Fresnel Mode Research Articles

Imaging magnetic microstructures with the use of electrons

AbstractThe paper presents investigations of magnetic microstructures by techniques which make use of electrons. In this context, we refer to the examples of magnetic microstructure images of cobalt monocrystals, Nd–Fe–B permanent magnet and a thin permalloy film, while the methods used are the type‐I magnetic contrast technique of scanning electron microscopy (SEM), the colloid‐SEM method and the Fresnel mode of transmission electron microscopy (TEM). It is shown that the SEM type‐I magnetic contrast and the colloid‐SEM method have quite different probing depths and consequently provide useful complementary information on the magnetic microstructure at the surface of bulk materials which exhibit an out‐of‐plane component of magnetization. It is also demonstrated that TEM is a powerful tool for investigating the magnetic microstructure of thin magnetic films. Some improvements over previous results are presented. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Quantitative Fresnel Lorentz microscopy and the transport of intensity equation

Imaging of the magnetic structure of thin films by the Fresnel mode of Lorentz microscopy has been re-evaluated in terms of the Ampérian current density within a sample. The conditions for which this imaging can be treated as linear are discussed for quantitative application. Additionally, the consequences for magnetic phase reconstruction using the transport of intensity equation for defocused images are considered. While the range of applicability may initially appear rather limited examples of objects containing different spatial frequency components are used to illustrate the possibilities where relatively large defocus values may be used.

Remanent magnetization states and interactions in square arrays of 100-nm cobalt dots measured using transmission electron microscopy

Large area square arrays of circular cobalt dots, nominally 100nm in diameter and 20nm in thickness, were patterned using interference lithography. Magnetic remanent states were measured using off-axis electron holography in the transmission electron microscope (TEM). The results show that the dots are mostly single domain, although vortex states and multidomain configurations are occasionally observed. Significant magnetic interactions between adjacent dots result in variations in their magnetization direction away from the direction of the applied field. The suitability of such dots for data storage applications is discussed. Quantitative magnetic phase measurements were also obtained by applying the transport of intensity equation to images acquired using the Fresnel mode of Lorentz microscopy in the TEM. The consistency between the electron holography and transport of intensity equation (TIE) results is assessed.

Correlation between magnetization performance and magnetic microstructure of patterned permalloy films fabricated by microcontact printing

Permalloy films were patterned by two simple methods, direct etching and lift-off, which are based on the microcontact printing method. Several kinds of patterns, including chessboard, stripe, grid, and rectangular dot patterns, have been produced. These patterned permalloy films reveal varied magnetization behavior that originate from the differences in the shape of the pattern-consisting elements. The magnetic structure of the chessboard pattern film was investigated by Fresnel and Foucault modes of Lorentz transmission electron microscopy. It is found that the square elements of the patterned film exhibit vortexlike magnetization closures, although some connections between the elements remained due to insufficient etching. The macroscopic magnetization process of the chessboard-patterned film was explained in reference to the in situ magnetization-distribution observation of the specimen along a minor magnetization loop. We also tried to control the rotational sense of the “vortices” in the square elements by using a gradient field.

Effect of Ga+ ion irradiation on the structural and magnetic properties of CoFe/IrMn exchange biased bilayers

The effects of focused beam Ga+ ion irradiation on the physical microstructure and magnetic properties of CoFe/IrMn exchange biased bilayers have been investigated by transmission electron microscopy. Only for Ga+ ion doses >1014 ions cm−2 was the microstructure detectably altered with increases in both the bilayer mean grain size and texture being observed. At larger doses (>1015 ions cm−2) radical alterations to the grain morphology and polycrystalline nature of the film were observed including formation of a remarkable needle-like phase at 3×1015 ions cm−2. The magnetic properties and magnetization reversal behavior of the bilayer were studied using the Fresnel mode of Lorentz microscopy. Ga+ ion doses >1013 ions cm−2 were found to progressively reduce the bias field strength and coercivity with the former reaching half original strength at 2×1014 ions cm−2. Higher doses were found to alter the reversal mechanism accompanied by progressive degradation of the magnetic properties. It is likely that the observed magnetic property changes are caused by intermixing of atoms between the various layers.

Observation of magnetic structures in Fe granular films by differential phase contrast scanning transmission electron microscopy

Differential phase contrast (DPC) scanning transmission electron microscopy was applied to investigate the magnetic structures of Fe granular films. The DPC images showed a cluster-like contrast of 200 nm in the lateral size, which could not be observed by Lorentz transmission electron microscopy in the Fresnel mode. The magnetization vector map reproduced from the DPC images indicated that a magnetization loop generally intersects several isolated particles of 20–40 nm in diameter. The films composed of densely dispersed magnetic particles showed larger magnetization loops, while more separately dispersed films tended to have smaller loops, or magnetization closures in one particle. The loop size is associated with the strength of magnetic interaction between magnetic particles.

Electron-optical phase shift of magnetic nanoparticles II. Polyhedral particles

A method is presented to compute the electron-optical phase shift for a magnetized polyhedral nanoparticle, with either a uniform magnetization or a closure domain (vortex state). The method relies on an analytical expression for the shape amplitude, combined with a reciprocal-space description of the magnetic vector potential. The model is used to construct two building blocks from which more complex structures can be generated. Phase computations are also presented for the five Platonic and 13 Archimedean solids. Fresnel and Foucault imaging mode simulations are presented for a range of particle shapes and microscope imaging conditions.

Microstructure of nanocrystalline FeZr(N)-films and their soft magnetic properties

The microstructure of nanocrystalline FeZr(N) films, deposited by DC sputtering in a Ar+N 2 atmosphere was studied in correlation with their soft magnetic properties. The micromagnetic properties of the films were investigated using the Fresnel mode of Lorentz microscopy.

Magnetic Domains and Domain Wall Pinning in Nanocrystalline Ni-P

Magnetic domain structures in a Ni-5at%P alloy have been examined using Lorentz microscopy in Fresnel mode in a JEOL 2010TEM. with electron diffraction and high resolution electron imaging, the Ni-P alloy material is seen to be of FCC structure and composed of nanometer-sized grains (< 4nm in diameter), which is about 2 orders less in size than that of a single magnetic domain.The TEM specimen was prepared using jet polishing method. Before introducing the specimen into the microscope, the objective lens was turned off in a free lens control mode to ensure that the domain structures in the specimen remain unaffected. The objective mini-lens was used to perform Lorentz imaging with out-focus method.Stripe domains were observed. The width of these stripes is about 0.2 micron. But the length of these domains varies, sometime up to several microns. The stripe domains are grouped, which are near parallel one to the other.

Dynamic magnetization observations and reversal mechanisms of sintered and die-upset Nd–Fe–B magnets

In situ TEM magnetizing experiments combined with Lorentz microscopy in Fresnel and Foucault mode were used to study local features of magnetization reversal in polycrystalline sintered and die-upset Nd–Fe–B magnets. The die-upset (DU) magnets featured well-aligned grain texture along the c-axis, while the sintered (SI) magnets have a much wider distribution of grain orientations. Despite the similarity of the hysteresis curves, we found their nanoscale behavior of the magnetization reversal was quite different. The predominant mechanism controlling reversal in DU-magnets was nucleation of reversed domains, preferentially at misaligned grain interfaces, followed by domain wall pinning at grain boundaries. In contrast, magnetization reversal in SI-magnet progressed first by nucleation and expansion of reversed domain dipoles predominantly in misaligned grains at a high magnetic field, and then by the major irreversible process via nucleation and splitting of magnetic domains near the grain boundaries of well-aligned grains at more negative fields. This process is completed with lateral expansion of newly created domain walls. A simple model for the strong pinning–trapping centers observed in DU-magnets is discussed.

In-Situ Tem Dynamic Magnetizing Experiments Used To Identify The Pinning Centers In Hard Magnets Re13.75fe80.25b6 (Re=Nd, Pr).

Abstract The new JEOL 3000F high resolution electron microscope at BNL has been optimized for Lorentz imaging. The necessary field-free environment around the sample is obtained by switching off the objective lens in the free-lens control mode, and the associated reduction in magnification is compensated for by a Gatan post-column image filter (GIF) at ∼ 20x magnification. Fresnel imaging is obtained by defocusing with objective mini-lens (OM). The use of low angle diffraction with an aperture located at the back focal plane makes it possible to obtain Foucault images. In-situTEM dynamic magnetizing experiments combined with Lorentz magnetrc microscopy both in Fresnel and Foucault modes were used to characterize the magnetic structure of some hard and relatively soft magnets, Nd13.75Fe80.25B6 and Pr13.75Fe80.25B6, prepared by different processing routes. The goal of these in-situexperiments was to develop a reliable and effective procedure to search for, identify, and classify the different pinning centers present in real magnets that accord with their “pinning power” versus applied magnetic field.

Lorentz And Interference Electron Microscopy On A Scanning Tem

Magnetic imaging can be performed both on conventional (projection) and scanning TEM (CTEM/STEM). The CTEM-based magnetic imaging, classified into Fresnel, Foucault, holography, and other modes, has proven successful by now in observing various magnetic objects, and has accordingly been well documented elsewhere. The STEMbased one has emerged relatively recently prompted by the rapid growth of current STEM technology.The most ubiquitous implementation of Lorentz microscopy on a STEM is the differential phase contrast (DPC) mode. Each scanning signal obtained in the DPC mode is linear to Lorentz deflection, thus reflects the magnitude of an in-plane component of magnetic induction integrated along the optical axis(Fig.l). The signals therefore suffice to generate integrated in-plane magnetic induction maps(Fig.2). Furthermore, by making use of the fact that the Lorentz deflection distribution across the image plane forms an irrotational (vortex-free) 2D vector field, a pertinent numerical image processing yields a function equivalent to the electron phase function representing magnetically distorted electron wavefront.

Confinement-induced enhancement or suppression of the resonant dipole–dipole interaction

By using the quantized Fresnel modes in the planar cavity, the resonant molecular interaction is theoretically described and the general interaction potential tensor is obtained. The resonant dipole-dipole (D–D) interaction or excitation transfer matrix element are found to be dramatically suppressed or enhanced by the surrounding planar boundaries. The analytic expressions of the resonant D–D interaction potential are obtained for a few limiting cases. The results thus obtained are closely related to various phenomena associated with the spatial confinement, such as confinement-induced phase transitions of liquids, resonant excitation transfer between molecules bound in membrane etc. In the case of the high-Q cavity, the reduction of the effective dimensionality is discussed. Secondly, the short-distance limit of the general resonant molecular interaction potential tensor and its frequency dependence are investigated. Numerical calculations of the high-Q cavity limit and the lipid-water system are presented to demonstrate the crucial role of the planar boundaries in the resonant D–D interaction. The relevances of the current investigation with the excitation transfer in the photosynthetic antenna system, coherent energy transfer between neighboring peptide bonds, and static D–D interaction in the molecular dynamics simulation study of the membrane-bound proteins are briefly discussed.

In Situ Transmission Electron Microscopy Studies of the Magnetization Reversal Mechanism in Information Storage Materials.

: The Foucault and Fresnel modes of Lorentz microscopy, together with a quantitative magnetization mapping technique, summed image differential phase-contrast imaging, were used to study the magnetization reversal mechanism of the sense layer in spin-valve structures exhibiting the giant magnetoresistance effect. In addition to studies of sheet film, lithographically defined spin-valve elements were investigated. A current can be passed through the element during magnetizing so that the effect of the applied current on the giant magnetoresistance and magnetization reversal mechanism can be studied. Results are presented for a number of different spin-valve structures.

Magnetisation reversal mechanisms of FeMn-biased spin-valves as a function of interlayer coupling strength and field orientation

The Fresnel mode of Lorentz microscopy has been used to study the magnetisation reversal mechanisms of ‘free’ layers in a range of spin-valves in which the anisotropies of both magnetic layers are parallel. Fields were applied in situ in a modified TEM at various angles to the biasing direction on samples with different coupling strengths between ‘free’ and ‘pinned’ layers. Three distinct reversal modes were identified, as predicted using a modified Stoner-Wohlfarth theory. However, whilst the model only permits reversal by coherent rotation of magnetisation, domain processes were observed to assist two of the modes. A magnetic phase diagram has been constructed to show how reversal varies according to the orientation of the applied field and the coupling strength.

Energy filtered Lorentz microscopy

Energy-filtered Lorentz microscopy (EFLM) combines a post-column energy filter with a high-resolution TEM, operated with the main objective lens switched off. In this paper we use a magnetostrictive material, Terfenol-D (Tb 0.73Dy 0.27Fe 1.95), to illustrate the benefits of zero-loss filtering on both standard and coherent Fresnel and Foucault modes. It is shown that the signal-to-noise ratio of Foucault images is improved by an order of magnitude. We present a detailed study of magnetic domain configurations in the vicinity of growth twins and show that the magnetic resolution of the system is better than 5 nm. Our observations compare favorably with micromagnetic models. We also show that magnetic induction mapping becomes a viable technique in thicker regions when the inelastically scattered electrons have been removed from the images. Finally, we present simulations of Fresnel and Foucault images, assuming two-dimensional periodic boundary conditions for the computation of the Aharonov-Bohm phase shift.

Suppression and enhancement of van der Waals interactions

The van der Waals interaction of a pair of anisotropic molecules near planar dielectric surfaces is studied by using the linear response formalism. The spatial correlation function (Green function) of the vacuum electric field in the presence of dielectric surfaces is obtained by using suitable Fresnel mode functions of the quantized electric field. In the short-distance limit, it is observed that the long-range interaction potential is significantly modified by the dielectric surfaces and strongly depends on the geometry of the two molecules near dielectric surfaces. When the two molecules are anisotropic, depending on the molecular alignments with respect to the surfaces, the van der Waals interaction is enhanced or suppressed by the existence of the surfaces. When the two molecules are in between two dielectric surfaces, the overall magnitude of the van der Waals interaction is suppressed in comparison to that in the free space because the vacuum electromagnetic field intensity is reduced by the transmissivities that are generally less than unity.

Magnetic coupling of Co layers through a Cu spacer layer

In this paper we present results of investigations of the magnetic coupling of Co layers through a Cu spacer layer. These were obtained by examining Co/Cu/Co sandwich layers by Lorentz microscopy in the Fresnel and Foucault imaging mode. Ferromagnetic (F) and antiferromagnetic (AF) coupling of the magnetic Co layers can be found for different Cu spacer layer thicknesses. For the F coupled sandwich layer twin walls have been observed and the Co layers are only F coupled throughout the specimen. For the mainly AF coupled sandwich layer we observed a partial F and AF coupling of the Co layers which can be explained by thickness fluctuations of the Cu interlayer. Although we have to expect thickness fluctuations of the Co spacer layer in the F and the mainly AF coupled system, we only observe partial F and AF coupling for the mainly AF coupled system and not for the F coupled system. Therefore AF coupling is much more sensitive to thickness fluctuations than F coupling.

Far-out-of-focus electron holography in a dedicated FEG STEM

Flexible operation of off-axis holography modes is achieved when a biprism is placed in the illumination system of a scanning transmission electron microscope, equipped with a field emission electron source. The separation of the two virtual sources created by the biprism can be varied by changing the voltage applied to the biprism or by simply changing the excitation of the condensor and/or objective lenses. Two distinct modes of holography are used to study the magnetic microstructure in thin magnetic films. In the absolute mode the phase difference changes linearly in regions of constant magnetization and thickness and the slope determines the magnitude of magnetization. In addition, this mode permits the determination of the mean inner potential of a solid of known geometry. In the differential mode the phase difference is constant in regions of constant magnetization, allowing a simple and straightforward determination of domain wall profiles. The contrast obtained in the holography modes is compared to well known contrast in the Fresnel and differential phase contrast modes of Lorentz microscopy. The combination of several scanning transmission electron microscopy based techniques presents a powerful tool for the investigation of magnetic microstructure.

Pattern formation and competition in photorefractive oscillators.

We introduce a general model of pattern formation in optical systems made of a cavity with an active medium as a photorefractive crystal fed by a pump. The model is based on the interplay of a diffractive equation for the optical field and a diffusive equation for the medium refractivity. The aim of the model is to describe a series of experiments which have shown mode competition (periodic or chaotic alternation) for low Fresnel numbers (F) and mode coexistence, leading to short range space correlations, for high F. For low F, a linear stability analysis provides the set of modes above threshold as a function of the transverse wave number. Due to the interplay of the optical and the diffusive interactions, different behaviors result depending on the thickness of the medium as compared to the optical absorption length and diffusion length. Including the leading nonlinearities compatible with the symmetry constraints, we introduce normal form equations which describe the time-dependent mode competition. In the case of a large number of modes (high F), nonlinear mode-mode interaction is equivalent to a self-induced noise. In this limit, the relevant feature to be compared with the experiment is the power spectrum.