Synthetic Antiferromagnetic Skyrmions

In this project we will stabilise small circular magnetic domains called skyrmions in chiral synthetic antiferromagnetic multilayers and study their current-driven dynamics. The project is based on two recent breakthroughs by our groups: our being able to stabilise skyrmions as a topologically protected structure (making them resistant to annihilation) in a suitably designed single chiral perpendicularly magnetised layer, and being able to move coupled topological defects (domain walls) at low current density in a simple in-plane magnetised synthetic antiferromagnet.

Whilst conventional skyrmions are interesting candidates for a variety of novel information storage and processing devices that offer the prospect of very low power operation, they are expected to move slowly at small sizes due to topological damping and are diverted at an angle to their current drive direction by the Magnus forces that lead to a skyrmion Hall effect. To realise their potential, we need to establish the optimal multilayer structure to support synthetic antiferromagnetic skyrmions that are small, highly mobile, and move in the direction of an electrical current drive. We need to find a reliable nucleation method to that can create synthetic antiferromagnetic skyrmions in a controlled manner for further study. We need to know how make synthetic antiferromagnetic skyrmions respond directly to spin current drives by balancing the Magnus forces on the two component skyrmions to reduce the skyrmion Hall angle to zero. Finally, we need to learn how to exploit the expected suppression of topological damping in order to move the synthetic antiferromagnetic skyrmions move at velocities far higher, and at smaller sizes, than for conventional skyrmions.

In this project we will prepare chiral magnetic multilayers that support synthetic antiferromagnetic skyrmions, image the skyrmion structures, and fabricate nanoscale devices in which we can measure current-driven skyrmion dynamics. We will combine our expertise with synthetic antiferromagnet multilayers with our proven ability to induce strong Dzyaloshinskii-Moriya interactions at interfaces to combine two coupled skyrmions with opposite polarity and chirality into a synthetic antiferromagnetic skyrmion that can be stabilised at room temperature, with their structure and motion under field imaged using state-of-the-art microscopy techniques. Next, we will study the nucleation of synthetic antiferromagnetic skyrmions at randomly occurring and deliberately introduced defects due to the application of stimuli including pulses of magnetic field or electrical current. We will then prepare skyrmion racetracks along which synthetic antiferromagnetic skyrmions can be propelled using current-driven torques from this optimised multilayer stack and image the skyrmion motion at moderate current densities in order to measure the skyrmion Hall angle and find the conditions when it is zero. We will go on to increase the current densities to seek high velocity skyrmion motion exploiting the suppression of topological damping that arises between coupled topological defects in synthetic antiferromagnets.

The results we shall obtain will not only lead to high impact publications and conference presentations by shedding light on the possibilities offered by this novel combination of materials, but also develop potentially valuable knowhow in the field of spintronics based on synthetic antiferromagnetic skyrmions for technological applications.

Magnetic skyrmions are extraordinary objects found in specially engineered chiral magnetic materials. Their small size, enhanced stability due to topology, and responsiveness to electrical currents make them suitable candidates for a wide variety of spintronic applications. The most widely-trailed device proposal is the racetrack memory, in which skyrmions are nucleated and shifted along a one-dimensional magnetic wire to represent a stream of digital bits. Nevertheless, as particle-like objects, skyrmions are not restricted to moving back and forth along a line: they can be moved beside, around, and past each other in two dimensions, permitting a variety of different information storage and processing schemes that operate using either Boolean logic or unconventional schemes such as neuromorphic computing. In all these schemes the benefits of a synthetic antiferromagnetic skyrmion that we shall realise, namely being able to move quickly, and in a well-controlled direction, are essential to efficient and reliable operation. Thus, the work we shall carry out will have impact in two areas identified by BEIS as among the Eight Great Technologies that 'support UK science strengths and business capabilities': these are "Advanced Materials" and "Big Data and Energy Efficient Computing", and matches well to the EPSRC Physics Grand Challenge "Nanoscale Design of Functional Materials".

Our project is inspired by the prospect of eventual impact in the ITC hardware sector, where spintronics is already a commercial success story that has enabled the huge amounts of extremely cheap data storage needed to provide social media, such as Facebook, Instagram, and Youtube, free to users. Nevertheless, the reality is that the world's server farms are consuming 30 billion watts of power. Furthermore, only about 10% of this energy is actually used for computation. The remainder is used to keep servers available should an urgent demand be requested, and to run cooling systems to dissipate this enormous amount of waste heat. As long ago as 2008, it was pointed out that the carbon footprint of the internet exceeds that of commercial air travel. As the rates of data production and consumption increase, this is clearly not sustainable. As the data volumes are unlikely to reduce, we need to search for new materials that will permit new devices and architectures to greatly more efficient use of energy. There are proposals now, based on established physical principles, for skyrmion-based spintronic devices such as racetrack memories, logic gates, and magnonic, rf, or neuromorphic devices. To realise these we require advances in materials science and the experimental study of skyrmions of the sort we propose here. Fast, dense non-volatile memories are a crucial technology to permit the development of normally-off/instantly-on computers to avoid the wasted energy of keeping idling servers ready for use. Low-power Boolean logic operations will reduce the energy requirements of information processing. Magnonic and rf devices will open up new prospects for high-bandwidth communications hardware and novel ways of processing information as the interferences of waves.

Further in the future, tremendous impact can be expected if neuromorphic computing realises its potential. At the time of writing the world's most powerful supercomputer is the Summit system at Oak Ridge National Laboratory in the US, which consumes 9.8 megawatts of power. In spite of this, its computational ability in most respects lies far below that of the human brain, which requires only 20 watts, about one million times less. This emphasises that point that moving away from conventional von Neumann architectures to those that mimic neural architectures found in the brains of animals can not only enhance the performance of computers in tasks at which they currently perform poorly, such as pattern recognition, but offers the prospect of truly radical reductions in power consumption.