Positive sign of the spin polarization of electrons emitted from Ni and prediction of the Stoner-Wohlfarth-Slater theory

Abstract

Abstract We describe a new apparatus for the measurement of the spin polarization P of field emitted electrons from Ni single crystal tips cleaned by controlled ultra high vacuum field evaporation. The [h k l] crystallographic direction is selected using a probe-hole with a 2 mm diameter while observing the field emission pattern on a screen. The quantization axis is defined by a static external magnetic field generated by a bakeable superconducting coil. The necessary conditions for a meaningful measurement of P for various crystallographic directions are described. Positive P values, i.e. magnetic moment parallel to the magnetization, have been observed for high-index, low work function faces (i.e. P ≈ 7% for the [321] direction, P ≈ 3.6% for the [301] direction). These observations are of crucial importance for the interpretation of the results of Tedrow and Meservey, using polycrystalline Ni-films in a superconducting tunneling experiment. Using the band structure calculation of Zornberg, we can show that, assuming s-tunneling only but including s-d hybridization, very large positive P values can be expected. Further inclusion of d-contribution is considered, and found to bring about reasonable agreement with experiment. We also point out the inadequacy of the usual field emission theory in the case of high-index crystallographic faces; in this case the condition k ‖ + G i = 0 , where k‖ is the parallel component of t he electron momentum, and Gi a vector of the two-dimensional surface reciprocal lattice, can be fulfilled by a large number of k values on the Fermi surface, including many with k ‖ ≠ 0 . This problem is investigated by using a scattering approach and its effect on the direction dependence of the spin polarization is discussed.