Magnetic Domain Imaging Research Articles

Imaging of buried 3D magnetic rolled-up nanomembranes.

Increasing performance and enabling novel functionalities of microelectronic devices, such as three-dimensional (3D) on-chip architectures in optics, electronics, and magnetics, calls for new approaches in both fabrication and characterization. Up to now, 3D magnetic architectures had mainly been studied by integral means without providing insight into local magnetic microstructures that determine the device performance. We prove a concept that allows for imaging magnetic domain patterns in buried 3D objects, for example, magnetic tubular architectures with multiple windings. The approach is based on utilizing the shadow contrast in transmission X-ray magnetic circular dichroism (XMCD) photoemission electron microscopy and correlating the observed 2D projection of the 3D magnetic domains with simulated XMCD patterns. That way, we are not only able to assess magnetic states but also monitor the field-driven evolution of the magnetic domain patterns in individual windings of buried magnetic rolled-up nanomembranes.

GHz magnetic field influence on magnetization reversal in amorphous microwires

AbstractThis work is devoted to the study of the magnetization reversal in the Co‐rich glass covered microwire with nearly‐zero magnetostriction in the presence of super high frequency (SHF) (about 1 GHz) circular magnetic field that is important for the elucidation of GMI mechanism. Magnetic domain imaging has been performed by means of magneto‐optical Kerr effect (MOKE) polarizing microscopy. Local hysteresis loops were obtained from the magneto‐optic intensity as a result of images processing. The increase of the SHF field amplitude causes the change of the magnetization reversal mechanism – the rotation of the magnetization dominates. Also the hysteresis loop has an asymmetric shape that confirms the competition between stable and meta‐stable helical magnetic states. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Uniaxial magnetic anisotropy in epitaxial Fe/MgO films on GaAs(001)

Magnetic and structural properties of Fe films grown on MgO buffered GaAs(001) substrates were investigated. Structural analysis using a transmission electron microscope shows that the Fe/MgO/GaAs system is fully epitaxial when MgO is grown at high temperature (~350°C). A two-fold uniaxial magnetic anisotropy along the Fe[100]//GaAs[1̄10] was found for the epitaxial Fe/MgO/GaAs system using magnetic hysteresis loop measurements and magnetic domain imaging. However Fe grown on room temperature deposited MgO shows amorphous island morphology with discrete boundary, and no magnetic anisotropy was found.

Antisymmetric magnetoresistance in SmCo5 amorphous films with imprinted in-plane magnetic anisotropy

We report on magnetoresistance measurements in SmCo5 amorphous films with a giant imprinted magnetic anisotropy. At low applied field parallel to the easy axis, the magnetoresistance exhibits a hysteretic, square, and antisymmetric shape. The antisymmetry in the magnetoresistance is a result of the non-uniform distribution of the magnetization direction over the sample in conjunction with the extraordinary Hall effect. Moreover, the combination of anisotropic magnetoresistance measurements and magnetic domain imaging demonstrates that the symmetry depends on the magnetization orientation with respect to the applied field.

Magnetic domain observation of FeCo thin films fabricated by alternate monoatomic layer deposition

FeCo thin films are fabricated by alternate monoatomic layer deposition method on a Cu3Au buffer layer, which in-plane lattice constant is very close to the predicted value to obtain a large magnetic anisotropy constant. The variation of the in-plane lattice constant during the deposition process is investigated by reflection high-energy electron diffraction. The magnetic domain images are also observed by a photoelectron emission microscope in order to microscopically understand the magnetic structure. As a result, element-specific magnetic domain images show that Fe and Co magnetic moments align parallel. A series of images obtained with various azimuth reveal that the FeCo thin films show fourfold in-plane magnetic anisotropy along ⟨110⟩ direction, and that the magnetic domain structure is composed only of 90∘ wall.

Nitrogen-vacancy-center imaging of bubble domains in a 6-Å film of cobalt with perpendicular magnetization

We employ a scanning nitrogen-vacancy-center microscope to perform stray field imaging of bubble magnetic domains in a perpendicularly magnetized Pt/Co/AlOx trilayer with 6 Å of Co. The stray field created by the domain walls is quantitatively mapped with few-nanometer spatial resolution, with a probe-sample distance of about 100 nm. As an example of application, we show that it should be possible to determine the Bloch or Néel nature of the domain walls, which is of crucial importance to the understanding of current-controlled domain wall motion.

スピン偏極低エネルギー電子顕微鏡

Low energy electron microscopy (LEEM) and photo-emission electron microscopy (PEEM) have been widely considered as powerful methods of real time observation with versatile functionality and high lateral resolution required for the dynamic measurements of surfaces and interfaces. Spin-polarized LEEM (SPLEEM), which is mainly described in this article, has a spin-polarized electron gun (spin gun) and makes detailed magnetic domain observation possible. We developed the spin gun having very high brightness, high spin-polarization and long life time which enabled us to conduct the dynamic imaging of magnetic domains as well. We adopted several innovative ideas to develop the high performance spin gun, e.g., the backside illumination of laser beam to a photo-cathode for high brightness, strained super lattices of GaAs/GaAsP for high spin-polarization and extremely high vacuum for long life time of photo-cathodes. Such novel SPLEEM has been applied to [CoNix]y system, one of the promising multi-layered materials for spintronics devices.

Energy-resolved magnetic domain imaging in TbCo alloys by valence band photoemission magnetic circular dichroism

We report magnetic domain imaging of a terbium cobalt (TbCo) alloy thin film with a high perpendicular magnetic anisotropy in one- and two-photon photoemission electron microscopy (PEEM). Both photoemission processes deliver a clear magnetic circular dichroism (MCD) whose strength is strongly energy dependent. Comparing the energy dependence of the MCD signal in one- and two-photon photoemission, we conclude that the magnetic contrast is mainly an initial state effect. Our results ultimately show that MCD contrast can be obtained in PEEM in valence band photoemission from a material supporting all-optical magnetization switching. This opens the way for the investigation of the all-optical switching process with simultaneous ultrahigh temporal and spatial resolution.

High-resolution magnetic-domain imaging by Fourier transform holography at 21 nm wavelength

Exploiting x-ray magnetic circular dichroism at the L-edges of 3d transition metals, Fourier transform holography has become a standard technique to investigate magnetic samples with sub-100 nm spatial resolution. Here, magnetic imaging in the 21 nm wavelength regime using M-edge circular dichroism is demonstrated. Ultrafast pulses in this wavelength regime are increasingly available from both laser- and accelerator-driven soft x-ray sources. We explain the adaptations concerning sample preparation and data evaluation compared to conventional holography in the 1 nm wavelength range. We find the correction of the Fourier transform hologram to in-plane Fourier components to be critical for high-quality reconstruction and demonstrate 70 nm spatial resolution in magnetization imaging with this approach.

Soft X-Ray Magneto-Optics: Probing Magnetism by Resonant Scattering Experiments

The most advanced X-ray sources (third generation synchrotrons, linear free-electron lasers and high-harmonic generation sources) widen the range of application of X-ray scattering techniques considerably. Beyond flux and brilliance, improvements in polarization tuneability, degree of coherence and selectable time-structure promoted new methods for investigating the electronic and magnetic properties of solids. The soft X-ray range (50-2000 eV) is well suited for studying magneto-optical effects in laterally confined submicron sized objects, either artificially built or self-assembled. First, by tuning the photon energy at a core resonance, one provides the X-ray scattering technique with element selectivity. Second, resonant excitations make the optical constants sensitive to the local magnetization by introducing large off-diagonal elements in the dielectric tensor; since magnetic effects are stronger when the core excitation produces a dipolar transition to final states involving the magnetic orbitals ( 3d for the first row TM; 4 f for RE), the most interesting resonances for X-ray magneto-optics [(2, 3) p → 3 d and (3, 4) d → 4 f] are all located in the soft X-ray region. Finally, the wavelengths corresponding to soft X-rays are very well suited for scattering studies of nanometer- to micrometer-sized magnetic structures. We will present the results of recent soft X-ray resonant scattering experiments, showing that the combination of element selectivity, magnetic sensitivity and structural analysis can help disentangling and understanding the magnetic properties of complex self-assembled periodic systems. Lastly, recent applications of coherent scattering to the X-ray holographic imaging of magnetic domains will be presented.

Manifestations in the magnetization of the hcp-Co nanowires due to interdependence of aspect ratio and c-axis orientation

Effect of interdependence of aspect ratio (AR) and c-axis orientation of the hcp-cobalt nanowires (NWs) on their magnetization behavior is reported in 40 and 100 nm diameter NWs. Experimental evidence of periodically modulated magnetic state viz. large transverse-susceptibility arising due to orientation of c-axis normal to NW-axis in 40 nm NWs and magnetic domain imaging is demonstrated, which disappears at low AR owing to randomly oriented c-axes. The 100 nm NWs exhibit a crossover in the easy-axis direction from longitudinal at high AR to transverse at low AR and are explained on the basis of competition between different anisotropic contributions.

Observation of Micro-Magnetic Structures by Synchrotron Radiation Photoelectron Emission Microscopy

Examples of the application of photoelectron emission microscopy (PEEM) combined with synchrotron radiation excitation are reviewed. In particular, the combination study of PEEM and X-ray magnetic dichroism spectroscopy is very valuable for magnetic domain imaging. According to the demands of the industry, such as the production of magnetic recording media, the application method becomes very important. In this review, several examples of studies using magnetic circular and/or linear dichroism spectroscopy for ferro- and antiferromagnetic materials are introduced. In addition, pump–probe time-resolved PEEM is introduced. These methods allow us to measure the dynamical motion of micro-magnetic structures in mesoscopic size magnets in response to the field pulse in the sub-nanosecond range.

Space- and time-resolved Seebeck and Nernst voltages in laser-heated permalloy/gold microstructures

Thermoelectric effects in microstructured permalloy (Py)/Au wires are investigated using space- and time-resolved measurements based on scanning focused laser heating. Supported by numerical simulations of the temperature distribution, we identify two major contributions to the laser-induced signals: (i) the Seebeck effect due to thermocouples of Py/Au and (ii) the anomalous Nernst effect (ANE) in Py with a coefficient of NANE≈1.6 μV/K. ANE-based magnetic imaging of magnetic domains and magnetization reversal is demonstrated with a lateral resolution on the μm scale.

Method for Single-Shot Coherent Diffractive Imaging of Magnetic Domains

In preparation for real space studies of magnetic domains in a pump-probe setup at free-electron laser sources, it is necessary to develop an imaging method compatible with the linearly polarized radiation available at these sources. We present results from a prototype experiment performed at the synchrotron source BESSY II, using a modification of existing phase retrieval techniques. Our results show that it is possible to image magnetic domains in real space using linear polarized light, and we introduce the concept of a reliability map of our reconstructions using Gabor transforms.

Magnetic Domains and Twin Boundary Movement of NiMnGa Magnetic Shape Memory Crystals

AbstractTime‐resolved metallographic optical microscopy techniques are used together with magnetic domain imaging to clarify the interaction between magnetic domains and twin boundary (TB) motion in magnetic shape memory NiMnGa single crystals. The magnetic field and stress induced magnetic domain formation is imaged by a magneto‐optical indicator film technique. Reversible TB motion is visualized up to high actuation speeds. From domain observation at adjacent crystal surfaces the fundamental volume magnetic processes during strain and field induced TB motions are derived. For magnetic field induced structural reorientations a concurrent absence of magnetic domain wall motion is found. In contrast, for strain induced reorientations processes, a complete rearrangement of the magnetic domain structure by the moving TB is observed. Dynamic actuation experiments on TB motion reveal non‐linear time effects on TB mobility. In addition to training effects, the maximum field induced strain increases with actuation speed. Both effects can be interpreted as the interaction of moving twin boundaries with local non‐fixed defects. The summarized results provide key information for the understanding of the connection of magnetic and crystallographic domains in magnetic shape memory alloys and for the optimization of devices for future technical applications.

Magnetic stripes and skyrmions with helicity reversals

It was recently realized that topological spin textures do not merely have mathematical beauty but can also give rise to unique functionalities of magnetic materials. An example is the skyrmion--a nano-sized bundle of noncoplanar spins--that by virtue of its nontrivial topology acts as a flux of magnetic field on spin-polarized electrons. Lorentz transmission electron microscopy recently emerged as a powerful tool for direct visualization of skyrmions in noncentrosymmetric helimagnets. Topologically, skyrmions are equivalent to magnetic bubbles (cylindrical domains) in ferromagnetic thin films, which were extensively explored in the 1970s for data storage applications. In this study we use Lorentz microscopy to image magnetic domain patterns in the prototypical magnetic oxide-M-type hexaferrite with a hint of scandium. Surprisingly, we find that the magnetic bubbles and stripes in the hexaferrite have a much more complex structure than the skyrmions and spirals in helimagnets, which we associate with the new degree of freedom--helicity (or vector spin chirality) describing the direction of spin rotation across the domain walls. We observe numerous random reversals of helicity in the stripe domain state. Random helicity of cylindrical domain walls coexists with the positional order of magnetic bubbles in a triangular lattice. Most unexpectedly, we observe regular helicity reversals inside skyrmions with an unusual multiple-ring structure.

Soft magnetic thin films: influence of annealing on magnetic properties

Soft magnetic materials are currently used in a variety of applications in electrical machines, sensors and elements of devices. If prepared in thin film form, they can be applied to micro- and nano-patterned devices. However, with respect to ribbons and bulk materials, thin films of the same composition usually display worse soft magnetic properties, thus requiring suitable solutions to restore high permeability and low coercivity. In this review, the magnetic properties of thin films prepared by sputtering are presented. Several compositions of soft magnetic materials are investigated, including Co-Fe-Si-B, Fe-Cu-Nb-Si-B, Fe-Si-B, Fe-Co-Nb-Si-P-B, Fe-Zr-Nb-Cu-B. Their amorphous-to-crystallization processes are studied by means of furnace annealing and Joule heating. Hysteresis loops and magnetic domain imaging, together with structural techniques, are used to follow the effects of annealing, which include stress relaxation and crystallization. The effects of magnetic field annealing are investigated and discussed.

Magnetic imaging by Fourier transform holography using linearly polarized x-rays

We present a method for imaging magnetic domains via x-ray Fourier transform holography at linearly polarized sources. Our approach is based on the separation of holographic mask and sample and on the Faraday rotation induced on the reference wave. We compare images of perpendicular magnetic domains obtained with either linearly or circularly polarized x-rays and discuss the relevance of this method to future experiments at free-electron laser and high-harmonic-generation sources.

A Comparison of Magnetic Domain Images Using a Modified Bitter Pattern Technique and the Kerr Method on Grain-Oriented Electrical Steel

Differences in the magnetic domain patterns of high permeability grain-oriented (HGO) electrical steel have been compared using a modified Bitter technique and the magneto-optical Kerr method. Distinct differences in the domain images were observed which have been attributed to the greater sensitivity of the Bitter technique to the degree of misorientation of the crystal grains. Electron backscatter diffraction (EBSD) analysis was also carried out to confirm the degree of crystal misorientation for the area under investigation. The results show that the Bitter technique is most effective in detecting the presence of low angle grain boundaries but less accurate than the Kerr method for determining the absolute direction of magnetization.

Element-Specific Magnetic Domain Imaging of (Nd, Dy)-Fe-B Sintered Magnets Using Scanning Transmission X-Ray Microscopy

We demonstrate an element-specific observation of magnetic domains in thermally demagnetized Nd-Fe-B and (Nd, Dy)-Fe-B sintered magnets using scanning transmission X-ray microscopy (STXM). Clear chemical and magnetic contrast images with the 30-nm spatial resolution were taken by STXM. Both maze-like magnetic domains and stripe magnetic domains with their widths of 200-300 nm are observed in both Nd-Fe-B and (Nd, Dy)-Fe-B sintered magnets. In both sintered magnets, multidomain structures are mostly formed within each grain-that is, magnetic domains are likely to be terminated at the grain boundaries. Stripe domains are originated from the grains with the (001)-axis misoriented to the sample normal. From the comparison between chemical and magnetic images, it is found that no clear magnetic domain is observed in Nd-rich phase at grain boundary triple points. Furthermore, it is also found that the interface between Nd2Fe14B phase and Nd-rich phase is chemically abrupt. Similar magnetic domain patterns are observed in Nd-Fe-B and (Nd, Dy)-Fe-B sintered magnets.