Terahertz light pulses speed up spin switching

Researchers from the Moscow Institute of Physics and Technology and their colleagues from Germany and the Netherlands have achieved material magnetization switching on the shortest timescales, at a minimal energy cost. They have thus developed a prototype of energy-efficient data storage devices. CREDIT: @tsarcyanide/MIPT Press Office

https://physicsworld.com/a/terahertz-light-pulses-speed-up-spin-switching/

A new technique to rapidly reverse a magnet’s polarity in a way that all of its spins coherently rotate could be used to develop more energy-efficient data storage devices and superfast computers in the future. The technique, which works by applying ultrashort pulses of terahertz-frequency light to the magnet, does not produce any waste heat and requires very little energy – just one photon per spin flip.

Modern-day computer hard drives encode data as binary zeros and ones by orienting the spins in magnetic materials using magnetic field pulses created by an electrical current. This process dissipates huge amounts of energy though (and is relatively slow). Indeed, today’s data centres consume between 2 and 5% of the world’s electricity and produce waste heat that, in turn, requires even more power, to cool the servers down.

Researchers in Russia, Germany, the UK and the Netherlands have now exploited a novel, unprecedented strong interaction between the electric field of terahertz light pulses and magnetic spins. The effect, which they discovered in 2016 in the antiferromagnetic material thulium orthoferrite, makes the spins oscillate with large amplitude. This interaction, they found, is still not strong enough to switch the orientations of the spins, however, even using the most powerful THz radiation sources.

To be able to switch spins with THz light, the team designed and fabricated a special nanosized antenna (made of gold) and placed it on top of the thulium orthoferrite magnet. The antenna possesses plasmonic modes (collective oscillations of the metal’s conduction electrons) that increase the coupling between light and the antenna.

Enhancing the local light field

The device collects and focuses light at THz frequencies and enhances the local light electric field by more than 10 times. “This electric field is now strong enough to steer the magnetization of all the spins over a potential energy barrier and into a new orientation, in just picoseconds,” explains team member Rostislav Mikhaylovskiy, formerly of Radboud University in the Netherlands and now at Lancaster University in the UK. “This is because the photon energies of THz radiation are comparable to the energy needed to align the spins in the magnet.”

And that is not all: the temperature of the magnet does not increase at all during switching since the process requires the energy of just one quantum of the terahertz light – a single photon – per spin, he adds.nt

“The speed of purely electrical spin switching is typically limited to the GHz range by capacitances and inductances in electronic circuitry,” explains team member Christoph Lange of Regensburg University in Germany. “Most importantly, however, electronics inherently suffer from Ohmic energy losses, and subsequent heating. The flow of data in modern-day systems is now so intense that this waste heat is already restricting the performance of data centres and supercomputing facilities. Our approach avoids this problem by replacing electric current with light pulses.”

Coherent spin switching

To prove that they had indeed observed coherent switching of all the spins in the magnet, the researchers monitored the spin orientation using the polarization rotation imprinted on a short optical pulse that is delayed relative to the THz pulse. “If the initial spin deflection is not sufficient for synchronous spin switching, we observe a sinusoidal, oscillating signal in the polarization rotation,” explains Mikhaylovskiy. “If, on the other hand, the spins are switched, we observe a ‘beating’ signature on top of the oscillations, which is the characteristic ‘fingerprint’ of the spins deflecting over a potential barrier into a neighbouring local potential energy minimum.”

Our work is a major milestone in the worldwide research effort towards complete control of spins by THz pulses, Mikhaylovskiy tells Physics World. “The technology we have developed could enable highly energy-efficient data storage at greatly increased speeds as compared to existing technology. What is more, the coherent dynamics made possible by the extremely low energy dissipation may even allow for quantum information processing based on solid-state spins at THz clock rates.”

The team, which also includes Stefan Schlauderer and Rupert Huber from Regensburg University, Alexey Kimel of Radboud University and Anatoly Zvezdin from the Russian Academy of Sciences, now plans to continue its research at the new ultrafast laser at Lancaster University and accelerators at the Cockroft Institute. These facilities are able to generate intense pulses of THz light and the new experiments will allow the researchers to determine the practical and fundamental speed and energy limits of magnetic recording using THz light pulses.

The research is detailed in Nature 10.1038/s41586-019-1174-7.

Energy-free superfast computing invented by scientists using light pulses

Using ultrashort pulses of light enables extremely economical switching of a magnet from one stable orientation (red arrow) to another (white arrow). This concept enables ultrafast information storage with unprecedented energy efficiency. Credit: © Brad Baxley (parttowhole.com)

https://phys.org/news/2019-05-energy-free-superfast-scientists-pulses.html

Superfast data processing using light pulses instead of electricity has been created by scientists.

The invention uses magnets to record computer data which consume virtually zero energy, solving the dilemma of how to create faster data processing speeds without the accompanying high energy costs.

Today's data centre servers consume between 2 to 5% of global electricity consumption, producing heat which in turn equires more power to cool the servers.

The problem is so acute that Microsoft has even submerged hundreds of its data centre services in the ocean in an effort to keep them cool and cut costs.

Most data are encoded as binary information (0 or 1 respectively) through the orientation of tiny magnets, called spins, in magnetic hard-drives. The magnetic read/write head is used to set or retrieve information using electrical currents which dissipate huge amounts of energy.

Now an international team publishing in Nature has solved the problem by replacing electricity with extremely short pulses of light—the duration of one trillionth of a second—concentrated by special antennas on top of a magnet.

This new method is superfast but so energy efficient that the temperature of the magnet does not increase at all.

The team includes Dr. Rostislav Mikhaylovskiy, formerly at Radboud University and now Lancaster University, Stefan Schlauderer, Dr. Christoph Lange and Professor Rupert Huber from Regensburg University, Professor Alexey Kimel from Radboud University and Professor Anatoly Zvezdin from the Russian Academy of Sciences.

They demonstrated this new method by pulsing a magnet with ultrashort light bursts (the duration of a millionth of a millionth of a second) at frequencies in the far infrared, the so called terahertz spectral range.

However, even the strongest existing sources of the terahertz light did not provide strong enough pulses to switch the orientation of a magnet to date.

The breakthrough was achieved by utilizing the efficient interaction mechanism of coupling between spins and terahertz electric field, which was discovered by the same team.

The scientists then developed and fabricated a very small antenna on top of the magnet to concentrate and thereby enhance the electric field of light. This strongest local electric field was sufficient to navigate the magnetization of the magnet to its new orientation in just one trillionth of a second.

The temperature of the magnet did not increase at all as this process requires energy of only one quantum of the terahertz light—a photon—per spin.

Dr. Mikhaylovskiy said: "The record-low energy loss makes this approach scalable.

Future storage devices would also exploit the excellent spatial definition of antenna structures enabling practical magnetic memories with simultaneously maximal energy efficiency and speed."

He plans to carry out further research using the new ultrafast laser at Lancaster University together with accelerators at the Cockroft Institute which are able to generate intense pulses of light to allow switching magnets and to determine the practical and fundamental speed and energy limits of magnetic recording.

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T-rays will 'speed up' computer memory by a factor of 1,000

http://www.nanowerk.com/nanotechnology-news/newsid=44801.php

(Nanowerk News) Together with their colleagues from Germany and the Netherlands, scientists at the Moscow Institute of Physics and Technology (MIPT) have found a way to significantly improve computer performance. In their paper published in Nature Photonics ("Nonlinear spin control by terahertz-driven anisotropy fields"), they propose the use of the so-called T-waves, or terahertz radiation as a means of resetting computer memory cells. This process is several thousand times faster than the magnetic-field-induced switching.“We have demonstrated an entirely new way of controlling magnetization, which relies on short electromagnetic pulses at terahertz frequencies. This is an important step towards terahertz electronics. As far as we know, our study is the first to make use of this mechanism to trigger the oscillations of magnetic subsystems,” says Anatoly Zvezdin of Prokhorov General Physics Institute and MIPT, a coauthor of the paper and a USSR State Prize-winning scientist heading MIPT’s Laboratory of physics of magnetic heterostructures and spintronics for energy-saving information technologies.

The figure shows thulium orthoferrite (TmFeO2) spin and lattice structure on the left and the T-ray-induced transitions between the energy levels of thulium ions (Tm3+), which trigger coherent spin dynamics (memory switching), on the right. (click on image to enlarge)The rapidly increasing amounts of digital data that have to be manipulated, along with the growing complexity of the computation tasks at hand, compel hardware designers to achieve ever higher computational speeds. Many experts believe that classical computation is currently approaching a limit, beyond which no further increase in data processing speed would be practicable. This motivates scientists all over the world to investigate possibilities of entirely different computer technologies. One of the weak spots in modern computers retarding their evolution is memory: it takes time to complete every set/reset operation for a magnetic memory cell, and reducing the duration of this cycle is a very challenging task.A group of scientists that includes Sebastian Baierl of the University of Regensburg, Anatoly Zvezdin, and Alexey Kimel of Radboud University Nijmegen (the Netherlands) and Moscow Technological University (MIREA) proposed that electromagnetic pulses at terahertz frequencies (with wavelengths of about 0.1 millimeter, i.e., between those of microwaves and infrared light) could be used in memory switching instead of external magnetic fields. A more familiar device that makes use of the terahertz radiation is the airport body scanner. T-rays can expose weapons or explosives concealed under a person’s clothing, without causing any harm to live tissues.To find out, whether T-rays could be used for convenient memory states switching (storing “magnetic bits” of information), the researchers performed an experiment with thulium orthoferrite (TmFeO2). As a weak ferromagnet, it generates a magnetic field by virtue of the ordered alignment of the magnetic moments, or spins of atoms in the microcrystals (magnetic domains). In order to induce a reorientation of spins, an external magnetic field is necessary.However, the experiment has shown that it is also possible to control magnetization directly by using terahertz radiation, which excites electronic transitions in thulium ions and alters the magnetic properties of both iron and thulium ions. Furthermore, the effect of T-rays proved to be almost ten times greater than that of the external magnetic field. In other words, the researchers have devised a fast and highly efficient remagnetization technique—a solid foundation for developing ultrafast memory.The scientists expect their “T-ray switching” to work with other materials as well. Thulium orthoferrite, which was used in the experiment, happens to be convenient for the purposes of demonstration, but the proposed magnetization control scheme itself is applicable to many other magnetic materials.“There was a Soviet research group that used orthoferrites in their studies, so this was always kind of a priority field for us. Our research can be seen as a follow-up on their studies,” points out Anatoly Zvezdin.

Read more: T-rays will 'speed up' computer memory by a factor of 1,000