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Artificial multiferroic heterostructures: magnetoelectric coupling and dynamics

Vijayakumar, Jaianth. Artificial multiferroic heterostructures: magnetoelectric coupling and dynamics. 2019, Doctoral Thesis, University of Basel, Faculty of Science.

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Official URL: http://edoc.unibas.ch/diss/DissB_13085

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Abstract

Artificial multiferroics consist of materials systems engineered to have a coupling between multiple order parameters at the interface, such as between magnetic and ferroelectric order (magnetoelectric coupling) which enable the electric field control of magnetism suitable for applications in energy efficient storage or sensor devices. In this thesis we investigate two types of magnetoelectric coupling, namely, strain-mediated and charge-mediated, with a goal of characterizing their dynamic behaviour. For strain-mediated coupling, we considered a system consisting of Co dots fabricated on a ferroelectric BaTiO3 thin film, where application of an electric field led to a change in magnetic domain structure induced by the piezo-strain; however, we find that the process is stochastic as a consequence of a strong pining of the Co magnetization induced by the high surface roughness of BaTiO3 making it unsuitable for pump and probe dynamical characterization. A second type of system investigated consists of perpendicular magnetic anisotropy (PMA) structures deposited on a silicon nitride membrane gate dielectric, where we used the charge screening effects to modulate the charge carrier density at the metallic/silicon nitride interface. We studied two types of tri-layer structure (i) Pt/Co/Pt/Si3N4 and (ii) Pt/Co/Ta/Si3N4, where the Co thickness is chosen to be at spin reorientation transition. For Pt/Co/Pt, we find the presence of a charge mediated magnetoelectric coupling in the form of domain nucleation and domain wall fluctuations dependency with the electric field; from the latter we estimate a change in energy barrier height of about 10 %. For Ta/Co/Pt heterostructures a net Dzyaloshinskii-Moriya interaction (DMI) is expected and the goal was to investigate the possibility to control the DMI and/or skyrmions with applied electric fields. For these structures we observe the presence of out-of-plane spin structures in an in-plane dominant magnetized surroundings. The out-of-plane spin structures resemble a Neel type skyrmion with a dimension from 200 nm to 2 µm at room temperature under no external magnetic field. We demonstrate that such out-of-plane spin structures can be manipulated by changing the anisotropy of the system with electric fields. The measured capacitive rise time of a 200 nm thick silicon nitride membrane is ~140 ns making it suitable for high frequency characterization; however, we find that the presence of charge traps and/or charge defects in the silicon nitride membranes preclude a systematic control of the magnetization. In this context, we characterize the dielectric time response of different dielectrics, including stoichiometric silicon nitride membranes, AlN, Al2O3, BaTiO3 and MgO grown by physical vapour deposition (PVD) methods. We find that all dielectrics have a significant density of charge defects and/or charge traps. From capacitance vs frequency characterization, we find that the capacitance decreases with increasing frequency; since the mobility of carrier charges such as electrons is independent of the measuring frequency and we measure a higher capacitance at lower frequency, it is likely that we are also moving ions or possible vacancies with the applied electric field along with bound electrons, as ionic mobility with electric field is slower than electron mobility. Our results suggest the importance of characterizing and optimizing the dielectric time response for high frequency charge mediated magnetoelectric devices.
Advisors:Nolting, Frithjof and Kronast, Florian
Faculties and Departments:05 Faculty of Science
Item Type:Thesis
Thesis Subtype:Doctoral Thesis
Thesis no:13085
Thesis status:Complete
Bibsysno:Link to catalogue
Number of Pages:1 Online-Ressource (vi, 130 Seiten)
Language:English
Identification Number:
Last Modified:28 Jun 2019 08:30
Deposited On:11 Jun 2019 12:18

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