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Surface Science

Volume 488, Issues 1-2
1 August 2001
Pages 90-98

DOI: 10.1016/S0039-6028(01)01109-8
PII: S0039-6028(01)01109-8
Copyright © 2001 Elsevier Science B.V. All rights reserved.

Spin-polarized electron emission induced by impact of protons and electrons on Cr/Fe(1 0 0)

R. Pfandzelter, , M. Ostwald and H. Winter

Institut für Physik, Humboldt-Universität zu Berlin, Invalidenstrasse 110, 10115 Berlin, Germany

Received 14 July 2000;  accepted 4 April 2001.  Available online 18 July 2001.

Abstract

We report on spin- and energy-resolved secondary electron emission, induced by impact of fast protons and electrons on ultrathin Cr films epitaxially grown on Fe(1 0 0). Based on a simple model, we extract spin- and energy-dependent probing depths from experimental polarization spectra. For grazing impact of protons the probing depth strongly depends on the energy of secondary electrons.

Author Keywords: Secondary electron emission measurements; Electron emission; Metal¯metal magnetic thin film structures; Magnetic surfaces; Single crystal epitaxy; Ion¯solid interactions; Iron; Chromium

Article Outline

1. Introduction
2. Experiment
3. Experimental results
4. Evaluation of data
5. Discussion
6. Summary and conclusions
Acknowledgements
References


 (15K)
Fig. 1. Intensity distribution (curves, right-hand ordinate) and spin polarization (symbols, left-hand ordinate) of electrons excited by (a) 25 keV protons and (b) 4 keV electrons. The spectra refer to the clean Fe(1 0 0) surface (bottom) and Cr coverages (±0.2 ML in each case) as indicated. The intensity distributions are normalized to the intensity at 20 eV. The origins are displaced vertically by constant amounts (- - -).

(9K)
Fig. 2. Energetic positions of intensity maximum () and fine structure in the spin-polarization curve () for impact of 4 keV electrons. The fine-structure positions are derived from maxima of the second derivative d2P/dE2 of smoothed polarization curves.

(11K)
Fig. 3. Measured spin polarization (symbols) of electrons excited by grazing impact of 25 keV protons on Cr/Fe(1 0 0) versus Cr overlayer thickness. Different symbols refer to values averaged over spectral ranges as indicated. The solid lines are results of calculations ((2), (4) and (5)) for layer-dependent magnetic moments from Fig. 5 and the following parameters (from bottom to top): s=a=0.5 ML, As=Aa=0, f=2.2; s=0.4×4.2 ML, a=0.4×2.9 ML, As=0.25×0.28, Aa=0.25×0.16, f=1.9; s=0.8×4.2 ML, a=0.8×2.9 ML, As=0.55×0.28, Aa=0.55×0.16, f=1.7; s=4.2 ML, a=2.9 ML, As=0.28, Aa=0.16, f=1.3.

(12K)
Fig. 4. Same as Fig. 3 for impact of 4 keV electrons. The calculations are performed with mean free paths s=4.2 ML, a=2.9 ML, f=1 and asymmetries As and Aa from Fig. 3. For the dashed line we assumed smaller mean free paths s=3 ML, a=2 ML.

(6K)
Fig. 5. Magnetic moment per atom at the topmost film layer versus total number of Cr layers on Fe(1 0 0), obtained from a fit to the data from Fig. 3 (10¯20 eV) according to (2), (4) and (5). Alloyed layers are marked by * (0* and 1* refer to the subsurface and surface layer after deposition of 1 ML of Cr, respectively).

(9K)
Fig. 6. Calculated spin polarization ((2), (4) and (5)) with layer-dependent magnetic moments from Fig. 5. (a) Variation of mean free paths s, a with constant asymmetries (As=0.28, Aa=0). (b) Variation of asymmetries As, Aa with constant mean free paths (s=4.2 ML,   a=2.9 ML).

(5K)
Fig. 7. Probing depths as function of electron energy for proton () and electron () excitation, respectively (left-hand ordinate). Asymmetry A of spin-dependent scattering cross-sections as function of electron energy (, right-hand ordinate).

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Corresponding author. Tel.: +49-30-2093-7693; fax: +49-30-2093-7899; email: pfandz@physik.hu-berlin.de
This document
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Journal Format-PDF (197 K)
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Surface Science
Volume 488, Issues 1-2
1 August 2001
Pages 90-98


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