Magnetic Domain Imaging Research Articles

Correlation of magnetic field and stress-induced magnetic domain reorientation with Barkhausen Noise

This paper investigates magnetic field and stress-induced magnetic domain reorientation and its correlation with Magnetic Barkhausen Noise (MBN). The magnetic domain dynamics and MBN of grain-oriented transverse and longitudinal electrical steels are studied by using time-resolved magneto-optical imaging. Time and frequency-domain characteristics of MBN signal are investigated for evaluation of stress state and magnetocrystalline anisotropy. Transitions between magnetic domain states with differing tensile stress and magnetic anisotropy directly influence the MBN signal. Its shown that 180° domain wall (DW) reorganization and DW-type conversion generate peaks in the MBN measurement for longitudinal and transverse electrical steel, respectively. The magnetic domain imaging reveals complicated domain arrangement processes in transverse electrical steel. The results provide a physical insight into bulk micromagnetic phenomena.

Extreme anti-reflection enhanced magneto-optic Kerr effect microscopy

Magnetic and spintronic media have offered fundamental scientific subjects and technological applications. Magneto-optic Kerr effect (MOKE) microscopy provides the most accessible platform to study the dynamics of spins, magnetic quasi-particles, and domain walls. However, in the research of nanoscale spin textures and state-of-the-art spintronic devices, optical techniques are generally restricted by the extremely weak magneto-optical activity and diffraction limit. Highly sophisticated, expensive electron microscopy and scanning probe methods thus have come to the forefront. Here, we show that extreme anti-reflection (EAR) dramatically improves the performance and functionality of MOKE microscopy. For 1-nm-thin Co film, we demonstrate a Kerr amplitude as large as 20° and magnetic domain imaging visibility of 0.47. Especially, EAR-enhanced MOKE microscopy enables real-time detection and statistical analysis of sub-wavelength magnetic domain reversals. Furthermore, we exploit enhanced magneto-optic birefringence and demonstrate analyser-free MOKE microscopy. The EAR technique is promising for optical investigations and applications of nanomagnetic systems.

High-frequency power loss mechanisms in ultra-thin amorphous ribbons

Soft magnetic amorphous materials with ultra-low power loss are highly desirable for high-frequency drive applications. The present work demonstrates the high-frequency power loss performance and underlying loss mechanisms in ultra-thin amorphous alloys. This is achieved by rapid-quenching amorphous alloys of Co-, CoFe- and Fe-rich systems, investigating their amorphous atomic structure, quantifying the saturation magnetostriction constants (λs), imaging magnetic domains at remanent magnetization, analyzing magnetization reversal from various magnetization levels, and finally, investigating the material loss performance over a broad frequency range (f = 50 kHz–2 MHz) at various excitation levels (Bm = 25–100 mT). The ultra-high performance of ultra-thin Co-rich amorphous ribbons, as compared to CoFe- and Fe-rich alloys, was attributed to the significantly low eddy current loss, due to the reduced thickness, and a minimal amount of excess loss, owning to minimal magnetoelastic contributions and magnetization reversal by rotation. The underlying loss mechanisms were analyzed by decomposing material loss into primary components and identifying the magnetization reversal mechanisms using minor hysteresis loops. In the Co-rich amorphous alloys, we suggest that magnetization reversal by rotation dominates, at least at low excitations, while in CoFe- and Fe-rich alloys domain wall displacement prevails and contributes significantly to the excess loss up to the MHz frequency range. Magnetization reversal by rotation in Co-rich alloys could be attributed to the zero/near-zero λs, and eventually low residual stress, leading to a homogeneous magnetic domain structure, as compared to the inhomogeneous “fingerprint-like” complex domains in highly magnetostrictive CoFe-rich alloys.

Effects of Laser Cutting on Microstructure and Magnetic Properties of Non-Orientation Electrical Steel Laminations

Non-oriented electrical steel (NOES) is commonly used as a core material in electric machines. These laminations are cut into the desired core shape that modifies the material properties near the cut edge, which consequently changes the magnetic performance of the core lamination. Magnetic property deterioration due to cutting has been reported in the literature but less attention has been given to the microstructural changes due to cutting and its relation to the magnetic properties. The present research focuses on the effects of laser cutting on microstructure and magnetic properties of NOES laminations. A number of lamination steel samples were laser-cut using an identical procedure to that expected on an industrial scale. Property measurements showed that the magnetic permeability of the samples decreased while the core losses increased. The core loss increase was significant, about 22% at 50 Hz and 1.5 T. This was attributed to the change in the micromagnetic characteristics of the sample due to cutting. The change in micromagnetics was investigated using magnetic domain imaging and distorted domain structure was observed near the laser-cut edge. Backscattered electron imaging and nanoindentation measurements were also used for further investigation of micromagnetic properties of the material.

Creating, probing and confirming tetragonality in bulk FeNi alloys

Effects derived from the simultaneous application of passive, uniform saturating magnetic field and tensile stress during long-time annealing utilizing a unique and simple processing tool, the MultiDriver furnace, are reported. In particular, the evolution of the crystalline structure and the magnetic domain configuration of severely deformed and subsequently annealed equiatomic FeNi alloys were revealed by high-energy synchrotron X-Ray diffraction and Magneto-Optic Kerr Microscopy. These alloys are known to undergo a first-order magnetic phase transformation to a chemically ordered tetragonal (L10) structure in astronomical timeframes. MultiDriver-annealed specimens are found to retain a tetragonal crystallographic state and texture that is similar to that of the precursor deformed state, with a smaller unit cell volume and a larger c/a ratio relative to those of control samples that were annealed under the same conditions but without magnetic field and stress drivers. Magnetic domain images echo these observations: while as-cast FeNi alloys exhibit domain patterns consistent with the presence of a stress-modified cubic magnetocrystalline anisotropy, large areas of domains with uniaxial anisotropy are present in MultiDriver-annealed samples. While no chemical order was demonstrated in these samples, a strategy is proposed for achieving the ordered state by employing a passive magnetic gradient in the MultiDriver furnace to enhance diffusion.

Magnetic domain structure and magnetization reversal in (Co/Ni) and (Co/Pd) multilayers

The temporal behavior of magnetization reversal mechanism and domain structures of (Co/Ni) and (Co/Pd) multilayers have been studied using magneto-optical Kerr effect microscopy and magnetometry measurements. The domain size and nucleation field of (Co/Ni) multilayers were found to be reduced as the number of bilayers increases. On the other hand, (Co/Pd)×10 multilayer has a much larger domain size, faster magnetization switching, and a higher nucleation field than that in (Co/Ni). From the imaging of magnetic domains, two different types were observed, namely, dendritic domains for (Co/Ni) and blocks type for (Co/Pd). From time dependence of magnetization reversal at a fixed applied magnetic field, it was revealed that the reversal mechanism in (Co/Ni) multilayer occurs by nucleation followed by domain propagation. Whereas, for (Co/Pd) multilayers, few large domains were observed, and they were found to expand at a faster rate until a complete saturation.

Random number generation using magnetic domain images of magneto-optical materials

Random numbers were generated from magnetic domain patterns of a magneto-optical (MO) material. A hundred thousand chaotic magnetic domain images were photographed by a transparent polarization microscope. Post data processing (binarization, frequency adjustment, exclusive-OR, and offset data reduction) was used to obtain a random number from the images. The generated random numbers passed the statistical test (NIST SP-800-22). This result could contribute to realizing a relatively simple and fast random number generator using MO materials.

Visualization of stray-field distribution by charged domain-walls in rare-earth substituted iron garnets

Magnetic-optical methods are used to investigate the magnetic microstructure of an epitaxial film of (Lu,Bi) substituted rare earth iron garnet. In particular, we focused our study on a bi-dimensional domain configuration, the Néel spike, which is induced and pinned at defects inside the sample or at its edges. Imaging of magnetic domains and domain-walls is obtained by direct magneto-optical interaction in the sample under study. The visualization of the stray-field distribution that is generated by single domain-walls is achieved by means of magneto-optical imaging with an indicator film. Furthermore, magnetostatic calculations were performed in order to correlate the net magnetic charge of the Néel spike domain and domain-walls with the measured stray-field. It results that the measured magnetic field intensity nearby the Néel spike tip is compatible with a Bloch-to-Néel wall transition.

Control of Domain Structure in Magnetic Microwires by Combination of Torsion and Tension Stresses

In this letter we study magnetization reversal and transformation of magnetic domain structures in glass-covered magnetic microwires. Studies were performed using the magneto-optical Kerr effect technique in the presence of the combination of tension and torsion mechanical stresses. Formation and reversible transformations of the surface helical structure, induced by the combined stress, were found. Comparative analysis of the magnetic domain images and hysteresis curves gives unambiguous explanation of the character of the magnetization reversal. Spiral and elliptic structures, as basic helical structures, exist in microwires depending on the values and direction of the applied stresses.

Additive printing of pure nanocrystalline nickel thin films using room environment electroplating

Given its high temperature stability, oxidation-, corrosion- and wear-resistance, and ferromagnetic properties, Nickel (Ni) is one of the most technologically important metals. This article reports that pure and nanocrystalline (Ni) films with excellent mechanical and magnetic properties can be additively printed at room environment without any high-temperature post-processing. The printing process is based on a nozzle-based electrochemical deposition from the classical Watt’s bath. The printed Ni film showed a preferred (220) and (111) texture based on x-ray diffraction spectra. The printed Ni film had close to bulk electrical conductivity; its indentation elastic modulus and hardness was measured to be 203 ± 6.7 GPa and 6.27 ± 0.34 GPa, respectively. Magnetoresistance, magnetic hysteresis loop, and magnetic domain imaging showed promising results of the printed Ni for functional applications.

Effect of rapid thermal annealing on the structure, microstructure and magnetic properties of Fe thin films

This paper reports the structure, microstructure and magnetic properties of as-deposited and rapid thermal annealed Fe thin films. Rapid thermal annealing was carried out at 550 °C for different time intervals such as 2, 5, 10, 20 and 30 min. The as-deposited and rapid thermal annealed films were found to exhibit A2 type bcc structure. However, microstructural studies indicated formation of Fe–Si phase for the annealed Fe films. Surface roughness estimated from atomic force microscopy studies displayed an increase in surface roughness with increase in annealing time. Room temperature magnetization studies showed a predominant in-plane magnetic anisotropy for all the films. In addition to this presence of strong out-of-plane anisotropy coupled with in-plane magnetization was clearly evidenced from the considerable increase in the out-of-plane coercivity with increase in annealing time. Magnetic force microscopy studies further confirmed the presence of out-of-plane magnetic anisotropy. Magnetic domain imaging studies showed long period band domain patterns with 180° domain wall for the as-deposited film. The band domains were found to get branched with increase in annealing time. From the hysteresis loops and domain imaging studies it was clear that the magnetization reversal is dominated by domain wall motion.

Vectorial, non-destructive magnetic imaging with scanning tunneling microscopy in the field emission regime

When a scanning tunneling microscope is operated at tip-target distances ranging from few nanometers to few tens of nanometers (Fowler-Nordheim or field emission regime), a new electronic system appears, consisting of electrons that escape the tip-target junction. If the target is ferromagnetic, this electronic system is spin polarized. Here, we use these spin polarized electrons to image magnetic domains in thin films. As two components of the spin polarization vector are detected simultaneously, the imaging of the local magnetization has vectorial character. The tip is nonmagnetic, i.e., the magnetic state of the target is not perturbed by the act of probing. We expect this spin polarized technology, which scales down scanning electron microscopy with polarization analysis by bringing the source of primary electrons in close proximity to the target, to find its main applications in the imaging of noncollinear, weakly stable spin excitations.

Micro-macro characteristics between domain wall motion and magnetic Barkhausen noise under tensile stress

One challenge of stress measurement by using Magnetic Barkhausen noise (MBN) is to build correlation model between MBN and micro magnetic characterization of the material. In particular, it highly requires a quantitative evaluation for micro-macro magnetic properties characterizing under the stress impaction. In this study, a correlation model between domain wall (DW) motion and MBN under tensile stress is established and promoted. Optical flow (OF) traces motion of magnetic domain images, which can quantify the status of DW motion during magnetization process. When applied field is in stress direction, tensile stress aligns the magnetic domains parallel to the stress direction and makes coercive field decrease. This can make a phenomenon vary of DW motion as well as MBN signal. The average velocity of DW motion (OF value) is proportional to MBN signal under tensile stress. Root mean square (RMS) and mean value are extracted from both DW motion velocity (OF value) and MBN signal to quantitatively analyze the changes under tensile stress to quantify the correlation between micro and macro magnetic parameters. In addition, the correlation model between DW motion and MBN is quantitatively analyzed in different locations to evaluate the repeatability of the correlation, as well as the effect of microstructure on MBN and DW motion under the tensile stress. The correlation coefficient of RMS and mean value show the highly correlation between DW motion and MBN, which in turn sheds deep understanding the micro-macro property-physic mechanisms. The proposed work has potential for interpretation of the statistical properties of MBN under different tensile stress by studying DW motion, which can be further applied for enhancing accuracy on stress measurement.

Visualization of Magnetic Domains in Electrical Steel Using High-Resolution Dark-Field Imaging

Electrical steel is a soft magnetic steel material used in electric devices such as transformers and motors. The performance of these electric devices is primarily related to the magnetic properties of electrical steel, and the assessment of the magnetic properties of electrical steel has been considered an important topic. We use neutron grating interferometry, which is an imaging technique for visualizing the magnetic domain of electrical steel as the evaluation of magnetic properties. The dark-field image provided by neutron grating interferometry shows a sensitive contrast with respect to the magnetic domain of electrical steel due to the small angle neutron scattering generated at the domain wall. The Talbot-Lau interferometer was installed, and the feasibility test of high-resolution dark-field imaging was conducted at cold neutron imaging beamline of the NIST Center for Neutron Research. The dark-field image of electrical steel was compared with the magnetic domain image observed by the Bitter pattern based on the magnetic powder method to prove the validity of neutron grating interferometry. The dark-field image visualizes the magnetic domains of electrical steel, more detailed domain walls regardless of laser-irradiated lines than Bitter pattern result. (Received January 25, 2019; Accepted April 22, 2019)

Flux-closure domains in high aspect ratio electroless-deposited CoNiB nanotubes

We report the imaging of magnetic domains in ferromagnetic CoNiB nanotubes with very long aspect ratio, fabricated by electroless plating. While axial magnetization is expected for long tubes made of soft magnetic materials, we evidence series of azimuthal domains. We tentatively explain these by the interplay of anisotropic strain and/or grain size, with magneto-elasticity and/or anisotropic interfacial magnetic anisotropy. This material could be interesting for dense data storage, as well as curvature-induced magnetic phenomena such as the non-reciprocity of spin-wave propagation.

Understanding all-optical spin switching: Comparison between experiment and theory

Information technology depends on how one can control and manipulate signals accurately and quickly. Transistors are at the core of modern technology and are based on electron charges. But as the device dimension shrinks, heating becomes a major problem. The spintronics explores the spin degree of electrons and thus bypasses the heat, at least in principle. For this reason, spin-based technology offers a possible solution. In this review, we survey some of the latest developments in all-optical switching (AOS), where ultrafast laser pulses are able to reverse spins from one direction to the other deterministically. But AOS only occurs in a special group of magnetic samples and within a narrow window of laser parameters. Some samples need multiple pulses to switch spins, while others need a single-shot pulse. To this end, there are several models available, but the underlying mechanism is still under debate. This review is different from other prior reviews in two aspects. First, we sacrifice the completeness of reviewing existing studies, while focusing on a limited set of experimental results that are highly reproducible in different labs and provide actual switched magnetic domain images. Second, we extract the common features from existing experiments that are critical to AOS, without favoring a particular switching mechanism. We emphasize that given the limited experimental data, it is really premature to identify a unified mechanism. We compare these features with our own model prediction, without resorting to a phenomenological scheme. We hope that this review serves the broad readership well.

Thickness-dependent magnetic domain structures of Co ultra-thin film investigated by scanning transmission X-ray microscopy

Thickness-dependent magnetic domain structure of ultrathin Co wedge films (0.3 nm–1.0 nm) sandwiched by Pt layers was investigated by scanning transmission x-ray microscopy (STXM) employing X-ray magnetic circular dichroism (XMCD), utilizing elliptically polarized soft x-rays and electromagnetic fields, with a spatial resolution of 50 nm. The magnetic domain images measured at the Co L3 edge showed the evolution of the magnetic domain structures from maze-like form to the bubble-like form as the perpendicular magnetic field was applied. The asymmetric domain expansion of a 500 nm-scale bubble domain was also measured when the in-plane and perpendicular external magnetic field were applied simultaneously.

Soft X-ray Ptychography for Imaging of Magnetic Domains and Skyrmions in Sub-100 nm Scales

Soft X-ray Ptychography for Imaging of Magnetic Domains and Skyrmions in Sub-100 nm Scales

Magnetic domain imaging of a very rough fractured surface of Sr ferrite magnet without topographic crosstalk by alternating magnetic force microscopy with a sensitive FeCo-GdOx superparamagnetic tip

We imaged the magnetic domain of an extremely rough surface (with a roughness of ∼1 μm) of the anisotropic Sr ferrite sintered magnet without any topographic crosstalk by alternating magnetic force microscopy (A-MFM) using a sensitive FeCo-GdOx superparamagnetic tip. The magnetic moment of the FeCo-GdOx superparamagnetic tip is driven by an external AC magnetic field applied out of the plane direction to the magnetic sample. The static magnetic field is from the rough fractured ferrite sample parallel to the direction of the external AC magnetic field and is imaged by modulating the magnetic moment of the superparamagnetic tip. By using the frequency demodulation phenomena, A-MFM can extract the magnetic signal without any topography crosstalk versus the conventional MFM method. The intensity and the polarity of the static magnetic field originate from highly rough fractured hard magnetic Sr ferrite samples, and these were successfully detected and identified. This technique with the as-fabricated FeCo-GdOx superparamagnetic tips gives information about the intensity as well as polarity of magnetic fields from the magnetic domain structure of very rough fractured magnetic materials without any topographic crosstalk. This is crucial for the development of high performance hard magnets and magnetic devices.

Magnetic domain structure imaging near sample surface with alternating magnetic force microscopy by using AC magnetic field modulated superparamagnetic tip

For magnetic domain imaging, with a very high spatial resolution magnetic force microscope, the tip-sample distance should be as small as possible. However, magnetic imaging near the sample surface is very difficult with conventional magnetic force microscopy (MFM) because the interactive forces between the tip and sample include van der Waals and electrostatic forces along with a magnetic force. In this study, we proposed alternating MFM which only extracts a magnetic force near the sample surface without any topographic and electrical crosstalk. In the present method, the magnetization of an FeCo–GdOx superparamagnetic tip is modulated by an external AC magnetic field in order to measure the magnetic domain structure without any perturbation from the other forces near the sample surface. Moreover, it is demonstrated that the proposed method can also measure the strength and identify the polarities of the second derivative of the perpendicular stray field from a thin film permanent magnet with a DC demagnetized state and remanent state.