One way to produce rapid (i.e. subnanosecond scale) changes of strain and, thus, induce magnetization changes is by using surface acoustic waves (SAWs), which are deformation (strain) waves. Now, imagine an iron rod being hammered in one side. When the rod is hit, a sound wave propagates the deformation along it. Similarly, a surface acoustic wave propagates a deformation, but only in the surface layer, similarly to waves in the ocean. In certain materials (piezoelectrics), which expand or contract when applying a voltage, SAWs can be generated through oscillating electric fields.
A group of researchers from the ALBA Synchrotron, the Institute for Materials Science of Barcelona (ICMAB-CSIC) and the University of Barcelona (UB), in collaboration with the Paul Scherrer Institut (Switzerland), the Johannes Gutenberg University Mainz (Germany) and the Paul Drude Institut (Germany) have developed a new experimental technique to quantitatively image these SAW and used them to modify the magnetization in nanoscale magnetic elements (the “surfers”) on top of the crystal. In principle, similar methods could be used to study how to manipulate nanoparticles and cells or to control chemical reactions by SAWs
The experiment was done at the CIRCE beamline of the ALBA Synchrotron, using the PhotoEmission Electron Microscope (PEEM), a cutting-edge tool for analysing thin films, surfaces, and interfaces as well as magnetic properties of nanomaterials.
Researchers prepared magnetic squares on top of a piezoelectric crystal. Using the time signal of ALBA accelerators as a reference, they were able to synchronize the SAW signal and the synchrotron light pulses. This system enables researchers to take images (frames or snapshots) of the sample when the strain wave passes through the sample, giving the possibility of studying the details of fast processes occurring at 500 MHz (repeating 500 hundred million times per second).
Results showed that the magnetic squares changed their properties under the effect of SAWs, growing or shrinking the magnetic domains depending on the phase of the SAW. Interestingly, the deformation did not occur instantaneously and there was a delay between the SAW and the magnetic changes (see Figure). Understanding how the magnetic properties can be modified on a fast time scale is key to design effective devices in the future.
Reference: “Direct imaging of delayed magneto-dynamic modes induced by surface acoustic waves” M. Foerster, F. Macià, N. Statuto, S. Finizio, A. Hernández-Mínguez, S. Lendínez, P. Santos, J. Fontcuberta, J. M. Hernàndez, M. Kläui, L. Aballe Nature Communications
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