Larmor pseudo-precession of neutron polarization at reflection

Classical Larmor precession (LP) of neutron polarization is considered as a result of the quantum interference between neutron spin states split in the magnetic field due to the Zeeman effect. The interference takes place if polarization is not collinear with the direction of the magnetic induction. At grazing incidence onto a magnetized film LP may occur not only in transmission through but also at reflection from the film. If the magnetic reflection potential of the film is much lower than that of the substrate then the interference between spin components of neutron wave results in anomalous LP with doubled LP phase shift. At reflection from a film whose magnetic potential is comparable with the nuclear one and with that of the substrate both spin components are totally reflected, but with a phase shift resulting in the Larmor pseudo-precession (LPP) of the neutron polarization vector. The LPP period is proportional, in contrast to LP, to the neutron wave vector component normal to the film. A Pilot experiment on a ⁵⁷Fe film deposited on sapphire substrate shows that one precession can be achieved for the film thickness of ∼100nm.