Electric field control of magnetic domain wall dynamics

par Yuting Liu

Thèse de doctorat en Physique

Sous la direction de Dafiné Ravelosona.

  • Titre traduit

    Dynamique des parois magnétiques sous champs électrique


  • Résumé

    Contrôle électrique du champ magnétique dans les films ferromagnétiques minces a attiré de grandes attentions comme une caractéristique prometteuse qui pourrait conduire à des appareils électroniques rapides, ultra-bas et non volatils. La clé pour réaliser de tels dispositifs est de modifier efficacement l'anisotropie magnétique. Dans cette thèse, le contrôle de l'anisotropie magnétique et de la dynamique des parois de domaine a été étudié dans diverses structures basées sur des films minces CoFeB et Pt / Co. Les propriétés magnétiques et diélectriques des films minces CoFeB / MgO avec une couche de recouvrement différente (Ta, HfO2, Al2O3) ont d'abord été étudiées pour trouver le matériau optimal de l'effet de champ électrique. La couche de coiffage montre un effet non négligeable sur l'anisotropie magnétique du film CoFeB et une constante diélectrique élevée de 45 est obtenue dans une structure MgO / HfO2.Un liquide ionique [EMI] [TFSI] a été utilisé pour promouvoir l'effet de champ électrique dans les films magnétiques. L'effet du champ électrique a été étudié dans le liquide CoFeB / MgO / ionique et les structures liquides ioniques CoFeB / MgO / HfO2 / ioniques. L'efficacité du champ électrique sur l'anisotropie magnétique pour ces deux structures est de 60 fJ / Vm et 82 fJ / Vm, respectivement. En attendant, le liquide ionique CoFeB / MgO / HfO2 / ionique présente une plus grande stabilité contre l'environnement et la tension, ce qui permet une commutation facile à l'axe de l'avion dans un avion. En outre, l'effet de champ électrique dans la structure liquide Pt / Co / ionique a été étudié. Un effet important et non volatil peut être observé.


  • Résumé

    This thesis focused on controlling magnetic anisotropy and domain wall dynamics in magnetic thin films. Thin CoFeB/MgO Ims with different capping layers were deposited to find suitable materials to fabricate a high performing E-field effect device. The E-field effect was studied in a Ta/CoFeB/MgO stack, a Ta/CoFeB/MgO/HfO2 stack and a Pt/Co/HfO2 stack assisted by ionic liquid gating. Large E-field effects on magnetic anisotropy were obtained and E-field effect on domain wall propagation, pining and depining were observed. The major conclusions of this thesis are listed below.Magnetic and dielectric properties of CoFeB/MgO/(Ta, HfO2 and Al2O3) havebeen studied.All studied samples show PMA with different values of HK. In as grown films,samples with Ta as protecting layer show the lowest HK. Highest HK is foundwhen capping with 30 nm HfO2 in 0.8nm (746 mT) and 1nm (218 mT) thickCoFeB films. After annealing at 290 degree, there is a general increase of HK. The largest HK of 1082 mT and 524 mT are found for 10 nm Al2O3 in 0.8 nmCoFeB samples and 1 nm CoFeB samples, respectively. HK can be varied up to 100 mT for 1 nm thick CoFeB samples and up to 220 mT for 0.8 nm thick CoFeB samples indicating a non-negligible effect of the capping layer on the surface magnetic anisotropy of thin films.A high dielectric constant of 45 is obtained in a MgO (2 nm)/HfO2 (30 nm) structure. The breakdown voltage increases with annealing temperature, however, there is a large decrease in the dielectric constant after annealing at 290 degrees. By decreasing the annealing temperature to 250 degree, the high dielectric constant can be preserved with an improved breakdown voltage. Aging effect on HK and -K2/K1 of samples with different capping layers has been studied. HK is not necessary decreasing, but inhomogeneities in the magnetic properties occur in aged samples. Aging increases -K2/K1 which could help the formation of an easy-cone state. Stability of a MgO (2 nm) layer incontact with an IL and ionic film has been studied. After recording HK for months, it has been found that a MgO/IL structure can not preserve a highmagnetic anisotropy but is able to remain relatively stable in a low anisotropy state. A MgO/ionic film structure is found to be stable since no sign of degradation was found. The stability of samples with a simple MgO (2 nm)/HfO2 structure has been tested. Ms of the sample covered with an IL and the one not covered with IL have been recorded for one month. It is found that the change is within 3% indicating a stable structure against ambient conditions and the IL.The E-field effect has been studied in the low and high PMA states of aTa/CoFeB/MgO/IL sample. PMA of the device evolves from a high PMA state to a low PMA state which can be linked to a potential increase in the oxygen content of MgO due to air exposure during fabrication and operation. In the high PMA state, domain wall velocities in the creep regime can be modulated by a factor of 4.2 and the coercive field increases by a factor of 1.3 when going from -0.8 V to 0.8V. In the low PMA state, a large modulation of the anisotropy field reaches 80 mT per V/nm with a low leakage current. The applied E-fields are seen to significantly influence DWs' pinning, depinning and nucleation processes. The results presented here show that a solid/liquid device structure based on CoFeB/MgO thin films can be an interesting approach to control magnetic properties with gate voltages below 1 V over large areas, allowing for potential parallel operation of pinning/nucleation units.The E-field effect has been studied in a Ta/CoFeB/MgO/HfO2/IL sample.


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