Ferroelectric and ferromagnetic materials can be used in memory applications in which the polarization, electric or magnetic, can be switched by external fields. Coupling these two types of order, as present in multiferroic materials, could lead to further functionalities in memory devices. In multiferroic materials, for example, the magnetization could be controlled by an electric field, or the electric polarization could be controlled by application of a magnetic field.

The second possibility motivated Mamoru Fukunaga at Tohoku University and colleagues in Japan1 to study thulium manganese oxide (TmMn2O5), a multiferroic compound. The researchers previously observed that cooling a specimen to below 5 K caused its polarization to ‘flop’ — rotate by 90° — from its natural b-axis alignment to be aligned with the structural a axis. Now, they have demonstrated that the application of a magnetic field along the third axis, the caxis, at these low temperatures causes the polarization to ‘flop’ back to be aligned with the baxis.

Fig. 1: Schematic structure of TmMn2O5. Arrows indicate the polarization ‘flop’.

The idea of applying a field along the caxis came from previous results showing that such a field had an effect on the polarization along the baxis. As Fukunaga explains, “both magnetic field and temperature variations can cause magnetic phase transitions, and the b-axis polarization changes as the magnetic phase changes. Such facts have already been reported in many studies on RMn2O5,” where R is a rare earth element such as thulium. However, the polarization flop has never previously been observed.

The observation gave Fukunaga and his colleagues an important insight into the origin of the electrical polarization. In particular, they found that the polarization flop coincides with a variation of spin chirality in the material, and that adjacent spins interact through a vector product, as in many other multiferroics.

With a view to novel memory devices, the results represent an important step towards the possible realization of a four-state memory device, in which the polarization could be flipped (rotated 180°) by the application of an electric field, and flopped through the application of a magnetic field.