New standard of spin detectors

Abstract

ABSTRACT This paper is a summary of experimental results obtained on spin-dependent transmission of hot electrons through thinferromagnetic films. The goal of this contribution is to demonstrate that very simple devices can be used for measuringthe spin polarization of a free electron beam. Both cases of free-standing magnetic metal foils and ferromagnetic metal /semiconductor Schottky diode are studied here and analyzed in terms of electron spin detection. In these experiments anelectron beam is injected from vacuum into the magnetic thin film with controlled energy and spin polarization. Thespin-dependence of the transmitted current, originating from the spin-filtering through the magnetic layer, yields a newand efficient way for measuring the incident electron beam polarization.Keywords: spin-polarized electrons, spin detector, hot electrons, spin-dependent transport, ferromagnetic thin films 1. INTRODUCTION The development of sources and detectors of spin-polarized free electrons in the past decades has given an access tovarious spin-dependent processes that occur in physics. With the use of spin-polarized electron probes, many progresseshave been done in condensed matter - especially in surface science - and in high-energy physics. Nevertheless, thepractical manipulation of the spin polarization of free electrons is not as usual as it is for photons. The famous Stern-Gerlach technique, which allows to select fully polarized free atom beams, fails for electrons because of their charge[1]. Consequently, different methods have been investigated and are still investigated for producing and analyzing spin-polarized electrons.Optical spin orientation in semiconductor photocathodes [2] is nowadays widely used for producing intense and highlypolarized electron beams [3]. In a semiconductor, the absorption of circularly polarized light generates spin-polarizedelectrons in the conduction band (optical pumping). By coadsorption of cesium and oxygen, the vacuum barrier of thesemiconductor surface can be lowered below the minimum of the conduction band in the bulk material (activation tonegative electron affinity), allowing photo-electron emission into vacuum [4]. Spin-polarized electron sources workingon this principle exhibit spin-polarization as high as 84% and quantum yield of 0.1% [5].For spin detection, the situation is not so favorable. Therefore, many efforts are still made to develop efficient andconvenient spin polarimeter [6], in particular in view of application to spin-resolved electron spectroscopy andmicroscopy techniques. An ideal electron spin detector would be similar to an optical linear polarizer: the electrons withthe desired polarization are selected while the others are rejected. In fact, all detectors developed so far rely on spin-dependent collision processes (exchange interaction or spin-orbit coupling). Consequently, they all suffer from a lowefficiency, orders of magnitude worse than in the ideal case, and from severe operation conditions (surface preparation,high voltages requirement, large and complex equipment).The Mott polarimeter, based on the spin-orbit interaction of electrons with high atomic weight materials, remains theconventional spin detector for standard measurements. Basically, the spin-polarized electron beam impinges on a thingold foil and the electrons, backscattered in two symetrical directions with respect to the incident beam, are collected oncounters. The current asymmetry between the counters gives the spin polarization component in the directionperpendicular to the scattering plane. But, traditional Mott detectors are cumbersome and inconvenient due to the highvoltage operation (typically 100 kV) and to the dependence of the spin discrimination efficiency with the scatteringangle.To compare the detectors one to the others, two quantities are generally used (even if many considerations have to betaken into account in practical measurements). The first one is the Sherman function S which characterizes the spinselectivity of the detector (scattering asymmetry). The second quantity is the figure of merit F which defines the squareof the signal-to-noise ratio. If I