Abstract
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.
Highlights
Insulators with magnetic spiral order are of particular interest because of their associated multiferroism [1,2,3,4,5] in which the breaking of inversion symmetry by the magnetic spiral drives long-range ferroelectric order
By considering a realistic model for multiferroic YBaCuFeO5, we propose an alternative mechanism for magnetic-spiral order, and 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 nonfrustrated 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
Summary
Insulators with magnetic spiral order are of particular interest because of their associated multiferroism [1,2,3,4,5] in which the breaking of inversion symmetry by the magnetic spiral drives long-range ferroelectric order. The reported values of Tspi range from 180 K to 310 K [11,12,13,14,15,16] depending on the preparation conditions, and it was recently shown [16] that Tspi and Q increase systematically with Fe3þ=Cu2þ occupational disorder These observations suggest that chemical disorder plays an essential role in stabilizing the magnetic spiral motivating our search for a microscopic mechanism by which disorder facilitates, or even drives, magnetic spiral order. We note that randomly oriented impurity bonds would have a spin-glass solution [18,19,20,21], whose magnetic order does not couple to a net electric polarization and does not lead to multiferroism This mechanism results in a Tspi of the order of a typical exchange coupling. Our Monte Carlo simulations for YBaCuFeO5 yield Tspi as high as 250 K, depending on the concentration and strength of the impurity bonds, in a manner that is consistent with the experimentally observed dependence of Tspi and qspi on the amount of Fe3þ=Cu2þ occupational disorder [16]
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