High-Resolution Microscopy Confirms Nanoscale Magnetic Properties

The top row shows electron phase, the second row shows magnetic induction, and the bottom row shows schematics for the simulated phase of different magnetic domain features in multilayer material samples. The first column is for a symmetric thin-film material and the second column is for an asymmetric thin film containing gadolinium and cobalt. The scale bars are 200 nanometers (billionths of a meter). The dashed lines indicate domain walls and the arrows indicate the chirality of the molecules. (Image credit: Berkeley Lab)

A research team from the Lawrence Berkeley National Laboratory (Berkeley Lab) have confirmed the presence of chirality in nanometer-thick samples of multilayer materials with a chaotic structure. The information could be used for the transmission and storage of data.

Since most electronic devices are reliant on the flow of charge of the electrons, researchers worldwide are frantically looking for innovative ways to transform electronics by developing materials and techniques to control other intrinsic electron traits, such as their spin and their orbits around atoms, which can be imagined to be similar to a compass needle adjusted to face in various directions.

Research believe that these properties could lead to more reliable, more rapid, and smaller data storage by enabling spintronics, one aspect of which is the application of spin current for the manipulation of domains and domain walls. Developing spintronics-driven devices that produce lesser heat and need less power compared to traditional devices is also now possible.

In a recent study, researchers from Berkeley Lab’s Molecular Foundry and Advanced Light Source (ALS) confirmed the presence of chirality in the transition regions (known as domain walls) between neighboring magnetic domains with opposite spins.

Researchers believe that chirality could be controlled in order to control magnetic domains and transmit zeros and ones as in traditional computer memory.

The samples consisted of an amorphous alloy of cobalt and gadolinium, interposed between ultrathin layers of iridium and platinum, which are known to have a strong impact on neighboring spins.

In present-day computer circuits, silicon wafers are commonly used based on a crystalline form of silicon, with a regularly ordered structure. In this study, the material samples used in the experiments were amorphous (or noncrystalline), indicating that their atomic structure was chaotic.

Experiments demonstrated that the magnetic properties of the domain walls exhibited a dominant chirality that could probably be flipped to its opposite. This flipping mechanism will be a crucial enabling technology for spintronics and different fields of research that are based on the spin property of the electron.

To achieve optimization of this chiral effect, the research team made efforts to determine the optimum concentration, thickness, and layering of elements, as well as other factors.

Now we have proof that we can have chiral magnetism in amorphous thin films, which no one had shown before.

Robert Streubel, Lead Author

Streubel also stated that the favorable outcome of the experiments paves the way for regulating certain characteristics of domain walls (for example, chirality) with temperature, and for swapping the chiral properties of a material using light.

Streubel stated that in spite of their chaotic structure, amorphous materials could also be produced to suppress some of the disadvantages of crystalline materials for spintronics applications.

The researchers adopted a special, high-resolution electron microscopy method at Berkeley Lab’s Molecular Foundry and performed the experiments in a purported Lorentz observation mode to image the magnetic characteristics of the material samples.

The outcomes were consolidated with those of an X-ray method at the ALS, called magnetic circular dichroism spectroscopy, to confirm the nanoscale magnetic chirality in the samples.

The Lorentz microscopy method adopted at the Molecular Foundry’s National Center for Electron Microscopy offered the tens-of-nanometers resolution needed to resolve the magnetic domain characteristics called spin textures.

This high spatial resolution at this instrument allowed us to see the chirality in the domain walls—and we looked through the whole stack of materials.

Peter Fischer, Co-Author

Fischer said that the highly precise, high-resolution experimental methods—in which X-rays and electron beams are used, for instance—now enable researchers to investigate complex materials that do not have a well-defined structure.

Fischer stated that the final research tool, which is imminent given the next generation of X-ray and electron probes, would enable researchers to directly view, at atomic resolution, the magnetic switching taking place in the interfaces of a material at femtosecond — one-quadrillionths of 1 second— timescales.

Our next step is therefore to go into the dynamics of the chirality of these domain walls in an amorphous system: to image these domain walls while they’re moving, and to see how atoms are assembled together."

Peter Fischer, Co-Author

The results of the Lorentz microscopy method were fed into a mathematical algorithm, which was tailor-made by Streubel, to determine chirality and domain wall types. Another difficulty was the optimization of the growth of the sample to realize the chiral effects with the help of a traditional method called sputtering.

In the future, it will be possible to apply the algorithm, as well as the experimental methods, to a complete set of sample materials, and it “should be generalizable to different materials for different purposes,” Streubel stated.

The team also believes that this research could help induce R&D in relation to spin orbitronics, where “topologically protected” (resilient and stable) spin textures known as skyrmions could possibly replace the propagation of tiny domain walls in a material and result in rapid and smaller computing devices with lower power needs compared to traditional devices.

The Molecular Foundry and the ALS are DOE Office of Science User Facilities. The U.S. Department of Energy’s Office of Science supported this study.

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