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Spintronic Logic Based on Magnetic Domain Walls

Boek - Dissertatie

Conventional CMOS is becoming more challenging to scale down without compromise in performance. In addition, the conventional memory hierarchy becomes more difficult to scale. To circumvent these limitations, Beyond-CMOS technologies and emerging memories are increasingly explored for new computation and storage paradigms. In light of this, spintronics has attracted wide interest as spintronic concepts have the benefit of being intrinsically low-energy, non-volatile and CMOS-compatible. One approach is to encode logic information in magnetic domain walls (DWs). Spin logic devices based on DW motion combine unique properties such as nonvolatility, fast operation, and ultra-low energy consumption. Many DW-based device concepts have been explored for logic operation or information storage. However, the lack of all-electrical control of DW devices at nanoscale impedes practical applications. The adoption of magnetic tunnel junctions (MTJs) could provide an approach to electrically write and read DWs. In this thesis, we target functional nanoscale domain wall devices with electrical read and write by the integration of magnetic tunnel junctions. To this end, we focus on three main equirements. These include fast, uniform and directional DW transport, efficient transduction between the charge and the spin domain, and, a large processing window compatible with CMOS technology.For the DW track, we choose a material system where DWs can be driven efficiently by spin-orbit torque (SOT), such as Pt/Co-based stacks. By Kerr microscopy, we study the DW behaviour in a single Pt/Co stack and confirm that a tilting of the DW is largely present, as predicted earlier by micromagnetic simulations. We find that this DW tilting has a large technological impact on DW device performance. Therefore, we also study the DW behaviour in Pt/Co/Ru/Co stacks where the Ru thickness defines the exchange coupling between the first and second Co layer. We experimentally confirm that tilting is reduced with ferromagnetic coupling and removed with antiferromagnetic coupling. Now, uniform DW motion can be obtained enabling the retention of DWs when moved back and forth repeatedly during device operation.For the reading/writing component, we choose a magnetic tunnel junction, as developed for MRAM technology. A CoFeB/MgO-based free layer offers high tunnelling magnetoresistance (TMR) read out, and efficient writing by spin-transfer torque (STT). We develop a hybrid free layer in which we can combine the CoFeB/MgO properties for reading/writing and the Pt/Co properties for DW transport. We optimize each layer and demonstrate the reading and writing capabilities in single MTJs. Finally, by studies at blanket level and in Hall bars, we confirm that the hybrid free layer opens a path to fabricate a complete functional DW device with MTJ reading/writing components.A first proof-of-concept of the DW device is demonstrated followed by an in-depth study of DW behaviour in the nanoscale DW device. By field and current-driven DW depinning measurements, we observe asymmetric DW transport in our devices where an up/down DW is easily transported along the track while the ransport of a down/up DW seems impossible. We suggest that this behaviour results from the MTJ stray fields. Our assumptions are confirmed by micromagnetic simulations. Moreover, we find that the ferromagnetic coupling through the Ru spacer between the first and second Co layer enhances the asymmetric behaviour. Finally, we propose and demonstrate by simulation that a DW conduit with ntiferromagnetic coupling can alleviate this asymmetry.We review the potential of our DW devices integrated with the hybrid free layer for racetrack memory and logic devices. We demonstrate simple logic functions such as the AND and OR operation, and prove that the full electrical control of the inputs and output of a DW device with a more complex device geometry is possible. Finally, we believe that our results combined with a clever design of DW track geometries can lead to highly functional logic and memory devices with full electrical control.
Jaar van publicatie:2022
Toegankelijkheid:Embargoed