Energy-loss Magnetic Chiral Dichroism Research Articles

Quantitative determination of site-specific magnetic structure in TEM

Determining the magnetic structure of material on a nanometer scale is fundamental for understanding its nano-scale magnetic property and developing nano-scale magnetic devices. Site-specific electron energy-loss magnetic chiral dichroism (site-specific EMCD[1],[2]) method is come up with to get the crystallographic site-specific magnetic information of nanostructures. By constructively using the dynamical diffraction conditions in EMCD experiments, we experimentally achieve the crystallographic site-specific magnetic structure of a nanostructure of NiFe2O4 as an example in transmission electron microscope, with its site-specific magnetic circular dichroism spectra, and its site-specific spin/orbital magnetic moments extracted. The site-specific EMCD method shows its unique capability for solving the crystallographic site-specific magnetic structure on nano-scale, compared with X-ray magnetic circular dichroism and neutron diffraction. This work may benefit the research and application of magnetic materials on a nanometer scale. Acknowledgement: This work is financially supported by National 973 Project of China (2009CB623701) and Chinese National Nature Science Foundation (11374174,51390471 ). This work made use of the resources of the Beijing National Center for Electron Microscopy.

Comment on “Quantized Orbital Angular Momentum Transfer and Magnetic Dichroism in the Interaction of Electron Vortices with Matter”

It was claimed (Lloyd et al., PRL 108 (2012) 074802) that energy loss magnetic chiral dichroism (EMCD) with electron vortex beams is feasible, and has even advantages over the standard setup with Bragg diffracted waves. In this Comment, we show that Lloyd et al. ignored an important constraint on the proposed selection rule for the transfer of angular momentum in the interaction, namely that it is only valid for an atom located in the very center of the vortex. As an experimental consequence, the EMCD signal will only be strong for extremely small nanoparticles of 1 to 2 nm diameter.

Quantitative experimental determination of site-specific magnetic structures by transmitted electrons

Understanding the magnetic structure of materials on a nanometre scale provides fundamental information in the development of novel applications. Here we show a site-specific electron energy-loss magnetic chiral dichroism method, first experimentally demonstrating that the use of transmitted electrons allows us to quantitatively determine atomic site-specific magnetic structure information on a nanometre scale. From one NiFe(2)O(4) nanograin in composite films, we extract its atomic site-specific magnetic circular dichroism spectra and achieve the quantitative magnetic structure information, such as site-specific total magnetic moments and orbital to spin magnetic moment ratios, by constructively selecting the specific dynamical diffraction conditions in electron energy-loss magnetic chiral dichroism experiments. The site-specific electron energy-loss magnetic chiral dichroism method shows its unique ability for solving the site-specific magnetic structure at nanoscale resolution, compared with X-ray magnetic circular dichroism and neutron diffraction. This work opens a door to meet the challenge of exploring the magnetic structures of magnetic materials at the nanoscale using transmitted electrons.

Simulation of magnetic circular dichroism in the electron microscope

As electron energy-loss spectroscopy (EELS) and x-ray absorption spectroscopy (XAS) probe the same transitions from core–shell states to unoccupied states above the Fermi energy, it should always be possible to apply the two techniques to the same physical phenomena, such as magnetic dichroism, and obtain the same information. Indeed, the similarity in the expression of the electron and x-ray cross-sections had been already exploited to prove the equivalence of x-ray magnetic linear dichroism and anisotropy in EELS, by noting that the polarization vector of a photon plays the same role as the momentum transfer in electron scattering. Recently, the same was proven true for x-ray magnetic circular dichroism (XMCD) by establishing a new TEM technique called EMCD (electron energy-loss magnetic chiral dichroism) (Schattschneider P et al 2006 Nature 441 486–8), which makes use of special electron scattering conditions to force the absorption of a circularly polarized virtual photon.The intrinsic advantage of EMCD over XMCD is the high spatial resolution of electron microscopes, which are readily available. Among the particular obstacles in EMCD that do not exist for synchrotron radiation, is the notoriously low signal and the very particular scattering conditions necessary to observe a chiral dichroic signal. In spite of that, impressive progress has been made in recent years. The signal strength could be considerably increased, and some innovations such as using a convergent beam have been introduced. EMCD has evolved into several techniques, which make full use of the versatility of the TEM and energy filtering, spectroscopy or STEM conditions (Rubino S 2007 Magnetic circular dichroism in the transmission electron microscope PhD Thesis Vienna University of Technology, Vienna, Austria).

Real-space simulations of spin-polarized electronic transitions in iron

After the advent of energy-loss magnetic chiral dichroism (EMCD) in 2006, rapid progress in theoretical understanding and in experimental performance was achieved, recently demonstrating a spatial resolution of better than 2 nm. Similar to the x-ray magnetic circular dichroism technique, EMCD is used to study atom specific magnetic moments. The latest generation of electron microscopes opens the road to the mapping of spin moments on the atomic scale with this method. Here the theoretical background to reach this challenging aim is elaborated. Numerical simulations of the ${L}_{3}$ transition in an Fe specimen, based on a combination of the density-matrix approach for inelastic electron scattering with the propagation of the probe electron in the lattice potential indicate the feasibility of single spin mapping in the electron microscope.

Circular dichroism in the electron microscope: Progress and applications (invited)

According to theory, x-ray magnetic circular dichroism in a synchrotron is equivalent to energy loss magnetic chiral dichroism (EMCD) in a transmission electron microscope (TEM). After a synopsis of the development of EMCD, the theoretical background is reviewed and recent results are presented, focusing on the study of magnetic nanoparticles for ferrofluids and Heusler alloys for spintronic devices. Simulated maps of the dichroic strength as a function of atom position in the crystal allow evaluating the influence of specimen thickness and sample tilt on the experimental EMCD signal. Finally, the possibility of direct observation of chiral electronic transitions with atomic resolution in a TEM is discussed.

Magnetic properties of FeCo alloys measured by energy-loss magnetic chiral dichroism

The energy loss magnetic chiral dichroism (EMCD) in a transmission electron microscope to study magnetic devices of reduced dimensions is a young and very promising technique. It relies on the study of electron energy loss spectra acquired in specific locations of the diffraction pattern. The sensitivity of the method to the magnetic properties is investigated on a series of FexCo1−x alloys. The spectra can reflect modifications of magnetic or structural properties and the iron-cobalt alloys present the advantage of having a crystal structure which remains body centered cubic on a wide composition range. A significant variation of the EMCD signal as a function of the composition of the alloy has been detected and attributed to changes in the magnetic moment.

A software package for the simulation of energy-loss magnetic chiral dichroism

In this work, the theoretical framework for energy-loss magnetic chiral dichroism (EMCD) is reviewed and a new, fast, and easy-to-use software package is presented. This software is used to simulate the dependence of the EMCD signal on various parameters such as sample thickness and sample tilt. The results are found to be in excellent agreement with other theoretical predictions and experimental data. Furthermore, it is shown that the simulations are fast enough for the planning of experiments “on-the-fly” and for live didactic demonstrations.

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.

Effect of spatial and energy distortions on energy-loss magnetic chiral dichroism measurements: Application to an iron thin film

The various distortions associated with the energy-loss magnetic chiral dichroism (EMCD) technique are important as these distortions can modify the dichroic signal and even supress it. The usefulness of measurement and correction procedures will first be demonstrated on a calculated datacube before applying to the experimental datacube of an iron sample. The application of the EMCD sum rules will give quantitative information on the magnetic moments in the film at a local scale.

Real space maps of magnetic moments on the atomic scale: Theory and feasibility

The recently discovered EMCD technique (energy loss magnetic chiral dichroism) can detect atom specific magnetic moments with nanometer resolution, exploiting the spin selectivity of electronic transitions in energy loss spectroscopy. Yet, direct imaging of magnetic moments on the atomic scale is not possible. In this paper we present an extension of EMCD that can overcome this limit. As a model system we chose bcc Fe. We present image simulations of the L 3 white line signal, based on the kinetic equation for the density matrix of the 200 kV probe electron. With actual progress in instrumentation (high brightness sources, aberration corrected lenses) this technique should allow direct imaging of spin moments on the atomic scale.

Distortion corrections of ESI data cubes for magnetic studies

Measuring magnetic properties at a nanometre scale could be achieved in a transmission electron microscope by using dedicated techniques. Among these, the energy-loss magnetic chiral dichroism has already proven its efficiency and needs improvements to be widely used. The energy spectrum imaging technique can be used to measure dichroism but some image treatments are necessary due to distortions. This paper deals with the corrections that need to be applied on the data to remove all distortions, especially drift and non-isochromaticity, and extract reliable information. The measure and correction procedures are developed on an artificial data cube containing the dichroic signal and some noise to prove the efficiency of the routines.

Real space maps of atomic transitions

Considering the rapid technical development of transmission electron microscopes, we investigate the possibility to map electronic transitions in real space on the atomic scale. To this purpose, we analyse the information carried by the scatterer's initial and final state wave functions and the role of the different atomic transition channels for the inelastic scattering cross section. It is shown that the change in the magnetic quantum number in the transition can be mapped. Two experimental set-ups are proposed, one blocking half the diffraction plane, the other one using a cylinder lens for imaging. Both methods break the conventional circular symmetry in the electron microscope making it possible to detect the handedness of electronic transitions as an asymmetry in the image intensity. This finding is of important for atomic resolution energy-loss magnetic chiral dichroism (EMCD), allowing to obtain the magnetic moments of single atoms.

Energy-loss magnetic chiral dichroism (EMCD): Magnetic chiral dichroism in the electron microscope

A new technique called energy-loss magnetic chiral dichroism (EMCD) has recently been developed [P. Schattschneider, et al.Nature441,486 (2006)] to measure magnetic circular dichroism in the transmission electron microscope (TEM) with a spatial resolution of 10 nm. This novel technique is the TEM counterpart of x-ray magnetic circular dichroism, which is widely used for the characterization of magnetic materials with synchrotron radiation. In this paper we describe several experimental methods that can be used to measure the EMCD signal [P. Schattschneider, et al.Nature441,486 (2006); C. Hébert, et al.Ultramicroscopy108(3), 277 (2008); B. Warot-Fonrose, et al.Ultramicroscopy108(5), 393 (2008); L. Calmels, et al.Phys. Rev. B76,060409 (2007); P. van Aken, et al.Microsc. Microanal.13(3), 426 (2007)] and give a review of the recent improvements of this new investigation tool. The dependence of the EMCD on several experimental conditions (such as thickness, relative orientation of beam and sample, collection and convergence angle) is investigated in the transition metals iron, cobalt, and nickel. Different scattering geometries are illustrated; their advantages and disadvantages are detailed, together with current limitations. The next realistic perspectives of this technique consist of measuring atomic specific magnetic moments, using suitable spin and orbital sum rules, [L. Calmels, et al.Phys. Rev. B76,060409 (2007); J. Rusz, et al.Phys. Rev. B76,060408 (2007)] with a resolution down to 2 to 3 nm.

Exchange of angular momentum in EMCD experiments

In energy loss magnetic chiral dichroism (EMCD) experiments a chiral electronic transition is induced that obeys the dipole selection rule for the magnetic quantum number Δ m = ± 1 or Δ L z = ± ℏ . The incident plane electron wave is inelastically scattered and is detected in the diffraction plane, i.e. again in a plane wave state. Naïve reasoning suggests that the angular momentum L z of the probe electron has not changed in the interaction since plane waves have 〈 L z 〉 = 0 . This leads to the seeming contradiction that angular momentum is not conserved in the interaction. A closer inspection shows that the density matrix of the probe has indeed 〈 L z 〉 = ± ℏ after a chiral interaction. However, 〈 L z 〉 is not conserved when the probe electron propagates further to the exit surface of the specimen because the rigid lattice breaks rotational symmetry. Thus, the angular momentum of the photo electron that is created in a chiral electronic transition stems from both the probing electron and the crystal lattice.

Energy loss magnetic chiral dichroism: A new technique for the study of magnetic properties in the electron microscope (invited)

The similarity between x-ray absorption near edge structure and electron energy loss near edge structure is well known. However, “exporting” x-ray magnetic circular dichroism (XMCD) to the transmission electron microscope (TEM) was considered impossible with present technology since electron probes possessing chirality (i.e., spin polarization) cannot be set up with sufficient intensity. But recently magnetically induced chiral electronic transitions were detected in the TEM. In analogy to XMCD we introduced the term EMCD (energy loss magnetic chiral dichroism). The mechanism builds upon the formal equivalence between the mixed dynamic form factor for inelastic electron scattering and the absorption cross section for x-rays. Experiments on the 3d ferromagnets show effects very similar to XMCD. Calculations based on the WIEN2K package are in good agreement with experiments. The recent improvement in signal strength and spatial resolution allows now to study atom specific spin and orbital moments on a scale of less than 10nm, an important progress for spintronics and nanomagnetism.

Optimal aperture sizes and positions for EMCD experiments

The signal-to-noise ratio (SNR) in energy-loss magnetic chiral dichroism (EMCD)—the equivalent of X-ray magnetic circular dichroism (XMCD) in the electron microscope—is optimized with respect to the detector shape, size and position. We show that an important increase in SNR over previous experiments can be obtained when taking much larger detector sizes. We determine the ideal shape of the detector but also show that round apertures are a good compromise if placed in their optimal position. We develop the theory for a simple analytical description of the EMCD experiment and then apply it to dynamical multibeam Bloch wave calculations and to an experimental data set. In all cases it is shown that a significant and welcome improvement of the SNR is possible.

Magnetic circular dichroism in electron energy loss spectrometry

The measurement of circular dichroism in the electron microscope is a new, emerging method and, as such, it is subject to constant refinement and improvement. Different ways can be envisaged to record the signal. We present an overview of the key steps in the energy-loss magnetic chiral dichroism (EMCD) experiment as well as a detailed review of the methods used in the intrinsic way where the specimen is used as a beam splitter. Lateral resolution up to 20–30 nm can be achieved, and the use of convergent beam techniques leads to an improved S / N ratio. Dichroic effects are shown for Ni and Co single crystal; as a counterexample, measurements were carried also for a non-magnetic (Ti) sample, where no dichroic effect was found.

Experimental application of sum rules for electron energy loss magnetic chiral dichroism

We present a derivation of the orbital and spin sum rules for magnetic circular dichroic spectra measured by electron energy loss spectroscopy in a transmission electron microscope. These sum rules are obtained from the differential cross section calculated for symmetric positions in the diffraction pattern. Orbital and spin magnetic moments are expressed explicitly in terms of experimental spectra and dynamical diffraction coefficients. We estimate the ratio of spin to orbital magnetic moments and discuss first experimental results for the Fe ${L}_{2,3}$ edge.

Sum rules for electron energy loss near edge spectra

We derive four sum-rule expressions for spectra measured in electron energy loss near edge structure experiments. These sum rules permit the determination of spin and orbital magnetic moments, spin-orbit interaction, and number of states, analogously to the sum rules of x-ray magnetic circular dichroism. The derivation of the sum rules is based on dynamical electron diffraction theory and the properties of the mixed dynamic form factor. The accuracy of the sum rules is tested by a complete evaluation of the thickness-dependent electron energy loss spectra for iron, cobalt, and nickel crystals. We find that the sum rules reproduce both spin and orbital moments with very good accuracy. Our results provide a foundation for the use of the energy loss magnetic chiral dichroism technique as a quantitative probe of element-specific magnetic properties.