Materials in which electric dipole order (ferroelectricity) is coupled to magnetism could be very useful in electronics. In these ‘magnetoelectric multiferroics’ an electric field changes the magnetic properties and vice versa.

Fig. 1: The spins on the Cu sites of CuO order into a spiral such as the one shown here between 213 and 230 K.

One type of material that has these properties is the spiral magnet (see Figure 1), because the low symmetry of the magnetism allows ferroelectricity. However, the magnetism usually occurs at temperatures that are too low to be practically useful.

Now, Tsuyoshi Kimura and colleagues1 at Osaka University in Japan and Bell Laboratories in the US propose looking for high-temperature spiral magnets in familiar territory: copper-based high-temperature superconductors.

Layers of copper and oxygen arranged on a square lattice are the building blocks of cuprate superconductors. The magnetic interaction between the spins on neighboring copper sites is exceptionally strong and magnetic order occurs near room temperature. The angle of Cu-O-Cu bond determines the type of spin-spin interaction. Copper spins point antiparallel to one another if the bond is straight, but along the same direction if the bond is 90 degrees.

In CuO—a common starting material for making high-temperature superconductors—all of the bonds are between these two limits and the copper spins adjust by forming a spiral. This spiral phase only occurs between 230 K and 213 K, forming a simpler magnetic structure at lower temperatures.

Although no one has observed ferroelectricity in CuO, Kimura and colleagues found that their CuO single crystals developed an electric polarization in the temperature range where the spiral phase occurred, but this polarization disappeared above 230 K or below 213 K, when the simpler magnetic phase became dominant. They were also able to flip the polarization of the crystal by changing the sign of the applied electric field—a key finding for any future applications.