Urs Staub, Swiss Light Source, Paul Scherrer Institut
With the need to produce smaller and faster electronic devices recently a group of materials, multiferroics attracted strong interest. In these materials, magnetism and ferroelectricity coexist and are coupled. The magnetoelectric (ME) effect in a solid, the induction of a magnetization M by electric fields and the induction of an electric polarization P by magnetic fields, was first predicted by Pierre Curie . After the ME effect was first confirmed experimentally in the 1960s, many magnetic materials were found to produce this effect. Nevertheless, observed ME effects are usually too small for practical applications. In recent years, materials where discovered, in which the coupling between ferroelectricity and (anti)ferromagnetism is huge and ferroelectricity is caused by the particular magnetic spin structure called magnetically driven multiferroics.
In many of these materials, the ferroelectricity is caused by a cycloidal spin structure (a structure where the spins rotate along the ordering wave vector). It was shown that when cooling the magnetically driven multiferroics of TbMnO3 in an electric field through the phase transition, then a single spin chirality of the cycloid could be selected with the handiness depending on the direction of the applied electric field .
In ErMn2O5 the spontaneous electric polarization P is induced by a non-collinear arrangement of magnetic moments leading to a magnetoelectric coupling which is gigantic. Here the ferroelectricity is caused by the magnetic exchange striction. The magnetic structure and the asphericity of the Mn states, driving the multiferroic behavior, are probed by resonant soft x-ray diffraction performed at the SLS at PSI. Applying a large electric field on the sample perpendicular to the ferroelectric direction, with a large component along the antiferroelectric direction, shows a significant effect on the magnetic structure as can be seen in Fig. 1. The clearly seen enhancement of the commensurate magnetic Bragg reflections indicates, that the electric field changes the magnetic structure. As the field is applied perpendicular to the ferroelectric direction, this does not reflect a change of the domains structure, it rather reflects an excitation of the magnetic structure. It is interesting that by applying electric fields and reversing it, a hysteresic behavior is observed see Fig. 2, which again does not reflect a simple domain repopulation. Such an unusual hysteresis indicates that the magnetic structure can be driven back in a different meta-stable magnetic state at zero applied electric field. All these experiments clearly show, that due to the recent discovery of the magnetically driven multiferroics, many interesting physical properties are observed and that controlled manipulation of magnetic structure may become feasible.
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[Released: November 2008]