  Multiferroism can originate from the breaking of inversion symmetry caused by
magnetic-spiral order. The usual mechanism for stabilizing a magnetic spiral is
competition between magnetic exchange interactions differing by their range and
sign, such as nearest-neighbor and next-nearest- neighbor interactions. Since
the latter are usually weak the onset temperatures for multiferroism via this
mechanism are typically low. By considering a realistic model for YBaCuFeO$_5$
we propose an alternative mechanism for magnetic-spiral order, and hence for
multiferroism, that occurs at much higher temperatures. We show using
Monte-Carlo simulations and electronic structure calculations based on density
functional theory that the Heisenberg model on a geometrically non-frustrated
lattice with only nearest-neighbor interactions can have a spiral phase up to
high temperature when frustrating bonds are introduced randomly along a single
crystallographic direction as caused, e.g., by a particular type of chemical
disorder. This long-range correlated pattern of frustration avoids
ferroelectrically inactive spin glass order. Finally, we provide an intuitive
explanation for this mechanism and discuss its generalization to other
materials.
