Awards - Gerrit van der Laan
Network Member shares Europe's Most Prestigious Physics Prize Agilent Technologies Europhysics Award for the Discovery of Magnetic X-ray Dichroism
Paolo Carra (ESRF, Grenoble, France), Gerrit van der Laan (Daresbury
Laboratory, Warrington, UK) and Gisela Schutz (Institute of Physics,
Wurzburg, Germany) have won the Agilent Technologies Europhysics Award for the
year 2000 for their Pioneering work in establishing the field of magnetic X-ray dichroism
.
The award is given for work leading to advances in the fields of electronic,
electrical and materials engineering which represents scientific excellence
.
The award ceremony took place on March 17th during the EPS Condensed Matter General Conference in Montreux, Switzerland. The Agilent Technologies Europhysics Award was formerly known as the Hewlett Packard Europhysics Award. Due to the recent split by Hewlett Packard into two separate companies (Hewlett Packard for the development sale of computer equipment and Agilent Technologies for the development and sale of equipment for systems and measures), this prestigious award is now given by Agilent Technologies. The prize is awarded every year for recent work by one or more individuals in the area of physics of condensed matter, specifically work leading to advances in the fields of electronic, electrical and materials engineering which represents scientific excellence. A complete list of prize winners over the years is given below. This list contains no less than seven Nobel prize laureates, namely von Klitzing (Physics, 1985), Binnig and Rohrer (Physics, 1986), Bednorz and Mueller (Physics, 1987), Kroto and Smalley (Chemistry, 1996).
Magnetic X-ray dichroism (MXD) is considered as one of the most important discoveries in the field of magnetism in the last decennial. Sum rules allow to separate the spin and orbital parts of the magnetic moment. They can be used to measure independently the orbital moment providing a useful insight into the microscopic origin of anisotropic magnetic properties, such as the magnetocrystalline effect, easy direction of magnetization, magnetostriction and coercivity of magnetic materials. The surface sensitivity combined with the element and site specificity of the technique make it immensely valuable in the study of magnetic thin films and multilayers. The interesting special magnetic properties characteristic of thin films derive from modifications in symmetry and coordination of the atoms primarily at the surface and interfaces. Materials with enhanced spin and orbital moments, perpendicular anisotropy and antiferromagnetic coupling between ferromagnetic layers across a non-magnetic spacer can all now be prepared - the latter two types of material are of great interest to the magnetic recording industry. Determination of the polarization of a non-magnetic metal at an interface with a ferromagnetic layer provides a useful insight into the coupling mechanism in such materials. An important part of the pioneering work in the field of MXD has been done by Theo Thole, who died on July 4th 1996 after a fatal fall down the stairs at his home. [1] Would he still have been alive, he certainly would have been sharing the Agilent award. A short - and far from exhaustive - history of MXD is presented below.
In 1985 Theo Thole, Gerrit van der Laan, and George Sawatzky [2] from the Rijks University Groningen (NL) predicted a strong magnetic dichroism in the M X-ray absorption spectra of magnetic rare-earth materials, for which they calculated the temperature and polarization dependence. A year later this MXD effect was confirmed experimentally by van der Laan et al. [3]. Using linearly polarized soft X-rays from the ACO storage ring at Orsay near Paris and total electron yield detection, the Tb M absorption edge of terbium iron garnet showed the predicted polarization and temperature dependence. The next year Gisela Schutz et al. [4] performed measurements using X-ray transition at the K edge of iron with circularly polarized X-rays, where the asymmetry in absorption is in the order of 10. This was shortly followed by the observation of magnetic EXAFS. [5] A theoretical description for the XMCD at the Fe K absorption edge was given by Ebert et al. [6] using a spin-polarized version of relativistic multiple scattering theory with relativistic effects and spin polarization treated on equal footing. This was soon also extended to include the X-ray magneto-optical Kerr effect. [7]
In 1990 Chen et al. [8] observed a large magnetic dichroism at the L edge of nickel metal. Also cobalt and iron were showing huge effects, which rapidly brought forward the study of magnetic 3 transition metals, which are of technological interest. Full multiplet calculations for 3 transition metal L edges by Thole and van der Laan [9] were confirmed by several measurements on transition metal oxides. First considered as a rather exotic technique, MXD has now grown out as an important measurement technique for local magnetic moments. Dedicated beamlines and insertion devices for polarized X-rays were constructed at all synchrotron radiation facilities worldwide supporting extensive research programmes.
MXD can be explained by electric multipole transitions from the core state to unoccupied valence states in combination with the Pauli exclusion principle. In 1988 Thole and van der Laan [10,11] presented a general analysis to explain the systematic trends observed in the branching ratio of the L edges in 3 transition metals and the M edges in rare earth systems. Using angular momentum algebra they showed that the branching ratio of the isotropic spectrum is proportional to the angular part of the spin-orbit operator in the ground state. In 1992, Thole, Carra, Sette, and van der Laan [12] derived a further sum rule showing that the X-ray magnetic circular dichroism signal integrated over the entire absorption edge is proportional to the orbital part of the magnetic moment. An additional sum rule was derived by Carra, Thole, Altarelli, and Wang [13], establishing the relation between the effective spin magnetic moment and the weighted difference in intensity over the two spin-orbit split edges. These sum rules have been confirmed by Wu and Freeman [14] and others using the thin film full potential linearized augmented plane wave energy band method and a tight binding analysis for Fe, Co and Ni metal. Carra et al. [15] extended these sum rules to electric multipole transitions. Sum rules in coupled operators were derived by van der Laan [16], who also showed that for metallic 3d transition metal systems the magnetocrystalline anisotropy energy is directly related to the anisotropic part of the spin-orbit interaction. [17] The expectation value of the spin-orbit interaction can be obtained using the sum rule for X-ray magnetic linear dichroism.
XMCD in transverse geometry, i.e. with the applied magnetic field perpendicular to the light helicity vector, gives the possibility to determine, element-specifically, the easy direction of magnetization. This was demonstrated by studying the changes in the magnetocrystalline anisotropy that occur when a 3 ML Co film was deposited onto a 33 ML thick Ni layer on Cu, inducing a spin reorientation from perpendicular to in-plane. [18]
Sum rules for X-ray magnetic scattering were derived by Luo, Trammell, and Hannon [19]. Circular dichroism in X-ray resonant magnetic scattering can be used to obtain the magnetization profile of magnetic patterns in thin films by using circular dichroism to recover the phase relation in X-ray resonant magnetic scattering. This has been demonstrated for single-crystalline FePd layers with striped magnetic domain pattern providing unambiguous evidence for the presence of magnetic flux closure domains. [20]
Magnetic dichroism is also present in core level and valence band photoemission. When the emitted photoelectron has no interaction with the system left behind, the dichroism is due to the angular dependent part of the electrostatic interaction between the core hole and localized valence electrons [21]. The general theory for dichroism in photoemission has been developed by Thole and van der Laan and is described in a series of four papers. [22] In Paper I the authors discuss the origin of the spin polarization and magnetic dichroism. The different ways to orient the polarizations of the magnetization, electric vector of the light and the spin of the photoelectron allow measurements of different kinds of correlations between the corresponding atomic properties: the valence spin, core hole orbital momentum and core hole spin, respectively. Fundamental spectra can be defined as those linear combinations of the polarized spectra that are directly connected to physical properties. They are the natural quantities to obtain the information about the many-electron system contained in the spectrum. Magnetic dichroism in core level photoemission, which gives the alignment between the valence spin magnetic moment and the core hole orbital moment requires both spin-orbit and electrostatic interactions. Therefore, the strength of this alignment can be expressed in terms of the magnitude of these interactions. In Paper II it was shown that for the emission from an incompletely filled localized shell, such as the 4f shell in the rare earths, the integrated intensities of the magnetic circular dichroism and spin spectrum are proportional to the ground state orbital and spin magnetic moment, respectively. In Paper III the angular dependence of the polarized photoemission is treated. The geometry can be separated from the physical properties and the angular dependence provides a way to measure higher magnetic moments. The interference term between the and emission channels allows to measure the odd magnetic moments with linearly polarized light. In angle integrated photoemission these magnetic moments can only be measured with circularly polarized light. Resonant photoemission is described in Paper IV with a discussion on core-hole polarization and a derivation of super sum rules.
Finally, to mention just one recent interesting development, the combination of high-resolution transmission X-ray microscopy based on the zone plate technique with X-ray magnetic circular dichroism, providing a huge contrast, is a new technique to image magnetic domain structures with a lateral spatial resolution down to 30 nm. [23]
The above gives only some of the highlights in the area of MXD and is by no means a complete overview. MXD is a rapidly expanding field and lots of new results can be expected in the forthcoming years.
REFERENCES
[1] Obituary, Theo Thole, 1950 - 1996, J. Synchrotron Rad. 3, 248 (1996).
[2] B.T. Thole, G. van der Laan, and G.A. Sawatzky,
Strong magnetic dichroism predicted in the M X-ray absorption spectra of
magnetic rare earth materials,
Phys. Rev. Lett. 55, 2086 (1985).
[3] G. van der Laan, B.T. Thole, G.A. Sawatzky, J.B. Goedkoop, J.C. Fuggle,
J.M. Esteva, R.C. Karnatak, J.P. Remeika, and H.A. Dabkowska,
Experimental proof of magnetic X-ray dichroism,
Phys. Rev. B 34, 6529 (1986).
[4] G. Schutz, W. Wagner, W. Wilhelm, P. Kienle, R. Zeller, R. Frahm, and G. Materlik,
Absorption of circularly polarized X-rays in iron,
Phys. Rev. Lett. 58, 737 (1987).
[5] G. Schutz, R. Frahm, P. Mautner, R. Wienke, W. Wagner, W. Wilhelm, and P. Kienle,
Spin-dependent extended X-ray-absorption fine-structure - probing magnetic short-range order,
Phys. Rev. Lett. 62, 2620 (1989).
[6] H. Ebert, P. Strange, and B.L. Gyorffy,
Theory of circularly polarized X-ray absorption by ferromagnetic Fe,
J. Appl. Phys. 63, 3055 (1988).
[7] H. Ebert,
Magneto-optical effects in transition metal systems,
Rep. Prog. Phys. 59, 1665 (1996).
[8] C.T. Chen, F. Sette, Y. Ma, and S. Modesti,
Soft-X-ray magnetic circular-dichroism at the L edges of nickel,
Phys. Rev. B 42, 7262 (1990).
[9] G. van der Laan and B.T. Thole,
Strong magnetic X-ray dichroism in 2p absorption spectra of 3d transition metal ions,
Phys. Rev. B 43, 13401 (1991).
[10] G. van der Laan and B.T. Thole,
Local probe for spin-orbit interaction,
Phys. Rev. Lett. 60, 1977 (1988).
[11] B.T. Thole and G. van der Laan,
Linear relation between X-ray absorption branching ratio and valence-band spin-orbit expectation value,
Phys. Rev. A 38, 1943 (1988).
[12] B.T. Thole, P. Carra, F. Sette, and G. van der Laan,
X-ray circular dichroism as a probe of orbital magnetization,
Phys. Rev. Lett. 68, 1943 (1992).
[13] P. Carra, B.T. Thole, M. Altarelli, and X. Wang, X-ray circular dichroism and local magnetic fields Phys. Rev. Lett. 69, 2307 (1993).
[14] R. Wu, D. Wang, and A.J. Freeman,
First principles investigation of the validity and range of applicability of the X-ray magnetic circular-dichroism sum-rule,
Phys. Rev. Lett. 71, 3581 (1993).
[15] P. Carra, H. Konig, B.T. Thole, and M. Altarelli,
Magnetic-X-ray dichroism - general features of dipolar and quadrupolar spectra,
Physica 192 B, 182 (1993).
[16] G. van der Laan,
Angular momentum sum rules for X-ray absorption,
Phys. Rev. B 57, 112 (1998).
[17] G. van der Laan,
Magnetic linear X-ray dichroism as a probe of the magnetocrystalline anisotropy,
Phys. Rev. Lett. 82, 640 (1999).
[18] H.A. Dürr, G.Y. Guo, G. van der Laan, J. Lee, G. Lauhoff, and J.A.C.
Bland,
Element-specific magnetic anisotropy determined by transverse magnetic circular X-ray dichroism,
Science 277, 213 (1997).
[19] J. Luo, G.T. Trammell, and J.P. Hannon,
Scattering operator for elastic and inelastic resonant X-ray scattering,
Phys. Rev. Lett. 71, 287 (1993).
[20] H.A. Dürr, E. Dudzik, S.S. Dhesi, J.B. Goedkoop, G. van der Laan, M.
Belakhovsky, C. Mocuta, A. Marty, and Y. Samson,
Chiral magnetic domain structures in ultrathin FePd films,
Science 284, 2166 (1999).
[21] G. van der Laan,
Magnetic-circular-dichroism in core level photoemission of localized magnetic systems,
Phys. Rev. Lett. 66, 2527 (1991).
[22] B.T. Thole and G. van der Laan,
Spin polarization and magnetic dichroism in photoemission from core and valence states in localized magnetic systems,
Phys. Rev. B 44, 12424 (1991); ibid 48, 210 (1993); ibid 49, 9613 (1994); J. Phys.: Condens. Matter 7, 9947 (1995).
[23] P. Fischer, T. Eimuller, G. Schutz, P. Guttmann, G. Schmahl, K. Pruegl,
and G. Bayreuther,
Imaging of magnetic domains by transmission X-ray microscopy,
J. Phys. D: Appl. Phys. 31, 649 (1998).
APPENDIX: COMPLETE LIST OF PRIZE WINNERS OVER THE YEARS
2000, Montreux,
P. Carra (I), G. van der Laan (NL), G. Schuetz (D),
For Pioneering Work in Establishing the Field of Magnetic X-ray Dichroism.
1999, London,
C. Glallti (F), M. Reznikov (IS),
For Developing Novel Techniques for Noise Measurements in Solids Leading to
Experimental Observation of Carriers with a Fractional Charge.
1998, Grenoble,
M. T. Rice,
For Original Contributions to the Theory of Strongly Correlated Electron
Systems.
1997, Leuven,
A. Fert (F), P. Gruenberg (D), S. S. P. Parkin (GB),
For Discovery and Contribution to the Understanding of the Giant
Magnetoresistance Effect in Transition-Metal Multilayers and for
Demonstrations of its Potential for Technological Applications.
1996, Stresa,
R.H. Friend,
Pioneering work on Semiconducting Organic Polymer Materials and
Demonstrating of an Organic Light Emitting Diode.
1995, Telford,
Yakir Aharonov (IL), Michael V. Berry (GB),
For Introduction Fundamental Concepts in Physics that have profound Impact
on Condensed Matter Science.
1994, Madrid,
D.R. Huffman (USA) , W. Kraetschmer (D), H.W. Kroto (GB), R.E. Smalley (USA),
New Molecular Forms of Carbon and their Production in the Solid State.
1993, Regensburg,
B.L. Altshuler (RU), A.G. Aronov (RU), D.E. Khmelnitskii (RU), A.I. Larkin
(RU), B. Spivak (RU),
Theoretical Work on Coherent Phenomena in disordered Conductors.
1992, Prague,
G. Ertl (D), H. Ibach (D), J. Peter Toennies (D),
Pioneering Studies of Surface Structures, Dynamics and Reactions through the
Development of Novel Experimental Methods.
1991, Exeter,
K. Bechgaard (DK), D. Jerome (F),
For the Synthesis of a New Class of Organic Metals and the Discovery of
their Superconductivity and Novel Magnetic Properties.
1990, Amsterdam,
R. Car (I), M. Parrinello (I),
A Novel and Powerful Method for the ab-initio Calculation of Molecular
Dynamics.
1989, Nice,
F. Steglich (D), H.-R. Ott (CH), G.G. Lonzarich (GB),
Pioneering Investigations of Heavy-Fermion Metals.
1988, Budapest,
J.G. Bednorz (D), K.A. Mueller (CH),
Discovery of High-Temperature Superconductivity.
1987, Helsinki,
I.K. Yanson (USSR),
Point-Contact Spectroscopy in Metals.
1986, Stockholm,
F. Mezei (H),
Neutron Spin Echo Spectroscopy.
1985, Berlin,
J. Als-Nielsen (DK), M. Pepper (GB),
The Experimental Study of Low Dimensional Physics.
1984, Prague,
G.K. Binnig (D), H. Rohrer (CH),
For building the Scanning Tunnelling Microscope.
1983, Lausanne,
A.F. Silvera (NL),
Atomic and Solid Hydrogen.
1982, Manchester,
K. von Klitzing (D),
Experimental Demonstration of the Quantized Hall Resistance.
1980, Leeds,
O.K. Andersen (DK), A.R. Miedema (NL),
Original Methods for the Calculation of the Electronic Properties of
Materials.
1979, Paris,
E.A. Ash (GB), J.H. Collins (GB), Y.V. Gulaev (USSR), K.A. Ingebrigtsen (N),
E.G.S. Paige (GB),
The Physical Principles of Surface Acoustic Wave Devices.
1978, York,
Z.I. Alferov (USSR),
Heterojunctions.
1977, Leeds,
W.E. Spear (GB),
Amorphous Silicon Devices.
1976, Heidelberg,
W. Helfrich (D),
Contributions to the Physics of Liquid Crystals.
1975, Bucharest,
V.S. Bagaev (USSR), L.V. Keldysh (USSR), J.E. Pokrovsky (USSR), M. Voos (F),
The Condensation of Excitons.