Researchers in Germany, the United States and Russia have shown how a single laser pulse can create patterns of skyrmions with a density determined by an applied magnetic field. The field-of-view of the X-ray microscope used to image the skyrmions (shown within the dashed lines) is 1 μm in diameter. [Image: MBI]
In the search for ever smaller and faster information technology, skyrmions seem just the ticket. These tiny magnetic vortices could potentially allow both the size and energy consumption of data storage devices to be shrunk compared with existing magnetic hard drives.
But for skyrmion-based technology to become reality, scientists must first show they can properly control the generation, movement and deletion of individual vortices. A German-led collaboration has now shown how such control might be achieved by using laser pulses and a magnetic field to create and annihilate skyrmions in two types of ferromagnetic multilayer (Appl. Phys. Lett., doi: 10.1063/5.0046033).
Toward rapid data encoding in tiny spaces
Skyrmions are extremely stable, particle-like disturbances in a field, within which the field vectors all point towards or twist around a single point. Their stability could potentially enable data to be encoded in much smaller regions of a magnetic material than is possible by simply relying on the parallel or antiparallel orientation of atomic magnetic moments.
Most skyrmions studied to date have been made electrically, typically by exploiting torques generated from nanosecond-long pulses of spin-polarized current. But laser pulses offer a quicker and potentially more efficient means of skyrmion production. Over the last three years, several groups have shown how to create skyrmions in ferromagnetic multilayers by exposing the material to a single laser pulse lasting just a few femtoseconds (10–15 s).
In the latest work, Kathinka Gerlinger of the Max Born Institute for Nonlinear Optics and Short Pulse Spectroscopy in Berlin and colleagues investigated two different cobalt–platinum-based multilayers already seen as contenders for magnetic devices—Co/Pt and Pt/CoFeB/MgO. They imaged the materials using X-rays from the PETRA III synchrotron source in the DESY lab near Hamburg, applying individual pulses from a 1030-nm fiber laser and a magnetic field from an electromagnet.
Magnetic and optical control knobs
To demonstrate how skyrmions could be written and erased in these multilayers, the researchers employed a four-step process. First they magnetically saturated the materials using a field of a few tens or hundreds of milliteslas. They then irradiated the materials with a single laser pulse under an applied field to break the symmetry, yielding a single skyrmion. The next step was to nudge the field up a bit, keeping the skyrmion intact. Then, by applying a second laser pulse at this higher field strength, the team was able to annihilate the skyrmion—and complete the cycle.
The researchers found that they could vary the density of skyrmions created in this way by altering the applied field. This allowed the team to generate dense arrays of vortices at relatively low fields and single skyrmions within a roughly 1-µm field-of-view at higher fields. The researchers found that the skyrmion density varies linearly with the applied field for both materials. That linear variation provides what the research team describes as “a very direct and easy handle to control the skyrmion density.”
Looking at the effect of changes to the laser pulses, the researchers confirmed previous results showing a sharp threshold imposed by the pulses’ fluence (or irradiance per unit area). Below a certain fluence, it took them thousands of pulses to generate skyrmions; once over that threshold, the density of skyrmions quickly rose as the fluence went up.
This ability to control the properties of skyrmions by tuning the magnetic field and laser pulses, the researchers argue, suggests that the materials could be used to create “an all-optical skyrmion writer and deleter.”
Toward novel applications in computing
The group observed another intriguing fluence-related effect by subjecting the samples to a series of laser pulses and imaging the skyrmions generated by each pulse, but without saturating the materials in-between pulses. When the fluence was just above the threshold for skyrmion nucleation, the spatial pattern of skyrmions remained largely intact from one pulse to the next. But at higher fluences there was no correlation between the position of skyrmions following successive pulses.
The researchers say that this randomization of skyrmion patterns could potentially be exploited in stochastic computing. Based on probability rather than arithmetic, such computing establishes the prevalence of 1s or 0s in bit streams to carry out certain operations, such as multiplication, more easily than digital processors can. But it relies on different streams being entirely uncorrelated with one another. A single laser shot well above the threshold for skyrmion nucleation, the team reckons, could enable very fast “reshuffling” of the inputs to probabilistic logic gates.
What’s more, Gerlinger and colleagues say that one of the multilayers—Pt/CoFeB/MgO—could also be used to make what is known as an “optospintronic skyrmion racetrack.” By exploiting both electrical and laser pulses, they were able to create, move and annihilate skyrmions in a device exposed to a constant magnetic field.
However, the researchers caution that currently their devices are laboratory demonstrations and that producing working products will require, among other things, miniaturization and integration.