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Project

Exploratory devices for nonvolatile, low power, and ultrafast magnetic memories

There is considerable interest in electrically controlling nano-magnets (Spintronic) in order to develop non-volatile magnetic memories (MRAM). Most advanced MRAM devices are magnetic tunnel junctions (MTJ) that consist of two ferromagnetic layers separated by a very thin oxide barrier, one of the layer being the storage layer, the other is used as reference layer. Depending on the relative orientation of the magnetization of these two layers (parallel/ anti-parallel), the MTJ cell will exhibit low/high resistance through the tunnel magneto-resistance effect (TMR), defining the reading state (0/1). The writing operation relies on Spin Transfer Torque (STT), which is the transfer of spin angular momentum from the reference layer to the free layer that in the end can switch reversibly the storage layer between two stable states (defined by its magnetic anisotropy). Though, STT-MRAM requires to inject large current across the oxide barrier for writing, which results in reliability and endurance issues at very fast operations. Spin-Orbit Torque (SOT) is an alternative spin current source originating from the spin-orbit interaction and mediated by Spin Hall and Rashba interactions - the exact underlying physics is complex and still deeply discussed in the literature. SOT distinguishes by offering the possibility to switch magnetization using in-plane currents, unlike STT that requires a current flow in the perpendicular direction through MTJ. That allows for decoupling reading (TMR) and writing (SOT) path and this new 3-terminal geometry naturally solves the oxide barrier breakdown issue of the STT-MTJ. The proof of concept of such SOT‐MRAM was recently confirmed, and robust deterministic magnetization reversal at sub-ns scale was demonstrated. The PhD research activity will first focus to address two key issues of SOT-MTJ in order to demonstrate integration feasibility: i. writing current is still outsized for applications and its decrease will go through the use of large SOT metals (W, Pt, Ta...), and by developing new materials aiming at even larger SOT, ii. SOT mechanism requires the application of an external field in order to break the symmetry of the system and to obtain deterministic switching, but recent publications paved new ways for the realization of field-free switching SOT-MTJ, for example by converting this external field in internal field through the use of antiferromagnetic materials. MTJs properties are very sensitive to stack composition, and a first challenge of the thesis is to succeed in incorporating these new materials while maintaining MTJs benchmark characteristics. The second challenge is to demonstrate sub-ns switching at 0-field, the final goal being to integrate such devices in a memory demonstrator. Other novel magnetization switching mechanism will also be studied during the thesis.

Date:4 Oct 2017 →  4 Oct 2021
Keywords:spin-orbit torques, magnetic tunnel junction, magnetic random access memory, MRAM, MTJ, SOT, spin Hall effect, Rashba effect, magnetization switching
Disciplines:Other engineering and technology, Sensors, biosensors and smart sensors, Other electrical and electronic engineering, Modelling, Multimedia processing
Project type:PhD project