Experimental Methods
NEXAFSNear
Edge X-ray Absorption Fine Structure
SEXAFS
Surface Extended X-ray Absorption Fine Structure
Local adsorption sites, dynamics, pair potentials
of low Z atoms and molecules
We exploit the soft X-ray range (~100-1500 eV) using X-ray absorption
spectroscopy at BESSY storage ring. We use Near Edge X-ray Absorption Fine
Structure (NEXAFS) and Surface Extended X-ray Absorption Fine Structure
(SEXAFS) techniques. As the names suggest, these techniques are directly related
to near edge (approx. 30 eV above the edge) and far edge (several hundreds
of eV) features in the X-ray absorption spectrum. All experiments are carried
out in UHV. We measure with total electron yield and fluorescence yield detection
modes. We have the possibility to go down to 30 K, which gives us an opportunity
to study bond dynamics.
The NEXAFS, where intense features and/or resonances dominate the spectrum
one can get information of preferential orientation of chemisorbed or physisorbed
molecules and an indirect determination of intramolecular bond lengths.
We carry out such studies on O2, N2, various hydrocarbons
adsorbed on different metal surfaces. From SEXAFS (surface version of its
bulk analogue EXAFS), we not only try to answer the two basic questions
in surface crystallography: What atoms are there and how are they arranged,
but also try to understand the dynamics of adsorbate-substrate bonds. It is
possible to determine change of EXAFS Debye-Waller factor and the asymmetric
part of the pair distribution function at different temperatures from this
technique. For both NEXAFS and SEXAFS, we also perform multiple scattering
calculations.
FMR
Ferromagnetic Resonance
Temperature dependence of magnetic anisotropy,
magnetic relaxation g-factor
Ferromagnetic resonance is one of the best techniques to determine the
magnetic anisotropy constants up to sixth order. Surface/interface and volume
magnetic anisotropies in ultrathin films can be quantitatively determined
as a function of temperature with a resolution better than microeV. One major
advantage of this technique is the low excitation energy (microwave quanta)
by which the magnetic ground state properties are probed. Also, the dynamic
behaviour i.e. the magnetic relaxation rate can be probed in a time window
of typically few hundred picoseconds.
The FMR signal is measured by monitoring the microwave losses as a function
of the applied dc-field. For our UHV-studies 1, 4 and 9 GHz spectrometers
have been employed. The measured signal is proportional to the imaginary
(absorptive) part chi" of the complex microwave susceptibility chi = M||/h
mw = chi' + i chi" where M|| is the total rf magnetization
component parallel to the rf magnetic field component hmw which
is perpendicular to the applied dc-field H0. Evaluation of the
Landau-Lifshitz equation of motion which describes the time dependence
of the magnetization vector in the presence of a microwave field, dc- field
and any intrinsic anisotropy field typically yield a Lorentzian lineshape.
Integration of the signal allows the determination of the total sample magnetization
in absolute units.
XMCD
X-ray Magnetic Circular Dichroism
MEXAFS
Magnetic Extended X-ray Absorption Fine Structure
Element specific spin/orbital momentum and magnetization,
local magnetic disorder
These techniques are measures of the difference in the absorption of
left and right-circularly polarised photons at the absorption edges of the
constituent elements. We use both total yield and fluorescence yield detection
modes. The data are recorded by keeping the helicity of the incident light
fixed and reversing the direction of remanent magnetization by means of
a pulse driven electromagnet.
From XMCD, one can draw information about spin and orbital contributions
to the local magnetic moment, Curie temperature and nature of coupling. The
beauty of this technique lies in its site and element specificity. We
study changes in the above mentioned magnetic properties for ultrathin films
of transition metals (Fe,Co,Ni,Cu), with a cap layer i.e. a bi-layer and
tri-layers e.g. two ferromagnetic layers separated by a non-magnetic spacer.
MEXAFS adds magnetic selectivity to the now matured EXAFS technique and can
be called as its spin-dependent cousin. It has opened a new field of research
allowing to investigate static and dynamic magnetic phenomena from a site
selective point of view. We carry out temperature dependent MEXAFS measurements
at the L-edges of transition metal thin/thick films (e.g. Fe,Co) to study
the magnetic short-range order and gain an insight into the dynamic magnetic
phenomena.
MI
Mutual Inductance, ac-susceptibility
Para to ferromagnetic phase transition of single
and trilayers
The magnetic susceptibility c
=dM/dH is one of the basic magnetic observables. We have set-up a classical
alternating current mutual induction apparatus, which measures in UHV
in situ the susceptibility of ultrathin films. The non-magnetic substrate
with the magnetic film is placed inside a quartz finger which is part of
a UHV chamber equipped with conventional methods of preparation and characterization
of the sample. The coil setup is mounted ex situ around the quartz
finger. We can measure the temperature dependence (80 - 600 K) as well as
the dependence on external static magnetic fields up to 3 kA/m. chi is a
diverging quantity at the para-/ferromagnetic phase transition. A precise
measurement of chi(T) allows a precise determination of TC, which
varies with thickness and surface morphology for example. The driving field
is applied in the film plane which enables us to measure huge values of
chi up to 20 000 corresponding to the Curie-Weiss divergence at the Curie
temperature. An absolute measurement of chi is achieved by calibration
with Gd-sulphate. Another interesting topic is the measurement of the susceptibility
related critical exponent gamma.
STM Scanning
Tunneling Microscopy
Structure and morphology of metallic monolayers
and substrate surfaces
STM is one of the most powerful techniques for visualizing surfaces
in real space with even atomic resolution. The images are produced by the
interaction of an atomically sharp W tip with the surface. The structure
of ultrathin films directly influences their magnetic properties. Moreover,
it is very important to know the substrate morphology before evaporating
films. We characterise various substrates like Cu, W, Re. We find the crystallographic
direction and the density of the atomic steps from which we can estimate
the miscut of the substrates. This is very important since the magnetic
anisotropy is directly related to the direction and density of the steps.
We produce nanostructures on the substrates by controllable Ar+
bombardment. You might have noticed one of them as the background of the
first page of this web site. Evaporation of magnetic thin films on these
is expected to lead to structures with novel properties.
MOKE
Magneto-Optical Kerr Effect
Magnetization of ultrathin single- and trilayers
MOKE is one of the most widely used techniques for magnetic characterisation
of ultrathin films with sensitivity of one atomic layer. The measured quantities
are the Kerr rotation or ellipticity which are proportional to the magnetization
of the samples. By using the quartz finger technique, we have the unique
possibility to measure MOKE in situ under large external laboratory
fields of 20 kOe in the temperature range 80 to 600 K. The application of
a small oscillatory field (< 1 Oe) give us the opportunity to study ac-susceptibility.
This novelty is the so called ac MOKE technique. Thus, one has an in situ
magnetometer/susceptometer. With this powerful tool, we can draw conclusions
about the magnetic properties, ferromagnetic domains or interlayer coupling
effects and to study the critical behaviour of ultrathin films even in the
vicinity of the Curie temperature.
VSM
Vibrating Sample Magnetometry
Magnetization of magnetic layers protected against
oxidation
VSM is a classical static magnetometry technique.
A VSM measures the difference in magnetic induction between in region of
space with and without the specimen. It therefore gives a direct and absolute
measure of the magnetization. The exact knowledge of the magnetization values
is a crucial input in the FMR data analysis. Our VSM can measure in a field
up to 50 kOe, realized with a superconducting magnet, at temperatures from
1.2 K to >300 K. The measurements are performed ex situ on samples
with a protective cap layer against oxidation. Magnetic hysteresis loops
and temperature dependent magnetization curves are recorded for ultrathin
films and magnetic superlattices.
UHV-SQUID
SQUID-Magnetometry in UHV (in
situ)
A novel high-TC SQUID-measurement
in ultrahigh vacuum with monolayer sensitivity
Since the summer of 1999 we constructed a novel
in situ magnetization measurement: a commercial high-TC
superconducting quantum interference device is incorporated in an ultrahigh
vaccum chamber in which magnetic monolayers can be prepared, characterized
and directly measured with the SQUID. The high-TC SQUID is as
sensitive as a regular SQUID, i.e. measurements down to single atomic layers
of Fe, Co and even Ni are possible. The additional advantage is that only
lN2 -cooling is necessary. This saves a lot of space and makes
the handling very easy. The sample is mounted on an usual UHV-manipulator
which allows evaporation of Fe, Co, Ni, etc. and structural analysis (LEED,
AES, etc.). In addition a lHe-cooling allows to vary the sample temperature
between 25 and 300 K. The most striking advantage is the absolute calibration
of the magnetization, independent of the measured element (Fe, Co, Ni, etc.).
First results are reported in
[238, 251] .
(last update: 12.08.2003)