Observation of Electron Polarization in Photoemission

Abstract

0 0 0 25-JJJ 0 a O 20-V) M LJJ K I-V) CI UJ lO-0 0 20 40 60 80 IOO TEMPERATURE, K )20 I40 FIG. 1. Temperature dependence of the yield stressin single-pass zone-refined tungsten. Specimens prepared by electrolytic dissolution, without plastic deformation. All specimens were of the same orientation.in which tests could be performed at various temperatures between 4 and 120'K. This temperature range is a relatively unexplored temperature regime for plastic deformation experiments. Some measurements of the yield stress for tungsten crystals are shown in Fig. 1, where the conditions of the test are given. In discussing these results, it is important to emphasize that the observed behavior for the yield stress at low temperatures is not associated with twinning. An extensive microscopic examination of specimens deformed in this temperature range reveals no twinning. Secondly, the stress-strain curves show no characteristic yield drops, such as that associated with twinning. Ne take the absence of any observed twinning as an indication that some basic atomic resistance is rate determining for the motion of the dislocations. It is important for our context, not whether this involves dislocation splitting or kink generation, but simply that a large barrier to dislocation motion exists.In general the sharp temperature dependence from 80 to 20 K insures the presence of the steeply varying potential required if tunneling is to occur at substantial rates. Secondly, the ternperature independent region (between 4 and 20'K) is another characteristic of tunneling processes. 'In summary, then, we report some observations of the mechanical properties of tungsten crystals, which are consistent with quantum mechanical tunneling of dislocations.We have found that photoelectrons from ferromagnetic gadolinium show spin polarization, the preferential direction of the magnetic moment being parallel to the magnetization of the sample. Our experiment yields a degree of polarization P = (5.27 + 0.70) %. P did not vary significantly over the energy range from 2.8 to 5.0 eV of the incident light.Photoemission from solids is a three-step process: (i) By absorbing a photon, the electron goes into an excited state; (ii) it diffuses through the solid and may reach the surface; (iii) it crosses the surface barrier if its energy is high enough and no scattering occurs. The purpose of this Letter is to give experimental evidence that during this process the spin direction of the electron is at least partly conserved. This means that a new method has been found for the investi-gation of spin polarization in electronic energy bands and also a new way of producing polarized electron beams.The electron spin polarization of photoelectrons was measured by means of Mott scattering. Ferromagnetic Gd was chosen because it is known to be a very efficient photoemitter with a low work -function. ' In addition, one knows from positron-annihilation experiments that a nonvanishing electron spin polarization exists in the 597

Spintronics: Fundamentals and applications

“…7 Unless explicitly specified, we shall use the terms ''nonmagnetic'' and ''paramagnetic'' interchangeably, i.e., assume that they both refer to a material with no long-range ferromagnetic order and with Zeeman-split carrier spin subbands in an applied magnetic field. 8 It has been shown that electrons photoemitted from ferromagnetic gadolinium remain spin polarized (Busch et al, 1969).…”

Can spin-polarized photoemission measure spin properties in condensed matter?

“…More than four decades ago Siegmann et al [5] have shown that the spin-polarization of electrons photoemitted from a magnetic sample is related to its magnetization state. The 1980ies and early 90ies saw numerous studies related to the magnetism of surfaces and ultrathin films by spin-polarized photoemission by a small number of research groups.…”

Polarized electrons from ferromagnets

“…На рис. 4 показан фотокатод вместе с электронно-оптической систе-мой 26 . При такой геометрии из-за большого размагничивающего фактора необходимы большие поля для намагничивания образца.…”

“…Моттовский детектор регистрирует лишь поперечную, т. е. перпендикулярную к направлению скорости поляризацию пуч-ка. Поэтому в случае намагничивания образца перпендикулярно к поверхности приходится преобразовывать продольную* поляризацию пучка в поперечную с помо-щью цилиндрического конденсатора 26 . Кроме того, с помощью этого конденсатора пучок электронов отклоняют от направления освещения и отсеивают фотоэлектроны с первых электродов электронно-оптической: системы.…”

“…S считалось равным 0,31 ± 0,01 26 . Это несколько меньше эксперименталь-ного значения (0,36 ± 0,01), найденного для этих значений энергии и углш рассеяния в работе 38 .…”