Spin-polarized Scanning Electron Microscopy Research Articles

Modular nanomagnet design for spin qubits confined in a linear chain

On-chip micromagnets enable electrically controlled quantum gates on electron spin qubits. Extending the concept to a large number of qubits is challenging in terms of providing large enough driving gradients and individual addressability. Here, we present a design aimed at driving spin qubits arranged in a linear chain and strongly confined in directions lateral to the chain. Nanomagnets are placed laterally to the one side of the qubit chain, one nanomagnet per two qubits. The individual magnets are “U”-shaped, such that the magnetic shape anisotropy orients the magnetization alternately toward and against the qubit chain even if an external magnetic field is applied along the qubit chain. The longitudinal and transversal stray field components serve as addressability and driving fields. Using micromagnetic simulations, we calculate driving and dephasing rates and the corresponding qubit quality factor. The concept is validated with spin-polarized scanning electron microscopy of Fe nanomagnets fabricated on silicon substrates, finding excellent agreement with micromagnetic simulations. Several features required for a scalable spin qubit design are met in our approach: strong driving and weak dephasing gradients, reduced crosstalk and operation at low external magnetic fields.

Magnetic-field-application system at high temperatures for spin-polarized scanning-electron-microscopy measurement

To conduct operando measurements, we previously developed a sample-heating system and, for this study, developed a magnetic-field-application system for spin-polarized scanning electron microscopy measurement. A Sm2Co17 permanent magnet placed below a sample, which works with a resistive heater, is used to apply a magnetic field of more than 20 kA/m perpendicular to a sample at up to 200 ℃. A specially designed magnetic shield is attached close to the sample to avoid decreasing the accuracy of secondary polarization detection due to the stray field from the magnet. With this system, we observed the reformation of a magnetic domain structure of Fe (001) at 200 ℃ by applying a magnetic field of 20 kA/m.

Operando observation of magnetism in HDD writing heads by spin-polarized scanning electron microscopy.

Operando observation using spin-polarized scanning electron microscopy (spin SEM) has been demonstrated by detecting changes in the magnetization in the writing head of a hard disk drive (HDD) during operation. A current-applying system developed for use in the sample stage of a spin SEM enables imaging of the magnetization changes in the writing head of an HDD while the writing head is activated. Focused ion beam (FIB) technology is used to fabricate electric contacts between the head terminals and the sample holder electrodes. Tungsten film is deposited by FIB technology on the insulator around the writing head to prevent electrostatic charge buildup in the insulators during SEM measurement. This system is well suited for studying the characteristics of writing heads in HDDs in an activated state.

Magnetization Analysis by Spin-Polarized Scanning Electron Microscopy.

Spin-polarized scanning electron microscopy (spin SEM) is a method for observing magnetic-domain structures by detecting the spin polarization of secondary electrons. It has several unique abilities such as detection of full magnetization orientation and high-spatial-resolution measurement. Several spin-SEM experiments have demonstrated that it is a promising method for studying various types of magnetic materials and devices. This review paper presents several spin-SEM observations to demonstrate the capability and potential of spin SEM.

Achiral tilted domain walls in perpendicularly magnetized nanowires

Perpendicularly magnetized nanowires exhibit distinct domain wall types depending on the geometry. Wide wires contain Bloch walls, narrow wires N\'eel walls. Here, the transition region is investigated by direct imaging of the wall structure using high-resolution spin-polarized scanning electron microscopy. An achiral intermediate wall type is discovered that is unpredicted by established theoretical models. With the help of micromagnetic simulations, the formation of this novel wall type is explained.

Spin-Polarized Scanning Electron Microscopy (Spin SEM) and its recent progress

Spin-Polarized Scanning Electron Microscopy (Spin SEM) and its recent progress

Magnetism in grain-boundary phase of a NdFeB sintered magnet studied by spin-polarized scanning electron microscopy

The magnetism in the grain-boundary phase of a NdFeB sintered magnet was measured by spin-polarized scanning electron microscopy (spin SEM). A sample magnet was fractured in the ultra-high-vacuum chamber to avoid oxidation, and its magnetizations in the exposed grain-boundary phase on the fracture surface were evaluated through the spin polarization of secondary electrons. Spin-SEM images were taken as the fracture surface was milled gradually by argon ions, and the magnetization in the grain-boundary phase was quantitatively obtained separately from that of the Nd2Fe14B phase. The obtained magnetization shows that the grain-boundary phase of this magnet has substantial magnetization, which was confirmed to be ferromagnetic.

スピン偏極走査電子顕微鏡

Spin-Polarized Scanning Electron Microscopy (Spin SEM) is one way for observing magnetic domain structures taking advantage of the spin polarization of the secondary electrons emitted from a ferromagnetic sample. This principle brings us several excellent capabilities such as high-spatial resolution better than 10 nm, and analysis of magnetization direction in three dimensions. In this paper, the principle and the structure of the spin SEM is briefly introduced, and some examples of the spin SEM measurements are shown.

Sample heating system for spin-polarized scanning electron microscopy

A sample-heating system for spin-polarized scanning electron microscopy (spin SEM) has been developed and used for microscopic magnetization analysis at temperatures up to 500°C. In this system, a compact ceramic heater and a preheating operation keep the ultra-high vacuum conditions while the sample is heated during spin SEM measurement. Moreover, the secondary-electron collector, which is arranged close to the sample, was modified so that it is not damaged at high temperatures. The system was used to heat a Co(1000) single-crystal sample from room temperature up to 500°C, and the magnetic-domain structures were observed. Changes of the domain structures were observed around 220 and 400°C, and these changes are considered to be due to phase transitions of this sample.

Spin-polarized scanning electron microscopy

Spin-polarized scanning electron microscopy (spin-SEM) is a magnetic domain observation method. In spin-SEM, polarization of secondary electrons emitted from a sample in a scanning electron microscope is detected by a spin detector and used as a signal for forming an image. The characteristics of spin-SEM are detection of all three magnetization vector components, which leads to the detection of the magnetization vector direction, high spatial resolution of around 3 nm and applicability to samples with rough or even 3D surfaces. Spin-SEM combined with other imaging methods using an electron probe beam such as scanning Auger electron microscopy for imaging element distribution and electron backscattering diffraction microscopy for imaging crystal direction distribution provides additional information that is important to study the magnetism. Spin-SEM with these excellent characteristics has a broad range of applications from basic research to applied research and developments in various industries.

ハードディスクドライブ用磁気記録ヘッドにおける磁化挙動解析手法の実際と将来展望

With the increase in the recording density of hard disk drives, the narrower magnetic pole and read-sensor widths in writing and reading heads are required, and techniques used for the magnetic behavior analysis in a nanoscale area is essential. The observation of the in-plane domain structure of the writer pole for a perpendicular recording (PMR) head was performed by electron holography in order to elucidate the mechanism of the pole erasure originating from the instability of the magnetic domain state. The results revealed that the stability of the domain structure is strongly related to the domain wall trapping, and the pole erasure can be suppressed by realizing the stable domain structures. With respect to a current perpendicular to plane (CPP)-giant magneto-resistive (GMR) head, which has been promising candidates of a next-generation reading head element, the insertion of non-magnetic materials, such as Cu, between the ferromagnetic pinned and free layers is known to increase the MR ratio. For realizing the practical use of the CPP-GMR head, effects of the insertion of materials on the increase in the MR ratio were investigated by X-ray magnetic circular dichroism (XMCD). The results revealed that XMCD is a powerful technique to obtain information on electronic states and magnetic moments with the help of the theoretical electronic band calculation. The potential application of secondary cantilever resonance magnetic force microscopy and spin-polarized scanning electron microscope to the in-situ magnetic imaging of PMR heads will be also discussed.

ChemInform Abstract: Electron Microscopy and the Study of Microstructure and Domain Structure of Magnetic Materials

Abstract Five different electron optical techniques to characterize magnetic domains and domain walls in ferromagnetic materials are reviewed. It is shown that one can choose one or more of these techniques to solve a given problem depending on the nature of the information needed, the resolution level desired and the sophistication of the instrument available. Lorentz microscopy in a transmission electron microscope is a clear choice for most problems although spin-polarized scanning electron microscopy or magnetic force microscopy are better suited to non-destructive surface studies, and differential phase contrast microscopy is preferable for quantitative study of domains in thin specimens. Electron holography is capable of providing contour maps corresponding to lines of force distribution as well as quantitative data on flux distribution. In the near future, it should become possible to combine a few of these techniques with computer simulation to obtain three-dimensional domain wall distributions in bulk ferromagnets.

High-resolution spin-polarized scanning electron microscopy (spin SEM)

We have developed spin-polarized scanning electron microscopy (spin SEM) with a 5-nm resolution. The secondary electron optics is very important, as it needs to transfer a sufficient number of secondary electrons to the spin polarimeter, due to the low efficiency of the polarimeter. The optics was designed using a three-dimensional (3D) simulation program of the secondary electron trajectories, and it achieves highly efficient collection and transport of the secondary electrons even though the distance between the sample and the objective lens exit of the electron gun remains short. Moreover, the designed optics enables us to obtain clear SEM images in the spin SEM measurement and to precisely adjust the probe beam shape. These functions lead to images with high spatial resolution and sufficient signal-to-noise (S/N) ratios. This optics has been installed in an ultra-high vacuum (UHV) spin SEM chamber with a Schottky-type electron gun for the probe electron beam. We observed recorded bits on a perpendicular magnetic recording medium and visualized small irregularities in the bit shapes around the track edges and bit boundaries. The high resolution of 5 nm was demonstrated by observing the smallest domain composed by a single grain in the recording medium.

A compact surface decontamination system for surface-sensitive magnetic imaging

A surface decontamination system for application in surface sensitive magnetic imaging tools, such as a spin-polarized scanning electron microscope, is described. Adsorbed contaminant is chemically decomposed with the use of active oxygen in a compact vacuum chamber mounted in a microscope. The present method is especially suitable for fragile magnetic surfaces of complex oxide materials to which the conventional physical etching widely used for magnetic metals could cause serious structural damage. We have succeeded in detecting magnetic signals from a decontaminated oxide surface and visualizing domain images with sufficient magnetic contrast to analyze detailed domain structures.

Analysis of Magnetization Mechanism for NdFeB Magnet Using Spin-Polarized Scanning Electron Microscopy (Spin SEM)

The magnetization process of Nd-Fe-B bonded magnets prepared by the Hydrogenation-Disproportionation-Desorption-Recombination(HDDR) process was studied by observing magnetic domain structures using spin-polarized scanning electron microscopy (spin SEM). The positional relationship between the grain boundaries and the magnetic domain walls is clearly imaged. The result revealed that these magnets contain grains with a multi-domain structure in the thermally demagnetized state. The domain wall motions in grains with multi-domains are dominant in weak magnetic fields in the initial magnetization process, whereas fraction of the grains with a single domain increases with further increasing the magnetic field. The coercivity variations in these grains were also visualized.

Control of Domain Wall Polarity by Current Pulses

Direct observation of current-induced propagation of purely transverse magnetic domain walls with spin-polarized scanning electron microscopy is reported in Fe30Ni70 nanowires. After propagation, the domain walls keep their transverse nature but switch polarity in some cases. For uniform Ni70Fe30 wires, the effect is random and illustrates domain-wall propagation above the Walker threshold. In the case of Ni{70}Fe_{30}/Fe wires, the transverse magnetization component in the wall is entirely determined by the polarity of the current pulse, an effect that is not reconciled by present theories even when taking into account the nonuniform Oersted field generated by the current.

Large area magnetic domain imaging of magnetite films with Spin‐SEM

Abstract The surface crystallographic structure and magnetic structure of magnetite thin films epitaxially grown on a MgO(001) substrate were investigated by means of a scanning tunnelling microscope (STM), magnetic force microscope (MFM) and Spin‐polarized scanning electron microscopy (Spin‐SEM). The STM investigation and MFM measurement were performed in‐situ and in an ambient condition, respectively. After exposing the sample to air, a Spin‐SEM image was acquired. Spin‐dependent contrasted SEM images could be obtained after cleaning the sample surfaces. The cleaning procedure is annealing the sample at 250 °C in an ultra high vacuum (UHV) below 3 x 10–8 Pa for 3 hours. The STM observation indicates the thin films have antiphase domains and the observed MFM images reflect a cease in micromagnetic domain structures. Spin‐SEM images indicate that large‐scale magnetic inhomogeneity of a few μm is the inhomogeneity of the perpendicular magnetic moments of the films. Due to the spatial resolution limitation of our Spin‐SEM system, the correlation between the fine and coarse magnetic structures of the thin films induced by the antiphase domain boundaries is not clear, however, we have suggested the annealing process alone in a UHV makes the spin configuration of the thin films visible using a conventional Spin‐SEM system. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Recent progress in spin-polarized scanning electron microscopy

Recent progress in spin-polarized scanning electron microscopy

Direct Observation of Domain-Wall Configurations Transformed by Spin Currents

Direct observations of current-induced domain-wall propagation by spin-polarized scanning electron microscopy are reported. Current pulses move head-to-head as well as tail-to-tail walls in submicrometer Fe20Ni80 wires in the direction of the electron flow, and a decay of the wall velocity with the number of injected current pulses is observed. High-resolution images of the domain walls reveal that the wall spin structure is transformed from a vortex to a transverse configuration with subsequent pulse injections. The change in spin structure is directly correlated with the decay of the velocity.

Microscopy of magnetic transition in a layered manganiteLa2−2xSr1+2xMn2O7(x=0.32)

Temperature-dependent magnetic structures in ${\mathrm{La}}_{2\ensuremath{-}2x}{\mathrm{Sr}}_{1+2x}{\mathrm{Mn}}_{2}{\mathrm{O}}_{7}$ $(x=0.32)$ are derived from real-space observation based on three-dimensional magnetization vector analysis using spin-polarized scanning electron microscopy. Below $80\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, a flux closure structure is formed at the $ab$ surface, indicating ferromagnetic inter-bilayer coupling with magnetization along the $c$ axis. The closure domains are turned into larger ones with irregular shapes above $80\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The drastic domain reconstruction results from the change in inter-bilayer coupling to antiferromagnetic one, accompanied by the magnetization inclined toward the $ab$ plane.