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Multiferroic magnetic spirals induced by disorder

Markus Müller (PSI Villigen)

Multiferroism, such as magnetically induced electric polarisation, can originate from the breaking of inversion symmetry caused by magnetic-spiral order. Such spirals usually arise due to competing nearest and next-to-nearest neighbor exchange interactions and thus only occur at low temperatures. Here, we will present a different mechanism that works at much higher temperatures since it involves only nearest neighbor exchange. The main ingredient are frustrating bonds randomly introduced along a single crystallographic direction, as found in materials like YBaCuFeO5, where spiral order at high temperatures was indeed reported experimentally. Our theoretical model predicts a direct proportionality between the density of frustrating bonds, the ordering temperature and the spiral wavevector. We further show that spin orbit coupling at impurities induces a non-trivial tilt of the easy plane, which ensures that the spiral order couples to electric polarization.