Hans Lidbaum

Two-, three-, and four-component magnetic multilayer onion nanoparticles based on iron oxides and manganese oxides.

Magnetic multilayered, onion-like, heterostructured nanoparticles are interesting model systems for studying magnetic exchange coupling phenomena. In this work, we synthesized heterostructured magnetic nanoparticles composed of two, three, or four components using iron oxide seeds for the subsequent deposition of manganese oxide. The MnO layer was allowed either to passivate fully in air to form an outer layer of Mn(3)O(4) or to oxidize partially to form MnO|Mn(3)O(4) double layers. Through control of the degree of passivation of the seeds, particles with up to four different magnetic layers can be obtained (i.e., FeO|Fe(3)O(4)|MnO|Mn(3)O(4)). Magnetic characterization of the samples confirmed the presence of the different magnetic layers.

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.

Imprinting layer specific magnetic anisotropies in amorphous multilayers

We demonstrate how layer specific in-plane magnetic anisotropy can be imprinted in amorphous multilayers. The anisotropy is obtained by growing the magnetic layers in the presence of an external field and the anisotropy direction can thereby be arbitrarily chosen for each of the magnetic layers. We used Co68Fe24Zr8 and Al70Zr30 layers as building blocks for demonstrating this effect. The imprinting is expected to be obtainable for a wide range of amorphous materials when grown at temperatures below the magnetic ordering temperature.

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.

Asymmetry of the two-beam geometry in EMCD experiments

We analyse theoretically the influence of the asymmetry of the two‐beam geometry on quantitative measurements of the energy‐loss magnetic chiral dichroism. Our simulations indicate that this asymmetry is not very strong inside or close to the Thales circle, but in other regions of the diffraction plane it can hinder an accurate extraction of the orbital to spin moment ratio.

Structure of Fe-Co/Pt(001) superlattices : a realization of tetragonal Fe-Co alloys

The structural properties of a tetragonally distorted Fe1−xCox alloy, in the form of Fe1−xCox/Pt(001) superlattices with x = 0.64, have been investigated experimentally. The study follows recent theoretical predictions on the enhanced uniaxial magnetocrystalline anisotropy of such alloys with specific combinations of chemical composition and tetragonal distortion. The ratio between out-of-plane and in-plane lattice parameters in the Fe0.36Co0.64 layers, c/a, was found to vary between 1.18 and 1.31, depending on the thickness ratio between the alloy and the spacer. This covered the range of interest c/a = 1.20–1.25 in the previous calculations and should be compared to c/a = 1 in the original bcc alloy lattice. Simulations of x-ray diffraction patterns as well as density functional calculations support the derivation of the Fe0.36Co0.64 lattice parameters.

Magnetic and transport properties of Ni81Fe19∕Al2O3 granular multilayers approaching the superparamagnetic limit

The magnetic and transport properties of Ni81Fe19∕Al2O3 granular multilayer films were studied in relation to their structural properties as the nominal thickness t of the permalloy (Ni81Fe19) layer was varied near the percolation limit: in the range of 8⩽t⩽16A while keeping the nominal thickness of the Al2O3 layers constant at 16A. A good structural quality of the multilayers was demonstrated by low angle x-ray reflectivity measurements, and transmission electron microscopy showed the transition from continuous permalloy layers separated by aluminium oxide layers for t=16A to metal grains dispersed in the insulator at t=8A. Magnetization measurements showed the gradual transition from ferromagnetic layers to superparamagnetic clusters and grains that successively become blocked as the temperature decreases. A strong correlation between transport and structural properties was observed in the temperature (T) dependence of the electrical resistance measured with the current in the plane in the range of 2⩽T⩽...

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.

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 ...