When a neutron hits a wall, it either can propagate inside the medium or, if its momentum lies below a critical value, is totally reflected. Scientists at the Max-Planck-Institut für Metallforschung in Stuttgart found that, in the latter case, the neutron tunneling wave travelling below the surface is split into two components whenever the medium is magnetic (Physical Review Letters 81, 116 (1998)).
Neutrons with thermal energies are used in solid-state physics to investigate structural, magnetic and dynamic properties of bulk materials. The interaction of neutrons with matter is twofold, since they carry a magnetic moment and experience both nuclear and magnetic potentials. However, these interactions are rather small, and the depth in which information can be gained in a scattering experiment is typically in the order of centimeters.
Researchers from the Max-Planck-Institut für Metallforschung tried to enhance the surface sensitivity of neutrons for the exploration of magnetic properties of thin films and interfaces. They used the quantum-mechanical tunneling effect in order to create an exponentially decaying wave travelling in a skin of only a few nanometers in thickness. Tunneling occurs whenever the (perpendicular) momentum transfer of the neutron is too small to overcome the potential wall represented by the surface of the material. Tunneling states inside the medium can be observed via Bragg reflection at lattice planes lying perpendicular to the surface; the diffraction process represents only a small perturbation of the neutron state.
Unexpected results were found in an experiment on a thin iron film carried out at the high flux reactor of the Institute Laue-Langevin in Grenoble (France): an unpolarized neutron beam was directed onto the surface of the film under grazing incidence, and the neutron tunneling state beneath the surface was found to be split into two components whenever the film was magnetized. The splitting of the beam - due to the Zeeman effect induced by the magnetic potential of the film - was found to be very sensitive to small magnetic stray fields of nanometer dimensions. The rather fundamental effect of birefringent tunneling can therefore be applied to study subtle magnetic phenomena in magnetic thin films and interfaces of technological interest.
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