Laser determines the future: surprising butterfly effect found in ultrafast magnets
Researchers from the FOM Foundation and Radboud University Nijmegen together with Russian colleagues have found a 'butterfly effect' in the world of ultrafast magnetism. The researchers saw that a brief magnetic pulse determines the direction of the magnetisation that occurs later. Interestingly, the entire system can be controlled with this minimal disruption, just like a butterfly which flaps its wings can ultimately cause a tornado thousands of kilometres away. This butterfly effect also provides a conceptually new way of storing and processing information. The researchers published these results on 9 April online in Physical Review Letters. This article has been selected as an Editor's suggestion.
For the experiments, the researchers made use of crystalline materials (rare earth orthoferrites). In these materials the direction of the magnetisation spontaneously rotates by 90 degrees upon heating, a so-called magnetic phase transition. If the magnetisation rotates to the left, then the north pole comes to lie on top and if the magnetisation rotates to the right, the south pole comes on top. Without external influences, the rotational direction is entirely random. However, if the material is placed in a magnetic field then the direction of rotation can be controlled.
To their amazement the researchers saw that this direction could also be controlled by exposing the material to a magnetic pulse of 0.06 picoseconds (made with the help of a laser) two picoseconds before the rotation starts (a picosecond is 10-12 seconds). This pulse initiates minimal, but decisive vibrations of the magnetisation direction. Despite their small amplitude, these vibrations ensure that the left and right rotational directions are no longer equivalent: the symmetry is broken. Consequently the magnetisation acquires a strong preference for one of the two rotational directions even before the phase transition occurs (Figure 1). Furthermore, the researchers could easily control the preferred direction by adjusting the phase of the vibrations via the direction of the magnetic pulse.
FOM PhD student Johan de Jong: "It is quite unique that we can control the entire system, the magnet, with such a small disruption. In the future we hope to use this approach to gain more control over other ultrafast processes as well."
Reference
'Coherent control of the route of an ultrafast magnetic phase transition via low-amplitude spin precession', J.A. de Jong, I. Razdolski, A.M. Kalashnikova, R.V. Pisarev, A.M. Balbashov, A. Kirilyuk, Th. Rasing, and A.V. Kimel, Phys. Rev. Lett. 108, 157601 (2012). DOI: 10.1103/PhysRevLett.108.157601.
Further information
Johan de Jong and Alexey Kimel +31 (0)24 365 30 26.