United States

We report on the imaging domain wall structure of amorphous Fe/Gd multilayer thin films with dendritic stripe domains. Micromagnetic calculations suggest in both stripe and skyrmion phases the domain walls have a hybrid structure where the center of the domain wall is Bloch-like with random helicity transitioning to Néel-like walls at the surfaces with fixed helicity.&lt;br/&gt;&lt;br/&gt;To experimentally probe the domain structure we combine transmission electron microscopy in Lorentz mode (L-TEM) and scanning electron microscopy with polarization analysis (SEMPA). L-TEM images are processed to show the magnetic induction through the bulk of the sample, while SEMPA images show magnetization at the top surface of the sample. In this way, 3-dimensional reconstructions of the surface magnetization can be realized[1].&lt;br/&gt;&lt;br/&gt;The SEMPA instrument, based on an Auger microscope, leverages spin-polarized secondary electrons scattered from the top few nanometers of a sample[2]. These secondaries are accelerated through the column and Mott scattered from a gold target in order to reveal the left-right and up-down polarization for both in-plane and out-of-plane electrons.&lt;br/&gt;&lt;br/&gt;We observe and characterize the the chirality of Néel-like domains closure caps at the surface of Fe/Gd following the convention established by Chen et al[3]. The z-components of magnetization for several domains are shown in Fig. 1. A comparison of the average magnetization vectors in the plane of the sample is shown in Fig. 2, where the difference in L-TEM and SEMPA observations indicate an out-of-plane, twisting component.&lt;br/&gt;&lt;br/&gt;We have thus confirmed Bloch wall formation in the bulk topped with Néel caps at the surface.

![](https://assets.underline.io/uploads/markdown_image/1/image/2f064e2fb92d0b646f15fbcf73cf3382.jpg)

Fig. 1: Hybrid domains in amorphous Fe/Gd with arrows showing reconstructed in-plane and colors showing in- (blue) and out-of-plane (red) magnetization.

![](https://assets.underline.io/uploads/markdown_image/1/image/6ef24c67176adf710147a1a02e566c08.jpg).

Fig. 2: Angle of in-plane magnetization relative to increasing out-of-plane magnetization (α) showing expected π/2 radians average from L-TEM, indicating Bloch walls, and larger deviations averaging 3π/4 radians from SEMPA, indicating Néel caps.

2022 Joint MMM-INTERMAG

Characterization of Hybrid Domain Wall Structure and Surface Chirality for Amorphous Fe/Gd Thin Films

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INTERMAG 2021

We report on the imaging domain wall structure of amorphous Fe/Gd multilayer thin films with dendritic stripe domains. Micromagnetic calculations suggest in both stripe and skyrmion phases the domain walls have a hybrid structure where the center of the domain wall is Bloch-like with random helicity transitioning to Néel-like walls at the surfaces with fixed helicity. To experimentally probe the domain structure we combine transmission electron microscopy in Lorentz mode (L-TEM) and scanning electron microscopy with polarization analysis (SEMPA). L-TEM images are processed to show the magnetic induction through the bulk of the sample, while SEMPA images show magnetization at the top surface of the sample. In this way, 3-dimensional reconstructions of the surface magnetization can be realized[1]. The SEMPA instrument, based on an Auger microscope, leverages spin-polarized secondary electrons scattered from the top few nanometers of a sample[2]. These secondaries are accelerated through the column and Mott scattered from a gold target in order to reveal the left-right and up-down polarization for both in-plane and out-of-plane electrons. We observe and characterize the the chirality of Néel-like domains closure caps at the surface of Fe/Gd following the convention established by Chen et al[3]. The z-components of magnetization for several domains are shown in Fig. 1. A comparison of the average magnetization vectors in the plane of the sample is shown in Fig. 2, where the difference in L-TEM and SEMPA observations indicate an out-of-plane, twisting component. We have thus confirmed Bloch wall formation in the bulk topped with Néel caps at the surface.

![](https://assets.underline.io/uploads/markdown_image/1/image/2f064e2fb92d0b646f15fbcf73cf3382.jpg)

Fig. 1: Hybrid domains in amorphous Fe/Gd with arrows showing reconstructed in-plane and colors showing in- (blue) and out-of-plane (red) magnetization.

![](https://assets.underline.io/uploads/markdown_image/1/image/6ef24c67176adf710147a1a02e566c08.jpg).

Fig. 2: Angle of in-plane magnetization relative to increasing out-of-plane magnetization (α) showing expected π/2 radians average from L-TEM, indicating Bloch walls, and larger deviations averaging 3π/4 radians from SEMPA, indicating Néel caps.

technical paper

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The welds of steel structures, used in the bridges and roadway steel decks, are stressed areas where fatigue cracks are prone to occur. Once a fatigue crack occurs in a weld, it spreads and becomes larger, increasing the risk of collapse of the structure. Therefore, it is important to detect fatigue cracks in welds at an early stage. However, the surfaces of the structures are coated to protect the structure itself and the landscape. This makes it difficult to detect cracks using the conventional methods of visual inspection and magnetic particle inspection. In this study, a sensor probe adapted to the weld shape of the structure was developed for detecting cracks with the surface coating using the unsaturated AC magnetic flux leakage (USAC-MFL). This method utilizes a small AC magnetic field in the unsaturated region of the initial magnetization for steel and measures the magnetic flux leakage due to the crack. We have previously reported that the crack detection of steel plates was realized with low power and high sensitivity using the USAC-MFL [1]. To evaluate the performance of the developed sensor probe, 20 samples of welded joints with an average coating thickness of 125 and 250 μm were prepared. The slits were located in three positions: in the rib, the weld, and the base (Fig. 1). A total of 70 slits, ranging from 1 mm to 5 mm depth, were measured. The differential signals from the two sensors were recorded and analyzed. The same measurements were performed in the non-slit areas to compare the signals of the slits. Figure 2 shows the result measured at the slits positioned in the weld. The correlation coefficient of slit depth and differential intensity was 0.89, indicating a strong correlation. The same tendency was obtained for the slits in the rib and base. The differential signal intensity of the slit was larger than that of the non-slit area, even 1 mm depth. These results suggest that the USAC-MFL and the developed sensor probe can be used to detect cracks in the welds of steel structures at an early stage with the surface coating.

![](https://assets.underline.io/uploads/markdown_image/1/image/0d1693854f1cfd481032ccf0b4002494.jpg)

Fig. 1 Welded joint sample with surface coating and the sensor probe.

![](https://assets.underline.io/uploads/markdown_image/1/image/0bd665c2474845ff0803bb88336e9692.jpg)

Fig. 2 Dependence of differential signal intensity and slit depth measured in the weld.

Crack detection for welded joint with surface coating using unsaturated AC magnetic flux leakage

This work presents a novel system to image formation permeability of the reservoir in situ downhole. Colloidal magnetite fluid with high magnetic permeability is injected from the surface into the subsurface formation. The magnetic fluid permeates through the formation some radial distance ΔR over a given period of time, which is directly related to the formation permeability. A sensor array which measures the radial distribution of magnetic permeability surrounding the tool is deployed downhole during this injection process. By comparing the magnetic permeability radial distribution surrounding the sensor before/during/after fluid injection, the system is able to directly measure phase permeability of the surrounding formation in situ. The sensor array measures the radial magnetic permeability distribution of the formation via mutual inductance between pairs of solenoid coils axially distributed along a downhole tool. As the magnetic permeability of the material surrounding the solenoid coils increases, the mutual inductance between coils increases. Eddy current effects on mutual inductance measurements can be minimized by reducing the interrogation frequency of the measurement such that electrical conductivity and permittivity changes have no effect on the measurement of interest, e.g. the measurement is insensitive to the movement of brine and hydrocarbon and sensitive only to the movement of the magnetic permeability doped fluid. Measuring mutual inductance across the array of solenoid coils distributed along the tool enables the system to map the radial distribution of magnetic permeability surrounding the sensor. A laboratory prototype of this sensor system was built and evaluated for its sensitivity to changes in magnetic permeability distributions surrounding the tool. Results from these laboratory tests are compared to analytical predictions of the tool response. Magnetic permeability doped fluid of various concentrations are measured with the sensor array, and measurement results are compared to the predicted magnetic permeability of the various concentrations of doped fluid.

![](https://assets.underline.io/uploads/markdown_image/1/image/a8837a4474972e06f1721268e9344f16.jpg)

Magnetic Permeability Sensor Array Prototype To Evaluate Reservoir Phase Permeability in-Situ Downhole

Effect of individual magnetic nanoparticle on magnetization reversal of ferromagnetic layers is well know [1,2 ]. Here we present effect of ferromagnetic surface of the magnetoresistive sensor on magnetic susceptibility of nanoparticles (NPs), deposited on the sensor surface. Deposition of CoFe2O4 nanoparticles on CoFeB/Ta/CoFeB platform, containing two ferromagnetic nanolayers, changes blocking temperature of the NPs. Magnetic dipole interaction between the NPs and the platform effects frequency and field dependences of the blocking temperature of the NPs. Effective barrier of magnetization reversal and blocking temperature of NP, coupled with ferromagnetic platform, are higher, than in NP deposited on diamagnetic substrate. We observed feedback of magnetic susceptibility of cobalt ferrite cubic nanoparticles on magnetic dipole interaction between NPs and ferromagnetic bilaere CoFeB/Ta/CoFeB platform. The main magnetic characteristics of dynamical magnetization reversal in NPs are changed, when they are deposited on the surface of the ferromagnetic platform. Change of the blocking temperature of NPs under the magnetic dipole interaction with platform is caused by increase of the constant of effective magnetic anisotropy of the NPs up to 15-20%. Micromagnetic modeling predicts change of platform magnetization under individual NP up to 10 % that is similar with changes of the blocking temperature, effective magnetic anisotropy and other phenomenological parameters directly determined in experiments (Fig.1).The variations of the magnetizations of the two ferromagnetic layers under alternative magnetic field were visualized by micromagnetic modeling. Obtained results have principal importance for correct data collecting in lab-on-chip devices, where concentration of the NPs or magnetically labeled biology objects should be measured. Change of the AC susceptibility of NPs caused by their magnetic dipole interaction with the platform can valuably affect estimated number of nanoparticles in real devices.

![](https://assets.underline.io/uploads/markdown_image/1/image/654e8026a3211609a42279c3b16cd777.jpg)

Fig.1. Calculated orientations of local magnetization in top and bottom layers of the CoFeB/Ta/CoFeB platform in presence of ferromagnetic NP on its surface in external magnetic field.

Feedback of magnetic nanoparticles susceptibility to their magnetic dipole interaction with ferromagnetic GMR sensor

The linear magneto-optic Kerr effect (LinMOKE) as well as the quadratic MOKE (QMOKE) are important magneto-optic effects of fundamental physics but also powerful tools for thin-film sample characterization, e.g., to analyze magnetocrystalline anisotropies via vectorial magnetometry [1] or to sense the crystallographic ordering in Heusler compounds [2]. In order to separate and study the dependencies of LinMOKE and QMOKE on the crystallographic direction, the so-called eight-directional method can be used [3]. For each crystallographic in-plane direction, a magnetic field is applied in eight in-plane directions (every 45°) and the responses are added up or subtracted depending on the odd and even nature of LinMOKE and QMOKE with respect to the magnetization. So far,this method or similar ones have been utilized to characterize (001)-and (011)-oriented thin films of cubic crystal structure [4,5]. In addition, new setups for QMOKE spectroscopy have been realized for (001)-oriented thin films [6]. Within this contribution, we apply the eight-directional method to sputter-deposited Ni(111) thin films and report on a strong three-fold anisotropy in longitudinal MOKE (LMOKE), see Fig. 1. This anisotropy can be explained by theory as an optical interplay of elements in the permittivity tensors of first and second order in M, effectively creating cubic MOKE contributions, i.e., MOKE of third order in M. Furthermore, we studied Ni(111) thin films with and without several multiple (111) phases of 60° in-plane rotation (twinning) which have been characterized by off-specular x-ray diffraction texture maps. We observed substantial reduction of QMOKE oscillations in a twinned sample compared to a thin film with almost no twinning. This indicates that the LMOKE anisotropy truly is of crystallographic origin in the ferromagnetic layer and is not due to other, e.g., interface effects.

![](https://assets.underline.io/uploads/markdown_image/1/image/ee68c6112eec764f164a4c5214b5abe1.jpg)

QMOKE anisotropy measurement of a 20-nm thick Ni film on MgO(111) at a wavelength of 406 nm. The threefold anisotropy of longitudinal MOKE (∝ ML in black), transverse MOKE (∝ MT in green) and QMOKE (∝ MLMT in blue, as well as ∝ MT2-ML2 in red) are clearly visible.

Quadratic and third-order magneto-optic Kerr effect in Ni(111) thin films with and without twinning

Fourier transform holography (FTH) is a lensless imaging technique that uses a known reference in the sample to retrieve the object of interest in one single step of calculation (i.e. it does not require an iterative method), overcoming the phase problem inherent to other techniques such as coherent diffraction imaging and ptychography. In the case of an extended reference, a linear differential filter is applied and the object can simply be obtained by calculating the inverse Fourier transform. This approach is known under the name of HERALDO [1]. By exploiting the x-ray magnetic circular dichroism effect, this technique has shown to be useful to image magnetic textures by providing a projection (sometimes two) of the magnetization in 2D layers [2-5]. In this work, we go further and use FTH to image the magnetic structure of a 3D object. To be sensitive to all three components of the magnetization (perpendicular as well as in-plane), we use a sample setup with two perpendicular slits as complementary references (Fig. 1). By tilting the sample around the axes defined by the two slits, we obtain a dual set of projections which gives information about the internal configuration of the sample [6]. We discuss the different problems that can arise in the experiment, such as shadowing effects and a reduced angular range for the tilting of the sample, and how these can be conquered. We test the method in a 1μm thick Fe/Gd multilayer with maze-like magnetic pattern and image a circular window of diameter 5μm. Finally, we use this projection set as input to the tomography algorithm that we developed [7] to reconstruct the vector field structure of the whole object.

![](https://assets.underline.io/uploads/markdown_image/1/image/3d094f6eccc80616b54ad2113433005c.jpg)

Fig. 1: Fourier transform holography: The coherent x-ray beam illuminates the whole sample. A circular window is milled into the gold layer, which masks the magnetic sample, to allow passing through the x-rays. Two slit references are also milled across the mask and the sample. After filtering the diffraction pattern and applying the inverse Fourier transform, the magnetic projection is recovered (magnetic contrast is obtained by x-ray magnetic circular dichroism). Tilting the sample around two axes allows us to probe all three components of the magnetization.

Imaging the Magnetization Vector with Fourier Transform Holography

Magnetic nanoparticles are powerful tools in biomedical applications, where they are employed in both diagnosis and therapy. A prerequisite for the efficiency of these applications are precisely characterized particles. Several magnetic measurement techniques are established, which all have in common that they measure the magnetic response of the particles exposed to an externally applied magnetic field, which may change the magnetic state of the particles [1]. Therefore, Thermal Noise Magnetometry (TNM) has been developed, a magnetic nanoparticle characterization technique that does not rely on an external magnetic excitation [2]. It detects amplitude variations in the net magnetic moment of the sample caused by thermal fluctuations. TNM measurements have been proven to be feasible, and complementary to other characterization techniques due to their diminutive impact on the sample [2, 3]. Two mechanisms are responsible for the directional fluctuations of the magnetic moments. Brownian fluctuations occur if the particles are free to move in the sample, i.e. when they are suspended in a solution. The hydrodynamic size of the particles and the viscosity of the fluid influence the Brownian fluctuation rate. In parallel, the magnetization of each particle can also switch in the frame of the particles themselves, called Néel switching. The Néel switching rate is determined by the volume of the particles’ magnetic core and anisotropy constant. Additionally, the timescales of both mechanisms also depend on the temperature since the fluctuations are induced by the thermal energy in system. TNM experiments are typically performed at room temperature, but the particles are characterized for biomedical applications at body temperature. From preliminary results, a considerable effect of temperature variations in TNM is visible, influencing the particles’ fluctuation rate distributions and noise spectrum. As expected from the theory, the fluctuation rates shift towards higher values for higher temperatures, which leads to a decrease in the noise power in the considered frequency range.

![](https://assets.underline.io/uploads/markdown_image/1/image/cb671696c9bc121f01d6d3494d47ebc4.jpg)

Power spectrum densities for Perimag® particles at different temperatures.

The Effect of Temperature Variations in Thermal Noise Magnetometry

Since the discovery of monolayer graphene in 2004, immense exploration of 2D materials with the main goal of minimizing electronic devices towards atomic level is in progress. The 2D materials family includes a huge range of materials- from insulators, semiconductors, metals to superconductors. In 2017, two independent groups demonstrated stable magnetic ordering in 2D materials (CrI3 and Cr2Ge2Te6) 1,2 bringing ferromagnetic materials into the 2D family. Since then, 2D magnetism has been discovered in various vdW materials, for example CrX3 (X= Cl, Br, I) 3,4 among which CrBr3 is the most air stable in the CrX3 family. Magnetic and optical properties of bulk CrBr3 have been studied since the 1960s. Based on the various optical measurements, it is believed that CrBr3 has an energy gap in the range of 1.68 – 2.1 eV5,6. These controversial results have indicated that the electronic properties of CrBr3 are not well explored. In this poster, I will present the STM/S result of both thin and thick CrBr3 flakes with the density fluctuation theory (DFT) calculations to reveal a small energy gap to be 0.57 eV ± 0.04 eV7. This uncovering of the small energy gap will solve the controversy and is the key to better understand the electronic properties of CrBr3.

Scanning Tunneling Microscopy and Spectroscopy (STM/S) Reveals Small Energy Gap in CrBr3

Magnetic van der Waals (vdW) materials have emerged as promising candidates for spintronics applications, especially after the discovery of intrinsic ferromagnetism in monolayer vdW materials. Here, we report a real-space imaging study of itinerant ferromagnet Fe3GeTe2 and the enhancement of its Curie temperature well above ambient temperature [1]. We later found implanting Ga into Fe3GeTe2 could greatly enhance the Fe3GeTe2 Curie temperature by almost 100% [2]. Magnetic skyrmions are topological spin textures, which usually exist in noncentrosymmetric materials where the crystal inversion symmetry breaking generates the so-called Dzyaloshinskii-Moriya interaction. Using photoemission electron microscopy and Lorentz transmission electron microscopy, we investigated and stabilized Néel-type magnetic skyrmion in vdW ferromagnetic Fe3GeTe2 on top of (Co/Pd)n without the need of an external magnetic field.

Magnetic Domains in 2D van der Waals Material Fe3GeTe2

Magnetic metal nanoparticles dispersed in non-magnetic (carbon) matrix are
of great importance both from a scientific point of view and in connection
with numerous applications in biomedicine, spintronics, catalysis, sensors,
etc. [1-3]. Especially promising are synthesis of novel functional nanoparticles with a core-shell architecture for improving their working magnetic
characteristics due to the possibility of combining materials with different
magnetic properties based on these magnetic nanoparticles [4-5]. The
magnetic interactions between the core and the shell provide an additional
degree of freedom for tuning the magnetic characteristics [6]. Nanoparticles
based on iron and iron oxide with a core-shell architecture are effective
intermediaries for heat transfer in an AC magnetic field [7-8]. In current
work, Fe-Fe3O4 core-shell nanoparticles were synthesized by solid-phase
pyrolysis of iron phthalocyanine (FePc), followed by annealing in an oxygen
atmosphere. Changing the pyrolysis conditions, it is possible to vary sizes of
nanoparticles from 5 to 100 nm and core-shell ratio. The quantitative composition of the core-shell was controlled also by changing the concentration of
oxygen. Complex structural analyses of these materials are obtained using
HRTEM, STEM, XRD and Raman spectroscopies. Magnetic properties of
synthesized materials have been studied with a Quantum Design MPMS XL
SQUID magnetometer. To study the structure of the obtained nanoparticles, HAADF-STEM images were taken, confirming the Fe-Fe3O4 core-shell
architecture of the nanoparticles. Figures 1, and 2 show a HAADF-STEM
image with elemental mapping of a single chosen nanoparticle and XRD
pathern of the (Fe-Fe3O4)@C sample obtained by synchrotron radiation
(λ = 0.98782 Å) correspondingly. This work was supported by European
Union’s Horizon 2020 research and innovation program under grant No
857502 (MaNaCa). The work at CSULA supported by NSF CREST Grant
#HRD-1547723. M. Farle acknowledges support by government of the RF
(No 075-15-2019-1886)



Synthesis and Characterization of Fe-Fe3O4 Core-Shell Nanoparticles Dispersed in Carbon Matrix

Magnetorelaxometry imaging (MRXI) [1] enables a quantitative and specific localization of magnetic nanoparticles (MNPs) inside living organisms [2] which is required by several novel therapeutic approaches such as magnetic hyperthermia therapy and magnetic drug targeting. MRXI involves aligning the MNPs’ magnetic moments using electromagnetic coils and subsequently recording their decaying net magnetization (relaxation). The recorded signals are connected to the MNP iron masses inside the discretized region of interest (ROI) via a system of linear equations [3]. The MNP distribution is recovered by solving the corresponding ill-posed inverse problem. However, the ROI discretization is limited to rather large voxel sizes. Finer voxel grids increase the number of unknowns and aggravate accurate reconstructions of MNP distributions. To date, finer voxel resolutions with reliable reconstruction accuracies are still necessary to obtain clinically relevant MRXI reconstructions. Standard MRXI provides only information about the MNP distributions and every voxel can contain MNPs with equal probability. Obviously, this is not realistic in clinical practice due to the different anatomical structures inside a human body. Accompanying traditional medical imaging techniques such as CT are necessary to gain anatomical information about the ROI. Here, we develop a multimodal reconstruction using CT data as prior for MRXI reconstructions, enabling a distinction between voxels with impossible, improbable and probable MNP occurrences. We test the approach on a cylindrical ROI (see Fig. 1) containing objects that produce different radiographic density (meat, wood, water, etc.) as well as MNPs. CT and MRXI measurements were conducted and the incorporation of the CT data allows us to reconstruct MNP distributions more accurately with half the voxel edge lengths than with mere MRXI (see Fig. 2). Our multimodal approach demonstrates that resolution and quantification accuracy of MRXI substantially benefits from anatomical data provided by alternative techniques such as CT or MRI.

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