Andreas Liebig

Quantitative magnetic information from reciprocal space maps in transmission electron microscopy

One of the most challenging issues in the characterization of magnetic materials is to obtain a quantitative analysis on the nanometer scale. Here we describe how electron magnetic circular dichroism (EMCD) measurements using the transmission electron microscope can be used for that purpose, utilizing reciprocal space maps. Applying the EMCD sum rules, an orbital to spin moment ratio of mL/mS=0.08+/-0.01 is obtained for Fe, which is consistent with the commonly accepted value. Hence, we establish EMCD as a quantitative element-specific technique for magnetic studies, using a widely available instrument with superior spatial resolution.

Reciprocal and real space maps for EMCD experiments

Electron magnetic chiral dichroism (EMCD) is an emerging tool for quantitative measurements of magnetic properties using the transmission electron microscope (TEM), with the possibility of nanometer resolution. The geometrical conditions, data treatment and electron gun settings are found to influence the EMCD signal. In this article, particular care is taken to obtain a reliable quantitative measurement of the ratio of orbital to spin magnetic moment using energy filtered diffraction patterns. For this purpose, we describe a method for data treatment, normalization and selection of mirror axis. The experimental results are supported by theoretical simulations based on dynamical diffraction and density functional theory. Special settings of the electron gun, so called telefocus mode, enable a higher intensity of the electron beam, as well as a reduction of the influence from artifacts on the signal. Using these settings, we demonstrate the principle of acquiring real space maps of the EMCD signal. This enables advanced characterization of magnetic materials with superior spatial resolution.

Fine-tuning of the spin-density-wave state in Cr/V heterostructures via hydrogen uptake

We show that the magnetic state in rather thick Cr films can be finely tuned via hydrogen uptake into adjacent vanadium layers at rather low hydrogen pressures. By changing the hydrogen concentration and, hence, the electronic structure in the V layers, it is possible to affect the global properties of spin-density waves (SDWs) in Cr layers, including the SDW period and the Néel temperature. We provide direct experimental evidence that hydrogen uptake into V layers can be used to switch between incommensurate and commensurate SDW states in a reproducible way.

Hydrogen assisted growth of Fe/V superlattices

The functionality of magnetic heterostructures is strongly affected by the interface structure and morphology as well as by the crystalline quality. Using Fe/V superlattices, serving as model systems for exchange coupling and for superconducting spin valves, we show that the excellent interfacial properties can be further improved by hydrogen assisted growth. The role of the hydrogen thereby is twofold. It reduces thickness fluctuations and improves the crystalline quality. Sputter deposited Fe/V superlattices grown at a partial pressure of 2 × 10−6 mbar are atomically flat and possess a residual resistivity ratio which is significantly improved as compared to those of conventionally grown samples.

EMCD in the TEM – Optimization of signal acquisition and data evaluation

The method of electron magnetic circular dichroism (EMCD) was recently proposed anddemonstrated by Peter Schattschneider et al. [I]. The EMCD signal consists in differences of L3 andL2 edge intensi ...

Structural coherence and layer perfection in Fe/MgO multilayers

A series of Fe/MgO multilayers was grown on single-crystal MgO(001) substrates at different temperatures using magnetron sputtering. The structural quality of the samples was investigated by x-ray reflectometry, x-ray diffraction and transmission electron microscopy. The results show a strong dependence of the structural quality on the growth temperature. Although good epitaxial layers are obtained at 165 °C, the sample does not exhibit any superlattice diffraction peaks. This effect is shown to be related to a continuous variation of the distance between the Fe layers as well as between the MgO layers.

The influence of weak links on magnetic ordering in layered structures

We discuss the magnetic ordering in layered magnets with two different inherent ordering temperatures. A [V(12 ML)Fe(2 ML)/V(12 ML)(/Fe(3 ML)]70 superlattice with antiferromagnetic interlayer exchange coupling is used for this purpose. The more rapid decrease of the magnetic moment in the 2 monolayer thick Fe layers influences the ordering in a non-trivial way. A distribution of the effective ordering temperature is inferred, originating from a more rapid decrease of the magnetic moment in the near-surface region, as compared to the interior of the sample. The results clearly demonstrate the need to consider all energy and length scales when addressing ordering in materials with finite extension.

Morphology of amorphous Fe{sub 91}Zr{sub 9}/Al{sub 2}O{sub 3} multilayers: Dewetting and crystallization

Amorphous Fe91Zr9/Al2O3 multilayers grown by magnetron sputtering have been studied using x-ray reflectometry, x-ray diffraction, Rutherford backscattering spectrometry, and transmission electron m ...

Role of asymmetries for EMCD sum rules

The recent derivation of sum rules for electron energy loss near edge spectra (ELNES) [1,2] provided the theoretical foundation for extracting spin and orbital magnetic moments from ELNES spectra that are measured under particular scattering conditions — a phenomenon named electron magnetic chiral dichroism (EMCD) [3]. This theory predicts that the ratio of orbital to spin magnetic moments can directly be obtained from EMCD spectra, when these are constructed as a difference from two symmetrical measurements.

Experimental conditions and data evaluation forquantitative EMCD measurements in the TEM

The recently demonstrated technique electron energy-loss spectroscopy (EMCD) [I)opens new routes for characterization of magnetic materials using transmission electronmicroscopy. The technique enab ...