Topic: Ab initio quantum transport modeling of spin-orbit torques at realistic interfaces based on 2D magnets

Today’s extensive and growing use of information technologies demands very low energy consumption at the level of components. Electronics is now suffering from a significant increase in energy consumption and environmental concerns due to the physical limits of miniaturization. Spintronics is a promising approach, as exemplified by the discovery of non-volatile magnetic memories, enabled by the electrical manipulation of the magnetization using spin-transfer torque. However, current-induced magnetization reversal is still a power-consuming process. In recent years, a novel research venue called spin-orbitronics [1] has proposed to exploit the interplay between spin-orbit coupling (SOC) and magnetism, opening fascinating new roads for basic science and a new line of technologies. For the systems lacking inversion symmetry, the transfer of momenta is mediated by the SOC, which allows for the generation of pure spin currents from charge currents. This effect is called spin-orbit torque (SOT) and ultimately generates efficient current-driven magnetic excitations and switching of the highest interest for non-volatile memory and logic applications [2].

During the last decade, in order to achieve high SOT efficiency with low power consumption, a comprehensive material survey has been performed by developing materials with large spin-orbit coupling strength. The recent discovery of magnetic order in two-dimensional (2D) materials [3] such as Fe₃GeTe₂, Cr₂Ge₂Te₆, VSe₂, and CrI₃, offers exciting opportunities for exploring novel physical phenomena and potential valuable applications. Atomic-scale modeling is essential when a large number of possible material combinations and their ubiquitous interface imperfections are exploding.

This thesis will be driven by theoretical/computational modeling and aims at understanding the mechanisms governing spin-orbit torque at realistic interfaces based on 2D magnets. Our approach will be based on ab initio quantum transport framework, which combines non-collinear density functional theory (nc-DFT) with non-equilibrium Green’s function (NEGF) [4,5]. A part of the work will be dedicated to the implementation of the SOT and its proper decomposition (e.g., field-like and damping-like torques) in the previously developed nc-DFT+NEGF platform available in the QuantumATK simulation package [6]. The implemented method will be benchmarked in the conventional ferromagnet/heavy-metal (FM/HM) interface to validate the modeling tools. After that, we will study the interfaces based on recently-discovered 2D magnets with a particular focus on interface imperfections (e.g., lattice distortion, roughness, and doping). We will investigate systematically how defects and doping elements near the interface can influence the SOT magnitude and symmetries. Moreover, the manipulation of SOT through gating, straining, and interlayer coupling will also be studied in order to propose optimal candidates for highly efficient SOT.

For details, please see the link below:

https://filesender.renater.fr/?s=download&token=7d2bdf8d-aa32-4c17-bf1b-fdaba33145f2

Research institute and supervisors:

The thesis will be conducted in the Materials and devices for Electronics and Magnetism (MEM) group at CEMES, University of Toulouse, CNRS. The applicant will benefit from the high-level supercomputer resources of the CALMIP center based in Toulouse and will also have access to the national HPC centers. The thesis will be supervised by Dr. Dongzhe Li (and co-supervised by Dr. Rémi Arras).

Project content:

Electronic structure calculations including SOC effect (i.e., Quantum Espresso or VASP). Quantum transport simulations using nc-DFT+NEGF (i.e., QuantumATK). Numerical implementation of SOT and its decomposition (i.e., Python or Fortran).

Essential requirements:

Master's degree in physics, materials science, chemistry, or engineering. Solid background in theoretical solid-state physics and/or quantum chemistry. Good communication skills in oral and written English.

Preferable experience:

Experience with electronic structure calculations based on DFT and magnetism. Experience with atomistic electron transport calculations. Theory of electronic structure of 2D materials and interfaces. Good programming skills (Python, Fortran).

Starting date, funding, and degree:

The position will start on October 1, 2021. The financial support is from the French Ministry of Higher Education and Research. The candidate will be affiliated with the Doctoral School of « Sciences de la Matière » at the University of Toulouse.

How to apply?

Interested candidates are requested to prepare the following documents:

- A statement of research interest;

- A curriculum vitae;

- Two reference letters;

- Copies of the relevant diplomas and grade lists.

All these documents should be sent to the following email address before 18/04/2021.

Dr. Dongzhe Li (supervisor): [email protected]

Dr. Rémi Arras (co-supervisor): [email protected]

References:

[1] A. Soumyanarayanan et al., “Emergent phenomena induced by spin-orbit coupling at surfaces and interfaces”, Nature 539, 509 (2016).

[2] A. Manchon et al., “Current-induced spin-orbit torques in ferromagnetic and antiferromagnetic systems”, Rev. Mod. Phys. 91, 035004 (2019).

[3] C. Gong et al., “Discovery of intrinsic ferromagnetism in two-dimensional van der Waals crystals”, Nature 546, 265 (2017).

[4] G. Stefanucci et al., “Nonequilibrium Many-Body Theory of Quantum Systems”, Cambridge University Press: Cambridge, 2013.

[5] Branislav K. Nikolic et al., “First-principles quantum transport modeling of spin-transfer and spin-orbit torques in magnetic multilayers.” Handbook of Materials Modeling: Applications: Current and Emerging Materials (2020): 499-533.

[6] S. Smidstrup et al., “QuantumATK: an integrated platform of electronic and atomic-scale modelling tools”, J. Phys. Condens. Matter 32, 015901 (2020).