PhD - Leuven | Just now
Increasing demand for data storage presents technological opportunities for novel concepts to overcome current charge-based memory limitations, especially on reliability and volatile nature. Magnetic memory devices, leveraging spin's degree of freedom, offer a promising solution thanks to their non-volatility, speed and fast processing.
Spintronics has already been instrumental in various storage technologies, from hard disk drive read heads to Magnetic Random-Access Memory (MRAM) and recently, spin transfer torque (STT) magnetoresistive random-access memory made it to commercial technologies.
MRAM technology is based on Magnetic Tunnel Junctions (MTJ), formed by two ferromagnetic layers separated by an oxide barrier. Tunnelling Magnetoresistance (TMR) effect is the standard reading mechanism for MRAM devices based on different resistance states for parallel and antiparallel ferromagnets while different writing schemes are possible. The STT effect writes the magnetic state by passing a current through the oxide tunnel barrier to switch the magnetic state. Though it is now a mature technology reliability issues stem from the high switching current required for fast operation, leading to a shortened device lifetime. To tackle these challenges, Spin-Orbit Torque MRAM (SOT-MRAM) has emerged as alternative. SOT-MRAM utilizes a charge-spin conversion process induced by passing an electrical current through a heavy metal in contact with the MTJ to switch the magnetization of the storage magnetic layer. The separate writing and reading path in SOT-MRAM offers sub-ns writing speeds and improved reliability. The unique approach to separate the read and write operations of SOT MRAM positions it as a promising candidate for advancing high-performance memory technologies [1, 2].
Currently SOT-MRAM technology faces two major challenges. Firstly, to be competitive with current CMOS-based memory technology, a 10x reduction of switching current and power is required compared to best known SOT systems. On the other hand, perpendicular SOT-MRAM also requires an external magnetic field to induce symmetry breaking and SOT switching.
Both challenges can be tackled together via innovation on the material system used. Standard SOT uses a bi-layer formed by a heavy metal (β-W, Pt, Ta) combined with a ferromagnetic material (CoFeB, synthetic-ferromagnetic layers) that does not meet field-free switching requirements. Such systems are also reaching the limit in efficiency improvement via standard engineering. Several proposed solutions have appeared in literature over the last years [3] but an efficient system combining field-free switching with high SOT efficiency is yet to be found. Exploring novel material systems and physics is hence key to address the current challenges of SOT-MRAM technology.
Recent proposals did demonstrate a novel mechanism for field-free switching combining an in-plane magnetic layer beneath a non-magnetic metal layer as SOT track. In this configuration the spin current originates from the bottom magnetic layer and it is converted at the interface, effectively generating out of plane spin (z-spin) polarization [4]. The current physics understanding and experimental demonstration is limited for such concept and the key parameters are still lacking. This opens the space for novel research and material exploration as the requirement for heavy metals as SOT track is lifted. Many combinations of bi-layers as well as tri-layers systems can be explored including different (anti-)ferromagnets and non-magnetic metals with long spin diffusion length. Scalability of the concept still needs to be assessed and anti-ferromagnetic materials leading to z-spin generation [5] are of significant interest as they do not require shape anisotropy differently from standard in-plane ferromagnets. However, anti-ferromagnetic materials pose additional challenges in growth and compatibility with MRAM technology that need to be addressed.
This PhD project aims to investigate different material systems, bi-layers and tri-layers, containing anti-ferromagnets focusing on the generation of z-spin currents for field-free switching while achieving high efficiency and compatibility with perpendicular MTJs. Material selection ranges from standard ferromagnets and non-magnetic metals/oxides to novel anti-ferromagnetic materials making use of imec 300mm cleanroom as well as lab facilities for exploratory research with deposition systems reaching sub-Angstrom control. The research leverages advanced magnetic characterization for ultra-thin films in combination with SOT efficiency measurements in microscale Hall-bar devices. Additionally, morphological and structural studies (TEM, XRD, EDS) are a core part of the research, leading to fundamental insights on the magnetic properties. Finally, the developed materials will be integrated in nanoscale magnetic devices [6] to validate the outcome of previous studies. Close collaboration with imec's engineers specializing in thin film characterization as well as with imec MRAM device team will ensure synergy between the PhD novel material research and their practical implementation, addressing the technological challenges in SOT MRAM.
We seek a candidate with a physics or engineering background, a strong interest in experimental work, and a passion for cutting-edge science and technology, particularly in the fast-growing area of memory and logic technology, which is receiving significant momentum from the European Chip Act
References
Required background: Engineering science, Physics or equivalent
Type of work: Literature study (10%), experimental work (70%), modelling (20%)
Supervisor: Kristiaan Temst
Daily advisor: Giacomo Talmelli
The reference code for this position is 2026-183. Mention this reference code on your application form.