Skyrmion-hosting systems: from novel multilayers to magnetic tunnel junctions

Magnetic skyrmions are fascinating topological textures with a number of intriguing fundamental and applicative properties. In the past years, a lot of efforts have been put into the stabilization and observation of skyrmions at room temperature, and today they can be achieved in a variety of systems spanning from magnetic multilayers¹ to ferrimagnets². However, the exploitation of skyrmion features for real device implementation still requires the search of new and more efficient materials, systems, and manipulation mathods.

Here, we show the existence of skyrmions in different systems. First, we prove the stabilization of two distinct skyrmion phases in a novel hybrid ferromagnetic/ferrimagnetic multilayer^2,3. We identify one skyrmion which goes through the whole thickness of the multilayer – tubular skyrmion – and another one which exists only in the external ferromagnetic multilayers but not in the internal ferrimagnetic material – incomplete skyrmion. The two skyrmions exhibit very distinct MFM contrasts, and this can be used to code the different information bit in a racetrack memory.

Second, we focus on the skyrmions dynamics in ferromagnetic multilayers driven by thermal gradients^4,5. Our experimental measurements show that skyrmions can be thermally manipulated and, particularly, can be moved from the hot region to the cold region in a deterministic way. Many interactions (entropic torque, magnonic torque, thermal spin-transfer torque) can be responsible for such skyrmion motion. To disentangle the different contributions, we perform micromagnetic simulations which suggest that the entropic torque could be the major factor in promoting the hot-to-cold motion.

Eventually, we deal with the achievement of skyrmions in a magnetic tunnel junction (MTJ)⁶. We design an innovative system where a standard ferromagnetic multilayer is coupled with a standard CoFeB-based MTJ. In particular, the multilayer acts as “skyrmion generator” for the MTJ free layer. We can measure a TMR of about 20%, and each skyrmion provide a resistance change of about 50 ?. The existence of skyrmions in MTJ paves the way not only to an efficient detection method, but also to applications, such as skyrmion oscillators, detectors, memristors, which are still only theorized.