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.
Journal
Physical Review Letters