Optical Control of Magnetism
International team of researchers measures magnetic switching processes in the femtosecond range and provides answers to fundamental questions regarding magnetism
Kaiserslautern/Jülich, 12 March 2012 - Magnetic switching processes form the basis for processing and storing information. Magnetic media store data in tiny magnetic areas, which are usually written to using magnetic pulses. However, light pulses could perform the same task much faster. Such pulses can already be produced today with a duration of less than a trillionth of a millisecond (10-15 seconds). An international team of researchers has now provided a decisive contribution to improve our understanding of how magneto-optical switching can be controlled. Physicists from Forschungszentrum Jülich and State Research Center OPTIMAS in Kaiserslautern as well as from the University of Colorado and the National Institute of Standards and Technology in Boulder, USA, report on this in the current edition of the high-impact journal PNAS.
Usually, the elementary magnets of a magnetic metal or an alloy do not react independently of each other. Physicists refer to the corresponding force as exchange coupling. It is the reason, for example, why iron loses its magnetic properties at 768 °C, nickel at just 360 °C, and an alloy of the two materials, known as permalloy, loses these properties at 580 °C. Up to now, it was unclear whether the respective magnetic elements in an alloy of two magnetic metals exhibit different magnetic properties on very short time scales or whether they always behave synchronously. This is not just of interest to scientists but also for applications because decoupled systems react faster than coupled systems.
The researchers have now proved that the two magnetic elements Fe and Ni in permalloy briefly behave asynchronously. They believe that this discovery is instrumental for future spin dynamics investigations involving complex materials and that it could help to speed up magnetic switching processes significantly in the future.
Their findings were made possible using a method they developed themselves. It allowed the scientists to observe magnetic switching processes for the first time with a temporal resolution of a few femtoseconds (trillionths of a millisecond). They heated the permalloy with ultrashort laser pulses and showed that its two components reacted to this sequentially: nickel loses its magnetic properties 18 femtoseconds after iron. The time delay corresponds approximately to the energy of the above-mentioned exchange interaction (energy-time equivalent), which is the main physical cause of magnetism.
The experimental setup is known as a pump-probe experiment. It involves triggering the demagnetization of the probe using a pulse of infrared laser light lasting 25 femtoseconds. Using further pulses of soft X-ray light lasting no longer than 10 femtoseconds generated by what is known as a high-harmonics light source, they measured magnetic reactions that were registered as a spectrum on a CCD camera. Experts were initially unsure as to whether optical effects in such a measurement setup could distort the results or not. However, the researchers succeeded in excluding this possibility, as explained recently in Physical Review X.
Probing the timescale of the exchange interaction in a ferromagnetic alloy
Mathias et al.
PNAS 2012 (published ahead of print March 12, 2012)
Ultrafast Demagnetization Measurements Using Extreme Ultraviolet Light: Comparison of Electronic and Magnetic Contributions
Chan La-O-Vorakiat et al.
PHYSICAL REVIEW X 2, 011005 (2012)
Viewpoint: Spin-Sensitive Optics
Physics 5, 11 (2012)
Prof. Dr. Claus M. Schneider, Forschungszentrum Jülich, Peter Grünberg Institute, Electronic Properties
tel. +49 2461 61-4428
Angela Wenzik, Forschungszentrum Jülich
tel. +49 2461 61-6048