Studying Nanoscale Magnetism and its Dynamics with Soft X-ray Microscopy Page: 2 of 6
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ray optics can be fabricated by state-of-the-art nanotechnology
tools, such as e-beam lithography [6].
FZPs can be designed and therefore customized for specific
purposes and applications by a set of parameters, such as
- Ar, which is the outermost ring diameter
- N, the number of zones, and
- X, the photon wavelength at which the FZP is operating
These three parameters also determine the performance of
the FZP in an X-ray microscope, namely
- the spatial resolution is largely given by Ar
- the focal length, which is -4N(Ar)2/k and
- the spectral bandwidth. -1/N.
The most advanced FZPs for soft X-ray microscopy have
achieved a spatial resolution better than 15nm [7], and the
current developments seem to make the 10nm spatial
resolution regime become feasible in the near future. The
spectral bandwidth has an impact to the time resolution in soft
X-ray microscopy, which is given by the length of the X-ray
pulse where the FZP is operating. Current typical FZPs have N
between 500 and several 1000's, therefore the lower fsec time
regime is already feasible with regard to the X-ray optics in
soft X-ray microscopy.
B. Magnetic fullfield soft X-ray microscopy
The optical setup of the full-field soft X-ray microscope
endstation XM-1, where the data presented in this review have
been obtained is shown in Fig. 1 and described in detail
elsewhere [8].
Condenser
zone plate
Plane
mirror Applied
Bending magnetic
- AILS Bendingfil
Magnet
Pinhole
Mutual Indexing System
with kinematic mounts Sample M r
stage 2one
Spate 1
Soft xray
sensitiveS
,~zzzzz~Visible light
m croscopeFig. 1. Optical setup of the full-field high resolution soft X-ray
microscope XM-1 at the ALS in Berkeley, CA
Synchrotron radiation emitted from a bending magnet at the
Advanced Light Source (ALS) in Berkeley impinges on a
condenser zone plate (CZP). The CZP together with a pinhole
close to the sample provides a hollow cone illumination of the
sample and acts as linear monochromator due its wavelength
dependent focal length. With N-50,000 and an outermost zone
width of -40nm typical focal lengths of 20-30cm can be
obtained.. A particular wavelength is chosen by moving the
CZP along the optical axis of the synchrotron beam and a
spectral resolution of about leV at 500-1000eV can be
achieved. Bending magnet radiation viewed at an angle to the
orbital plane of the storage ring provides circular polarization
and therefore an aperture upstream the CZP allows to choosecircular polarization of different helicity with a typical degree
of circular polarization between 60-70%.
X-ray magnetic circular dichroism (XMCD), i.e. the
dependence of the photo absorption coefficient on the relative
orientation of the projection of the magnetization of the
ferromagnetic sample onto the photon propagation direction
provides magnetic contrast. In the vicinity of element specific
resonant X-ray absorption edges e.g. for 3d transition metals
such as Fe, Co, Ni large XMCD values up to 25% are
observed [9].
Illuminating a ferromagnetic sample which exhibits in the
groundstate magnetic domains [10], i.e. areas where the
magnetization direction varies, with circular polarized X-rays
at a photon energy corresponding to absorption edges of a
certain ferromagnetic components there is a locally varying
transmitted photon intensity.
A second Fresnel zone plate, the microzone plate (MZP),
downstream the specimen images the transmitted photons onto
a 2dimensional charge coupled device (CCD) detector with a
spatial resolution provided by the outermost zone width Ar.
The required illumination time is determined by several
factors:
- source intensity
- efficiency of the optical elements
- optical thickness of the sample
- quantum efficiency of the detectors.
Since about 1000 photons are required per CCD pixel to yield
a sufficient signal-to-noise ratio, the current intensity per X-
ray pulse in 3rd generation synchrotron sources is not sufficient
to allow for single shot imaging. Typical illumination times are
in the second regime and with current X-ray optics a field of
view of about 10pm can be achieved.
As a pure photon-in/photon-out based technique magnetic
fields of in principle any strength and pointing in any direction
can be applied during the recording of X-ray images. At XM-1
typical magnetic fields up to 2-3kOe in perpendicular
geometry and about 1-2kOe along the plane of the sample can
be applied.
Both samples with perpendicular and in-plane anisotropy can
be investigated. To image in-plane components the sample has
to be tilted at an axis perpendicular to the photon beam
propagation [11]. Typically the tilt angle a is 300, i.e. the
effective thickness increases by 1/cos a thereby reducing the
magnetic contrast by a factor 2.
C. Time resolved magnetic soft X-ray microscopy
Synchrotron storage rings such as the ALS are inherently
pulsed X-ray sources. The electrons having a typical energy of
1.9GeV circulate in so-called bunches at a velocity close to the
speed of light. The typical bunch length corresponds to about
70ps, therefore the emitted X-ray flashes have the same length.
This can be used to include time resolution into soft X-ray
microscopy and therefore to investigate spin dynamics on the
sub-100ps time scale in nanoscale magnetic elements [12].
However, as mentioned above the current available photon
intensity per bunch is not sufficient to allow for single shot
imaging and therefore a stroboscopic pump-probe scheme has
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Mccall, Monnikue M & Fischer, Peter. Studying Nanoscale Magnetism and its Dynamics with Soft X-ray Microscopy, article, May 1, 2008; Berkeley, California. (https://digital.library.unt.edu/ark:/67531/metadc896098/m1/2/: accessed April 24, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.