Magnetic X-ray Reflectivity

The scope of the thesis is to demonstrate the feasibility to
examine magnetization profiles of thin films and multilayer systems
via magnetic soft and hard x-ray reflectivity. The focus here is on
3d transition metals, which are used mainly for development of
numerous noval magnetic devices, that are both technologically and
scientifically interesting. Complementary to Neutron diffraction,
which is the standard tool for the examination of magnetic
structures in matter, magnetic x-ray diffraction permits to study
small samples and exhibits better Qz-resolution due its small and
only slightly divergent beam. The biggest advantage is its element
specificity, which enables one to probe different magnetic sites
separately. The method of magnetic x-ray reflectivity combines the
strong magnetic circular dichroism (MCD) effect, significantly
enhancing the magnetic sensitivity of x-rays, with the technique of
conventional specular reflectivity, a well established tool for the
structural studies of the chemical makeup of thin films and
artificial multilayer systems. The theory of resonant magnetic
scattering within dipole approximation combined with the specular
reflectivity condition suggests that the strongest effects are in
the lower incident angle regime using circularly polarized x-rays.
By using soft and hard x-rays structures on a scale of a few to
several hundreds of Å are probed, which is the dimensions of the
thicknesses of the layers of most thin film and multilayers
systems. In order to retrieve quantitative information from the
measured magnetic reflectivity curves, an approach for visible
light magneto-optical effects based on known dielectric tensors of
the sample has been adopted and applied for soft and hard x-ray
resonant scattering. Sample absorption and polarization changes in
the sample are accounted for. Besides the structural composition,
the thickness of the individual layers and the index of refraction,
also the magnetic spin configuration can be chosen with arbitrary
moment direction and magnitude by modifying the off-diagonal terms
in the dielectric tensor. The magnetic optical constants, which
determine the magnitude of the magnetic moments, are experimentally
determined via MCD absorption measurements and then retrieving the
real part through the Kramers-Kronig transformation of the measured
imaginary part. This is shown in this work for several 3d
transition metals and edges. The simulations are sensitive to a
variety of different spin configurations: spiral spin structures,
magnetic dead layers and of collinear alignment. Experimentally the
magnetic reflectivity of 3d transition metals has to distinguish
between the two available possible absorbtion edges, L and K, lying
in different x-ray regions. The L-edges are situated in the soft
x-ray region and exhibit large enhancements of the magnetic cross
section, while the K-edges lie in the hard x-ray regime and show
much smaller effects. In spite of this handicap, the latter can be
important due to the much larger penetration depth and better
Qz-resolution. The X13 beamline at the NSLS at Brookhaven National
Laboratory consisting of two branches for soft and hard-x ray
operations, respectively, uses an elliptical polarized wiggler
(EPW), which produces circularly polarized x-rays in the orbit
plane and allows fast switching between left and right circular
polarization. Lock-in detection is used to improve the
signal-to-noise ratio at the soft x-ray branch and single photon
detection at the hard x-ray branch to measure the magnetic signal.
The EPW and the experimental setup was commissioned to demonstrate
the feasibility of magnetic x-ray experiments. Especially at the
hard x-ray beamline branch the small magnetic effects, less than
0.1% of the charge scattering, were possible to detect. In order to
satisfy the need for high flux the CMC-CAT beamline at the APS in
Argonne was used for magnetic hard x-ray reflectivity, providing an
undulator beamline where the high flux of linear polarized photons
was converted into circular polarization via a diamond phase plate,
delivering much higher flux and better circular polarization. The
sample used to demonstrate the feasibility of the method of
magnetic reflectivity consists of two multilayer structures of
Fe/Cr on top of each other, where the iron spins of the upper are
ferromagnetically and of the lower antiferromagnetically coupled,
representing an exchange bias system. The sample was characterized
with conventional x-ray reflectivity and MOKE measurements in order
to accurately determine the structural composition and magnetic
configuration (hysteresis loops), respectively. Magnetic
reflectivity experiments on the L-edges at the X13A beamline showed
strong magnetic effects, which could be clearly identified as
ferromagnetic and antiferromagnetic Bragg peak contributions and
simulation confirmed the collinear alignment and full magnetization
of the iron spins throughout the iron layers. Energyand magnetic
field dependent measurements complete the picture. By tuning the
x-ray energy to the chromium L-edge, a signal 20 times weaker
compared with iron, demonstrates that the weak magnetic moment in
the chromium layers could be detected. Especially the AFM
contribution shows strong effects which could be qualitatively and
quantitatively evaluated. Simulation show clearly that the magnetic
moment is concentrated at the interfaces and could be approximated
to a magnetic layer with an effective thickness of about 0.5 Å
assuming a step function in the magnetization profile. Soft x-ray
data usually suffer from strong absorption and the limited Qz-range
and resolution and therefore the use of hard x-rays seems desirable
to probe the whole sample. Magnetic hard x-ray reflectivity
measurements on the Fe/Cr double multilayer carried out at the CMC
beamline by switching the magnetic field on the sample show clear
magnetic Bragg reflection at the ferromagnetic structural peaks.
They are very well reproduced by simulations and thus confirm the
collinear alignment of the iron spins. In order to probe the AFM
spin configuration the helicity of the photon beam has to be
switched with constant magnetic field. In spite of complications in
the reflectivity spectra it was possible to extract the relative
orientation of the AFM to FM spin configuration in the two
multilayers. In summary the work showed for the example of an Fe/Cr
double multilayer that magnetic soft and hard x-ray reflectivity
can be applied to retrieve information about the magnetization
profile of thin magnetic films and multilayer, and can compliment
polarized neutron scattering.