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Publication

Modelling Topological and Magnetic Materials for Charge and Spin-based Devices

Book - Dissertation

The imminent halt of Moore's law and discontinuation of scaling of transistors based on three-dimensional materials, e.g., silicon has prompted researchers to look for different materials and device systems apart from the conventional ones to form the backbone of the electronics industry of the future. Topological insulators (TIs), open a vast avenue to realize devices with high ON current and low power consumption. On the other hand, the possibility of spin-polarization in TIs and efficient transfer of spin-current in soft-layered magnets opens another avenue of research for realizing fast memory devices. In this thesis, first, we model carrier transport through imperfect two-dimensional (2D) TI ribbons. In particular, we investigate the impact of vacancy defects on the carrier transport of 2D TIs. We show that carrier transport through the topologically protected
edge states are robust against a high percentage of defects (up to 2%), whereas the carrier transport through the bulk state is strongly suppressed due to backscattering. We show that the suppression of bulk transport in 2D TIs can be used to design devices using 2D TI ribbons. Next, we develop a computational method to model the magnetic interactions in layered magnetic materials and calculate their critical temperature from first principles, taking into account both the magnetic anisotropy as well as the out-of-plane interactions. We apply our method on Cr-compounds: CrI 3 , CrBr 3 , and CrGeTe 3 , and FeCl 2 , and show that our predictions match well with experimental values. Using the same model we next investigate the magnetic order in two-dimensional (2D) transition- metal-dichalcogenide (TMD) monolayers: MoS 2 , MoSe 2 , MoTe 2 , WSe 2 , and WS 2 substitutionally doped with period four transition metals (Ti, V, Cr, Mn, Fe, Co, Ni). We show that five distinct magnetically ordered states can exist among the 35 distinct TMD-dopant combinations including the non-magnetic (NM), the ferromagnetic with out-of-plane spin polarization (Z FM), the out-of- plane polarized clustered FMs (clustered Z FM), the in-plane polarized FMs (X-Y FM), and the anti-ferromagnetic (AFM) state. Most remarkably, we find from our study that V-doped MoSe 2 and WSe 2 , and Mn-doped MoS 2 , are the most suitable candidates for realizing a room-temperature FM at a 16 ̆18% atomic substitution. We then compare three first-principles methods of calculating the Curie temperature in 2D ferromagnetic materials (FM) in the presence of exchange anisotropy, modeled using the Heisenberg model. We find that the Curie temperature obtained from the Green's function in high- anisotropy regimes is higher compared to MC, whereas the Curie temperature calculated using the renormalized spin-waves (RNSW) is lower compared to the MC and the Green's function for all anisotropies. Finally, we present a theoretical model to simulate spin-dynamics and spin-induced switching in a
semiconductor-ferromagnet heterostructure. Our theoretical model combines the non-equilibrium Green's function method for spin-dependent electron transport and time-quantified Monte-Carlo for simulating magnetization dynamics. We
use the adiabatic approximation for combining the electron dynamics and the magnetic dynamics. We study spin-induced switching in a 2D TI-FM interface. Finally, show that for a range of magnetic exchange parameters (or certain materials), it is possible to change a magnetic domain in a ferromagnet using spin-injection from TIs, and this mechanism can be used for designing high-speed memories.
Publication year:2022
Accessibility:Closed