Discovery of a new class of multiferroic materials
For next generation data storage devices the fast switching of magnetic states by applying electrical fields (voltage) will be of high importance. This switching could be realized in so-called magnetoelectric multiferroics. In general, multiferroic materials can be simultaneously electrically polarized (ferroelectric) and magnetically ordered. However, typical multiferroics with high ferroelectric ordering temperature do not exhibit a strong coupling of magnetic and electric properties. In such materials, also the magnetic ordering temperature and the ferroelectric onset temperatures are very different from each other. This already indicates the strong independence of these two phenomena in such multiferroics. Therefore, any macroscopic magnetoelectric response in multiferroics with conventional ferroelectricity is, if existent at all, very small. A few years ago, it was a great surprise to observe the magnetic control of the ferroelectric polarization in a manganese oxide (TbMnO3) below 28 K. In such magnetoelectric multiferroics, the ferroelectricity arises from the magnetic structure itself. Therefore, the ferroelectric properties are intimately coupled to the magnetic properties resulting in large magnetoelectric effects which are desirable for possible future applications. Often, spiral magnetic structures give rise to ferroelectric properties in these magnetoelectric multiferroics. These spiral magnetic structures typically arise from the competition beween exchange interactions of neighbouring and next-nearest neighbouring spins in a way such that it is difficult to align all spins parallel or antiparallel (ferro- or antiferromagnetically). This effect is called frustration.
A team from the Max Planck institute in Dresden in collaboration with ILL scientists found a new class of magnetoelectric multiferroics with high critical temperature - transition metal oxyhalides with Melanothallite structure. Powder neutron diffraction measurements of Cu2OCl2 reveal a magnetic structure with promising properties for magnetoelectric effects, and, a ferroelectric polarization appears simultaneously with the magnetic order at 70 K. Hence, all prerequisites for magnetoelectric multiferroicity appear in Cu2OCl2. These findings also point to the importance of sizeable exchange interactions within a frustrated magnetic structure for the emergence of magnetoelectric multiferroicity with high critical temperature. Furthermore, it links the field of multiferroics with another important field of contemporary solid state physics - that of high-temperature superconducting cuprates where frustration effects are also discussed in recent years.