My scientific research began in the area of optical spectroscopy during my studies toward a Master of Science degree at Washington State University where I was a Fulbright scholar in 1969-71. I studied the optical spectra of rare earths in various crystals, with the goal of determining the energy levels of the electronic ground state configuration and explaining the observed fine structure by crystal electric field calculations.
My Ph. D. thesis at the Technical University in Munich, Germany, during 1971-74 involved Mössbauer spectroscopy on the magnetic coupling and spin relaxation of rare earth impurities in various non-magnetic metals. At the time, such coupling phenomena were of great interest i n conjunction with such topics as the RKKY coupling, the Kondo effect, and the existence of giant magnetic moments.
During my postdoc days at Lawrence Berkeley Laboratory (1975-77) I used photoemission spectroscopy in all its variants (XPS, UPS, ARPES, Ph.D.) with emphasis on angle resolved photoemission studies and band mapping of noble metals. I was fortunate to experience the early days of synchrotron radiation experiments at SSRL. This exciting time constituted the beginning of my long love of synchrotron radiation research, which to this day has remained my main scientific focus.
My independent scientific career started as a staff scientist at SSRL. Upon my arrival in 1977, I decided to work in the challenging soft x-ray region, which at the time was largely inaccessible, owing to the unavailability of ultra-smooth optics and the omnipresent carbon contamination on the optical elements in grating monochromators which depressed the photon flux in the 250 - 1000 eV range. In the 1000 - 2000eV range, where crystal monochromators can be used it was difficult to obtain monochromator crystals with suitable d-spacings which could withstand the incident x-ray power. Earlier synchrotron radiation research had therefore focused on the ultraviolet region below about 40 eV, where near normal incidence optics could be used, and on the x-ray region above 4 keV, where conventional monochromator crystals like Si were readily available and the radiation could be brought into air through berillium windows. My goal as a staff scientist at SSRL was to open up the soft x-ray region which provides access to the K absorption edges of the chemically important elements carbon, nitrogen, oxygen, aluminum, silicon, and sulfur, to the important L absorption edges of the 3d metals, especially the magnetic materials iron, cobalt and nickel, and the M edges of the rare earths.
The results in the graph below show the success of this effort. The left column demonstrates the progress that was made in the late '70s with the grazing incidence grating monochromator "grasshopper" to produce soft x-rays in the range above the carbon K-edge (280 eV). The first breakthrough came from using high quality mirrors. The second from cleaning the optics of carbon contamination. However, the flux still deteriorated with time because the intense synchrotron radiation cracked hydrocarbons and deposited them on the o ptics (Appl. Optics 19, 3911 (1980)). The right column demonstrates the results obtained with the first ultra high vacuum double crystal monochromator "Jumbo" in the early '80s (Nucl. Instr. Meth. 195, 115 (1982)). As as staff scientist I was responsible for both monochromators and led the team that built Jumbo. These monochromators at SSRL led to pioneering studies in the soft x-ray region by many groups and this work stimulated the planning for the high brightness Advance d Light Source (ALS) in Berkeley.
My research at SSRL (1977-81) and later at EXXON (1981-85) focused on the development of the surface EXAFS and NEXAFS techniques for the determination of the geometric arrangement and bonding of atoms, molecules and thin organic films on surfaces. These experiments concentrated on surface complexes formed by C, N, O and S atoms and molecules, which constitute the most important elements in catalytic reactions. In the process of developing reliable x-ray based techniques for surface investigations, I also explored photon stimulated ion desorption and later contributed to the development of x-ray emission spectroscopy for the atom-specific investigation of the surface chemical bond. At the IBM Almaden Research Center (1985-99) my surface science interests shifted to the study of orientation and relaxation phenomena at polym er surfaces, in particular, the origin of liquid crystal alignment on such surfaces. My interest in magnetism research was stimulated by the strong magnetism research programs at Almaden and by managing the Department of Magnetic Materials and Phenomena from 1991 to 1994. During that time I began to use soft x-rays for magnetic spectroscopy and spectro-microscopy of magnetic thin films and interfaces which constitutes the major part of my research program today. The studies are carried out with polarized soft x-ray synchrotron radiation which offers access to the most important L absorption edges of Fe, Co and Ni. Among the unique capabilities of such x-ray magnetic dichroism studies are their elemental, chemical and magnetic specificity and the ability of magnetic imaging with x-ray photoemission electron microscopy (XPEEM< /a>) , presently offering a lateral resolution of 20nm (2nm in the near future).
Below I will briefly discuss the main milestones and areas of scientific research, give examples of some work, and list the most important publications associated with my research efforts as an independent scientist.
1978: Introduction of the SEXAFS technique. A group at Bell Labs (in the hard x-ray range) and I (in the soft x-ray range, Phys. Rev. B18, 4132 (1 978)) pioneered the surface extended x-ray absorption fine structure (SEXAFS) technique as a tool for exploring surface structures. Over the following years I developed SEXAFS from a conceptual into a powerful mature technique for the study of chemisorbed low-Z atoms such as C, N, O and S. Highlights of the work are given in Phys. Rev. Lett. 43, 1882 (1979), Surf. Science 117, 503 (1982), Phys. Rev. Lett. 49, 142 (1982) , Phys. Rev. Lett. 55, 1468 (1985), and Surf. Science 177, 114 (1986). Todate SEXAFS has been used to determine more than 100 adsorbate geometries on surfaces. Only LEED has determined more surface structures. I have written a comprehensive review of the technique, published in "X-Ray Absorption: Principles, Applications, Techniques of EXAFS, SEXAFS and XANES", Edit. D. C. Koningsberger and R. Prins (Wiley, 1988).
First SEXAFS data 12/5/1977
SEXAFS Review - 1988
1980-1983: Photon Stimulated Ion Desorption (PSID). During this time period I explored PSID from surfaces, following core electron excitation, as a structural tool (Phys. Rev. Lett. 45, 1870 (1980)), identified desorption mechanisms such as multielectron excitations (Phys. Rev. Lett. 47, 1300 (1981)), and x-ray induced electron stimulated desorption (Surf. Science 134, 547 (1983)).
1981: Introduction of the NEXAFS technique. After recording the first near edge x-ray absorption fine structure (NEXAFS) spectra of chemisorbed molecules (Phys. Rev. Lett. 47, 381 (1981) and Phys. Rev. B26, 4111 (1982), I developed NEXAFS into a powerful technique for the study of simple and complex mo lecules bonded to surfaces and of thin organic films (polymers). I demonstrated that NEXAFS can determine the precise orientation of molecules on surfaces, their intra-molecular bond lengths and their bond hybridization. Today, NEXAFS has been employed to determine the molecular orientation and bond lengths of more than 200 molecular adsorption systems. From such studies we have learned how to link the molecular chemisorption geometry to the geometry and molecular orbital structure o f the free molecule. The technique is extensively covered in my monograph "NEXAFS Spectroscopy" which was published in 1992. Today NEXAFS is increasingly used for the study of polymer surfaces and thin films and in conjunction with microscopy, so called "spectro-microscopy" , for the study of polymers and biological macromolecules.
First NEXAFS data 12/6/1980
1994-1997: X-Ray emission spectroscopy of molecules bonded to surfaces. This work carried out at the Advanced Light Source (ALS) in Berkeley in collaboration with Anders Nilsson of Uppsala University (today a Professor at SSRL) established x-ray emission spectroscopy as a powerful new tool for the study of the surface chemical bond (Phys. Rev. Lett. 78, 2847 (199 7), Appl. Phys. A 65, 147 (1997)). Systematic studies of a variety of chemisorption systems revealed a new atom based picture of the surface chemical bond, showing the inadequacy of conventional text book pictures. X-ray emission spectroscopy can be viewed as the experimental analogue of the Linear Combination of Atomic Orbitals (LCAO) scheme.
Element specific and polarization dependent XES spectra of glycine on Cu(110)
1996-present: The surface structure of polymers. This work utilizes surface
sensitive NEXAFS spectroscopy to probe the alignment (Macromolecules
31, 1942 (1998); ibid 34, 1128 (2001)) and
relaxation (Macromolecules 30, 7768 (1997))
of molecular groups at polymer surfaces. A
highlight of this work is the solution
of a 90-year-old puzzle, the origin of
liquid crystal (LC) alignment on rubbed polymer films. Rubbing is an important
technological process, used in flat panel displays to align LCs on the rubbed
surface, yet the alignment process remained a puzzle ever since its discovery
in 1906. NEXAFS work revealed the microscopic origin of the LC alignment process
(J. Elect. Spec. Rcl. Phenom. 98-99,189(1999),
Science 292, 2299 (2001)) and led to
new materials (diamond-like carbon) and processes (ion beam irradiation) for
use in tomorrow's flat panel displays (Nature
411, 56 (2001)). The scientific understanding provided by the NEXAFS work
constituted not only a device enabling step but promises to redirect the $20
Polarization dependent NEXAFS spectra of rubbed BPDA polyimide (PI) revealing preferred bond and orienta tions relative to the rubbing direction.
NEXAFS spectra of untreated PI, ion beam irradiated PI (PI+IB), amorphous carbon.The IB creates a-carbon at the surface. The fact that it aligns LCs led to the idea of using ion beam irradiated a-carbon for LC alignment.
1991-present: Magnetic thin films and interfaces. This work utilizes polarized soft x-rays for the study of magnetic materials and phenomena, in particular the x-ray magnetic circular (XMCD) and linear (XMLD) dichroism techniques. Such studies have proven the existence of induced interfacial magnetic moments in non-magnetic films such as Cu, when adjacent to a ferromagnet (Phys. Rev. Lett. 72, 1112 (1994)), and more generally demonstrated the existence of enhanced and reduced moments in ultrathin magnetic films and their dependence on crystallographic structure (IBM. J. Res. Develop. 42, 73 (1998)). Through a unique separation of spin and orbital moments, XMCD has provided an unprecedented clear picture of the origin of the magnetocrystalline anisotropy in thin film multilayers (Ph ys. Rev. Lett. 75, 3748 (1995); Phys. Rev. Lett. 75, 3752 (1995), J. Magn. Magn. Mater. 200, 470 (1999)). Examples of XMCD studies are shown in the pictures below.
Magnetic moments in a multilayer wedge showing a dead Fe layer at a thickness of about 12 Å
Results linking magnetocrystalline and orbital moment anisotropy
The XMCD and XMLD techniques can also be used for imaging. My work has provided the first ferromagnetic (Science 259, 659 (1993)) and antiferromagnetic (Phys. Rev. Lett. 83, 1862 (1999), Science 287, 1014 (2000)) images with x-rays which are shown below.
The first magnetic i mages recorded with x-rays
Magnetic imaging with x-rays offers many advantages over conventional imaging techniques. By tuning the photon energy one can select different elements or layers, polarization control can distinguish between ferromagnetism and antiferromagnetism, and the limited sampling depth of electron yield detection opens the door for getting unique interfacial information for thin film multilayers. Below is a picture of recent results that combine antiferromagneti c and ferromagnetic imaging. It demonstrates how the antiferromagnetic domain structure of the clean NiO(100) surface changes upon Co deposition and how the ferromagnetic domain structure of Co follows that of NiO. Within each antiferromagnetic stripe domain there are two ferromagnetic Co domains with opposite spin directions, as illustrated in the icon of the figure.
Domai n structure in antiferromagnetic NiO
and ferromagnetic Co deposited on top
Such x-ray photoemission electron microscopy (XPEEM) studies have provided significant new insight into interfacial magnetic phenomena such as exchange bias (Nature 405, 767 (2000)).
The next picture below illustrates how our studies have not only revealed the correspondence of the domain structure in an antiferromagnet and an adjacent ferromagnet, but were also able to determine the magnetic structure of the all important interface between them. Indeed, uncompensated spi ns at the interface hold the key to the exchange bias puzzle (Phys. Rev. Lett. 87, 247201 (2001); Phys. Rev. Lett. 91, 017203 (2003)). Some of them are anchored in the antiferromagnet (Phys. Rev. Lett. 92, 247201 (2004)) and their direction is therefore pinned in space. They give rise to the exchange bias effect. A more detailed discussion of the topic of exchange bias is given under Research Programs and under Highlights on the home page.
Artist's rendition of the magnetic domain structures in a
ferromagnetic (blue) / antiferromagnetic (green) sandwich
The shown domain structures are those measured by PEEM for Co (blue) on NiO(100) (green). The in terface layer shown in brown is formed by interdiffusion and contains signatures of both the ferromagnetic and antiferromagnetic domains. It contains uncompensated spins and forms the link between the ferromagnetic and antiferromagnetic spin structures. A small fraction of the interfacial spins are pinned by anchoring in the antiferromagnet. These give rise to the pinning of the magnetization in the ferromagnet, i.e. the exchange bias phenomenon.
My present and future research topics are discussed in detail under Research Programs, Synchrotron Techniques and Highlights on the home page. Below I only give some pictures associated with these experiments.
The next picture below is an illustration of an innovative experiment conceived by Hans Christoph Siegmann. It explores ultrafast magnetic switching with the shortest and strongest field pulses available today. They are produced by a relativistic electr on bunch in the Stanford linear accelerator (LINAC) at SLAC. (Nature 428, 831 (2004); Phys. Rev. Lett. 94, 197603 (2005)). The caption explains the picture. More can be found under Research Programs and Highlights.
Artist's rendition of the ultrafa
st magnetic switching
experiment using the SLAC LINAC
Shown is a picture of the 2-mile-long linear accelerator at SLAC and superimposed as a black line is the path of an electron bunch traveling down the LINAC. The relativistic electron bunch is surrounded by a circular magnetic field as indicated by the red field lines. In the rest frame of the sample, shown in purple and located at the end of the LINAC, the electron bunch has a bunch length that can be changed from picoseconds down to less than 100 femtoseconds. When the beam traverses the uniformly magnetized sample, it writes a magnetic pattern into it whose shape and intensity distribution gives important information on the magnetization dynamics which follows the field excitation. Today, such experiments provide the shortest and strongest field pulses and have established a speed limit for the magnetic switching process. Since in computer data storage, magnetic bits are also switched by a magnetic field pulse, the SLAC experiment also shows the existence of a speed limit for writing magnetic bits in data storage. For more information, please see Highlights on the home page. The next picture illustrates the development of a new lensless magnetic technique. It utilizes Fourier transform holography to record a hologram from the sample and reconstruct the sample image by simple Fourier transformation of the hologram (Nature 432, 885 (2004)). More can be found under Research Programs and Highlights.
Illustration of lensless magnetic imaging
The x-ray beam from an undulator source with variable polarization is incident on a pinhole that redefines the source. The central part of the Fraunhofer pattern of the pinhole then il luminates a mask that consists of a ``sample hole" and a ``reference hole". The scanning electron microscopy (SEM) image on the lower left shows a close-up of the two holes which weredrilled into a Au film by a focused ion beam. In the shown case the mask and sample are integrated, as shown above the SEM image. The magnetic domain structure within the pinhole opening, recorded by a scanning transmission x-ray microscope (STXM) is shown on the right top. The experimentally recorded hologram of the sample by a CCD detector is shown in false color on the lower right.
One of the unique and most exciting capabilities of synchrotron radiation is the study of nanoscale structures on ultrafast timescales. My group has pioneered such time-dependent imaging experiments on nanoscale magnetic materials (Science 304, 420 (2004)). We have used both PEEM microscopy and scanning transmission x-ray microscopy (STXM) at the Advanced Light Source.
In the figure below we show an example (Phys. Rev.Lett. 96, 217202 (2006)). Using STXM we studied the time evolution of the magnetization of a 4 nm thick sensor layer (light blue), buried inside a nanopillar of 100 nm x 150 nm cross section. The time dependent images were taken as a function of delay time of the 70 ps long x-ray “probe” pulses relative to spin polarized current (“pump”) pulses injected into the layer. The current pulses of 200 ps rise time were generated by a pulse generator and wer e spin polarized by either transmission through (current flow from bottom to top in the shown pillar) or reflection from (current flow from top to bottom) a ferromagnetic polarizing layer (dark blue) in the pillar. As shown in the figure, we alternated 4 ns long positive and negative pulses to excite and reset the magnetization direction in the sensor layer. The images a) – i) were recorded at different delay times relative to the current pulse sequence, as indicated. They reveal that the magnetization in the sensor layer switches in a non-uniform fashion through creation of a magnetic vortex and its lateral shift. The magnetic C-state produced by the lateral shift of the vortex is metastable and may straighten out into a uniform magnetic state.
The switching process is initiated by the circular Oersted field which accompanies the charge current through the pillar and is then shifted by the exchange torque due to the spin current. The experiment clearly demonstrates the breakdown of the so-called macro-spin approximation which assumes that the magnetization is uniform across the nanoscale sensor layer and rotates as a whole during the switching process.
Switching of a nanoscale magnetic layer by injection of a spin polarized current
Much of the magnetism research described above is part of a book I have written with Hans Christoph Siegmann.
Magnetism: From Fundamenta ls to Nanoscale Dynamics
Series: Springer Series in Solid-State Sciences, Preliminary entry 180
Joachim Stöhr and Hans Christoph Siegmann
May 2006, Approx. 840 p. 400 illus., Hardcover
The book is a comprehensive treatment of dif ferent aspects of the broad field of magnetism. It covers historical aspects, fundamental aspects all the way to state-of-the-art problems and research in contemporary magnetism. It particularly distinguishes itself from other books on “magnetism” in its treatment of spin polarized electron physics, the discussion of spin polarized transport and the novel technique of x-ray dichroism for the study of magnetic materials.
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