Spin polarization of field-emitted electrons from Fe, Co, Ni, and rare-earth metals

The spin polarization of field-emitted electrons is investigated at 10-K emitter temperature and magnetic field strengths up to 6 T. Field emitters are electrolytically etched Fe, Co, and Ni tips of polycrystalline material and W tips coated by vapor deposition with Fe, Co, Ni, and rare earths. Uncoated W tips show no polarization. The maximum polarization obtained so far is + 80% and + 47% for Fe and Co, respectively (the "+" sign indicates predominant majority spins). Often the polarization is found to vary beyond control in magnitude and occasionally even in sign, depending upon, e.g., surface preparation, emission spot, tip crystallography, adsorption. Clean Ni tips show (-3.5 ±{} 1.5)% in the vicinity of $〈111〉$ and (0 ±{} 1.5)% in the vicinity of $〈100〉$. In both cases polarization increases with time up to + 15% by hydrogen adsorption. All ferromagnetic rare-earth metals give rather high electron spin polarization, clean Gd up to + 53%, EuO and EuS show up to + 90%. The temperature dependence of the polarization follows approximately the saturation magnetization, as checked with Ni, Gd, EuO.