Spin Dependent Electron Mean Free Path in Ferromagnets
The electron mean free path (MFP) plays a fundamental role in all applications of electron spectroscopic techniques in surface science. It determines the probing depth of the various methods. In the low-energy range used (typically below 1 keV) energetic electrons in solids are strongly scattered leading to a strong attenuation of the intensity with a l/e attenuation length on the order of only a few atomic distances. In ferromagnetic materials, at least in principle, the mean free path can be expected to be spin dependent due to the spin dependence of the scattering processes. A spin dependent mean free path obviously will affect the measured polarizations in spin polarized spectroscopies, e.g., in photoemission. While there is general agreement that the mean free path of electrons in ferromagnets is indeed spin dependent at low energies the underlying mechanism is controversial. One point of concern is whether the main effects are due to a spin dependence of elastic or inelastic scattering. In this lecture I will summarize the evidence for a spin dependent electron mean free path in 3d transition metals. Some of the evidence is based on a direct measurement of spin dependent attenuation in ultrathin films whereas the evidence from other methods, like spin polarized secondary electron emission spectroscopy and spin polarized electron energy loss spectroscopy, is more indirect but agrees qualitatively with the results of the direct measurements. Spin polarized electron energy loss spectroscopy suggests that the main spin dependent scattering mechanism is due to Stoner excitations.
KeywordsFree Path Inelastic Scattering Spin Asymmetry Mean Free Path Attenuation Length
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