Connection Between Ferroelectricity and Magnetic Structure at the Microscopic Level

In a magneto-electric material, a magnetic field can induce a ferroelectric moment-a displacement of the ions that creates an electric field. Similarly, an electric field can induce a change in the material's magnetic structure. These materials have caught the attention of technologists who are interested in developing them as future data storage devices: it is much easier to make a compact storage system that can be switched electrically, rather than with the current system of magnetic read/write heads.

Unfortunately, relatively few magneto-electric materials exist, which is why Daisuke Okuyama of the RIKEN Advanced Science Institute, Wako, and Yuichi Yamasaki of the University of Tokyo and colleagues are aiming to better understand the connection between ferroelectricity and magnetic structure at the microscopic level in TbMnO3. TbMnO3 is one of the most well-studied magneto-electric materials.

At low temperatures, a magnetic field can rotate the ferroelectric polarization from pointing along the c-axis of this material to pointing along the a-axis. To really understand this effect, however, the researchers needed a microscopic picture that explains why magnetism and ferroelectricity are connected. This in turn required knowing what the magnetic structure looked like-both in zero and high magnetic fields.

Okuyama, Yamasaki and colleagues were confronted by the problem that the best experimental technique for resolving a material's magnetic structure-neutron diffraction-cannot be performed at high magnetic fields. They therefore devised a clever alternative*: they studied a similarly structured material, Gd1-xTbxMnO3, which they believe has the same magnetic structure in zero magnetic field that TbMnO3 has at high magnetic field.

After careful analysis of over 150 neutron diffraction peaks, the team has determined that at temperatures close to ~20 K (~ -253 °C) the manganese (Mn) spins in Gd1-xTbxMnO3 spiral in the a-b crystallographic plane. The team has also shown that the formation of this spiral-like magnetic phase occurs at the same temperature that the material develops a ferroelectric moment. Based on comparisons of this spin structure with that of TbMnO3 at zero field, in which the spins spiral in the b-c plane, the team argues that the electric polarization rotates 90 degrees in TbMnO3 in the presence of magnetic field because the field changes the sense of rotation of the spiral spin structure.

Armed with the knowledge of how a magnetic field changes the electric polarization in TbMnO3, Okuyama says that: “The control of the magnetic structure with an electric stimulus is our next target.”

• Yamasaki, Y., Sagayama, H., Abe, N., Arima, T., Sasai, K., Matsuura, M., Hirota, K., Okuyama, D., Noda, Y. & Tokura, Y. Cycloidal spin order in the a-axis polarized ferroelectric phase of orthorhombic perovskite manganite. Physical Review Letters 101, 097204 (2008).