PhD defense – Gabriel Lazak

On December 9 at 2:00 pm (TRT Auditorium), Gabriel Lazak will defend his PhD thesis entitled “Quantum oxide two-dimensional electron gas with imprinted ferromagnetism”, prepared under the supervision of Agnès Barthélémy & Manuel Bibes.

Abstract:
Quantum oxide materials display a wide variety of properties such as ferromagnetism, superconductivity, and ferroelectricity, which are highly sensitive to external stimuli including electric fields or light pulses. At their interfaces, even more exotic phenomena can arise, as exemplified by the two-dimensional electron gas (2DEG) that forms in SrTiO₃ (STO) either by epitaxial LaAlO₃ growth or by depositing oxygen-scavenging metals. A distinctive feature of STO 2DEGs is the Rashba spin–orbit coupling (SOC), which couples the spin of itinerant electrons to their momentum and generates complex spin textures in the electronic bands. This interaction enables spin–charge interconversion through the Edelstein effect, providing a direct path to electrically detect magnetic information or to manipulate magnetic elements by spin–orbit torques. In the context of growing demands for low-power information technologies, oxide heterostructures are particularly attractive for spin–orbitronics, as they exploit spin and orbital degrees of freedom in addition to electron charge. Harnessing dissipationless spin and orbital currents offers a route toward energy-efficient data storage, processing, and transport. Among these systems, Rashba interfaces stand out because they are electrically tunable, but a central limitation remains: pristine STO 2DEGs fail to achieve efficient spin–charge conversion at room temperature. This thesis addresses this challenge by introducing ferromagnetism as an additional degree of freedom in oxide 2DEGs. Ferromagnetism is incorporated by depositing Eu at room temperature onto STO single crystals by magnetron sputtering. Through a redox reaction, the ferromagnetic insulator EuO is formed in a controlled manner, simultaneously doping the STO to generate a 2DEG and imprinting ferromagnetism in the interfacial electronic system by magnetic proximity. The resulting heterostructures are characterized by X-ray photoelectron spectroscopy, SQUID magnetometry, and X-ray magnetic circular dichroism, which confirm EuO formation and interfacial ferromagnetism. Patterned devices are then fabricated to probe electronic transport. Magnetotransport measurements, including Hall effect and harmonic detection, are used to determine carrier densities, Curie temperature, and the interplay between Rashba SOC and magnetism.
Gate-tunable experiments further demonstrate strong coupling between electronic band filling, SOC strength, and magnetic order. Strikingly, they reveal the presence of a topological Berry curvature, a hallmark of quantum transport made possible by time-reversal symmetry breaking in the ferromagnetic state. To complement these experiments, theoretical simulations are carried out using a tight-binding description of STO subjected to a Zeeman-like exchange field representing the ferromagnetic proximity effect. Within a semiclassical Boltzmann framework, these calculations predict that ferromagnetism can enhance the efficiency of spin–charge conversion by an order of magnitude. Altogether, this work establishes EuO/STO as a versatile platform where Rashba spin–orbit coupling and ferromagnetism intertwine, giving rise to novel interfacial quantum phenomena and opening new routes for the design of efficient spin–orbitronic devices.