Magnetic Dichroism in Core-Level Photoemission

142 Pages·2000·2.178 MB·English

by  Kai Starke (auth.)

Kai Starke
Magnetic Dichroism
in Core-Level
Photoemission
With 46 Figures
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Springer Tracts in Modern Physics
Volume 951
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Dr. habil. Kai Starke
Freie Universitiit Berlin
Institut ffir Experimentalphysik
Arnimallee 14
D-14195 Berlin, Germany
E-mail: [email protected]
Physics and Astronomy Classification Scheme (PACS): 32.7o.C, 61.66.B, 68.35, 68.55,
71.2o, 71.22, 71.7o, 73.2o, ,mJ.o1.57 75.2o.En, 75.25.+z, 75.3o.Et, 75.3o.Pd, ,gG.o5.57
75.5o.Vv, 75.7o.-i, 75.7o.Ak, 75.7o.C, 78.4o.K, 79.6o, 81.15
ISSN oo81-3869
ISBN 3-540-66268-5 Springer-Veflag Berlin Heidelberg New York
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Starke, Kai: Magnetic dichroism in core level photoemission/Kai Starke. - Berlin; Heidelberg; New ;kroY Barcelona;
Hong Kong; London; Milan; Paris; Singapore; Tokyo: Springer: 2o00. (Springer tracts in modern physics; Vol. )951
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Preface
The joy of insight is a sense of involvement and awe,
the elated state of mind that you achieve
when you have grasped some essential point;
it is akin to what you feel on top of a mountain
after a hard climb,
or when you hear a great work of music.
Viktor Weisskopf
This volume is a review of a rapidly developing field in thin-film magnetism.
Between 1990, when magnetic dichroism in core-level photoemission (MDPE)
was discovered, and today, the attitude of researchers in the field has changed
from surprise to a full appreciation of the physical basis. Most important for
the present level of understanding was the early participation of theorists,
to whom nature was kind enough to reveal some of its underlying structure.
A wealth of primary research literature has now been published on MDPE,
and it is our aim to make a selection of it available to an interested broader
audience. Special emphasis is laid here on the application of MDPE as an
element-specific magnetization probe for low-dimensional rare-earth systems.
After an introduction to core-level photoemission with a brief description
of chemical binding-energy shifts, the basic role of exchange and spin-orbit
interaction in the occurrence of magnetic dichroism is described in detail.
Transparent LS-coupling pictures are developed, which explain MDPE as a
consequence of selection rules in dipole transitions from magnetically oriented
states. They predict particularly large magnetic dichroism effects for the well-
resolved multiplet lines of shallow rare-earth core levels. Experimental chal-
lenges and their state-of-the-art solutions, e.g. for preparing atomically sharp
rare-earth interfaces, are described along with synchrotron radiation sources
for circularly polarized soft x-rays.
We review early MDPE experiments on closed p shells of ferromagnetic
iron to demonstrate that both spin-orbit coupling and intra-atomic exchange
interaction are necessary for magnetic dichroism. The example of Gd 4d shows
how MDPE can give insight into the core-level electronic structure which
could hardly be obtained otherwise.
To my wife Martina,
and my children Nils, Muriel,
and Olivia
VIII Preface
The main part of Magnetic Dichroisra in Core-Level Photoemission is
devoted to rare-earth 4f shells. The sharp and often well-resolved 4f photoe-
mission lines exhibit huge dichroic effects, which are comparable with or even
larger than those known from x-ray absorption. In relevant cases, 4f photoe-
mission line intensities are large for only one magnetization direction, relative
to the incoming circular-polarized light, but nearly absent for the other. A
detailed comparison with recent intermediate-coupling calculations by Thole
and van der Laan - explained in the commonly used LS-coupling scheme -
shows that the basic observations are quantitatively well described by atomic
multiplet theory.
Resonant MDPE is analyzed in great detail at d ~ 4f excitation thresh-
olds. It is shown that the angular-momentum character of the x-ray absorp-
tion states involved is directly reflected by the sign of the associated x-ray
absorption magnetic circular dichroism (XMCD) spectra. In addition, we dis-
cuss the issue of spin-flip excitations in resonant photoemission.
Chapter 7 covers the application of MDPE as an element-specific sensor
for surface and interface magnetization studies. The availability of intense po-
larized light makes it a fast alternative method to established spin-resolving
techniques. Using the core-level chemical shifts at rare-earth metal surfaces,
we discuss the potential of MDPE to probe the orientation of the top-layer
magnetization with respect to adjacent 'bulk' atomic layers. Novel experi-
ments on rare-earth interfaces illustrate the feasibility of magnetometry with
atomic-layer resolution, a unique merit of MDPE by which it provides layer-
by-layer coupling strengths from rare-earth interfaces.
An appendix provides practical advice for experimentalists who want to
apply MDPE at 4f shells for magnetic analysis. The formulae given shall
facilitate the choice of the most appropriate measurement geometry.
It is a pleasure to thank the many collegues whose spirit of collabora-
tion has made the present volume possible. In particular, I am grateful to
G. Kaindl for his scientific advice and continuous support, and for the con-
genial atmosphere in his group at the Freie Universit~t Berlin, of which I
have been a member since 1991. I owe considerable debt to the group's for-
mer and present PhD students, E. Arenholz, .S Bode, A. V. Fedorov, A.
Y. Grigoriev, F. Heigl, F. Hiibinger, E. Navas, the 'multivalent' Zhiwei Hu,
and .S Gorovikov (St. Petersburg State University). I would like to thank L.
Baumgarten (Forschungszentrum Jiilich) for his help in the initial stage of
this work. The numerous atomic multiplet calculations of G. van der Laan
(Daresbury Laboratory) and his theoretical support over the years have be-
come indispensable. In 1994-95 I benefited much from the generous support
by the Deutsche Forschungsgemeinschaft, and I would like to thank C. T.
Chen (Bell Labs) and P. D. Johnson (Brookhaven Nat. Lab.) for their hos-
pitality during that time. Special thanks go to P. Jensen (Freie Universit~t
Berlin), C. .S Fadley (Lawrence Berkeley Nat. Lab.), and J. Henk (Max-
Planck-Institut Halle) for our stimulating collaboration.
Berlin, September 1999 Kai Starke
Contents
o1 Introduction .............................................. 1
2. Theoretical Considerations ............................... 3
2.1 Atomic Multiplet Theory ................................ 4
2.1.1 Selection Rules and Total Final States .............. 4
2.1.2 Photoemission Final States and Line Strength ....... 8
2.2 Simple Cases ........................................... 10
2.2.1 Gd 4f Photoemission: The Role of Final-State
Spin-Orbit Interaction ............................ 10
2.2.2 Tb 4f Photoemission: The sST/2 Component ......... 13
2.3 Geometries for Magnetic Dichroism ....................... 14
2.3.1 Circularly Polarized Light ......................... 16
2.3.2 Linearly Polarized Light ........................... 18
2.4 Binding Energy and Chemical Shift ....................... 19
2.4.1 Core-Level Binding Energy in Rare-Earth Metals ..... 20
2.4.2 4f Photoemission and Surface Core-Level Shift ....... 25
3. Experimental Techniques ................................. 29
3.1 Sample Preparation ..................................... 29
3.1.1 Rare-Earth-Metal Evaporators ..................... 30
3.1.2 Wide-Temperature-Range Tungsten-Crystal Holder ... 31
3.1.3 Structural and Magnetic Characterization
of Rare-Earth-Metal Films ......................... 33
3.2 X-Ray Magnetic Circular Dichroism
as a Complementary Method ............................. 37
3.3 Beamlines for Circularly Polarized Soft X-Rays ............. 39
4. Closed Shells ............................................. 43
4.1 Analytical Single-Particle Model .......................... 44
4.2 Transition Metals: Fe 2p and Fe 3p ....................... 45
4.3 Rare-Earth Metals: Gd 4d ............................... 50
X Contents
. Rare-Earth 4f Shells ...................................... 55
5.1 Gd Metal .............................................. 55
5.2 Heavier Rare-Earth Metals: Tb and Dy ................... 57
5.3 Intermetallic Compounds: TbFex ......................... 59
6. Magnetic Circular Dichroism in Resonant Photoemission . 65
6.1 A Primer on Fano's Theory ............................. 66
6.2 Gd 4d-4f Giant Resonance .............................. 69
6.2.1 Identification of X-Ray Absorption Lines ............ 71
6.2.2 Magnetic Cicular Dichroism
in Resonant 4d-4f Photoemission ................... 73
6.3 Gd 3d-4f Resonance .................................... 76
6.3.1 High-Resolution X-Ray Absorption Magnetic
Circular Dichroism ............................... 77
6.3.2 Spin Flip in Resonant Photoemission ............... 79
. Magnetic Dichroism in Photoemission
as a Near-Surface Magnetic Probe ........................ 85
7.1 Magnetic Surface Reconstruction ......................... 86
7.1.1 Wb(0001) ........................................ 89
7.1.2 Gd(0001) ........................................ 91
7.2 Atomic-Layer Resolution at Heteromagnetic Interfaces:
Eu/Gd(0001) .......................................... 96
7.2.1 An Atomically Sharp Rare-Earth Interface ........... 96
7.2.2 Layer-Dependent Magnetic Coupling of Ordered
Eu Layers ....................................... 100
7.2.3 Thicker Eu Films on Gd(0001) ..................... 107
8. Conclusions and Future Directions ........................ 111
A. Angle-Resolved Photoemission from f Shells: Gd .......... 115
References .................................................... 121
Glossary of Acronyms ........................................ 131
Index ......................................................... 133
1. Introduction
Magnetic coupling in layered metallic structures has become a key issue in
thin-film magnetism since the discovery of interlayer exchange coupling in
the mid-1980s. In multilayers built from different rare-earth elements, it was
found that exchange coupling is mediated over many nonmagnetic atomic
layers. Such long-range oscillatory coupling was also observed in artificially
layered transition-metal films, and from then on basic and applied research
in magnetism aimed at understanding and tailoring the properties of mul-
tilayers. Efforts were exceptionally quickly rewarded with observations of
new scientific phenomena and advances of practical value. 1 Among these,
'giant magnetoresistance' has currently the strongest impact on information
technology. Similarly important are multilayers with perpendicular magnetic
anisotropy, which show promise as 'blue' magneto-optical media. In all these
multilayers with alternating magnetic and 'nonmagnetic' elements, atomic
layers which are located directly at interfaces play a decisive role.
One of the most versatile methods for studying multicomponent magnetic
systems is magnetic circular dichroism in x-ray absorption (XMCD). This
technique is based on the fact that the absorption of circularly polarized x-
rays depends on the relative orientation of the photon momentum and sample
magnetization. Through the involvement of inner-shell electrons, XMCD is
an element-specific technique, as has been demonstrated in numerous stud-
ies Schfitz et al., 1987; Chen et al., 1990; Ebert et al., 1991b; Samant et
al., 1994. Moreover, XMCD can probe magnetic moments in a quantitative
way, on the basis of theoretical 'sum rules' which connect the XMCD signal
with the size of the orbital and spin magnetic moments Thole et al., 1992;
Carra et al., 1993. With these merits, XMCD has become a magnetometry
tool which is widely used at many 'dedicated' synchrotron beamlines supply-
ing circularly polarized x-rays for magnetism studies around the world. The
concomitantly growing need for intense circularly polarized x-ray sources re-
sulted in new insertion-device designs of undulators and wigglers, optimized
1 The improved sensitivity of present-day magnetic read heads si based on 'giant
magnetoresistance' (GMR) in exchange-coupled Co/Cu multilayers. In 5991 IBM
purchased the patent on GMR (obtained in 6891 by .P Griinberg, -nezsgnuhcsroF
trum Jiilich, Germany) - a fortunate example of fast technology transfer and
monetarily rewarded scientific efforts.