Single domain wall in FeNi film on Si as a neutron spin flipper

A permalloy film on Si is divided over its thickness into two opposite domains by passing a DC “wall current” of 20 A through the film. The position of the resulting domain wall is adjusted inside the film thickness by an in-plane magnetic field. To investigate the flipping properties we perform 3D-analysis of the polarisation vector of the transmitted beam (λ=0.2nm) around loops at 10–1000 Hz with this field, with the film positioned under 3–15∘15∘ to the beam.At all frequencies the domain wall annihilates before reaching either surface. At 7∘7∘, with restricted amplitude of this field (30 A/m), the net precession of the polarisation vector after the film can be varied between -π-π and +π+π without hysteresis, with a depolarisation of 0.85. The response of the domain wall is quick enough to follow the required 1/tt time dependence of the applied field when the film is used as ±π±π-flipper at a pulsed source operated at 20–50 Hz. The geometry allows that a beam of cross-section 20×10mm2 can be intercepted and flipped. This demonstrates that the film can be used for such a flipper.The annihilation of the domain wall and the depolarisation are attributed to disturbing magnetic/non-magnetic nuclei on both surfaces. Eliminating their influence requires much stronger wall current fields, only possible using thicker films.