Multiferroic Propellers

Abstract

Viewpoint: Multiferroic Propellers Maxim Mostovoy, Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4, Groningen 9747 AG, The Netherlands Published February 6, 2012 | Physics 5, 16 (2012) | DOI: 10.1103/Physics.5.16 Helicoidal magnetic order induces a record high electrical polarization. Giant Improper Ferroelectricity in the Ferroaxial Magnet CaMn7O12 R. D. Johnson, L. C. Chapon, D. D. Khalyavin, P. Manuel, P. G. Radaelli, and C. Martin Phys. Rev. Lett. 108, 067201 (2012) Published February 6, 2012 | PDF (free) +Enlarge image Figure 1 APS/Carin Cain Figure 1 (a) Helicoidal spin spiral in which the spin rotation axis and the induced polarization are parallel to the spiral wave vector. (b) Cycloidal spin spiral in which spins (red arrows) rotate around an axis normal to the spiral wave vector Q. The induced electric polarization P is normal to both the wave vector and the spin rotation axis. Green curve is the cyloid. (c) Propellerlike structure of Mn-O octahedra in CaMn2O7, which changes the rotation direction when the sample is turned around. Control of spin ordering in magnetic insulators with an applied electric field (also known as the magnetoelectric effect) can significantly reduce the power consumption of memory devices, but with no mobile charges present, it would seem to be an impossible task. Encouragingly, it was recently discovered that some magnetic orders induce an electric polarization, which couples spins to electric field. So far, the electrical polarization in such magnetic ferroelectrics (also called multiferroics) tends to be small and the Néel magnetic transition temperature is usually well below liquid nitrogen temperature. Now, however, Roger Johnson and co-workers at the University of Oxford, UK, with collaborators in France, report in Physical Review Letters on achieving giant polarization in CaMn2O7. The measured polarization is the highest measured magnetically induced polarization, persisting up to a Néel temperature of 90K. Remarkably, this polarization appears to be induced by a long-period helicoidal (or proper-screw) spin spiral [see Fig. 1(a)], in which spins rotate around the spiral wave vector [1]. This discovery represents an important development for the field of magnetic ferroelectrics, as large polarization is crucial for electric manipulation of spins. It confirms earlier estimates of polarization from studies of polycrystalline samples [2].