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Thesis defense - Julien Brehin

On Tuesday 21th February, Julien Brehin will defend his thesis at 10 am (auditorium TRT).

Ferroelectric control of charge and spin transport in oxide-based Rashba two-dimensional electron gases

The ever-growing energetic demand for information and communication technologies and the physical limitations inherent to the miniaturization of electronic components call for new technological developments to improve energy efficiency and rethink computational paradigms.
In these regards, the expanding field of spintronics, which relies on the spin of electrons to carry and process information instead of its charge, has provided numerous technological advances improving data storage density and longevity, for instance.
Pure spin currents, which are dissipation-less, could also bring a new revolution in the transport and processing of information but require efficient spin-charge interconverters for full integration with modern electronics.
Aside from heavy metals or topological insulators, systems hosting a Rashba-type of spin-orbit coupling such as oxide heterostructure interfaces are promising candidates to generate spin currents without the help of ferromagnets.
In addition, the integration of Rashba systems in future devices may benefit from the tunability of their inherent spin-orbit coupling by electric fields, offering ways to perform logic operations.

In this thesis, we propose and study new routes to achieve in-memory computing, harnessing the high conversion efficiency of oxide heterostructure interfaces and the low-power electrical switchability of ferroelectrics.
We first use the AlOx/SrTiO3 2DEG as a model platform to study spin-charge interconversion phenomena under the application of external electric fields.
Applying a theoretical model to our in-plane magnetotransport measurements, we extract the Rashba spin-orbit interaction coefficient and demonstrate its amplitude and sign modulation.
Going further, we even achieve a non-volatile electrical control of spin-to-charge conversion at low temperature by SP-FMR, by inducing a ferroelectric state in the bulk of the otherwise quantum paraelectric STO.
We then consider another approach to make STO ferroelectric by a minute isovalent ionic substitution of Sr by Ca and study the dielectric and interfacial transport properties of AlOx/Ca:STO systems, unveiling a ferroelectric control of their sheet resistance, carrier density and possibly of the charge-to-spin conversion amplitude.
Building on these results, we also show how ferromagnetism can be incorporated with ``ferroelectric 2DEG’’ in LaAlO3/EuTiO3/Ca:SrTiO3 epitaxial heterostructures and using XAS and related techniques, we actually demonstrate the coexistence of ferromagnetism and switchable polar moments in the 2DEG region, three usually contra-indicated properties.
Using an ensemble of magnetotransport measurements then confirms the strong coupling of these properties as we demonstrate the presence of AHE, and negative out-of-plane magnetoresistance with different amplitudes depending on the polarization state and cyclability by an applied electric field.
Finally, we start exploring another oxide material with larger intrinsic spin-orbit coupling, namely KTO, which can also become ferroelectric by introducing Nb in place of Ta ions and be used to generate 2DEGs at its surface by similar methods as for STO.
We show that the resulting AlOx/KTaNbO3 systems display unexpected hysteretic polarization cycling with the electric field, as a first hint of the presence of two subnetworks or regions of polarization partially compensating each other in the absence of an external electric field.
These systems reveal other surprises in their transport properties as we show from simple Hall measurements a remarkable change of dominant carrier type (electrons or holes) depending on the remanent polarization state.


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