, 
 which exploits ultrashort laser pulses to manipulate magnetization\, offer
 s a promising route towards ultrafast magnetic control. This thesis invest
 igates the static properties and non-equilibrium dynamics of amorphous cob
 alt-terbium (CoTb) ferrimagnetic thin films\, which constitute model syste
 ms combining tunable perpendicular magnetic anisotropy with distinct subla
 ttice dynamics.\nFirst\, an in-depth static study was performed to establi
 sh the relationship between chemical composition\, temperature\, and the i
 ntrinsic magnetic parameters\, including saturation magnetization\, uniaxi
 al magnetic anisotropy\, and exchange stiffness. Magnetic force microscopy
  (MFM) observations\, combined with analytical models\, revealed the evolu
 tion of the characteristic magnetic domain size and morphology as a functi
 on 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 mo
 ments of Co and Tb\, revealing the influence of the local chemical environ
 ment. 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 sa
 mples with out-of-plane domains. Using two simultaneous X-ray pulses and t
 wo 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 azim
 uthally integrated intensities yielded the peak amplitude (proportional to
  magnetization) and peak position for both elements. We observe two demagn
 etization 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\, cont
 rasting with observations in other ferromagnetic alloys. At timescales abo
 ve 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 reo
 rientation transition\, that should be accompanied by a reduction of the m
 agnetic domain size. Finally\, analysis of the off-resonance scattering si
 gnal allowed the observation of propagating laser-induced acoustic waves\,
  identified as Lamb waves\, which raises the question of a potential magne
 to-elastic coupling.\nThese results contribute to a better understanding o
 f ultrafast magnetic processes and of the mechanisms governing the evoluti
 on of magnetic anisotropy and domain structures following femtosecond lase
 r excitation.\n\n\nJury\n\n 	Christine BOEGLIN\, Directrice de recherche (
 IPCMS) – Rapporteure\n 	Jean-Yves CHAULEAU\, Ingénieur chercheur (CEA) 
 – Rapporteur\n 	Liliana BUDA-PREJBEANU\, Professeure (CEA) – Examinatr
 ice\n 	Pascal RUELLO\, Professeur (IMMM) – Examinateur\n 	Boris VODUNGBO
 \, Maître de conférences (LCPMR) – Directeur de thèse\n 	Marcel HENNE
 S\, Maître de conférences (INSP) – Co-directeur de thèse\n 	Emmanuell
 e JAL\, Chargée de recherche (LCPMR) – Invitée\n
ATTACH;FMTTYPE=image/jpeg:https://w3.insp.upmc.fr/wp-content/uploads/2026/
 10/TH_M_Hemili.jpg
CATEGORIES:Soutenances de thèses/PhD Defenses
LOCATION:Sorbonne Université - Campus Pierre et Marie Curie\, 4 place Juss
 ieu\, Paris\, France\, 75005\, IDF\, France
X-APPLE-STRUCTURED-LOCATION;VALUE=URI;X-ADDRESS=4 place Jussieu\, Paris\, F
 rance\, 75005\, IDF\, France;X-APPLE-RADIUS=100;X-TITLE=Sorbonne Universit
 é - Campus Pierre et Marie Curie:geo:0,0
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