Photoemission Electron Microscopy

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Lawrence Berkeley National Laboratory

Advanced Light Source
Andreas Scholl
last update: 01/6/2015



Current projects:

  • Imaging of magnetic nanostructures.
  • P-sec magnetization dynamics.
  • Properties of multiferroic materials.
  • Chemistry of materials, minerals and bio-minerals..
  • Surface chemistry.


Thermal spin ice

Artistic representation of a square artificial spin ice array. Elongated magnetic nano-elements are patterned from a magnetic thin film and interact with each other through their magnetic stray field at each vertex. The resulting “islands” resemble magnetic macro-spins, consisting of a large number of atomic spins (depiction inside magnifier glass). Under certain conditions it is possible to “flip” the direction of the magnetization of such islands by temperature induced thermal fluctuations. By means of these fluctuations and tuning the island interactions the temperature dependence of the macro-spin configurations can be studied. Configuration deviations from those depicted, possess effective magnetic charges.

V. Kapaklis, U.B. Arnalds, A. Farhan, R. Chopdekar, A. Balan, A. Scholl, L.R. Heyderman, and B. Hjörvarssonet, Thermal fluctuations in artificial spin ice, Nature Nanotechnology 9, 514 (2014).

Two-dimensional nano-patterned arrays provide a new route for the investigation of geometric frustration in magnets. Artificial spin ice structures, mimicking crystalline spin ice materials, can be designed and imaged directly using real-space techniques. These systems therefore provide an ideal platform for the investigation of ordering in systems with degenerate ground states and the related excitations. Furthermore, in artificial spin ice the arrangement of the moments arising from elongated single-domain nano-patterned magnetic islands can lead to excited states that possess magnetic charges, analogous to the monopole excitations reported for rare-earth pyrochlores.

One particularly interesting case is that of systems exhibiting frustration, where competing interactions cannot be simultaneously satisfied. This results in a degeneracy of the ground state and intricate thermodynamic properties. The archetypical frustrated physical system is water ice, and similar physics can be mirrored in nano-magnetic arrays, by tuning the arrangement of neighboring magnetic islands and magnetic materials, referred to as artificial spin ice. Thermal excitations in such systems resemble magnetic monopoles. Our recent work demonstrates how such arrays can be designed in order to support thermal fluctuations. Using magnetic imaging employing synchrotron x-ray photoemission electron microscopy and by changing the temperature we showed that the arrays undergo a transition from a frozen to a dynamic state depending on the array geometry and material. In the dynamic state the array temperature dictates the distribution of energy states. This approach provides a direct control on the population and dynamics of the thermal excitations and magnetic charges. In the future, this will enable the utilization of magnetic charges in novel technologies such as magnetic cellular automata. Finally, the physics of emergence can also be studied for the first time in artificial magnetic systems.

From the press release: "Thermal fluctuations in artificial spin ice"

A magnetic skyrmion consists of a central in-plane vortex and the surrounding out-of-plane spins. The N=0 and N=1 states are characterized by parallel and antiparallel alignment of the vortex core relative to the surrounding out-of-plane spins, respectively.

Skyrmion PEEM images after the application of an in-plane magnetic field pulse (skyrmion core annihilation process) for (a) N=0 and (b) and N=1. The central vortex is surrounded by out-of-plane nickel spins as indicated by the yellow symbols. A lower critical field is needed to annihilate the core for N=0 compared to N=1.

J. Li, A. Tan, K.W. Moon, A. Doran, M.A. Marcus, A.T. Young, E. Arenholz, S. Ma, R.F. Yang, C. Hwang, and Z.Q. Qiu, "Tailoring the topology of an artificial magnetic skyrmion," Nat. Commun. 5, 4704 (2014).

Sometimes, the spins in a magnetic material will form tiny swirls that can move around like particles. The spins themselves stay put—it's the pattern that moves. These quasiparticles have been dubbed "skyrmions," after British physicist Tony Skyrme, who described their mathematics in a series of papers in the early 1960s. Now, over 50 years later, scientists are intrigued by the possibility that skyrmions could play a key role in spintronics—electronics that employ spin to carry and maniupulate information. The research group using X-PEEM created artificial skyrmions in 30-nm-thick cobalt disks on 30-monolayer nickel, all on a copper substrate. This structure created two magnetic regions: the area in/under the cobalt disk (in-plane magnetization), and the surrounding nickel (out-of-plane magnetization). This was important because skyrmions can be characterized by a skyrmion number (N) equal to 0 when the core and surrounding spins are parallel, and 1 when they are antiparallel. By applying various out-of-plane magnetic fields, the researchers were able to control the formation of the two states in this system.

To study the skyrmions' topological nature, the researchers annihilated the skyrmions by applying an in-plane magnetic pulse that pushed the core out of the disk, converting the vortex into a simple magnetic domain. Photoemission electron microscopy (PEEM) measurements at Beamline 11.0.1 revealed that N=1 skyrmions (antiparallel) required a higher critical field for annihilation than N=0 skyrmions (parallel). This distinct behavior for the two states suggests a topological effect of the magnetic skyrmions in the core annihilation process.The exploration of the world of magnetic skyrmions is just beginning, but it already reveals that these particle-like spin configurations not only hold promise for ultracompact data storage and processing, but they may also open up entirely new areas of study in the emerging field of quantum topology.

Science Highlight: "Skyrmion Behavior Revealed by Two X-Ray Studies"

Diamondoids are attached to the surface of a Co/Pd multilayer which exhibits magnetic domains that are 120nm wide. The two images in the top row are shown to  compare PEEM images obtained from samples without and with diamondoid layer. The improvement of the spatial resolution in the presence of the diamondoid layer is clearly visible. The images have been obtained using a low sample voltage (10 kV) and the high transmission mode of the microscope to simulate imaging conditions for radiation sensitive samples. The study showed that the diamondoid cover improves the performance of PEEM over a wide range of imaging conditions.

Ishiwata, H., Y. Acremann, A. Scholl, E. Rotenberg, O. Hellwig, E. Dobisz, A. Doran, B.A. Tkachenko, A.A. Fokin, P.R. Schreiner, J.EP. Dahl, R.MK. Carlson, N. Melosh, Z.X. Shen, and H. Ohldag, "Diamondoid coating enables disruptive approach for chemical and magnetic imaging with 10 nm spatial resolution," Applied Physics Letters 101(16), 163101 (2012).

Diamondoids are nanoparticles made of only a handful of carbon atoms, arranged in the same way as in diamond, forming nanometer sized diamond crystals. Previously, researchers at the ALS demonstrated the fascinating capability of these tiny little diamonds to act as a monochromator for electrons. In short, a thin layer of diamondoids deposited on a metal surface will first capture the electrons ejected from the surface below due to its negative electron affinity. These electrons, which are emitted from the metal with a wide variety of kinetic energies – or colors – are then re-emitted by the diamondoid layer but with a very narrow energy distribution. This property, which is unique to diamondoid, is believed to enable the development of a new generation of electron emitters with unprecedented properties.

In Photoemission Electron Microscopy (PEEM), electrons emitted from a sample due to x-ray irradiation are used to obtain images of the chemical or magnetic properties of a surface with high spatial resolution. However, chromatic aberrations limit the resolution typically to 20 nm due to the wide energy distribution of the emitted electrons. While it is possible to add expensive and complex correction elements to the microscope, this experiment demonstrates that the energy spread can instead be reduced right at the source by using an inexpensive, simple coating of diamondoids. Using this approach, the resolution of the PEEM3 microscope at ALS Beamline 11.0.1 improved significantly and researchers were able to attain chemical information from 10 nm Au nanoparticles. The study exemplifies how a problem not suited to conventional engineering solutions can be solved with the help of the unique properties of a nanoparticle.

Science Brief: "Diamondoids Improve Electron Emitters"

Component mapping of ACC•H2O, ACC, and calcite in spicules from the Californian sea urchin Strongylocentrotus purpuratus. (A) XANES-PEEM image of 3 spicules embedded in epoxy, polished to expose a cross-section, and coated with 1 nm of platinum. (B) Red, green, and blue (RGB) map displaying the results of component mapping, in which each component is color-coded. The box indicates the region magnified in (C). (C) Zoomed-in portion of the RGB map in (B), where each 15-nm pixel shows a different color. Pure phases are R, G, or B, while mixed phases are cyan, magenta, or yellow. The white line shows the positions of the 20 pixels from which the spectra in (D) were extracted. (D) Sequence of 20 XANES spectra extracted from 15-nm adjacent pixels along the white line in (C).

Y.U.T. Gong,  C.E. Killian,  I.C. Olson,  N.P. Appathurai,  A.L. Amasino,  M.C. Martin,  L.J. Holt,  F.H. Wilt,  P.U.P.A. Gilbert, “Phase Transitions in Biogenic Amorphous Calcium Carbonate,” Procs. Natl. Acad. Sci. USA 109,  6088-6093 (2012).

Sea urchin spicules are single-crystalline calcite structures with smooth, rounded shapes, significantly different from the flat faces and sharp edges of calcite formed geologically or synthetically. Though many other biominerals exhibit this morphological character, sea urchin spicules are an ideal system for studying biomineral formation mechanisms because they contain 99.9% calcite (CaCO3) by weight and only 0.1% intracrystalline proteins.

In principle, sea urchins could build spicules by depositing amorphous precursor phases, which can be morphed into any shape, and letting them slowly crystallize. The three CaCO3 phases present in sea urchin spicules (ACC•H2O, ACC, and calcite) are spectroscopically distinct, enabling two-dimensional component mapping. Using x-ray absorption near edge structure (XANES) spectroscopy and photoelectron emission microscopy (PEEM) at ALS Beamline 11.0.1, researchers have directly observed each of these three phases in cross-sections of sea urchin spicules caught in the act of crystallizing. These data provide the first experimental evidence that sea urchins form spicules by first depositing ACC•H2O onto the surface of the forming spicule, then ACC•H2O dehydrates to form ACC, and finally the ACC crystallizes to form calcite.

Science Brief: "Imaging the Formation of Sea Urchin Spicules"


Two images are taken with opposite incident photon helicity at each of the characteristic absorption energies for manganese (Mn, left) and iron (Fe, right) at the surface of Fe2MnGa. There is zero (grey) contrast in the Mn image and strong (black/white) contrast for the Fe, indicating that the surface magnetization comes from the Fe. One mangetostructural domain boundary is observed to sweep through the bulk of the material while the other is pinned.

C.A. Jenkins et al. "Temperature-induced martensite in magnetic shape memory Fe2MnGa observed by photoemission electron microscopy," Appl. Phys. Lett. 100, 032401 (2012).

Conventional shape memory materials, such as the commercially available Nitinol (an alloy of nickel and titanium used in microsensing, actuation, and medical devices), undergo a phase transformation with cooling or heating when large areas of a sample distort along a single axis, and where the atomic-unit cell “stretching” from a cube to a rectangular prism occurs. In contrast, magnetic shape memory (MSM) materials are much more rare but have an advantage: The axis of magnetic anisotropy is coupled to the direction of stretching, so a perfect MSM crystal can be made to flex and bend reversibly by applying an external magnetic field. The research team used the photoelectron emission spectromicroscope on ALS Beamline 11.0.1 to acquire nanoscale, element-specific images of a crystallographic boundary in Fe2MnGa, a new MSM compound. With heating and cooling, the boundaries between some magnetic domains are observed to sweep back and forth, while others are stationary because the local direction of stretching is incompatible with domain-wall motion. Knowledge of this kind of magnetostructural domain wall will be useful in tailoring the properties of MSM crystals for robotic and medical applications.

Science Brief: "Crystallographic Boundary in a Magnetic Shape Memory Material"

Effects of external magnetic field

Magnetic domain images of a 2-μm-diameter CoO(3-nm)/Fe(12-nm)/Ag(001) disk under different conditions. (a) The local magnetic field produced by the Fe film surrounding the disk (H ~ 32 Oe) shifted the vortex core position from the disk center. (b) After the application of an external 50-Oe magnetic-field pulse, the Fe film changed from a single-domain state to a multidomain state, and the local magnetic field produced by the Fe film went from ~32 Oe to ~0 Oe. The Fe vortex core remained in its off-center position, showing the exchange-bias effect of the CoO vortex. (c) After the sample was warmed to 300 K (above the CoO Néel temperature), the exchange-bias effect vanished, and the Fe vortex core moved back to the center.

J. Wu, D. Carlton, J.S. Park, Y. Meng, E. Arenholz, A. Doran, A.T. Young, A. Scholl, C. Hwang, H.W. Zhao, J. Bokor, and Z.Q. Qiu, "Direct observation of imprinted antiferromagnetic vortex states in CoO/Fe/Ag(001) discs," Nat. Phys. 7, 303 (2011).

Magnetic materials are characterized by the ordering of electron spins, with nearest-neighbor spins parallel to each other in ferromagnetic (FM) materials and antiparallel to each other in antiferromagnetic (AFM) materials. As the size of a magnetic system is reduced to micron scale, it has been shown that the spins in an FM microstructure can curl around to form a magnetic vortex state. While there has been intensive activity in the study of vortex states in FM disks, there has been no direct observation of such states in an AFM microstructure, although theory predicts many interesting and unique properties for the AFM vortex state. Recently, a research team from Berkeley, Korea, and China has taken the first direct image of an AFM vortex in multilayered magnetic disk structures using x-ray magnetic linear dichroism (XMLD) and photoemission electron microscopy (PEEM) at ALS Beamlines 4.0.2 and 11.0.1 , respectively. The experiments observed two types of AFM vortices, one of which has no analogue in FM vortices.

Science Highlight: "Direct Imaging of Antiferromagnetic Vortex States"

Direct-write process

Left: Schematic of the AFM tip and substrate geometry and the chemical synthesis of Si and Ge nanostructures. Right: Precursor molecules for Si (diphenylsilane) and Ge (diphenylgermane).

NEXAFS spectra

Left: Representative electron-yield image (x-ray energy = 1222 eV) of Ge structures written from diphenylgermane. Center: NEXAFS spectrum acquired from one of the Ge structures on the left. The spectrum shows the onset of the 1218-eV edge that corresponds to the Ge L3 excitation. Right: Spectra acquired at the onset of the C K-edge (285.7 eV) for Ge (red), Si (green), and C (black) nanostructures. This data validates that at most, only trace amounts of C is present in the Si and Ge structures.

J.D. Torrey, S.E. Vasko, A. Kapetanovic, Z. Zhu, A. Scholl, and M. Rolandi, "Scanning probe direct-write of germanium nanostructures," Adv. Mater. 22, 4639 (2010); S.E. Vasko, A. Kapetanovic, V. Talla, M.D. Brasino, Z. Zhu, A. Scholl, J.D. Torrey, and M. Rolandi, "Serial and parallel Si, Ge, and SiGe direct-write with scanning probes and conductive stamps," Nano Lett. 11, 2386 (2011).

Nanostructured materials (nanowires, nanotubes, nanoclusters, graphene) are attractive possible alternatives to traditionally microfabricated silicon in continuing the miniaturization trend in the electronics industry. To go from nanomaterials to electronics, however, the precise one-by-one assembly of billions of nanoelements into a functioning circuit is required—clearly not a simple task. An interdisciplinary team from the University of Washington, in collaboration with the ALS and the Pacific Northwest National Laboratory, has devised a strategy that could make this task a little easier. They have demonstrated the ability to directly "write" nanostructures of Si, Ge, and SiGe with deterministic size, geometry, and placement control. As purity is essential for electronic-grade semiconductors, the resulting patterns were carefully evaluated for carbon contamination using photoemission electron microscopes at ALS Beamlines 7.3.1 and 11.0.1.

Science Highlight: "Direct-Write of Silicon and Germanium Nanostructures"

SEM micrographs of the tooth tip of a sea urchin. The overgrown and co-oriented calcite rhombohedra are colored in green, mis-oriented rhombohedra are colored in blue. Mis-oriented rhombohedra appear at the edges of the plates, which the sea urchin broke while grinding

This PIC map, acquired with X-PEEM on a tooth cross-section, shows the outlines of 2 plates (p), approximately 30 fibers (f), and the polycrystalline matrix (m) cementing plates and fibers together. The grayscale indicator shows qualitatively that gray levels correspond to different angles between the linear polarization vector (dark vertical line) and the c-axis (gray line).

Christopher E. Killian, Rebecca A. Metzler, Y. U. T. Gong, Ian C. Olson, Joanna Aizenberg, Yael Politi, Fred H. Wilt, Andreas Scholl, Anthony Young, Andrew Doran, Martin Kunz, Nobumichi Tamura, Susan N. Coppersmith, and P. U. P. A. Gilbert, "Mechanism of Calcite Co-Orientation in the Sea Urchin Tooth", J. Am. Chem. Soc., 2009, 131 (51), pp 18404–18409

Sea urchin teeth are remarkable and complex calcite structures, continuously growing at the forming end and self-sharpening at the mature grinding tip. The calcite (CaCO3) crystals of tooth components, plates, fibers, and a high-Mg polycrystalline matrix, have highly co-oriented crystallographic axes. This ability to co-orient calcite in a mineralized structure is shared by all echinoderms. However, the physico-chemical mechanism by which calcite crystals become co-oriented in echinoderms remains enigmatic. Here, we show differences in calcite c-axis orientations in the tooth of the purple sea urchin (Strongylocentrotus purpuratus), using high-resolution X-ray photoelectron emission spectromicroscopy (X-PEEM) and microbeam X-ray diffraction (μXRD). All plates share one crystal orientation, propagated through pillar bridges, while fibers and polycrystalline matrix share another orientation. Furthermore, in the forming end of the tooth, we observe that CaCO3 is present as amorphous calcium carbonate (ACC). We demonstrate that co-orientation of the nanoparticles in the polycrystalline matrix occurs via solid-state secondary nucleation, propagating out from the previously formed fibers and plates, into the amorphous precursor nanoparticles. Because amorphous precursors were observed in diverse biominerals, solid-state secondary nucleation is likely to be a general mechanism for the co-orientation of biomineral components in organisms from different phyla.

Co and Fe magnetic domains as a function of NiO thickness from Co(2 nm)/NiO/F(15 ML)/Ag(001). The correspondence between domain contrast and spin orientation is shown in the schematic drawing as a reference. Arrows represent the spin orientations. The result shows that the Co and Fe have a 90° coupling for d(NiO)<2.0 nm and a collinear coupling for d(NiO)>2.0 nm.

J. Wu, J. Choi, A. Scholl, A. Doran, E. Arenholz, Y. Z. Wu, C. Won, Chanyong Hwang, and Z. Q. Qiu, "Element-specific study of the anomalous magnetic interlayer coupling across NiO spacer layer in Co/NiO/Fe/Ag(001) using XMCD and XMLD", Phys. Rev. B 80, 012409 (2009)

Co/NiO/Fe trilayers are grown on Ag(001) substrate using molecular-beam epitaxy and investigated by element-specific magnetic domain images using x-ray magnetic circular dichroism and x-ray magnetic linear dichroism techniques. By comparing the Co, Fe, and NiO magnetic domain images, we identify that the anomalous Co-Fe interlayer coupling from a 90° coupling to a collinear coupling with increasing the NiO film thickness is due to a transition from a collinear to 90° coupling at the NiO/Fe interface while retaining a 90° coupling at the Co/NiO interface. Uncompensated Ni spins are found at the Co/NiO interface but are absent at the NiO/Fe interface. No evidence of spiral NiO spin structure is found in this Co/NiO/Fe sandwich.

X-PEEM color-coded composite map of human serum albumin (HSA) adsorbed on polystyrene-polylactide (40:60 PS-PLA, 0.7 wt %) thin films

Bonnie O. Leung, Adam P. Hitchcock, Rena Cornelius, John L. Brash, Andreas Scholl, and Andrew Doran, "X-ray Spectromicroscopy Study of Protein Adsorption to a Polystyrene−Polylactide Blend", Biomacromolecules, 2009, 10 (7), pp 1838–1845

Synchrotron-based X-ray photoemission electron microscopy (X-PEEM) was used to study the adsorption of human serum albumin (HSA) to polystyrene-polylactide (40:60 PS-PLA, 0.7 wt %) thin films, annealed under various conditions. The rugosity of the substrate varied from 35 to 90 nm, depending on the annealing conditions. However, the characteristics of the protein adsorption (amounts and phase preference) were not affected by the changes in topography. The adsorption was also not changed by the phase inversion which occured when the PS-PLA substrate was annealed above Tg of the PLA. The amount of protein adsorbed depended on whether adsorption took place from distilled water or phosphate buffered saline solution. These differences are interpreted as a result of ionic strength induced changes in the protein conformation in solution.

PEEM image of wear track

Konicek, A., D. Grierson, P.UPA. Gilbert, G. Sawyer, A. Sumant, and R.W. Carpick, "Origin of Ultralow Friction and Wear in Ultrananocrystalline Diamond," Physical Review Letters 100(23), 235502 (2008).

Robert Carpick led a collaboration with researchers from Argonne National Laboratories, the University of Wisconsin-Madison and the University of Florida to determine what makes diamond films such slippery customers, settling a debate on the scientific origin of its properties and providing new knowledge that will help create the next generation of super low friction materials.

The Penn experiments, the first study of diamond friction convincingly supported by spectroscopy, looked at two of the main hypotheses posited for years as to why diamonds demonstrate such low friction and wear properties. Using a highly specialized technique know as photoelectron emission microscopy, or PEEM, the study reveals that this slippery behavior comes from passivation of atomic bonds at the diamond surface that were broken during sliding and not from the diamond turning into its more stable form, graphite. The bonds are passivated by dissociative adsorption of water molecules from the surrounding environment. The researchers also found that friction increases dramatically if there is not enough water vapor in the environment.

From Azonanotechnology: "What Makes Diamond Films Slippery"

Mother of pearl

On the left is a visible light microscopy image of a polished nacre surface, on the right is a simulated layer of nacre via the theoretical model of nacre formation.

Rebecca Metzler, Dong Zhou and Susan Coppersmith, of the University of Wisconsin, and Andreas Scholl, Andrew Doran, Anthony Young, Martin Kunz and Nobumichi Tamura,“Gradual Ordering in Red Abalone Nacre.”, Journal of the American Chemical Society 130, 17519, (2008)

Under the probing polarized light of PEEM-3, Gilbert and her colleagues obtained measurements of aragonite crystal orientation, tablet size and tablet stacking direction. These measurements revealed that different orientations of the aragonite crystals result in different tablet growth rates. In turn, these different table growth rates give rise to self-ordering, which progressively selects faster and faster growing tablets. Furthermore, this self-ordering takes place gradually over a distance of approximately 50 microns.

In addition to its iridescent beauty, mother of pearl, or nacre, the inner lining of the shells of abalone, mussels and certain other mollusks, is also renowned for an amazing strength and toughness that has been a long-standing mystery. Now, scientists have brought to light a new aspect of nacre’s nanostructural architecture using the polarized x-ray beams and nanoscale imaging capabilities of the Advanced Light Source (ALS).

LBNL Press Release: "Mother of Pearl Secret Revealed"

3D and cross-sectional schematic diagrams of the coplanar epitaxial electrode device.The structure is grown on a strontium titanate substrate and has strontium ruthenate electodes. Right: In-plane PFM images showing the set of two stripe-like ferroelectric domains (black and brown contrast) running at 45º to the in-plane contacts. The domains switch by 90º for a device relative to the as-grown state following electrical switching and back again after the second switching.

Top: PEEM images of the ferromagnetic domain structure in the as-grown state, after the first electrical switch, and after the second electrical switch. Bottom: Schematic diagrams of the observed magnetic contrast in the PEEM images, where the black, gray, and white arrows indicate the magnetization directions of the domains.

Y.-H. Chu, L.W. Martin, M.B. Holcomb, M. Gajek, S.-J. Han, Q. He, N. Balke, C.-H. Yang, D. Lee, W. Hu, Q. Zhan, P.-L. Yang, A. Fraile-Rodríguez, A. Scholl, S.X. Wang, and R. Ramesh, "Electric-field control of local ferromagnetism using a magnetoelectric multiferroic," Nature Mater. 7, 478 (2008).

BFO is an antiferromagnetic, ferroelectric multiferroic with a Néel temperature of about 370ºC and a Curie temperature near 820ºC, respectively. Like a ferromagnet, which has a spontaneous magnetic dipole moment, a ferroelectric has a spontaneous electric dipole moment or electric polarization. The researchers developed a simple heterostructure device on a strontium titanate substrate to achieve deterministic control of ferromagnetism in a film of the cobalt–iron compound Co0.9Fe0.1 with an in-plane electric field applied to an underlying BFO layer.

In-plane PFM images of the BFO layer for this device structure in the as-grown state and after the application of electrical field pulses revealed the presence of a set of two stripe-like ferroelectric domains running at 45° to the in-plane contacts that switch by 90° back-and-forth in a repeatable fashion. Analysis of the intensity distribution in the PEEM images of the Co0.9Fe0.1—based on x-ray circular dichroism contrast at the cobalt L-edge—in the same sequence enabled the group to determine the direction of its local magnetization.

Comparing the two techniques revealed strong correlation between the ferroelectric domains in the BFO and ferromagnetic domains in the Co0.9Fe0.1, mediated by the collinear coupling between the magnetization in the ferromagnet and the projection of the antiferromagnetic order in the multiferroic. In short, the matching of the domains suggests the ability to control ferromagnetism with an applied electric field. This represents a significant advance in the field of multiferroics, as it marks the first demonstration of a possible room-temperature application to utilize multiferroic materials in a novel new device.

Science Highlight "Electric-Field Control of Local Ferromagnetism with a Magnetoelectric Multiferroic"

A 300-nm-thick epitaxial film composed of ferrimagnetic CFO pillars embedded in BFO matrix, with a relative volume ratio of 35/ 65, was prepared by pulsed laser deposition on (001) oriented SrTiO3.

Ferromagnetic (top) and element-sensitive image (bottom) at the Co L3 edge. The solid line encloses an unchanged region of the sample, the dotted line encloses an area that was switched using a charged atomic force microscope tip. Several nano pillars are marked by circles.

T. Zhao, A. Scholl, F. Zavaliche, H. Zheng, M. Barry, A. Doran, K. Lee, M. P. Cruz, and R. Ramesh, "Nanoscale x-ray magnetic circular dichroism probing of electric-field-induced magnetic switching in multiferroic nanostructures," Appl. Phys. Lett. 90, 123104 (2007).

Multiferroic materials blending magnetic and electric orders have become an exciting research topic in recent years due to the broad range of potential applications and the intriguing science behind the phenomenon.

The magnetic structure as well as its response to an external electric field were studied in ferrimagnetic CoFe2O4 nanopillars embedded in an epitaxial ferroelectric BiFeO3 film using photoemission electron microscopy and x-ray magnetic circular dichroism. Magnetic switching was observed in both Co and Fe magnetic sublattices after application of an electric field. About 50% of the CoFe2O4 nanopillars were measured to switch their magnetization with the electric field, implying an elastic-mediated electric-field-induced magnetic anisotropy change.

Magnetic domain images of Fe(wedge)/Ni(10.6 ML)/ Cu(001) for (a) as grown sample and (b) after applying a nearly in-plane magnetic field pulse. (c) Zoom-in image of the bubble domains in the SRT region.

The bubble domains change to the stripe domains after increasing the temperature to 370 K. The stripe domains remain after cooling to room temperature.

J. Choi, J. Wu, C. Won, Y. Z. Wu, A. Scholl, A. Doran, T. Owens, and Z. Q. Qiu, "Magnetic Bubble Domain Phase at the Spin Reorientation Transition of Ultrathin Fe/Ni/Cu(001) Film," Physical Review Letters 98, 207205 (2007).

Magnetic domain phases of ultrathin Fe/Ni/Cu(001) were studied using photoemission electron microscopy at the spin reorientation transition (SRT). The authors observed a new magnetic phase of bubble domains within a narrow SRT region after applying a nearly in-plane magnetic field pulse to the sample. By applying the magnetic field pulse along different directions, the authors found that the bubble domain phase existed only if the magnetic field direction was inclined less than 10 degrees relative to the sample surface. A temperature dependent measurement showed that the bubble domain phase became unstable above 370 K.

Comparison of X-PEEM images (top row) using linear dichroism and piezo force microscope images (bottom row) showing a correlation between the antiferromagnetic and ferroelectric domain structure in BiFeO3. The images were taken at two angles, rotated by 90°.

Temperature dependence of the dichroism contrast across the Neel temperature of BiFeO3, proving the magnetic origin of the X-PEEM contrast. The remaining contrast is of ferroelectric origin.
T. Zhao, A. Scholl, F. Zevaliche, K. Lee, M. Barry, A. Doran, M.P. Cruz, Y.H. Chu, C. Ederer, N.A. Spaldin, R.R. Das, D.M. Kim, S.H. Baek, C.B. Eom, and R. Ramesh, "Electrical control of antiferromagnetic domains inmultiferroic BiFeO3 films at room temperature," Nature Materials 5, 823 (2006)

Multiferroic materials, which offer the possibility of manipulating the magnetic state by an electric field or vice versa, are of great current interest. In this work, the authors demonstrate electrical control over the antiferromagnetic domain structure in a single-phase multiferroic material at room temperature. High-resolution images of both antiferromagnetic and ferroelectric domain structures of (001)-oriented multiferroic BiFeO3 films reveal correlation between ferroelectric and antiferromagnetic domains, indicating a strong coupling between the two types of order. The ferroelectric structure was measured using piezo force microscopy, whereas X-ray photoemission electron microscopy was used to detect the antiferromagnetic configuration. Antiferromagnetic domain switching induced by ferroelectric polarization switching was observed, in agreement with theoretical predictions.

90° rotation of the NiO antiferromagnetic axis with NiO thickness on a Fe(001) substrate.

(a) Domain nucleation, (b) shearlike rotation, (c) rigid rotation model. The data supports model (a)

M. Finazzi, A. Brambilla, P. Biagioni, J. Graf, G.-H. Gweon, A. Scholl, A Lanzara, and L. Duo, "Interface Coupling Transition in a Thin Epitaxial Antiferromagnetic Film Interacting with a Ferromagnetic Substrate," Phys. Rev. Lett. 97, 097202 (2006).

NiO thin films grown epitaxially on single crystalline Fe films exhibit a change in the interface coupling from parallel to perpendicular at about 1.8 nm thickness. This instability of the antiferromagnet between a parallel and a perpendicular magnetic state could be the origin of the often contradictory evidence about the nature of the interface coupling between ferromagnets and antiferromagnets. The instability may be driven by frustration of the antiferromagnet at steps and defects at the surface of the ferromagnet, leading to the generation of vortex-like antiferromagnetic structures that destroy the uniaxial anisotropy at a critical thickness. For sufficiently thick AFM films the vortices are
expected to coalesce, with the result that the spins in the topmost layers of the AFM film would eventually realign along an easy anisotropy axis. This new anisotropy axis, however, needs not to be parallel to the ferromagnetic direction.

Hysteresis loop of Fe/PdMn measured with the applied magnetic field 45 degrees to the bias direction.

PEEM images of exchange-biased Fe/PdMn at (top) point A on the descending and (center) point B on the ascending hysteresis loops for H applied in the iron [110] direction.
P. Blomqvist, K.M. Krishnan, and H. Ohldag, "Direct imaging of asymmetric magnetization reversal in exchange-biased Fe/MnPd bilayers by x-ray photoemission electron microscopy," Phys. Rev. Lett. 94, 107203 (2005).

The phenomenon of exchange bias has transformed how data is read on magnetic hard disks and created an explosion in their information storage density. However, it remains poorly understood, and even the fundamental mechanism of magnetic reversal for exchange-biased systems in changing magnetic fields is unclear. By using x-ray photoemission electron microscopy at the ALS to directly image the magnetic structure of an exchange-biased film, a team from the University of Washington and the Stanford Synchrotron Radiation Laboratory has identified separate magnetic-reversal mechanisms in the two branches of a hysteresis loop. This advance in fundamental understanding will provide new insights for developing the next generation of information storage and sensing devices where exchange bias is expected to play a critical role.

Science Highlight "Direct Imaging of Asymmetric Magnetization Reversal"

The interlayer coupling between PEEM image of the magnetic domains of Fe/Ni(5ML)/Cu(001). The stripe domain width decreases as the Fe thickness increases towards to the spin reorientation transition at 2.7 ML. (b) A zoom-in image of the magnetic stripes in the box of (a). (c) Stripe domain width versus Fe film thickness. The solid line depicts the theoretical fitting.
Wu, Y. Z., C. Won, A. Scholl, A. Doran, H. W. Zhao, X. F. Jin, and Z. Q. Qiu: "PEEM Study of Coupled Magnetic Sandwiches", Phys. Rev. Lett.93, 117205 (2004).

Ultrathin magnetic films a few atoms thick occupy a scientific "sweet spot" at the intersection of theory and application. Potentially lucrative as a medium for high-density data storage, such films are also of fundamental interest because of their low dimensionality, enabling scientists to study systems that model two-dimensional magnetic behavior. Nanostructures of several ultrathin magnetic layers can be engineered to explore many interesting phenomena, including the formation of elongated (stripe) magnetization domains. With the ALS's photoemission electron microscope, PEEM-2, researchers from the ALS, UC Berkeley, and China looked at stripe domains in magnetic sandwiches of cobalt, copper, and iron/nickel. The results revealed a hidden universal dependence of the stripe domain width on variables such as film thickness and external magnetic field.

Science Highlight "Stripe Domains in Coupled Magnetic Sandwiches"

A magnetic field (purple) applied to a ferromagnet /antiferromagnetic bilayer rotates the magnetization of the ferromagnet (blue) and creates a domain wall in the antiferromagnet (green), an exchange spring.

PEEM images show the magnetic coupling between NiO and Co domains. Arrows indicate the NiO AFM axes and Co magnetization directions (left). The rotation angle of the magnetization at the surface of the antiferromagnet is plotted as function of the applied field (right).

A. Scholl, M. Liberati, E. Arenholz, H. Ohldag, and J. Stöhr, "Creation of an Antiferromagnetic Exchange Spring," Phys. Rev. Lett. 92, 247201 (2004).

In the ongoing quest for faster and more efficient magnetic data storage, designs for devices such as read heads in computer hard drives are mostly produced through a trial-and-error process, combining thin magnetic films with different properties. To speed up this search for better materials, researchers are striving for a better
understanding of the microscopic structure and interactions between ferromagnet and antiferromagnet layers. Researchers from the ALS, Stanford University, and Italy have now solved a piece of this puzzle using an x-ray magnetometer at the ALS. They proved that antiferromagnets in contact with ferromagnets form an exchange spring system. An exchange spring combines the maneuverability of magnetically soft materials with the permanence of magnetically hard materials.

Spectroscopic measurements were performed at ALS BL 4.0.2

Science Highlight "Direct Imaging of Asymmetric Magnetization Reversal"

Laser pulses (red) generate a current pulse, resulting in a magnetic field that initiates the magnetization dynamics. X-ray pulses (blue) probe the sample at 100-picosecond time intervals. The electron image is detected by the photoemission electron microscope.

A time-resolved PEEM movie shows the vortex motion over a period of 8 ns after the driving field pulse. The movie shows the original magnetic dichroism image (left) and a gradient image (right), with enhanced contrast of domain walls and vortex core.
S.-B. Choe, Y. Acremann, A. Scholl, A. Bauer, A. Doran, J. Stöhr, and H.A. Padmore, "Vortex-driven magnetization dynamics," Science 304, 420 (2004).

The data rate in modern disk drives will soon surpass 1 GHz. Subnanosecond magnetic-field pulses like those of a write head initiate magnetization precession, a gyroscopic motion of the magnetization around an applied field (like a wobbling top). An ALS–Stanford–Berlin group has used a new time-resolved x-ray photoemission imaging technique to resolve the motion of magnetic vortices, peculiar magnetic structures that appear in micron-size magnetic patterns, in response to an excitation field pulse. Analysis of the observed gyrating trajectory of the core on such short time scales suggests the precession is induced by a handedness or chirality in the magnetization pattern, thereby demonstrating that handedness plays an important role in the dynamics of microscopic magnets.

Science Highlight "Picosecond Magnetization Dynamics"

Sketch of a mixed polymer brush comprising ydrophilic and hydrophobic homopolymers.

The PEEM images show inverted contrast (arrows) at x-ray energies specific for PSF (e) and PMMA (f), indicating an exchange of hydrophilic and hydrophobic polymers at the surface after toluene exposure.
. Minko, M. Müller, D. Usov, A. Scholl, C. Froeck, and M. Stamm, "Lateral versus Perpendicular Segregation in Mixed Polymer Brushes," Phys. Rev. Lett. 88, 035502 (2002).

The chemical separation of mixed polymers into microphases represents a powerful and inexpensive tool for the fabrication of nanostructures. An international team comprising researchers from Germany and the Advanced Light Source has explored changes in the surface chemical structure of mixed polymer brushes exposed to different solvents. A brush consists of polymer chains chemically attached to a substrate. The team's observations, made with the photoemission electron microscope PEEM-2 at the ALS and an atomic force microscope (AFM), provide guidance for creating novel materials that adapt to their environment by changing their surface properties.

Science Highlight "Segregation in Mixed Polymer Brushes"

Antiferromagnetic domains on NiO(001) in an area 12 µm across. The colored arrows indicate the projections of the antiferromagnetic axes in the surface plane for four types of domains. Domains with identical in-plane projections (e.g., those marked with red and blue arrows) can be distinguished by examining their orientation out of the surface plane, as illustrated in the sketch at the bottom for the area in the dashed box. The green line represents a domain wall where the spins are in-plane.
H. Ohldag, A. Scholl, F. Nolting, S. Anders, F.U. Hillebrecht, and J. Stöhr, "Spin reorientation at the antiferromagnetic NiO(001) surface in response to an adjacent ferromagnet," Phys.
Rev. Lett. 86, 2878 (2001).

One of the vexing mysteries facing researchers in magnetic materials is the origin of the exchange-bias effect in which an antiferromagnetic layer pins the magnetization of an adjacent ferromagnetic layer so that it doesn't reverse in an external magnetic field. Building on earlier work with the photoemission electron microscope (PEEM) on Beamline at the Advanced Light Source, a German-American collaboration has taken an important step toward unveiling the secret of exchange bias by observing that spins near a nickel oxide antiferromagnet's surface reorient after deposition of a cobalt ferromagnetic layer. This discovery rules out models of exchange bias based on the common assumption that the spin configuration at the surface of the antiferromagnet is the same as that in its interior (bulk).

Science Highlight "Antiferromagnetic Spin Reorientation"

Antiferromagnetic domains imaged usin x-ray magnetic linear dichroism while approaching the magnetic ordering temperature.

Exchange coupling of a Co feromagnetic film on a LaFeO3 antiferromagnetic film visualized by x-ray magnetic circular and linear dichroism.
F. Nolting, A. Scholl, J. Stöhr, J.W. Seo, J. Fompeyrine, H. Siegwart, J.-P. Loquet, S. Anders, J. Lüning, E.E. Fullerton, M.F. Toney, M.R. Scheinfein, and H.A. Padmore, "Direct observation of the alignment of ferromagnetic spins by antiferromagnetic spins," Nature 405, 767 (2000). A. Scholl, J. Stöhr, J. Lüning, J.W. Seo, J. Fompeyrine, H. Siegwart, J.-P. Loquet, F. Nolting, S. Anders, E.E. Fullerton, M.R. Scheinfein, and H.A. Padmore, "Observation of antiferromagnetic domains in epitaxial thin films," Science 287, 1014 (2000).

Researchers from the ALS, IBM, and Arizona State University have taken a major step toward the solution of a long-standing problem in magnetic multilayers: identifying the mechanism of directional coupling between spins in an antiferromagnet and those in an adjacent ferromagnet. Known as exchange bias, thiscoupling plays a key role in magnetic devices based on the giant magnetoresistance (GMR) effect. Using the photoemission electron microscope at the ALS (PEEM2), the group obtained x-ray magnetic dichroism images that revealed the magnetic structure on both sides of the interface between a thin layer of ferromagnetic cobalt grown on antiferromagnetic lanthanum iron oxide (LaFeO3), as well as local remanent hysteresis loops for individual ferromagnetic domains. The experiments may lead to a definitive understanding of the elusive mechanism of exchange biasing.

Science Highlight "PEEM2 Reveals Spin Alignment in Magnetic Layers"