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Motivated by the recent observations of incommensurate magnetic order andelectric polarization in YBaCuFeO5 up to temperatures TN2 as high as 230K [1,2]we report here for the first time a model for the incommensurate magneticstructure of this material that we complement with ab-initio calculations ofthe magnetic exchange parameters. Using neutron powder diffraction we show thatthe appearance of polarization below TN2 is accompanied by the replacement ofthe high temperature collinear magnetic order by a circular inclined spiralwith propagation vector ki=(1/2, 1/2, 1/2+-q). Moreover, we find that thepolarization approximately scales with the modulus of the magnetic modulationvector q down to the lowest temperature investigated (T=3K). Further, weobserve occupational Fe/Cu disorder in the FeO5-CuO5 bipyramids, although apreferential occupation of such units by Fe-Cu pairs is supported by theobserved magnetic order and by density functional calculations. We calculateexchange coupling constants for different Fe/Cu distributions and show that,for those containing Fe-Cu dimers, the resulting magnetic order is compatiblewith the experimentally observed collinear magnetic structure (kc=(1/2, 1/2,1/2), TN2 > T > TN1 = 440K). Based on these results we discuss possible originsfor the incommensurate modulation and its coupling with ferroelectricity.

Incommensurate magnetic structure, Fe/Cu chemical disorder and magnetic interactions in the high-temperature multiferroic YBaCuFeO5