Multiferroics hit "home run"

8 February 2012

In an article highlighted on the home page of the Physical Review Letters web site, Roger Johnson and co-workers report the highest magnetically-induced ferroelectric polarisation ever observed in a multiferroic, bringing these materials one step closer to applications in information technology. Magnetism and ferroelectricity are widely used properties to store digital information - for example in computer hard disks and chip-and-pin cards. Usually, these two properties are mutually exclusive, but in multiferroics they are coupled together, potentially opening the way to faster and more energy-efficient technologies to read and write digital information. Unfortunately, all materials so far discovered only work at very low temperatures, usually well below liquid nitrogen temperature, and their electrical polarisation is very small, making such materials unsuitable for applications. In this work, performed at the Clarendon Laboratory with collaborations with ISIS and Diamond (UK), and ILL and CNRS (France), Johnson et al. show that the compound CaMn7O12 displays "giant" magnetically induced polarisation persisting up to 90 K - the best performances ever achieved. Intriguingly, the helical magnetic structure of CaMn7O12 should not allow for a ferroelectric polarisation at all, according to previous theories. However, Johnson et al. demonstrate that CaMn7O12 possesses a crystal structure with a special propeller-like "ferroaxial" distortion (see figure), capable of coupling to the magnetic structure and generating electrical polarisation. Although other explanations are still possible, the strong magnetoelectric coupling found in CaMn7O12 will undoubtedly stimulate the search for other ferroaxial ferroelectrics to see if they too exhibit such a large polarization.

Reference: R. D. Johnson et al., Phys. Rev. Lett. 108, 067201 (2012)