  Motivated by the recent observations of incommensurate magnetic order and
electric 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 magnetic
structure of this material that we complement with ab-initio calculations of
the magnetic exchange parameters. Using neutron powder diffraction we show that
the appearance of polarization below TN2 is accompanied by the replacement of
the high temperature collinear magnetic order by a circular inclined spiral
with propagation vector ki=(1/2, 1/2, 1/2+-q). Moreover, we find that the
polarization approximately scales with the modulus of the magnetic modulation
vector q down to the lowest temperature investigated (T=3K). Further, we
observe occupational Fe/Cu disorder in the FeO5-CuO5 bipyramids, although a
preferential occupation of such units by Fe-Cu pairs is supported by the
observed magnetic order and by density functional calculations. We calculate
exchange coupling constants for different Fe/Cu distributions and show that,
for those containing Fe-Cu dimers, the resulting magnetic order is compatible
with the experimentally observed collinear magnetic structure (kc=(1/2, 1/2,
1/2), TN2 > T > TN1 = 440K). Based on these results we discuss possible origins
for the incommensurate modulation and its coupling with ferroelectricity.
