Birefringent Bragg diffraction of evanescent neutron states in magnetic films

When monochromatic neutrons impinge onto a surface below the critical angle for total external reflection, evanescent (tunneling) neutron wave fields emerge in a thin film below the surface. We show by theory and experiment on an Fe(001) film that this scheme leads to an unusual quadruplet structure of the evanescent Bragg diffraction when unpolarized neutrons tunnel into a ferromagnetic single crystal film. The observed Bragg splitting leads to a complete autoseparation of all neutron spin states and spin-flip processes without applying any spin analysis. We demonstrate that this surface phenomenon is most sensitive to perpendicular moments and indicate its potential for future applications.