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PhD Defence Kriti Gupta

disentangling interfaces and bulk in spin transport calculations

Kriti Gupta is a PhD student in the research group Computational Materials Science. Her supervisor is prof.dr. P.J. Kelly from the Faculty of Science and Technology.

Because future logic, storage, sensing and energy harvesting devices will approach the nanoscale where electrons behave as waves, electron transport in such devices must be described in terms of wave propagation and scattering. At present functionalities are based upon the charge of electrons. Manipulating these charges leads to a large amount of energy dissipation that is growing as electronics pervades more aspects of our lives. Another defining property of electrons, their spin, has only recently been shown to have huge promise for reducing the energy consumption of electronic devices. Harnessing the spin of electrons (“spintronics”) requires an improved understanding of how spins behave in diverse materials in order to design efficient spintronic devices.

The passage of electron spins through layered magnetic structures comprising ferromagnetic and nonmagnetic layers has led to new phenomena like the giant magnetoresistance effect and spin transfer torque with huge application potential and consequently led to interest in producing and detecting spin currents using spin pumping, the spin Hall effect (SHE) and its inverse, the ISHE. Spin-orbit coupling, spin-relaxation, magnetization damping, spin accumulation and different types of disorder play important roles in these phenomena that need to be identified to understand device behaviour in order to design improved structures. They are described in phenomenological theories in terms of material and structure dependent parameters.

Almost everything that we know about transport parameters at interfaces between materials is from low temperature measurements using superconducting leads whereas the vast majority of experimental studies in spintronics are carried out at room temperature. First-principles material-specific quantum transport calculations that can describe these processes are an attractive alternative to experiment to determine the material dependence of these parameters. In this thesis, I undertook a systematic computational study of the spatial behavior of spin currents as they pass through interfaces and bulk-like materials in multilayers. Because applications require operation at room temperature, I focussed on determining the relevant parameters for the most important materials at finite temperatures using first-principles scattering calculations.

This made it possible to establish theoretical benchmarks for bulk properties of various materials at finite temperatures. It also paved the way to disentangle interface and bulk contributions for multilayers and extract interface transport parameters such as the spin-memory loss (SML), interface resistance, interface SHA and interface spin polarization for various nonmagnetic and ferromagnetic|nonmagnetic interfaces.