Research Highlights

Quantum tricks with laser light teach magnetic devices how to think ultra-fast

A USA-Greece collaboration of researchers at the U.S. Department of Energy’s Ames Laboratory & Iowa State University and at the University of Crete & the Foundation for Research & Technology Hellas (FORTH) in Greece have found a new way to create small magnets by using short laser light pulses. In this way, they were able to switch magnetism at least 1000 times faster than in current magnetic devices. Magnetic switching is used to encode information in hard drives, magnetic memories, and other computing devices. The discovery reported in the April 4 issue of Nature opens the door to terahertz (1012 Hertz) memory speeds and moves magnetic switching to the fast lane.

Ames Laboratory physicist Jigang Wang and his team demonstrated experimentally that a magnetic structure can be changed within quadrillionths of a second (femtosecond) by using ultra-short laser pulses. They observed a transition from anti-ferromagnetic to ferromagnetic ordering – i.e. from anti-parallel to parallel magnetic spin alignment – in colossal magneto- resistive materials, which are promising for use in next-generation RAM memory, hard disk, and logic devices. University of Crete physics professor Ilias Perakis and his group in Greece showed that the experimental results prove the existence of a previously unobserved very early temporal regime of “Quantum Femto-Magnetism”.

The property of electrons called spin, responsible for magnetism, is used in electronic and magnetic devices to encode information into bits: 0 and 1 correspond to different values of the total electron spin. “A challenge facing magnetic writing, reading, storing, and computing is speed. Technology demands that the speed of electronic devices must increase by several orders of magnitude. We showed that we can meet the challenge to make magnetic switches think ultra- fast, in the femtosecond–quadrillionths of a second– range by using quantum ‘tricks’ with ultra- short laser pulses” say Wang and Perakis. The researchers were able to magnetize the material 1000 times faster than the fastest current magnetic memories, thus opening a new direction for implementing a magnetic switch that operates at terahertz speeds or even faster.

In current magnetic storage and magneto-optical recording technology, magnetic fields or continuous laser light is used. For example, photo-excitation causes atoms in ferromagnetic materials to heat up and vibrate, and the vibration, with the help of a magnetic field, causes magnetic flips. These flips are part of the process currently used to encode information.

“The speed of such thermal magnetic switching processes is limited by how long it takes to vibrate the atoms, and by how fast a magnetic field can reverse magnetic regions” say Wang and Perakis. “With such thermal methods it is difficult to push the speed beyond the gigahertz range (109 Hertz) switching speed limit of today’s magnetic writing/reading technology.” So, some scientists have turned their attention to colossal magnetoresistive (CMR) materials because they are highly responsive to external fields used to write data into memory, but do not require heat to trigger magnetic switching.

“Colossal Magnetoresistive materials are very appealing for use in new technologies. But first we need to understand more about how exactly they work” say Wang and Perakis. “We had to figure out a way to make these materials reveal their secrets. And we found one. We decided to turn our attention to what happens during very short periods of time, when the laser pulse still interacts with magnetic moments in CMR materials and laser heating is not significant. That means that, during femtoseconds, we must describe the magnetism by using quantum mechanics. We called this “Quantum Femto-Magnetism” ”.

The very high energies in accelerators such as CERN reveal new physics. The same is true for very short times-femtoseconds and attoseconds-accessible with state-of-the-art lasers. “We tried to understand how magnetism changes when illuminated by ultra-short laser pulses, by taking snapshots of the total spin and watching how it changes in real time. In this way, we learned about the physical processes that create the magnetism in these materials and were even able to control them,” say Wang and Perakis.

Wang’s experimental team specializes in using ultra-fast spectroscopy, which he likens to high- speed strobe photography, because both use an external pump of energy to trigger a quick snapshot that can be re-played afterwards. In ultra-fast laser spectroscopy, a short pulse of laser light is used to excite a material and trigger a measurement, all on the order of femtoseconds.

So what does light have to do with magnetism? “A laser pulse can do much more than simply heat up the material. The electric field makes electrons oscillate in a controlled way. Electrons are 10,000 times lighter than atoms, so their motion during very short time intervals is governed by the laws of quantum mechanics instead of Newton’s. As they jump to their neighboring atoms, driven by the laser electric field and quantum mechanics, they change the fundamental processes that create magnetism. In this way, we showed, both experimentally and theoretically, that the magnetic order is switched during the 100-femtosecond-long laser pulse. During that time interval the laser creates quantum mechanical superpositions of different spin states, which result in local magnetic regions whose size increases abruptly with laser intensity. This means that switching occurs by manipulating spin and charge quantum-mechanically, before the atoms have time to complete their oscillations” says Perakis. “Then the second laser pulse comes in and ‘sees’ a huge photo-induced magnetization, with an excitation intensity threshold behavior, developing immediately after the first pump pulse”. In some sense, this experiment confirms the ancient Greek philosopher Plato’s famous saying: “The beginning is half of everything”. Here, the beginning lasts for quadrillionths of a second.

The fast switching speed and huge magnetization that Wang observed meet both requirements for applying CMR materials in ultra-fast, terahertz magnetic memory and logic devices. “Our strategy and concept is to use all-optical quantum methods to achieve magnetic switching and to control magnetism. This lays the groundwork for seeking the ultimate switching speed and capabilities of CMR materials, a question that underlies the entire field of spin-electronics,” say Wang and Perakis. “Our hope is that this means someday we will be able to create devices that can read and write information faster than ever before. The role of physics is to open new directions for technology. And our discovery of “Quantum Femto-Magnetism” may have just opened a new one”.

Tianqi Li, Aaron Patz, Jiaqiang Yan,Tom Lograsso, and Jigang Wang at Ames Laboratory and Iowa State University performed the experiments. Leonidas Mouchliadis and Ilias E. Perakis at the University of Crete and the Institute of Electronic Structure and Laser Foundation of FORTH in Greece developed the theory that interprets the experiments.

Reference: "Femtosecond switching of magnetism via strongly correlated spin–charge quantum excitations", by Tianqi Li, Aaron Patz, Leonidas Mouchliadis, Jiaqiang Yan, Thomas A. Lograsso, Ilias E. Perakis & Jigang Wang, Nature 496, 69–73 (04 April 2013).

April 2013

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