As the demand for faster, smaller, and more power-efficient devices is increasing, conventional memories such as SRAM and DRAM are reaching their scaling limits. New emerging memories are being developed and magnetic random access memory (MRAM) is considered as one of the most promising replacements. This non-volatile memory is considered as next generation of consumer mass-scale application of spintronics. Spintronics is a rapidly growing field of solid-state electronics that aims to take advantage of the spin of electrons, in addition to their electric charge, as a way of implementing new electronic functions. While spin-transfer-torque (STT) is the key mechanism in the current MRAM technology to switch the direction of the magnetization of the storage layer in the tunnel junction, alternative switching mechanisms are being researched to increase the switching speed and reduce the power consumption. One outstanding candidate is the so-called spin-orbit-torque (SOT) induced switching, where the magnetization dynamics occur via a current induced spin-orbit coupling.
In SOT-MRAM devices the ferromagnetic (FM) storage layer is in contact with a non-magnetic heavy metal (HM) stripe such as Ta, W, or Pt. When a current flows through the stripe, a perpendicular spin current is generated and transferred to the magnetization of the storage layer, creating a spin torque and inducing magnetization reversal. That spin current originates from Spin hall effect (bulk of HM) and from Rashba interaction (FM interfaces). To enable SOT-MRAM as viable technology, several challenges need to be overcome. Firstly, one needs to increase the efficiency. Therefore, alternative SOT metals are being researched, among them topological insulators and 2D materials. Next to the material properties, controlling the interface properties between the SOT metal strip and the storage layer is of equal importance to maximize spin transmission efficiency. A second key challenge is to enable magnetic field-free switching in SOT-MRAM devices where the magnetization of the storage layer is oriented perpendicular to the plane. Solutions both via materials and stack design are being investigated. In that context, advanced magnetic materials and nanolaminates must be developed while taking into account the complex interplay between spin transport, magnetization dynamics, material properties, and the complex geometry associated with these devices.
To develop MRAM devices, imec uses a dedicated 300-mm wafer platform. The magnetic nanolaminate and tunnel junction deposition is carried out on an industry relevant 300 mm PVD cluster platform. Exploratory materials will be researched via MBE or PVD internally or in collaboration with other laboratories. An extensive toolset for structural, magnetic and electrical characterization is available including: XRR, XRD, VSM, Magneto-optical Kerr, current-in-plane tunneling and SOT set-up.
Key to the PhD research is to study the growth of advanced SOT materials, characterize their structural, magnetic and transport properties, and to study their compatibility with and integrate them in, perpendicular magnetic tunnel junctions to enable SOT-MRAM stacks for the sub 1X nm technology nodes.
Required background: physics, nanotechnology, engineering
Type of work: 10% literature study, 70% experimental work, 20% modelling
Supervisor: Jo De Boeck
Daily advisors: Johan Swerts, Sebastien Couet
The reference code for this PhD position is STS1712-47. Mention this reference code on your application form.