Figure 1: Neutron diffraction data demonstrating the spin cycloid in a 100 nm BiFeO₃ film.

(a,b) Neutron diffraction reciprocal space maps of a 100 nm BFO film grown on (110)-oriented STO with a 20 nm conductive SRO intermediate layer in the and reversed sample orientation. In (a) satellite peaks emerge, while in (b) a single peak is present, indicative of an incommensurate spin structure. The black line in (a) was determined from the peak maxima and indicates the propagation direction of the spin cycloid (c) X-ray RSM of the (113) reflection demonstrates single-domain growth of the sample. (d) Neutron diffraction diagonal slice through the AFM reflections provides a comparison between the two orientations. The overall measured intensity of the signal is retained in both orientations, that is, the integrated intensity of the satellite peaks around is equal to the intensity. The error bars correspond to 1 s.d. of the count rate. (e,f) The neutron diffraction data reveals a unique [11] cycloid propagation direction in contrast to [10] as observed in bulk BFO, but with an almost identical spiral length of λ₀=64.7(10) nm as compared to bulk BFO. The cycloidal propagation direction is perpendicular to the direction of the electric polarization P[111] and the in-plane [10] direction, resulting in the [11] spin cycloid propagation direction which is slightly canted out of the film plane. The light green arrow denotes the direction of the incoming neutron beam.