Controlling the magnetic properties of materials is fundamental for developing memory, computing and other communication devices at the nanoscale. As data storage and processing are evolving quickly, researchers are testing different new methods to modify magnetic properties of materials. One approach relies on elastic deformation (strain) of the magnetic material to tune its magnetic properties, which has been shown to work well at slow speeds. This scientific area has attracted much interest due to its potential to write small magnetic elements with an electric voltage rather than current and thus avoiding energy losses. However, studies so far have mainly been done at very slow time scales (seconds to milliseconds).
Surface acoustic waves (SAWs)
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
Magnetic properties, frame by frame
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.
Figure: Left, top: Scheme of magnetic domains in a magnetic square without applying strain (arrows indicate magnetic directions and grey colour contrast). Left, bottom: magnetic domain configuration with strains, favouring horizontal magnetization (black and white domains). Right, top: Series of direct images (frames) taken at different timings between the strain wave (bright gray vertical line) and the imaging light. The waves pass through the magnetic square centre (white, 2 mm sides) between the third and fourth image (highlighted by red box) Right, bottom: Corresponding images with XMCD magnetic contrast showing the magnetic domains in the square. The maximum expansion of black and white domains is approximately between the fourth and fifth image, i.e. delayed with respect to the wave.
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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