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Shedding light on magnetoelectric coupling

Posted July 30, 2013
Scanning electron microscopy image of the sample corresponding to a top view on the nanopillar structure. Credit: Uni Duisburg

Scanning electron microscopy image of the sample corresponding to a top view on the nanopillar structure. Credit: Uni Duisburg

Effect opens up new possibilities for digital data storage
It is possible to control the electric properties of solids by magnetic fields by means of the so-called magnetoelectric coupling. This has been investigated by scientists from the University of Duisburg-Essen and the Institute for Complex Magnetic Materials of the HZB at the electron storage ring BESSY II. The effect can be used to develop new data storage media which are faster and more energy saving than today. The scientists published their results in the current issue of the journal “Nature Communications”.
Dr. Carolin Schmitz-Antoniak from the team of Prof. Heiko Wende at the University of Duisburg-Essen used a composite consisting of a few hundred nanometers long cobalt ferrite nanopillars embedded in a barium titanate matrix. The magnetostrictive nanopillars are deformed in an applied magnetic field, and the surrounding matrix is piezoelectric, i.e. it builds up an electric voltage under mechanical strain. The scientists deformed the nanopillars by applying a magnetic field and thereby created in this composite a mechanical stress to the matrix which finally exhibited an electric voltage.

The investigations, performed in collaboration with Dr. Detlef Schmitz from the Institute for Complex Magnetic Materials at BESSY II, proved successful. The experiments were performed with the high-field endstation at beamline UE46-PGM1 using also the unique possibility to rotate the high magnetic field relative to the direction of the incident soft x-ray radiation. Utilizing the combination of what is known as circular and linear dichroism, the scientists studied the magnetism and the electric polarization of the nanopillars and the matrix of the composite, respectively.

In addition, experiments with hard x-rays were performed in collaboration with Dr. Esther Dudzik and Dr. Ralf Feyerherm of the same HZB Institute at the MAGS beam-line. The resulting information about the crystal structure of the sample directly verified the deformation of the matrix by the applied magnetic field.

By analyzing all experimental results the researchers concluded how the electric polarization is controlled by magnetic fields. The effect is based on smallest deformations of the materials in the composite. If the magnetic field is applied along the longitudinal axis of the nanopillars, then the nanopillars shorten longitudinally. At the same time the nanopillars become thicker in order to conserve their volume. As a consequence the surrounding matrix is squeezed uniformly. In contrast, if the magnetic field is applied along a transverse axis of the nanopillars, then the nanopillars shorten along this axis whereas they expand at right angles to it. In this way the matrix is stretched along the magnetic field and compressed at right angles to it, resulting in an asymmetric polarization distribution which has not been observed in this system before.

The composite is relevant as a digital data storage medium because the electric polarization is maintained even when the magnetic field is switched off again. Therefore the researchers also developed a strategy to compress single nanopillars by electric current pulses along longitudinal and transverse axes to write information bitwise.

Source: Helmholtz Association of German Research Centres

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