Experimental Methods


NEXAFSNear Edge X-ray Absorption Fine Structure
SEXAFS Surface Extended X-ray Absorption Fine Structure

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

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

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

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

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

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

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)

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)