Quantitative assessment of EMCD signals in real and reciprocal space in single and multilayer samples

Recent quantitative measurements of the orbital to spin magnetic moment ratio $m_L/m_S$ in electron magnetic circular dichroism (EMCD) experiments have given a $m_L/m_S$ ratio that is larger than commonly accepted values. We demonstrate here that plural scattering may noticeably influence the $m_L/m_S$ ratio. An equation is derived which describes its influence as a function of the spectral integrals of the plasmon scattering region and zero-loss peak. The influence of the electron-plasmon scattering can be removed when electron energy-loss spectra of the ionization edge are deconvoluted by the low-loss signal. For a bcc-Fe sample we obtain $m_L/m_S=0.04$ after plasmon removal. We conclude that the plural scattering should be considered when extracting quantitative information from EMCD measurements.

Extended abstract of a paper presented at Microscopy and Microanalysis 2011 in Nashville, Tennessee, USA, August 7–August 11, 2011.

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.

The structural and magnetic properties of Co68Fe24Zr8/Al2O3 multilayers grown by using magnetron sputtering were investigated with x-ray reflectivity, transmission electron microscopy and magneto-optical Kerr effect. The Co68Fe24Zr8 form amorphous islands when the nominal thickness of the Co68Fe24Zr8 layers is 10 Å, exhibiting an isotropic superparamagnetic behavior. Continuous layers with mostly a nano-crystalline structure are instead formed when the nominal thickness of the Co68Fe24Zr8 layers is increased to 20 Å. The continuous layers exhibit random, inplane, magnetic anisotropy resulting from the growth process. However, induced uniaxial anisotropy is obtained when growing the sample in the presence of an applied magnetic field, regardless of the combination of amorphous and nano-crystalline material.

We briefly review the state-of-the-art electron magnetic chiral dichroism experiments and theory with focus on quantitative measurements of the atom-specific orbital to spin moment ratio ml/ms. Our approach of quantitative method, based on reciprocal space mapping of the magnetic signal, is described. We discuss additional symmetry considerations for ml/ms measurements, which are present due to dynamical diffraction effects. These lead to a preference for the 3-beam orientation of the sample. Further on, we describe a method of correcting asymmetries present due to imperfect 3-beam orientation—the so-called double-difference correction.

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.

The structural and magnetic properties of Co68Fe24Zr8/Al2O3 multilayers grown by using magnetron sputtering were investigated with X-ray reflectivity, transmission electron microscopy and magneto-optical Kerr effect. The Co68Fe24Zr8 form amorphous islands when the nominal thickness of the Co68Fe24Zr8 layers is 10 Å, exhibiting an isotropic superparamagnetic behavior. Continuous layers with mostly a nano-crystalline structure are instead formed when the nominal thickness of the Co68Fe24Zr8 layers is increased to 20 Å. The continuous layers exhibit random, in-plane, magnetic anisotropy resulting from the growth process. However, induced uniaxial anisotropy is obtained when growing the sample in the presence of an applied magnetic field, regardless of the combination of amorphous and nano-crystalline material.

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.

A method for depositing highly amorphous, iron-rich Fe100−xZrx thin films on to room temperature substrates is presented. The method involves co-depositing Fe and Zr on to an amorphous AlZr layer. Experimental proof that the structures are completely amorphous is given by transmission electron microscopy and polarized neutron reflectometry. The reflectometry measurements also give an indication of the impact that Fe crystallite impurities have on the magnetic structure and properties of amorphous FeZr. The results are consistent with previous investigations on bulk samples, which showed that crystalline impurities make the magnetic structure more non-collinear.

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.

The ratio of the orbital to the spin magnetic moment was determined for both Fe and Co in amorphous Co68Fe24Zr8 layers using x-ray circular dichroism. The investigations were performed on both thick Co68Fe24Zr8 layers as well as on amorphous Co68Fe24Zr8/Al70Zr30 multilayers grown by dc sputtering. Structural characterization was performed using x-ray reflectometry, x-ray diffraction, and transmission electron microscopy. X-ray circular dichroism, x-ray magnetic scattering as well as the magneto-optic Kerr effect were used to characterize the magnetic properties of the amorphous materials. The ratio of the orbital to spin moments in the single CoFeZr-layer sample was 0.012 +/- 0.005 for Fe and 0.078 +/- 0.005 for Co. Substantial reduction in the the ratio of the orbital to spin moments was observed with decreasing CoFeZr-layer thickness.