New techniques and instrumentation for the ...characterisation of electrostatic and magnetic phase...
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New techniques and instrumentation for the characterisation of magnetic and electrostatic fields in nanocrystals and working devices R.E. Dunin-Borkowski 1,2 , M. Beleggia 2 , T. Kasama 2 and G. Pozzi 3 1. Ernst Ruska-Centre, Institute for Microstructure Research, Forschungszentrum Jülich, 52425 Jülich, Germany 2. Center for Electron Nanoscopy, Technical University of Denmark, DK-2800 Kongens Lyngby, Denmark 3. Dipartimento di Fisica, Universita’ di Bologna, V.le B. Pichat 6/2, 40127 Bologna, Italy [email protected] Keywords: off-axis electron holography, Lorentz imaging, magnetic fields, electric fields, working devices Off-axis electron holography is a powerful technique that be used to record the phase shift of a high-energy electron wave that has passed through an electron-transparent specimen in the transmission electron microscope (TEM) [1]. The interpretation of the recorded phase shift can be used to obtain quantitative nanometre spatial resolution information about magnetic and long-range electrostatic fields in nanoscale materials and devices. Here, we describe recent progress and prospects for extending medium resolution off-axis electron holography to the measurement of weak and three-dimensional fields, for developing techniques for the quantitative measurement of local charge densities and magnetic moments from electron-optical phase images and for applying multiple stimuli to specimens during examination in the TEM. Figure 1 illustrates the application of a recently developed algorithm for the quantitative measurement of the magnetic moment of one or more nanocrystals from a phase image by making use of the relationship between the volume integral of the recorded magnetic induction and the true magnetic moment [2]. The relation can be utilized to study particles of arbitrary shape and magnetization state to yield a measurement of the magnetic moment that is free of most artefacts. The magnetic induction map shown in Fig. 1a, which was recorded from a circular assembly of 20-nm-diameter nanocrystals of Co using electron holography, shows a magnetic flux closure state. Figure 1b illustrates the application of the newly developed algorithm to the measurement of the magnetic moment of the same ring. The approach involves the evaluation of either a contour integral of the recorded phase around a circular path or the integral of the phase gradient bounded by the same loop. Figure 1c show the magnetic moment of the ring measured as a function of the out-of-plane magnetic field applied to the sample before hologram acquisition in zero field. Different values of applied field result in transitions between the formation of onion-like and flux closure states at remanence. Figure 2 illustrates the application of electron holography to the measurement of the charge distribution along a carbon nanotube that has a voltage applied to it in situ in the TEM. Figures 2a and 2b show defocused bright-field images of several nanotubes, while Fig. 2c shows a phase image of one of the nanotubes acquired using electron holography. The charge distribution along the nanotube can be determined by evaluating a contour integral of the phase gradient around a closed path or the Laplacian of the phase within the area bounded by the same loop. By using this approach, the nanotube in Fig. 2c was inferred to have two different charge densities along its length (2.3 and 4.5 e/nm). Figure 3 shows simulations that highlight one of the main factors that influences the characterisation of crystalline materials using electron holography. The dramatic effect of dynamical diffraction on the phase shift is shown in the form of multislice simulations of the phase shift of the 000 beam for crystals of Au and graphite, calculated as a function of specimen thickness for zone axis orientations for each material. For Au, the calculations show that the phase shift can be more than 50% lower than the kinematic prediction at a specimen thickness of only 2 nm. For graphite, the phase shift can be close to zero or negative for zone axes in the plane of the graphene layers. Surprisingly, for certain values of specimen thickness and tilt angle, the phase shift for graphite can be larger than for Au. Additional considerations for electron holography experiments include charging of the specimen as a result of electron beam irradiation and changes over time in the thicknesses of overlayers or adsorbates on the specimen surface. Both effects are particularly important when applying electron holography to the characterisation of electrostatic and magnetic phase shifts in supported metal particles. The difficulty of characterising electromagnetic fields in small crystalline particles is illustrated by the fact that a 2 nm ferromagnetic particle produces a step in phase of ~2π/1000 radians, while a single Bohr magneton in a uniformly magnetized sphere with a typical atomic diameter would be associated with a step in phase of only ~2π/10 5 radians. Nevertheless, our assessment of the relationship between the phase noise and the error in the measurement of the magnetic moment suggests that it may be possible to measure the magnetic moments of nanoparticles that are below 100 μB, both in projection and in three dimensions, if the effects of dynamical diffraction, charging of the specimen and the presence of adsorbates on the specimen surface are minimized [3].
Transcript of New techniques and instrumentation for the ...characterisation of electrostatic and magnetic phase...
New techniques and instrumentation for the characterisation of magnetic and electrostatic fields in nanocrystals and working devices R.E. Dunin-Borkowski1,2, M. Beleggia2, T. Kasama2 and G. Pozzi3 1. Ernst Ruska-Centre, Institute for Microstructure Research, Forschungszentrum Jülich, 52425 Jülich, Germany 2. Center for Electron Nanoscopy, Technical University of Denmark, DK-2800 Kongens Lyngby, Denmark 3. Dipartimento di Fisica, Universita’ di Bologna, V.le B. Pichat 6/2, 40127 Bologna, Italy [email protected] Keywords: off-axis electron holography, Lorentz imaging, magnetic fields, electric fields, working devices
Off-axis electron holography is a powerful technique that be used to record the phase shift of a high-energy electron wave that has passed through an electron-transparent specimen in the transmission electron microscope (TEM) [1]. The interpretation of the recorded phase shift can be used to obtain quantitative nanometre spatial resolution information about magnetic and long-range electrostatic fields in nanoscale materials and devices.
Here, we describe recent progress and prospects for extending medium resolution off-axis electron holography to the measurement of weak and three-dimensional fields, for developing techniques for the quantitative measurement of local charge densities and magnetic moments from electron-optical phase images and for applying multiple stimuli to specimens during examination in the TEM.
Figure 1 illustrates the application of a recently developed algorithm for the quantitative measurement of the magnetic moment of one or more nanocrystals from a phase image by making use of the relationship between the volume integral of the recorded magnetic induction and the true magnetic moment [2]. The relation can be utilized to study particles of arbitrary shape and magnetization state to yield a measurement of the magnetic moment that is free of most artefacts. The magnetic induction map shown in Fig. 1a, which was recorded from a circular assembly of 20-nm-diameter nanocrystals of Co using electron holography, shows a magnetic flux closure state. Figure 1b illustrates the application of the newly developed algorithm to the measurement of the magnetic moment of the same ring. The approach involves the evaluation of either a contour integral of the recorded phase around a circular path or the integral of the phase gradient bounded by the same loop. Figure 1c show the magnetic moment of the ring measured as a function of the out-of-plane magnetic field applied to the sample before hologram acquisition in zero field. Different values of applied field result in transitions between the formation of onion-like and flux closure states at remanence.
Figure 2 illustrates the application of electron holography to the measurement of the charge distribution along a carbon nanotube that has a voltage applied to it in situ in the TEM. Figures 2a and 2b show defocused bright-field images of several nanotubes, while Fig. 2c shows a phase image of one of the nanotubes acquired using electron holography. The charge distribution along the nanotube can be determined by evaluating a contour integral of the phase gradient around a closed path or the Laplacian of the phase within the area bounded by the same loop. By using this approach, the nanotube in Fig. 2c was inferred to have two different charge densities along its length (2.3 and 4.5 e/nm).
Figure 3 shows simulations that highlight one of the main factors that influences the characterisation of crystalline materials using electron holography. The dramatic effect of dynamical diffraction on the phase shift is shown in the form of multislice simulations of the phase shift of the 000 beam for crystals of Au and graphite, calculated as a function of specimen thickness for zone axis orientations for each material. For Au, the calculations show that the phase shift can be more than 50% lower than the kinematic prediction at a specimen thickness of only 2 nm. For graphite, the phase shift can be close to zero or negative for zone axes in the plane of the graphene layers. Surprisingly, for certain values of specimen thickness and tilt angle, the phase shift for graphite can be larger than for Au.
Additional considerations for electron holography experiments include charging of the specimen as a result of electron beam irradiation and changes over time in the thicknesses of overlayers or adsorbates on the specimen surface. Both effects are particularly important when applying electron holography to the characterisation of electrostatic and magnetic phase shifts in supported metal particles. The difficulty of characterising electromagnetic fields in small crystalline particles is illustrated by the fact that a 2 nm ferromagnetic particle produces a step in phase of ~2π/1000 radians, while a single Bohr magneton in a uniformly magnetized sphere with a typical atomic diameter would be associated with a step in phase of only ~2π/105 radians. Nevertheless, our assessment of the relationship between the phase noise and the error in the measurement of the magnetic moment suggests that it may be possible to measure the magnetic moments of nanoparticles that are below 100 µB, both in projection and in three dimensions, if the effects of dynamical diffraction, charging of the specimen and the presence of adsorbates on the specimen surface are minimized [3].
Figure 1. (a) Magnetic induction map recorded from a ring of 20 nm Co nanoparticles using electron holography in magnetic-field-free conditions. (b) Magnetic contribution to the recorded phase shift, showing circles of radius 115 and 160 nm, corresponding to the integration boundaries used to measure the magnetic moment of the ring. (c) Resulting measurements of the magnitude and orientation (arrows) of the moment of the ring, plotted as a function of the external out-of-plane field applied before recording induction maps at remanence, showing vortex and onion magnetic states.
Figure 2. (a) and (b) Bright-field TEM images, acquired at defocus values of ±3 mm, of bundles of single walled carbon nanotubes examined under an applied bias in situ in the TEM. (c) Reconstructed phase image acquired from the region indicated in (b), with 1.5 radian phase contours superimposed. The outline in (c) marks the region used to determine the charge on the nanotube bundle.
Figure 3. Dynamical calculations of phase shifts of the 000 beam plotted for the indicated zone axis orientations for (a) Au and (b) graphite for an accelerating voltage of 300 kV. Each calculation was performed using a multislice calculation with a single projected potential and no absorption. Kinematic predictions are also shown. Note the different specimen thickness ranges and vertical axes. 1. R.E. Dunin-Borkowski, M.R. McCartney and D.J. Smith, Encyclopedia of Nanoscience and
Nanotechnology, (American Scientific Publishers: Stevenson Ranch) 3 (2004) ,41. 2. M. Beleggia, T. Kasama and R.E. Dunin-Borkowski, Ultramicroscopy 110 (2010), 425. 3. We thank Alexander Wei and Stephan Hofmann for providing specimens and for discussions.
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