Terahertz goes magnetic : Controlling spins with ultrashort electromagnetic bursts
A European collaboration of scientists has succeeded in controlling the ultrafast precession of electron spins in antiferromagnets, which has heretofore been inaccessible.
The experiment employs so-called terahertz radiation (1 THz = 1012 Hz), which is currently attracting attention because of its use in body scanners in airports. Like many other applications, these scanners exploit the electric component of the terahertz wave. The present work shows that the magnetic component of terahertz radiation has important potential applications as well. They have reported their findings in the esteemed journal Nature Photonics. (Published: 21 November 2010 | Nature Photonics)
The spin of an electron can be considered as a never-ending rotation of the electron about its own axis (see figure). Since this rotation is accompanied by a ring-like electrical current, the electron has a magnetic moment, just like a compass needle. Spins are currently used as information carriers on magnetic hard disks, where a group of many electrons pointing up or down encodes a bit value of 0 or 1, respectively.
In order to write such magnetic information as fast as possible, the spins have to be manipulated at an accordingly fast pace. This is equivalent to turning a compass needle around very, very quickly. The most natural way to do this would be to apply a rapid series of magnetic ‘kicks’ in the appropriate direction using a pulsed magnetic field. So far, this has not been feasible due to the lack of sufficiently strong and fast magnets. Now, scientists of the Universities of Konstanz and Bonn, AMOLF, and the Fritz Haber Institute in Berlin have made an important progress in the ultrafast manipulation of spins. They used the latest laser technology in order to generate strong magnetic pulses in the terahertz frequency window.
In their first experiment, the researchers focused the terahertz pulses onto the antiferromagnet nickel oxide, in which neighboring spins point in opposite directions (see figure). As seen in the experimental data, the spins start to precess; such motion is similar to that of a spinning top, albeit at an unimaginably high speed of one terahertz, that is, one million times one million cycles per second. This high precession frequency is a unique feature of antiferromagnets where the spins are pulled back into their equilibrium direction by very strong forces.
In order to bring the precession to a halt, the researchers applied a second magnetic pulse at the exact moment when the speed of the spin and the torque of the magnetic field point in exactly opposite directions. Their experimental data demonstrate that a sequence of two terahertz pulses can start and stop the precession of spins on an ultrashort time scale (see figure). Most importantly, the team shows that the resonant terahertz driving force leaves other degrees of freedom of the antiferromagnet unexcited and, thus, exclusively addresses the electron spin dynamics. This concept provides a universal ultrafast handle on previously inaccessible magnetic excitations in the electronic ground state.
Reference
Coherent Terahertz Control of Antiferromagnetic Spin Waves,
Tobias Kampfrath, Alexander Sell, Gregor Klatt, Alexej Pashkin, Sebastian Mährlein, Thomas Dekorsy, Martin Wolf, Manfred Fiebig, Alfred Leitenstorfer, Rupert Huber.
Nature Photonics, published online 21 November 2010.
For further information please contact:
Tobias Kampfrath
Fritz Haber Institute of the Max Planck Society
Tel. +49 30 83853588
Email: kampfrathATfhi-berlin.mpg.de
Rupert Huber
Department of Physics, Universities of Konstanz and Regensburg
Tel. +49 941 9432070
Email: rupert.huberATphysik.uni-regensburg.de
Left: The basic principle: Electrons (blue spheres) carry a spin that can be seen as a persistent rotation of the electron about its own axis, resulting in a magnetic moment (blue arrows). In the experiment, spins are kicked by an intense terahertz magnetic transient (orange waveform), leading to a precession about their equilibrium directions, similar to a spinning top. Right: Experimental data: A first terahertz magnetic pulse triggers a precession, shown by the spin deflection (blue curve), which is switched off by a second pulse after 6.5 precession cycles.