Chapter 13 - Fundamentals of spin filtering in ferromagnetic metals with application to spin sensors

Abstract

This chapter describes ultrathin ferromagnetic layers as spinfilters, analogous to optical polarizers. Dealing with electrons offer more possibilities because the electron polarization possibly has both longitudinal and transverse components. The inelastic electron mean free path (IMFP) in metals, which characterizes the escape depth, plays a crucial role in these experiments, as in all electron spectroscopies. In transition metals, the IMFP variation is mostly determined by density of state effects, whereas the transition matrix elements introduce weaker corrections. Cobalt and iron thin films have been extensively studied in the chapter. The spin-dependent IMFP component is of particular interest as it mostly originates from electron-electron interaction and can be measured with accuracy. Spin-dependent electron transport of low-energy electrons in a ferromagnetic metal arises because majority- and minority-spin electrons have different relaxation channels due to different final densities of states in the d spin subbands. The chapter establishes simple relations between the number of holes in the d spin subbands and the IMFPs. This information is essential for the conception of devices based on spin filtering. In ballistic-electron devices like hot-electron transistors, only electrons that have undergone almost no energy loss are collected. An electron ballistically injected into the metal layer and that undergoes a change in its wave-vector direction will no longer travel along the device axis, and will thus travel a longer path. Finally, it will be lost due to inelastic scattering. This effect is well known and has supported undimensional transport equations in field-assisted photoemission. The manipulation of spin packets in solids, experiencing coherent spin precession, also brings technological challenges to new frontiers. The development of magnetic semiconductors at room temperature could be a crucial step towards spintronics. The absorption or emission of spin waves and their coupling with light waves may also lead to applications involving long wavelength excitations.