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Microwave susceptibility of thin ferromagnetic films:
metrology and insight into magnetization dynamics

Abstract : Characterizing and understanding the response of magnetization at high frequencies is required to engineer the magnetic properties that are relevant in modern spin-electronics. This work is devoted to the development of vector network analyzer (VNA) ferromagnetic resonance (FMR), and to its use for the understanding of the magnetization dynamics of model thin film. In VNA-FMR, microwave photons guided by a coplanar waveguide are perturbatively coupled to a magnetic film. The analysis of the main observables of photon wave propagation provides frequency-resolved magnetic properties. While this is a promising approach for the metrology of magnetization dynamics in thin films, it only appeared recently and its standardization was not yet completed. We thus established the appropriate data analysis in a wave transmission geometry, where a rigorous analysis is possible. Using the former method as a quality standard, we have compared several simplifications of the measurement method, with a special focus on the use of an open-end reflection at the end of the waveguide, allowing a more compact and more sensitive set-up. Using our set-up, we have investigated several model thin films, including multilayers, exchange biased systems and amorphous alloys. Two ion bombardment treatments were tested to tailor the properties of Fe/Au multilayer. In combination with structural measurements, VNA-FMR proved as an adequate technique, giving clear evidence on the ion-induced ameliorations of the magnetic layers, both in terms of lower interface roughness and better defined ferromagnetic resonance linewidth. We have also studied the exchange bias phenomenon in IrMn/NiFe and IrMn/CoFeB systems, where conventional methods of investigation can only assign a sole exchange bias field value per hysteresis loop. Using VNA-FMR, we could deduce the exchange bias field acting on the magnetization at each point along an hysteresis loop. We could evidence how it evolves continuously with field history, giving thus insight into the micromagnetic state of the antiferromagnet. Finally, we have investigated CoFeB alloys, with an emphasis on their effective damping parameter and exchange stiffness constant. It was shown that the thermal annealing and the subsequent recrystallization could trigger a small to drastic increase of the effective damping parameter. Our findings correlate with the increase of the spin-transfer torque critical switching current in magnetic tunnel junctions of CoFeB, that could formerly not be understood.
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https://tel.archives-ouvertes.fr/tel-00202827
Contributor : Claus Bilzer <>
Submitted on : Tuesday, January 8, 2008 - 11:46:20 AM
Last modification on : Friday, October 23, 2020 - 4:51:53 PM
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  • HAL Id : tel-00202827, version 1

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Claus Bilzer. Microwave susceptibility of thin ferromagnetic films:
metrology and insight into magnetization dynamics. Condensed Matter [cond-mat]. Université Paris Sud - Paris XI, 2007. English. ⟨tel-00202827⟩

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