A spin-polarized triplet metastable helium (He*) beam has been used as a probe for surface magnetism, but changes in the spin state during scattering from a surface remain unclear. In the present study, we explored this issue by constructing an apparatus that allows us to direct a spin-polarized He* beam to a surface and measure the spin polarization of He* scattered from the surface. Magnetic hexapoles were used for both the beam polarization and the spin analysis. The results of the spin-dependent He* scattering experiments on clean
Fe3O4(100), H-terminated
Fe3O4(100), benzene-adsorbed
Fe3O4(100), and nonmagnetic Cu(100) surfaces indicated that, although the spin direction of He* was mostly preserved during scattering from these surfaces, spin-flop scattering of surviving He* occurred with a probability up to approximately 0.1. Our results showed that the survival probability was higher when the spins of He* and the
Fe3O4(100) film were parallel, which can be understood based on the lower He* resonance ionization rate for this spin orientation. Based on our findings, we estimate that a nonpolarized He* gas becomes 10% spin-polarized after a single collision with a clean
Fe3O4(100) surface.
Diode laser optical pumping to produce a highly spin-polarized metastable He beam to be used in a spin-polarized metastable atom deexcitation spectroscopy experiment on magnetized surfaces is described. Efficient pumping of the beam is performed by means of an SDL-6702 distributed Bragg reflector diode laser which yields 50 mW of output power in a single longitudinal mode at 1083 nm, the resonance wavelength for the 23 S→23 P0,1,2 (D0, D1, and D2) transitions of He*. The light is circularly polarized by a quarter-wave plate, allowing easy change of the sense of atomic polarization. The laser frequency can be locked to the atomic transition for several hours by phase-sensitive detection of the saturated absorption signal in a He discharge cell. Any of the three transitions of the triplet system can be pumped with the laser but the maximum level of atomic polarization of 98.5% is found pumping the D2 line.
The use of a spin-polarized metastable helium (
He∗) atom beam has demonstrated that the survival probability (SP) of
He∗ during the scattering from ferromagnetic surfaces depends on the electron spin of
He∗. The spin asymmetry in the SP of
He∗ scattered from a clean
Fe/Cu(100) surface changed its polarity with
H2O adsorption. We argue that the reversal is caused by the change in the spin polarization of the surface electronic states at around the Fermi level.