Abstract:

Spin polarized electrons are used in condensed matter physics to characterize magnetic materials by scattering or tunneling measurements. Spin polarized electron beams are also frequently employed in high energy physics experiment. In the present work, we have generated spin polarized electron beams by vacuum tunneling in field emission from sharp tungsten tips coated with thin ferromagnetic films. The transverse spin polarization directions that have been observed in field emission from Co and Fe films on W(111) and W(001) tips correlate with the in-plane magnetization easy axes of films on macroscopic crystalline surfaces. Superparamagnetic fluctuations of tip magnetization are also evident in the polarization direction of emitted electrons for very thin films. The superparamagnetic limit to spin polarized field emission can be overcome and long-term stability of polarization can be restored by increasing film thickness modestly. Results of spin resolved field emission energy distribution measurements will also be presented that may help to understand the spin polarized vacuum tunneling process at a more fundamental level. Besides making progress towards highly spin polarized field emission sources, this work may also contribute to an understanding of the tunneling process in spin polarized scanning tunneling microscopy.