"Hot Spins" – New Dynamics Discovered in Magnetic Multilayers
Jülich/Kaiserslautern, 4 September, 2012 – Laptops, mobile phones, the Internet, sat navs, ticket vending machines – without the countless advancements in the area of information and communication technologies, modern society as we know it would be inconceivable. However, the amount of energy they consume is enormous and is on the rise. One possible solution is "Green IT" – new technologies that require less energy. A promising approach is to exploit the angular momentum or "spin" of electrons, which determines the magnetic properties of materials in addition to their charge. An international team of researchers has now discovered a new physical effect in a system of magnetic layers based on the transport of excited ("hot") spins that could open up new avenues for computer technologies (Nature Communications, DOI: 10.1038/ncomms2029).
Random-access memory with its innumerable tiny condensers acts as the short-term memory in computers and laptops. It buffers the programs and files currently in use. However, the condensers have to be recharged regularly to ensure that no data is lost. This costs time and energy, and if there is a power failure, the buffered data are lost for good.
If we were to use magnetic materials to store information instead, we would conserve a lot of energy. The data would then be saved until overwritten, meaning that they would not have to be refreshed at regular intervals and would not be lost in the case of a power failure. In addition, computers would become faster as the data would then be written with the aid of very short laser pulses. Such pulses can already be produced today with a duration of less than a trillionth of a millisecond (10-15 seconds).
However, in-depth understanding as to how the magneto-optical switching can be controlled is still lacking. Researchers from Jülich, Kaiserslautern, Sweden, and the USA have now discovered a new effect that provides an answer to one of the underlying questions, and which could simultaneously open up new avenues for applications.
Scientists from Forschungszentrum Jülich, the University of Kaiserslautern, the State Research Center OPTIMAS in Kaiserslautern, and from research institutions in Sweden and the USA investigated the effect of laser pulses on an ultrathin system of magnetic layers for the first time layer-selectively.
"To date, such studies have only been conducted for multilayers as a whole. It was impossible to individually analyse each layer, independent of all the other layers," explains Denis Rudolf, PhD student at Jülich’s Peter Grünberg Institute, named after the Jülich Nobel laureate and pioneer of spintronics research. "Using particularly short-wave pulses in the soft X-ray range, we were able to gain insights into spin dynamics right down to the deepest layers for the very first time."
In doing so, they made an astonishing discovery. In the past, measurements have shown that laser pulses can significantly reduce the magnetization of magnetic layers and multilayers for a short time. Various explanations had been proposed for this, including one suggesting that the material is heated by the pulse to such a degree that the magnetization is partly lost.
However, the researchers have now been able to show a transient increase in the magnetization for one particular case: when two magnetic layers in the multilayer investigated were initially parallel, the pulse caused the magnetization of the lower layer to increase, while that of the upper layer decreased as expected. When the magnetic layers were initially aligned in an antiparallel manner, however, the magnetization decreased as expected in both layers.
"This clearly points towards a new theory," said Prof. Martin Aeschlimann from the University of Kaiserslautern and OPTIMAS. According to this, the pulses provide the electron spins with energy. The resulting "hot" spins are more mobile causing "spin currents" to flow. "As a small number of spins in a magnetic material are always oriented in the opposite direction of the overall magnetization, they don’t get very far; only spins with the ‘correct’ orientation can noticeably diffuse. When they enter the neighbouring layer, they increase the existing magnetization if it is parallel, and decrease it if it is antiparallel."
This effect is still too weak for technical applications. The researchers are now trying to identify materials that develop stronger spin currents and ways of structuring the multilayers so that the spin currents can be selectively channelled. The aim is to channel so many spins from one layer into the neighbouring layer that the magnetization does not just increase or decrease, but can actually be flipped and thus allow a data bit to be written.
Ultrafast magnetization enhancement in metallic multilayers driven by superdiffusive spin current; Rudolf et al.; Nature Communications 2012,
Denis Rudolf, Electronic Properties (PGI-6),
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