Atomically thin van der Waals magnets are broadly perceived as the ubiquitously compact media for rapid data processing and next-generation magnetic data storage.
But real-time control of the magnetic state of these materials has been demonstrated to be quite challenging.
Now, an international research team headed by Delft University of Technology (TU Delft) has successfully used light to alter the anisotropy of a van der Waals antiferromagnet on demand. This breakthrough opens the door to a new, highly efficient method of data storage.
The thin atomic layers that constitute van der Waals magnets may appear to be very fragile; however, they can be around 200 times more robust than steel. But regrettably, this kind of mechanical strength does not essentially translate into powerful magnetic characteristics.
This is because, in two dimensions, the magnetic order of such magnets becomes particularly sensitive to heat.
Temperatures higher than the absolute zero (−273°C) trigger arbitrary changes in the direction of the microscopic spins, which can fully collapse the magnetic order. Hence, until their magnetic state can be controlled, the promises of atomically thin magnets would remain just that—promises.
The only method to offset the thermal agitations is to bind magnetic spins more to certain directions in the material than to other directions, or to induce 'magnetic anisotropy,' as physicists call it.
Doing so makes it more difficult for spins to alter their orientation, thus increasing their ordering temperature (called the Curie temperature) much above absolute zero. In other words, controlling anisotropy in low-dimensional magnets paves a direct route to managing their ordering temperature and, therefore, the magnetism itself.
During the analysis, the international research team, which included investigators from The Netherlands, Ukraine, and Spain, utilized very short pulses of light, which is a trillion times shorter than one second, to promote the magnetic anisotropy in a two-dimensional van der Waals antiferromagnet.
But why use light?
Because it’s a very convenient control knob. You can simply and swiftly turn it on and off and therefore manipulate the anisotropy on demand, which is exactly what we need if we want to start using these materials for efficient data storage.
Dr Andrea Caviglia, Delft University of Technology
Tuning the Color
By methodically altering the light color from visible to near-infrared, the researchers further discovered that not all types of light can create magnetic anisotropy. To promote this property, the color of light should correspond with the energy needed to alter the electron orbital state.
That is to say: To alter the way an electron spins around a positively charged nucleus.
Since the electron spin and its orbital movement are closely associated, the excitations of light induce anisotropy, which leads to a 2D spin-wave movement.
This motion is coherent—the whole spin ensemble moves in-phase at high frequencies. This is an elegant and at the same time virtually universal solution to manipulating magnetic anisotropy in practically any two-dimensional magnet.
Jorrit Hortensius, PhD Student, Delft University of Technology
The researchers demonstrated in this proof-of-principle experiment that anisotropy can be photoinduced for a small fraction of time, almost the same as the duration of the pulse of light. But for practical applications, the magnetic changes should be sustained for an extended period of time.
The team hopes that longer-duration light pulses may help reach this objective.
We hope that longer light pulses can even promote the magnetic order above the equilibrium ordering temperature, so that we can watch in real-time how the ordered state arises from magnetic chaos. This will certainly increase our understanding of magnetism in these van der Waals magnets.
Dr Dmytro Afanasiev, University of Regensburg
Afanasiev, D., et al. (2021) Controlling the anisotropy of a van der Waals antiferromagnet with light. Science Advances. doi.org/10.1126/sciadv.abf3096.