Imaging Of Domain Structures Research Articles

Imaging mechanism of piezoresponse force microscopy of ferroelectric surfaces

In order to determine the origin of image contrast in piezoresponse force microscopy (PFM), analytical descriptions of the complex interactions between a small tip and ferroelectric surface are derived for several sets of limiting conditions. Image charge calculations are used to determine potential and field distributions at the tip-surface junction between a spherical tip and an anisotropic dielectric half plane. Methods of Hertzian mechanics are used to calculate the response amplitude in the electrostatic regime. In the electromechanical regime, the limits of strong (classical) and weak (field-induced) indentation are established and the relative contributions of electroelastic constants are determined. These results are used to construct ``piezoresponse contrast mechanism maps'' that correlate the imaging conditions with the PFM contrast mechanisms. Conditions for quantitative PFM imaging are set forth. Variable-temperature PFM imaging of domain structures in ${\mathrm{BaTiO}}_{3}$ and the temperature dependence of the piezoresponse are compared with Ginzburg-Devonshire theory. An approach to the simultaneous acquisition of piezoresponse and surface potential images is proposed.

Visualization of 180° domain structures in uniaxial ferroelectrics using confocal scanning optical microscopy

Confocal scanning optical microscopy (CSOM) is used to visualize ferroelectric domain structure in materials with no natural optical contrast between domains. By applying an in-plane ac electric field, the orientation of the in-plane ferroelectric polarization may be determined. Experiments on LiNbO3 single crystals show high-quality images of domain structures. The domain contrast is found to depend on the ac field orientation and the incident light polarization. This investigation demonstrates the utility of CSOM for studying static and dynamic processes of domain structure and the possibility for studying domain kinetics on the nanoscale level.

Imaging of Domain Structures in a One-Component Lipid Monolayer by Time-of-Flight Secondary Ion Mass Spectrometry

Time-of-flight secondary ion mass spectrometry with high lateral resolution was used to image the domain structures of a phase-separated pure dipalmitoylphosphatidylcholine monolayer, which was transferred from the air−water interface of a Langmuir film balance to a gold surface. In the two-phase region characteristic domain structures became clearly visible in the image maps of different fragment ions (secondary ions) as well as the molecule ion. Depending on the fragment, the liquid-condensed (LC) domains exhibit both lower and higher secondary ion yields than the liquid-expanded regions of the monolayer. To understand the mechanism leading to this alternating contrast, we determined the intensities of some secondary ions at different surface pressures adjusted during transfer. This analysis showed that the secondary ion formation in the lipid headgroup region is much more sensitive to the physical state of the monolayer than in the acyl chains. We conclude that an increased electrostatic interaction am...

Imaging Mechanism and Quantification of Scanning Probe Microscopies on Ferroelectric Surfaces

AbstractIn the last few years a wide spectrum of non-contact, intermittent contact and contact scanning probe microscopies have been applied to imaging ferroelectric surfaces. The imaging mechanism in non-contact SPM is ultimately related to the total charge distribution on the ferroelectric surface, including both polarization and screening charges. Contact voltage modulation (piezoresponse) force microscopy (PFM) is sensitive to both local polarization via electromechanical coupling and surface charge via capacitive interactions. In the present research we analyze the electrostatic and electromechanical contrast in PFM using analytical solutions for the electrostatic sphere-dielectric plane problem and for the piezoelectric indentation problem. The contribution of electrostatic forces to the image is estimated. Variable-temperature PRI imaging of domain structures in BaTiO3 is performed and the temperature dependence of the piezoresponse is compared with the Ginzburg - Devonshire theory.

Application of a Retarding-type Mott Analyzer to Observation of Magnetic Domains

We developed a retarding-type Mott analyzer, of which the figure of merit and the size were improved as compared with our previously designed analyzers. The figure of merit of the new analyzer is sufficient to be used in a spin-polarized secondary electron microscope (SP-SEM) system. Equipping a UHV-SEM system with the analyzer and a secondary electron collector, we constructed an SP-SEM system. The proper operation of the system was confirmed by observing a single Fe crystal and we got clear images of domain structures on the Fe (001) surface. We also got a magnetic domain image of a patterned permalloy thin film.

Feature selection algorithm and cobweb correlation

In this paper a method of feature selection is proposed where two different measures of cumulative between-class distance are considered. The algorithm has been derived to select an optimal set of translations for computing of the correlation function between couples of pixels. The texture features obtained from the used model result from the local context evaluation on the selected set of image points. This approach has been inspired with one type of Haralick's parameter. The selection algorithm is addressed, and its good properties are discussed. The results are presented using the images of magnetic domain structures from optical microscopy.

Magnetic force imaging of domain structures for a (Pr/Nd)FeB alloy

Magnetic force microscopy (MFM) has been used to image the magnetic stray field from a (Pr/Nd)FeB hard magnetic alloy containing α-Fe regions. Images were acquired with three different sensing tips (CoCr, NiFe and Fe/SiO2 coated Si). The MFM images were compared to optical Kerr effect images from the same area. Kerr images of the (Pr,Nd)2Fe14B grains were similar to MFM images obtained using the CoCr tip, revealing a regular open-flux magnetic structure for those grains imaged on a plane perpendicular to the easy direction and stripe domains on those grains imaged on a plane containing the easy direction. MFM images indicated that the α-Fe particles were single domain, while Kerr images suggested a substructure within α-Fe. The low coercivity NiFe and Fe/SiO2 sensing tips provided images only of the magnetic domain boundaries. The difference in image characteristics between the CoCr and NiFe and Fe/SiO2 tips is believed to be associated with the extent to which the initial tip magnetisation is changed by the stray field from the sample.

Soft X-ray photoemission electron microscopy as an element-specific probe of magnetic microstructures

Photoemission electron microscopy has matured into a versatile analytical technique on mesoscopic length scales. Applications range from surface physics or chemistry to biology. When operated with circularly polarized light in the soft X-ray regime, however, photoemission microscopy offers a unique access to many aspects in surface and thin film magnetism by combining magnetic and chemical information. Exploiting the high brilliance and circular polarization available at a helical undulator beamline, the lateral resolution in the imaging of magnetic domain structures may be pushed well into the sub-micrometer range. By means of a newly designed photoemission microscope not only domains, but also domain walls can be selectively investigated under these circumstances. The high sensitivity of the technique yields a sizable magnetic contrast even from ultrathin films in the monolayer regime. The combination of chemical selectivity and information depth can be employed to investigate the magnetic behavior of buried layers and covered surfaces. This approach offers a convenient access to magnetic coupling phenomena in magnetic sandwich structures.

Magnetization reversal in ultrathin ferromagnetic films with perpendicular anisotropy

This paper presents a detailed study of the magnetization reversal dynamics in ultrathin cobalt films ( t Co = 6–12 A ̊ ) sandwiched by gold (1 1 1) layers, their magnetic anisotropy being perpendicular to the film surface. The domain wall (DW) velocity and nucleation rate are determined from direct time-resolved domain structure imaging. Measurements as a function of the applied magnetic field and temperature enable us to estimate the magnetic parameters, such as activation volumes, coercive fields, Gilbert damping parameters, etc., controlling the magnetization reversal. Depending upon the applied magnetic field value three different regimes are evidenced and studied. At low fields the DW pinning by structural inhomogeneities controls the thermally activated magnetization reversal dynamics. Above the propagation field value the dynamics is due to viscous DW motion. In higher fields a DW velocity breakdown is observed. The DW jaggedness and its change with the applied field and temperature is examined through the DW fractal dimension. A simple DW motion simulation taking into account an activation volume distribution explains qualitatively the data.

Evolution of magnetic microstructure in high-coercivity permanent magnets imaged with magnetic force microscopy

Magnetic force microscopy (MFM) has been shown to give high-resolution imaging of magnetic domain structures in a variety of high-coercivity permanent magnets [Folks et al., J. Magn. Magn. Mater. (in press)]. We show that this technique can be extended by the application of external fields during imaging, thus allowing direct observation of submicron microstructural evolution as a function of field. Electromagnets mounted on the MFM supplied fields up to 7 kOe laterally and 3 kOe vertically. In sintered materials, submicron processes such as depinning of domain walls at grain boundaries, domain fragmentation, and hysteresis were observed. MFM tips having very low coercivity highlighted domain walls, whereas higher-coercivity tips suffered unpredictable rotation of their magnetic moment due to both the sample and applied fields, leading to images which are difficult to interpret. For imaging of the finer-grained melt-quenched magnets, however, relatively high-coercivity tips were superior. These results show promise for the direct observation of the submicron-scale processes that dictate bulk magnetic properties, and the quantification of their field dependence.

Observation of the ferromagnetic domain structure in exchange-coupled antiferromagnetic/ferromagnetic thin films (abstract)

Exchange-coupled antiferromagnetic/ferromagnetic (AFM/FM) thin films are known to exhibit many interesting and potentially useful properties, including a unidirectional anisotropy.1 Despite nearly three decades of research, the nature of the AFM/FM interface exchange interaction is still not well understood, particularly in polycrystalline thin films. Simple predictions suggest an exchange-coupling strength 10–100 times stronger than generally observed. The models of Mauri et al.2 and Malozemoff3 resolve this inconsistency by considering domains and reversal within the AFM layer. Clearly, to understand this phenomena we need an accurate description of the AFM’s bulk and interface spin structure and its action under the influence of a coupled FM film. In an effort to better understand the AFM/FM system, we have been studying NiO/NiFe. In this report, we examine the magnetic domain structure and reversal process in the NiO/NiFe system by imaging of the NiFe magnetic domain structure. This is accomplished using our newly developed Kerr microscope. This microscope uses a phase detection method that allows high spatial resolution (0.5 μm) and good magneto-optic contrast. In addition, we can discriminate the magnetization vector components in the sample plane to develop a vector map of the magnetization. Kerr images indicate that the magnetic domain structure in the NiFe is complicated by the NiO interaction. The magnetization assumes a large scale “ripplelike” structure with domains walls perpendicular to the easy direction. This surprising result may result from a local dispersion in the NiO pinning direction. We will present a simple model of this interaction and relate it to the other current standard models.

Nonreciprocal differential detection method for scanning Kerr-effect microscopy

We describe an optical detection scheme for scanning Kerr-effect microscopy (SKEM) which does not require modification of the magnetic state of a sample for domain observation. The scheme exploits the nonreciprocal nature of the magneto-optic Kerr effect (MOKE) to distinguish polarization rotation due to sample magnetization from other spurious sources, such as sample roughness and birefringence of the microscope optics. We present SKEM images of domain structures in lithographically patterned Ni81Fe19 elements which demonstrate the imaging capabilities of the new detection scheme for materials with typical MOKE magnitudes.

Magnetic Domain Imaging with a Photoemission Microscope

ABSTRACTPhotoelectron emission microscopy (PEEM) has proven to be a versatile analytical technique in surface science. When operated with circularly polarized light in the soft x-ray regime, however, photoemission microscopy offers a unique combination of magnetic and chemical information. Exploiting the high brilliance and circular polarization available at a helical undulator beamline, the lateral resolution in the imaging of magnetic domain structures may be pushed well into the sub-micrometer range. Using a newly designed photoemission microscope we show that under these circumstances not only domains, but also domain walls can be selectively investigated. The high sensitivity of the technique yields a sizable magnetic contrast even from magnetic films as thin as a fraction of a single monolayer. The combination of chemical selectivity and information depth is successfully employed to investigate the magnetic behavior of buried layers and covered surfaces. This approach offers a convenient access to magnetic coupling phenomena in magnetic sandwiches.

Domain structure changes in a large pulse in-plane field

The influence of large magnetic in-plane field pulses was investigated in the domain structure of garnet films. For quantitative evaluation, correlation coefficients of domain structure images before and after the field pulse were calculated. Essential changes in domain structure form were observed in fields much lower than the anisotropy field, but for short rise times and low anisotropy fields only. It is shown that the effect can be related to quasistochastic oscillations of magnetization.

Optical second harmonic images of 90° domain structure in BaTiO3 and periodically inverted antiparallel domains in LiTaO3

By using the second harmonic generation (SHG) microscope, which we have recently developed, two-dimensional images of ferroelectric domain structure were successfully obtained by exploiting the inhomogeneous distribution of the quadratic nonlinear optical constant in specimens. 90° domain structure of BaTiO3 was observed with intensity contrast of the second harmonic wave. A periodically inverted antiparallel domain in LiTaO3 was also observed. This fact indicates that the SHG microscope provides a new method of observing antiparallel ferroelectric domain structure in a nondestructive way, which is especially important for characterizing the quasiphase matched devices for compact blue light lasers.

Imaging magnetic domains with the X-ray dichroism photoemission microscope

Few techniques are available to create images of magnetic domain structures at surfaces, due to the weakness of magnetic forces and the relative inefficiency of electron spin-detectors. We have introduced a new method for high-resolution mapping of magnetic domains, that measures the local magnetic moment and magnetization direction of individual elemental constituents of the sample. In addition, it is able to probe the magnetic properties of layers buried as much as 100 Å below the surface. The method uses the magnetic X-ray dichroism effect, in combination with the X-ray photoelectron emission microscope (X-PEEM), also called the X-ray secondary-electron microscope (XSEM). The principles involved in imaging magnetic domains using an X-ray photoelectron emission microscope are discussed, based on recent results using circularly polarized soft X-rays. Examples of applications of the technique include direct mapping of the oscillatory exchange coupling in transition metal sandwich structures, and imaging of recorded bit patterns in magnetic media. New results from perpendicular recording media are presented.

Quantification of micromagnetic structure from Lorentz micrographs

Defocused imaging of magnetic domain structures is a well-known technique to observe the micromagnetic structures in ferromagnetic thin films. Nevertheless, Lorentz microscopy images are rarely subject to a quantitative evaluation of micromagnetic parameters. In this paper, we offer a new method for quantitative evaluation of ripple wavelengths and ripple angle from Lorentz microscopy images carried out on soft magnetic Ni81Fe19 films. The work was carried out using a Philips CM30 electron microscope with a combined Twin/Lorentz lens.The experiments were performed on thin ferromagnetic films of a Ni81Fe19 alloy. Due to the internal magnetic induction within the specimens, the partial electron waves experience a phase shift proportional to the local in-plane magnetic induction , the specimen's thickness t and the lateral distance x from an arbitrarily chosen point of zero phase shift on the specimen. This phase shift can then be imaged using phase contrast methods similar to HREM. Since the phase shifts and the corresponding deflection angles can be very small, a large defocus is necessary to obtain contrast. This large defocus gives rise to an oscillating phase contrast transfer function for the spatial frequencies under observation as well as to a damping envelope for higher spatial frequencies.

Improved Foucault imaging of magnetic domains with a modified 400 kV transmission electron microscope

Modifications have been made to a 400 kV side-entry transmission electron microscope fitted with a low-field objective pole piece, in order to position apertures close to the back focal plane of the objective lens, as there is no direct access to the required position in the column. The modifications have facilitated developments in the imaging of magnetic domain structure in magnetic materials using the Foucault technique, for which the correct positioning of the objective aperture is crucial. All usual transmission electron microscopy facilities are retained, allowing a full range of specimen holders and imaging modes to be used along with TV recording and electron energy loss spectroscopy. Some initial results are presented from Fe/Cr and Co/Pt multilayer films, for which the Lorentz deflection angle is very small, and for which the Fresnel imaging mode is of limited use as the grain structure contrast masks the magnetic contrast. Initial results are also presented from NdFeB permanent magnet material for which the use of a high electron accelerating voltage is necessary.

Unexpected magnetization processes in YIG + Co films

Unidirectional properties (shifts of BH loop) were observed in (YCa) 3(FeCoGe) 5O 12 films grown on (100) GGG substrate. These shifts could be flipped by an external field (≈ 50 Oe) applied along the normal to the film or in the sample plane. The properties are similar to those of magnetic materials characterized by unidirectional anisotropy (e.g. exchange-coupled soft and hard magnetic system). The proposed domain structure model of a homogeneous sample with mixed magnetic anisotropy explains these peculiarities reasonably well. The formula for the estimation of a self-biasing field vector is given for a multi-easy axes magnet. The magnetization processes were investigated using the Faraday effect (domain structure image was analyzed visually and digitally), low frequency magnetic susceptibility technique and a vibrating sample magnetometer. The room temperature FMR spectra and the Faraday rotation spectra were analyzed, too. Microscopic theory and domain structure model are discussed for the explanation of the previously reported effects: (i) unexpectedly low and changeable domain wall velocity v ( v, which depends on the width of the applied field pulses, is in the approximate range 1 ßm/s-220 m/s); (ii) domain structure shape memory.

Direct imaging of order-disorder and antiphase domain structures in Ti3Al intermetallic compound

The Ti3Al intermetallic compound has long been recognized as potentially useful structural materials. It offers attractive strength to weight and elastic modulus to weight ratios. Recent work has established that the addition of Nb to Ti3Al ductilized this compound. In this work the fundamental problem of this alloy, i.e. order-disorder and antiphase domain structures was investigated at the atomic scale.The Ti3Al+10at%Nb alloys used in this study were treated at 1060°C and then aged at 700°C for 2 hours. The specimens suitable for TEM were prepared by standard jet electrolytic-polishing technique. A JEM-200CX electron microscope with an interpretable resolution of about 0.25 nm was used for HREM.The [100] and [001] projections of the α2 phase were shown in Fig.l.The alloy obtained consist of at least two phases-α2(Ti3Al) and β0 structures. Moreover, a disorder α phase with small volume fraction was also observed. Fig.2 gives [100] and [001] diffraction patterns of the α2 phase. Since lattice parameters of the ordered α2 (a=0.579, c=0.466 nm) and disorder α phase (a0=0.294≈a/2, c0=0.468 nm) are almost the same, their diffraction patterns are difficult to be distinguished when they are overlapped with epitaxial orientation relationships.