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Photoemission Electron Microscopy Ultrafast Dynamics

Research

Presentations:

Current projects:

  • Imaging of magnetic nanostructures.
  • Magnetic domains and interlayer coupling in films and multilayers.
  • Exchange bias phenomena and antiferromagnetic structures.
  • Spin transport.
  • P-sec magnetization in magnetic patterns.
  • Chemical and magnetic properties of complex materials.
  • Properties of multiferroic materials.
  • Chemistry of materials and minerals.
  • Morphology and surface chemistry of polymer films and protein adsorbat

Highlights:


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 7.3.1.1 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"