A review of the theoretical and experimental analyses of electron spin polarization in ferromagnetic transition metals: I. Field emission, photoemission, magneto-optic Kerr effect and tunneling

Abstract

Until recently studies of solid surfaces have used exclusively photons, atoms, ions, and unpolarized electrons as experimental probes. However, in the last few years polarized electron beams have been used to investigate the electronic and atomic properties of the bulk and surfaces of solids. This has resulted in the development and application of a number of different experimental techniques sensitive to the electron spin polarization (ESP). The experimental results have often been less than definitive and frequently conflicting. For example, the observation of ESP from the ferromagnetic metals by photoemission, field emission, and tunneling experiments lead to contradictory conclusions concerning the validity of the Stoner-Wohl-farth-Slater (SWS) band theory of ferromagnetism. Additional complication of this problem has been introduced by recent magneto-optical Kerr-effect measurements which tend to support the ESP predicted by SWS theory. In this review we present a critical analysis of these experiments and their theoretical interpretation. It is shown that the results of these experiments do not necessarily contradict each other nor any of the current one-electron theories of itinerant ferromagnetism. It is further shown how, based on the use of over-simplified models to describe the physical processes involved in the experiments, the opposite conclusion can be reached. Based on the analyses in this paper, improvements in the theoretical models are suggested. We also discuss, from a theoretical point of view, the relative advantages and the limitations of the different experimental techniques using ESP to probe the electronic and magnetic structures of solids and surfaces. A fundamental objective of this and the following paper is to help develop a theory of spinpolarized electron emission and tunneling phenomena which will provide the basis for a systematic ESP spectroscopy of surfaces, interfaces, and thin films. This has prompted a reexamination of the ferromagnetism of 3d-transition metals and of their surfaces. The present work leads to some observations concerning the current interpretation of itinerant ferromagnetism and possible refinements of the SWS theory to produce better agreement with the ESP sensitive experiments.