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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New route for switching magnets using light

Impression of an iron oxide crystal lattice. Red: spins of iron ions, Blue: oxygen ions. Green: electrons in their orbit responsible for the exchange interaction. The interaction keeps the spins aligned. A light pulse excites the electrons, changes the exchange interaction and thus releases the spins.
 http://phys.org/news/2015-09-route-magnets.html

An international team led by Radboud University physicists has discovered that reversing the poles of magnets must be possible without a heating or a magnetic field.. A strong pulse of light can have a direct effect on the strong quantum mechanical 'exchange interaction', therefore changing the magnetism (Nature Communications, 16 September 2015).

In 2007, Professor Rasing and his group at Radboud University showed for the first time that fast pulses of laser light can reverse the poles of magnets. This was a paradigm shift as, until then, physicists believed that light could never be strong enough to break the strong magnetic interaction forces. It can, however, and very local heating by the laser pulse in combination with differences in the response times of the constituent atoms can explain this phenomenon. The researchers have now discovered a new way in which light can manipulate magnetisation.

Directly on the electrons

In the article published by Nature Communications on September 16 the researchers show that the light can excite electrons, which in turn can directly influence the strength of the exchange interaction and therefore change the magnetisation. No heat is released in the process, which is good news for magnetic data storage applications as it means that the method requires little energy. Exchange interaction refers to the internal, quantum mechanical forces that make a magnet magnetic.

"We carried out our experiments in iron oxides, including hematite," says project leader Alexey Kimel. "The crystal structure of hematite is a good system to study this mechanism, as the iron ions are neatly separated by oxygen ions in the crystal lattice. Even so, exchange interaction takes place between the iron ions because the electrons interact through the oxygen ions. By exciting the electrons in the oxygen with a pulse of light, we can manipulate the exchange interaction between the iron spins, and perhaps even reverse their polarity in the near future."

Cool savings

Switching with no heat has the potential to revolutionise magnetic data storage. Huge amounts of heat are currently released in large data centres, and good cooling is becoming a big problem. Facebook, for example, is planning to build its new data centre in the north of Sweden for this very reason. "If we can store information using a new, cool method, data storage will be a lot cheaper," explains Kimel.

Measure what you do

The researchers also developed a magnetometer to measure the ultrafast changes they induce in a magnet. They use the freely propagating electromagnetic radiation in the Terahertz frequency range (1 THz = 1012 Hz) emitted by the spins of the magnet. By measuring the changes in this radiation, they are able to measure the effect of light on the magnetisation. "We have produced a magnetometer that measures at the femtosecond scale," says Rostislav Mikhaylovskiy, the first author of the article.

To be continued at FELIX and HFML

The researchers will conduct further studies into switching using light in the new FELIX laser lab and the adjacent HFML in Nijmegen. The strength of magnetic fields generated by HFML is comparable to that of the exchange interaction and the frequency of light waves generated by FELIX can be tuned to affect the electrons and change the strength of the exchange interaction in the most effective way. "This will certainly help us explore this mechanism in greater detail," says Theo Rasing.

 Explore further: Magnetization can surf on the top of a laser-induced sound wave

More information: "Ultrafast optical modification of exchange interactions in iron oxides." Nature Communications. 16 September 2015 DOI: 10.1038/ncomms9190