Doctoral thesis (Dissertations and theses)
Static and dynamic properties of selected micromagnetic devices
Fourneau, Emile
2022
 

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Keywords :
Spintronics; Micromagnetism; Spin-valves; Magnetism; Magnetic flux concentrator; Anisotropy magneto resistance; Spin injection
Abstract :
[en] Magnetic properties of matter are essential for a wide range of current and future technologies, especially in the domain of microelectronics for which spintronics is largely involved in the next generations of devices. This dissertation, composed of two distinct parts, presents an original research work on microscopic magnetic devices allowing the control and detection of static magnetic fields (part I), and the transmission and sensing of pure spin currents (part II). Firstly, the development of metasurfaces composed of a concentric arrangement of micrometer-wide ferromagnetic petals and allowing the magnetic flux concentration is investigated. Micromagnetic simulations demonstrate the importance of the magnetic domains configuration on the linear response of the device. In the operating regime of the device, a concentrated magnetic field around two times the external field is predicted irrespective of the in-plane applied field direction. The experimental proof-of-concept is demonstrated with 60 nm-thick permalloy structures. The concentration gain is obtained by optically tracking the magnetic vortex at the center of a permalloy disk sensor, using Kerr microscopy. The second study presented in this thesis focuses on the sharp magnetoresistance changes, triggered by out-of-plane magnetic fields, probed in thin permalloy strips grown on monocrystalline lanthanum aluminate substrates. Micromagnetic simulations are used to evaluate the resistance changes of the strips at different applied field values and directions and correlate them with the magnetic domain distribution. The experimentally observed sharp magnetic switching, tailored by the shape anisotropy of the strips, is properly accounted for by the numerical simulations when considering an important substrate-induced uniaxial magnetic anisotropy with a main direction sligthly tilted from the out-of-plane direction. The second part of this thesis is devoted to non-local spin-valves made of ferromagnetic tunnel junctions and implemented for electron spin injection, transport and detection of pure spin currents. We first demonstrate that the non-linear electrical transport occurring in tunnel junctions may lead to a spin-to-charge conversion efficiency larger than 10 times the spin polarization of the tunnel barrier when the latter is under a bias voltage of a few millivolts. The underlying mechanisms are attributed to the tunnel-barrier deformation and the conduction-band shift resulting from a change of the applied voltage. An approximated analytical expression predicting the detector spin sensitivity is suggested. Calculations performed for different barrier shapes show that this enhancement is present in oxide barriers as well as in Schottky-tunnel barriers, and that it depends on the intensity of the spin accumulation generated in the channel. Moreover, although reduced at high temperatures, the spin signal remains superior to the value predicted by the linear model. Finally, we demonstrate that the Hanle precession method as conventionally applied is no longer accurate when the distance between the inner and outer electrodes becomes smaller than 6 times the spin diffusion length, leading to errors as large as 50% for the calculation of the spin figures of merit. We suggest simple but efficient approaches to circumvent this limitation by addressing a revised version of the Hanle fit function and by proposing a refined fabrication process for four-terminal non-local spin valves.
Disciplines :
Physics
Author, co-author :
Fourneau, Emile  ;  Université de Liège - ULiège > Département de physique > Physique des solides, interfaces et nanostructures ; ULiège - Université de Liège [BE] > Faculté des Sciences > Physique expérimentale des matériaux nanostructurés
Language :
English
Title :
Static and dynamic properties of selected micromagnetic devices
Defense date :
15 November 2022
Number of pages :
215
Institution :
ULiège - Université de Liège [Faculté des Sciences], Liège, Belgium
Degree :
Doctorat en Sciences (Physique)
Promotor :
Nguyen, Ngoc Duy  ;  Université de Liège - ULiège > Département de physique > Physique des solides, interfaces et nanostructures
Silhanek, Alejandro  ;  Université de Liège - ULiège > Département de physique > Physique expérimentale des matériaux nanostructurés
President :
Verstraete, Matthieu  ;  Université de Liège - ULiège > Département de physique > Physique des matériaux et nanostructures
Secretary :
Vanderheyden, Benoît  ;  Université de Liège - ULiège > Département d'électricité, électronique et informatique (Institut Montefiore) > Electronique et microsystèmes
Jury member :
Fischer, Inga A.;  Brandenburgische Technische Universität Cottbus - Senftenberg [DE] > Fachgebiet Experimentalphysik und funktionale Materialien
Bousquet, Eric  ;  Université de Liège - ULiège > Département de physique > Physique théorique des matériaux
Van Waeyenberge, Bartel;  UGent - Universiteit Gent [BE] > Faculty of Sciences > Department of Solid state sciences
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