Researchers from Johannes Gutenberg-University Mainz, ALBA Synchrotron Light Facility and Tohoku University have identified quasiparticles called merons in a synthetic antiferromagnet for the first time, which could lead to new concepts for spintronics devices.
The spintronics field is still rather nascent as research is ongoing. Recent research has focused on structures called skyrmions as potential building blocks. These structures are quasiparticles made up of numerous electron spins and can be thought of as two-dimensional whirls (or “spin textures”) within a material. Skyrmions exist in many magnetic materials, including cobalt–iron–silicon and the manganese–silicide thin films in which they were first discovered. They are attractive spintronics candidates because they are robust to external perturbations, making them particularly stable for storing and processing the information they contain. At just tens of nanometres across, they are also much smaller than the magnetic domains used to encode data in today’s disk drives, making them ideal for future data storage technologies such as “racetrack” memories. Like skyrmions, merons are made up of numerous individual spins. Unlike them, their stray magnetic fields are miniscule, which would facilitate ultrafast operations and even higher information storage densities within a device. Until now, however, merons have only been observed in natural antiferromagnets, where they have proved difficult to analyze and manipulate.
The research team has now identified merons in synthetic antiferromagnets made from multilayer stacks of mutually coupled individual ferromagnetic layers. Unlike natural antiferromagnets, these synthetic materials can be prepared in a well-controlled way using established techniques such as sputter deposition.
This control enabled the team to adjust how the different layers interact, and thereby minimize their net magnetic moments. This gives the system advantages of both antiferromagnets (in which electron spins tend to align antiparallel to each other) and ferromagnets (which have parallel electron spins). Examples include not only low stray magnetic fields, but also stable homochiral textures and fast spin dynamics within a polycrystalline setting, explains Mona Bhukta, a PhD student at JGU and the study’s co-leader.
In their work, the scientists stabilized these spin textures in synthetic antiferromagnets with a very small easy-plane anisotropy (so that the preferred orientation of the magnetization lies in the film plane) and imaged their intricate structures by combining several imaging methods. The methods they used included magnetic force microscopy and scanning electron microscopy with polarization analysis as well as element-specific photoemission electron microscopy using X-ray magnetic circular dichroism.
Thanks to these imaging techniques, the team identified multiple different spin textures in the stacked material. This was not easy, as the researchers had to image the quasiparticles in a way that resolves all three components of the magnetization vector before they could unequivocally demonstrate the presence of merons. The researchers also developed an analytical model to elucidate the mechanisms that stabilize such structures in their system. The goal in this case was to determine the optimal thickness of each layer and identify the best “host materials” for merons.
As well as identifying merons, the team also observed related structures such as antimerons and topologically stabilized bimerons in their synthetic antiferromagnets. Unlike in skyrmions, the direction of the net magnetization and the emergent field produced by bimerons are mutually orthogonal, the scientists explains.
The researchers now plan to investigate the interaction between merons and external magnetic fields and electrical currents.