Skyrmions are small magnetic vortices that occur in many materials, including manganese silicide thin films (in which they were first discovered) and cobalt-iron-silicon. In the present work, iridium-cobalt-platinum was studied. They can be imagined as 2D knots in which the magnetic moments rotate about 360° within a plane.

Skyrmions could form the basis of future magnetic data storage technologies, explains co-team leader Christoforos Moutafis of the University of Manchester in the UK. Today's disk drives use magnetic domains (in which all the magnetic spins are aligned in the same direction) to store information, but there are fundamental limits to how tiny such domains can become and how tightly they can be packed together. Skyrmions, on the other hand, might be made much smaller and thus be used to create storage devices with much higher density. What is more, such a memory would retain information even when the power is switched off.

Another plus: thanks to their smaller sizes and shorter spacing between them, skyrmions could also be moved rapidly and efficiently along nanowires or other nanostructures in future “racetrack” memories. Such memories would not require any moving parts either, unlike the read-write heads in traditional hard disk drives, making these future devices much more robust.

The researchers led by Albert Fert, Vincent Cros and Nicolas Reyren of CNRS/Thales in France say that they have now imaged stable sub-100 nm single chiral magnetic skyrmions for the first time in technologically important cobalt-based multi-layered thin films. Until now, the structures had only been observed at ultralow temperatures.

Large Dzyaloshinskii–Moriya interactions

The films, which are made of a cobalt layer that is sandwiched between two heavy metals, were designed to produce so-called additive Dzyaloshinskii–Moriya (DM) interactions and a large value close to 2 mJm–2 in the case of iridium-cobalt-platinum (IrCoPt) asymmetric multilayers. The DM interaction is part of the spin-orbital interaction, which couples electronic orbital and intrinsic spin magnetism and endows skyrmions with additional stability, a smaller size, and allows them to travel more smoothly in nanostructures, explains Moutafis.

The researchers grew their samples using a technique called sputtering and produced stacks of 10 repetitions of an IrCoPt trilayer, with each trilayer being composed of a 0.6 nm thick Co layer sandwiched between 1 nm of Ir and 1 nm of Pt: Pt10|Co0.6|Pt1|(Ir1|Co0.6|Pt1)10|Pt3 (numbers are thickness in nm).

“We chose these two heavy metals based on results from previous experiments and theoretical predictions of opposite DM interactions for Co on Ir and Co on Pt,” adds Moutafis. “This choice corresponds to additive DM interactions at the two interfaces of the Co layers sandwiched between Ir and Pt.”

The team imaged its samples using scanning transmission X-ray microscopy (STXM), which involves firing short, high-energy pulses of soft X-rays at a material. These experiments were performed at the Swiss Light Source (SLS) at the Paul Scherrer Institute in Villigen, Switzerland, and at BESSY in Berlin, which are powerful sources of such X-rays. “The STXM technique allowed us to image small magnetic domains on the nanoscale thanks to the X-ray circular magnetic dichroism effect,” explains Moutafis, "and the experiments were performed with the help of our colleagues Jörg Raabe, head of the Microspectroscopy group and the X07DA (PolLux) STXM beamline at the SLS and Markus Weigand, beamline scientist at BESSY II's MAXYMUS.

Experimental breakthrough

The researchers concluded that the domains they observed were actually magnetic skyrmions stabilized by a large DM interaction. They came to their conclusion by studying how their domain size and behaviour change with magnetic fields.

“Observing nanoscale chiral magnetic skyrmions at room temperature is an experimental breakthrough,” Moutafis tells nanotechweb.org. “These quasi-particles could be used to develop novel devices for memory and logic applications and could lead to ultradense, non-volatile spintronics components for more energy-efficient future information technologies. One such technology is the skyrmion racetrack memory, proposed by Albert Fert in 2013.”

The research is detailed in Nature Nanotechnology doi:10.1038/nnano.2015.313.