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" |
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