Magnetic Defects Research Articles

Visualizing the interplay of Dirac mass gap and magnetism at nanoscale in intrinsic magnetic topological insulators.

In intrinsic magnetic topological insulators, Dirac surface-state gaps are prerequisites for quantum anomalous Hall and axion insulating states. Unambiguous experimental identification of these gaps has proved to be a challenge, however. Here, we use molecular beam epitaxy to grow intrinsic MnBi2Te4 thin films. Using scanning tunneling microscopy/spectroscopy, we directly visualize the Dirac mass gap and its disappearance below and above the magnetic order temperature. We further reveal the interplay of Dirac mass gaps and local magnetic defects. We find that, in high defect regions, the Dirac mass gap collapses. Ab initio and coupled Dirac cone model calculations provide insight into the microscopic origin of the correlation between defect density and spatial gap variations. This work provides unambiguous identification of the Dirac mass gap in MnBi2Te4 and, by revealing the microscopic origin of its gap variation, establishes a material design principle for realizing exotic states in intrinsic magnetic topological insulators.

Looking for structure in the cobordism conjecture

The cobordism conjecture of the swampland program states that the bordism group of quantum gravity must be trivial. We investigate this statement in several directions, on both the mathematical and physical side. We consider the Whitehead tower construction as a possible organising principle for the topological structures entering the formulation of the conjecture. We discuss why and how to include geometric structures in bordism groups, such as higher U(1)-bundles with connection. The inclusion of magnetic defects is also addressed in some detail. We further elaborate on how the conjecture could predict Kaluza--Klein monopoles, and we study the gravity decoupling limit in the cobordism conjecture, with a few observations on NSNS string backgrounds. We end with comments in relation to T-duality, as well as the finiteness conjecture.

Magnetic Tile Surface Defect Detection Methodology Based on Self-Attention and Self-Supervised Learning.

As the core component of permanent magnet motor, the magnetic tile defects seriously affect the quality of industrial motor. Automatic recognition of the surface defects of the magnetic tile is a difficult job since the patterns of the defects are complex and diverse. The existing defect recognition methods result in difficulty in practical application due to the complicated system structure and the low accuracy of the image segmentation and the target detection for the diversity of the defect patterns. A self-supervised learning (SSL) method, which benefits from its nonlinear feature extraction performance, is proposed in this study to improve the existing approaches. We proposed an efficient multihead self-attention method, which can automatically locate single or multiple defect areas of magnetic tile and extract features of the magnetic tile defects. We also designed an accurate full-connection classifier, which can accurately classify different defects of magnetic tile defects. A knowledge distillation process without labeling is proposed, which simplifies the self-supervised training process. The process of our method is as follows. A feature extraction model consists of standard vision transformer (ViT) backbone, which is trained by contrast learning without labeled dataset that is used to extract global and local features from the input magnetic tile images. Then, we use a full-connection neural network, which is trained by using labeled dataset to classify the known defect types. Finally, we combined the feature extraction model and defect classification model together to form a relatively simple integrated system. The public magnetic tile surface defect dataset, which holds 5 defect categories and 1 nondefect category, is used in the process of training, validating, and testing. We also use online data augmentation techs to increase training samples to make the model converge and achieve high classification accuracy. The experimental results show that the features extracted by the SSL method can get richer and more detailed features than the supervised learning model gets. The composite model reaches to a high testing accuracy of 98.3%, and gains relatively strong robustness and good generalization ability.

POSSIBILITY OF IDENTIFYING DEFECTS IN MASSIVE CAST STRUCTURES BY NON-DESTRUCTIVE TESTING METHODS

In the context of improving methods and means of non-destructive testing (NDT), along with increasing in formativeness, complex technical problems arise. For example, when examining structural elements with a service life of more than 20 – 30 years, modern NC tools detect defects that have not been rejected by the manufacturer. Assessment of the impact of such defects on the operability of the structure requires a comprehensive study using NC and strain gauge. The purpose of the work is to evaluate the parameters of defects in a cast part of a hydraulic stamping press with a service life of more than 60 years in order to make a decision on the possibility of its further operation. The defect was investigated using magnetic powder, eddy current and ultrasonic control methods. By means of strain gauges, mechanical stresses are measured directly during operation. The indicator pattern of the magnetic particle inspection defect has signs uncharacteristic for fatigue defects: branching, intermittent and broken (not smooth) character. To quantify the degree of smoothness of the indicator pattern, it is proposed to use a fractal dimension, which for the defect under study was 1.07. The estimated depth of the defect detected by the eddy current method was from 4 to 6 mm, by ultrasound – from 8 to 18 mm. The presence of an echo signal by a direct converter indicates the volumetric nature of the defect. After mechanical removal of the surface layer with a thickness of 2 mm in some areas, the width of the defect increased to 2 mm, and the depth to – 20 mm. On the inner side surface of the defect, a dense, dark scale characteristic of high temperatures is visually determined.The defect is classified as a casting shell with a depth of 20 mm. No traces of fatigue failure were found. Compressive mechanical stresses with an amplitude of no more than 41 MPa amounted to less than 20 % of the fatigue endurance limit of 35L steel. During the year, the average number of working cycles is 20 thousand, repeated examination of the press defect is recommended in 5 – 10 years (100 – 200 thousand cycles).

Tracking the evolution from isolated dimers to many-body entanglement in NaLuxYb1−xSe2

We synthesize homogeneous compositions of NaLu$_x$Yb$_{1-x}$Se$_2$, connecting non-magnetic NaLuSe$_2$ to the triangular lattice spin liquid candidate NaYbSe$_2$. Thermal and magnetic properties are studied as the system evolves from one with dilute magnetic defects to one of a dense magnetic lattice. The field and temperature dependent heat capacity show the carriers of entropy crossover from isolated magnetic ions to a correlated lattice borne from spin dimers. For the dilute system we estimate the single ion anisotropy $(g_\perp/g_\parallel =3.13)$ and also the dimer exchange couplings $J_\parallel(=5.4$~K) and $J_\perp(=9.6$~K), in order to draw comparison to the half-doped and full magnetic compounds.

Enhancement of pinning and the peak effect in Y1–xFexBa2Cu3Oy high-temperature superconductors

Complex structural and magnetic studies of polycrystalline samples of the Y1 –xFexBa2Cu3Oy high-Tc superconductor (x = 0, 0.02, 0.05) have been carried out in order to determine the effect of weak doping with a magnetic 3d ion on structural changes and the critical current density Jc. An increase in the Fe content results in a significant change in Jc and its field dependence. The dependences Jc(H) become nonmonotonic and demonstrate a pronounced peak (peak effect); as a result, the Jc increases strongly in wide temperature and magnetic field regions. The analysis of the field dependences of the pinning force reveals a scaling behavior. The parameters of the scaling function correspond to the point magnetic defects under strong effects of a thermally activated flux flow. Thus, the weak Fe doping (several molar percent) is a promising way to significant improvement of transport properties of high-Tc superconducting materials based on 123-type compounds.

Thermodynamic characteristics and magnetocaloric effect of a diluted graphdiyne monolayer with defects: A Monte Carlo study

Based on Monte Carlo simulation, we studied the thermodynamic characteristics and magnetocaloric effect of a diluted graphdiyne monolayer with defects. The changes of magnetization, magnetic susceptibility and internal energy induced by physical parameters were examined. In particular, the effects of crystal field, external magnetic field and defect concentration on magnetic entropy change, adiabatic temperature change and hysteresis behavior were studied in detail. In addition, we also gave the relative cooling power induced by different external magnetic field and defect concentration. Finally, we found that defect concentration, temperature and crystal field have similar effect on hysteresis loops.

Power-law dynamics in the spin-liquid kagome latticesSrCr8Ga4O19andZnCu3(OH)6Cl2

We consider the polarization function ${P}_{Z}^{\mathrm{exp}}(t)$ measured by the muon-spin relaxation ($\ensuremath{\mu}\mathrm{SR}$) technique for the ${\mathrm{SrCr}}_{8}{\mathrm{Ga}}_{4}{\mathrm{O}}_{19}$ and ${\mathrm{ZnCu}}_{3}{(\mathrm{OH})}_{6}{\mathrm{Cl}}_{2}$ spin-liquid systems. We show the functional form of ${P}_{Z}^{\mathrm{exp}}(t)$ to imply that, in the temperature range of order 0.1 K, the spectral-weight function $F(\ensuremath{\omega})$ of the magnetic correlations scales with $1/{|\ensuremath{\omega}|}^{1\ensuremath{-}x}$ ($0<x<1$) in the energy range of $\ensuremath{\hbar}\ensuremath{\omega}\ensuremath{\approx}1\phantom{\rule{4pt}{0ex}}\ensuremath{\mu}\mathrm{eV}$. We derive the parameters involved in $F(\ensuremath{\omega})$ from fits to available experimental data. Inelastic neutron scattering data probing $F(\ensuremath{\omega})$ in the millielectronvolt energy range are consistent with a more conventional behavior. These differences could be due to a variety of spin-dynamics mechanisms, i.e., intrinsic to the kagome layer or related to the magnetic defects that have been evidenced in these compounds, acting at different energies. New $\ensuremath{\mu}\mathrm{SR}$ measurements are proposed and theoretical developments are suggested to pinpoint the mechanisms at play.

Coupling between magnetic order and charge transport in a two-dimensional magnetic semiconductor.

Semiconductors, featuring tunable electrical transport, and magnets, featuring tunable spin configurations, form the basis of many information technologies. A long-standing challenge has been to realize materials that integrate and connect these two distinct properties. Two-dimensional (2D) materials offer a platform to realize this concept, but known 2D magnetic semiconductors are electrically insulating in their magnetic phase. Here we demonstrate tunable electron transport within the magnetic phase of the 2D semiconductor CrSBr and reveal strong coupling between its magnetic order and charge transport. This provides an opportunity to characterize the layer-dependent magnetic order of CrSBr down to the monolayer via magnetotransport. Exploiting the sensitivity of magnetoresistance to magnetic order, we uncover a second regime characterized by coupling between charge carriers and magnetic defects. The magnetoresistance within this regime can be dynamically and reversibly tuned by varying the carrier concentration using an electrostatic gate, providing a mechanism for controlling charge transport in 2D magnets.

Effect of alloying in monolayer niobium dichalcogenide superconductors

When sulfur and silicon are incorporated in monolayer 2H-NbSe2 the superconducting transition temperature, Tc, has been found to vary non-monotonically. This was assumed to be a manifestation of fractal superconductivity. Using first-principles calculations, we show that the nonmonotonic dependence of Tc is insufficient evidence for multifractality. A unifying aspect in our study are selenium vacancies in NbSe2, which are magnetic pair-breaking defects that we propose can be present in considerable concentrations in as-grown NbSe2. We show that sulfur and silicon can occupy the selenium sites and reduce the pair-breaking effect. Furthermore, when sulfur is incorporated in NbSe2, the density of states at the Fermi level and the proximity to magnetism in the alloy are both reduced compared to the parent compound. Based on our results, we propose an alternative explanation of the non-monotonic change in Tc which does not require the conjecture of multifractality.

Photoinduced 120-degree spin order in the Kondo-lattice model on a triangular lattice

We theoretically predict the emergence of 120-degree spin order as a nonequilibrium steady state in the photodriven Kondo-lattice model on a triangular lattice. In the system away from the half filling with ferromagnetic ground state, the photoexcitation of conduction electrons and the photoinduced renormalization of bandwidth cause reconstruction of the band structure and subsequent redistribution of the electrons through relaxations, which result in an electronic structure similar to that in the half-filled system at equilibrium, where the 120-degree spin order is stabilized. In this photoinduced 120-degree spin ordered phase, domains of different spin-ordered planes are formed, and vortices of spin chirality vectors called ${Z}_{2}$ vortices appear at points where multiple domains meet. We also discuss favorable conditions and experimental feasibility to observe the predicted photoinduced magnetic phase transition by investigating dependencies on the light parameters (amplitude, frequency, and polarization), the electron filling, the strength of Kondo coupling, and the effects of antiferromagnetic coupling among the localized spins. Photoinduced magnetic structures proposed so far have been limited to simple collinear (anti)ferromagnetic orders or local magnetic defects in magnets. The present work paves a way to the optical creation of complex noncollinear magnetisms as a global nonequilibrium steady phase in photodriven systems.

Magnetic tile crack defect detection based on Contourlet transform and singular value decomposition

ABSTRACT The magnetic tile is a permanent magnet made of ferrite material, which is shaped like a tile and mainly used in permanent magnet motor. The magnetic tile image taken by camera is with dark colour and uneven background brightness and texture, and the contrast between crack defect and background is low. In this paper, a non-destructive testing method based on machine vision technology is proposed for magnetic tile crack defect detection. We propose a new crack defect detection algorithm based on Contourlet transform and singular value decomposition (SVD). The algorithm first adopted Contourlet transform to decompose the original image, and the subband coefficients were decomposed by singular value decomposition, then according to the difference of singular values gradient, the principal singular values which would be set to zero were determined. Finally, the image was reconstructed with the reconstruction coefficients that were modified by inversing SVD, the background texture was eliminated and crack defect was obtained. To verify the effectiveness and superiority of the proposed algorithm, extensive experiments were carried out and compared with the traditional algorithms. Experimental results show that the proposed method can effectively detect crack defect with the accuracy rate of 94.29%, and outperforms traditional methods.

Dynamics of the order parameter statistics in the long range Ising model

We study the relaxation of the local ferromagnetic order in the transverse field quantum Ising chain with power-law decaying interactions \sim 1/r^\alpha∼1/rα. We prepare the system in the GHZ state and study the time evolution of the probability distribution function (PDF) of the order parameter within a block of \ellℓ when quenching the transverse field. The model is known to support long range order at finite temperature for \alpha\leq 2.0α≤2.0. In this regime, quasi-localized topological magnetic defects are expected to strongly affect the equilibration of the full probability distribution. We highlight different dynamical regimes where gaussification mechanism may be slowed down by confinement and eventually breaks. We further study the PDF dynamics induced by changing the effective dimensionality of the system; we mimic this by quenching the range of the interactions. As a matter of fact, the behavior of the system crucially depends on the value of \alphaα governing the unitary evolution.

Neutron irradiation induced magnetization and persistent defects at high temperatures in graphite

Structural as well as magnetization studies have been carried out on graphite samples irradiated by neutrons for over 50 years in the CIRUS research reactor at Trombay, India. Neutron diffraction studies reveal that the defects in irradiated graphite samples are not well annealed and remain significant up to high temperatures much greater than 653 K where the Wigner energy is completely released. We infer that the remnant defects may be intralayer Frenkel defects, which do not store large energy, unlike the interlayer Frenkel defects that store the Wigner energy. Magnetization studies on the irradiated graphite show ferromagnetic behavior that persists up to \ensuremath{\sim}850 K and a large additional paramagnetic contribution. Ab initio calculations based on the spin-polarized density-functional theory show that the magnetism in defected graphite is essentially confined to a single twofold-coordinated carbon atom that is located around a vacancy in the hexagonal layer.

Detection of Structural Magnet Defects for Permanent Magnet Synchronous Motors

This paper presents a novel method for the detection of magnet cracks or defects through back electromotive force (EMF) voltage, line current and vibration signals for radial flux permanent magnet (PM) synchronous motors. An integer slot PM motor with 12 poles is used in the study. Different types of faults including both radial and axial magnet crack defects with multiple magnet fault scenarios and multiple pole fault cases are studied in detail. In addition, the severity of magnet fault is also investigated. Random forest based magnet defect detection is utilized and a classification scheme is proposed in order to analyze the changes in the harmonics arising from single or multiple, radial or axial magnet defects. The results clearly show that the harmonics is altered in such a manner that it is possible to determine not only the magnet fault but the type of the magnet defect using motor voltage, current or vibration data.

Novel ferromagnetism and negative magnetoresistance in BaBiO3 nanoparticles

Materials showing semiconducting and magnetic properties have significant commercial value presently, particularly in the field of spintronics. We report a novel ferromagnetic behavior and a negative magnetoresistance in the non-magnetic semiconductor perovskite material BaBiO3 at room temperature. We also observe an associated exchange spring like magnetic behavior in the sample annealed at a higher temperature. We discuss these effects based on a model of defect induced magnetic centers and magnetic domains in the material. The enhanced magnetization and exchange spring like behavior observed in the sample annealed at a higher temperature of 800 °C is attributed to higher number of magnetic defects and two dominant sizes of magnetic clusters. Photoluminescence and energy dispersive X-ray spectra corroborate the explanation based on charge induced defects in the material. Interplay of Bi3+ and Bi5+ states in BaBiO3, leads to a semiconducting behavior at room temperature. From the temperature dependence of the resistivity two distinct activation energies of 0.21 eV and 0.14 eV above and below 256 K, respectively, have been measured.

From Thermodynamics to Information: Landauer’s Limit and Negentropy Principle Applied to Magnetic Skyrmions

Magnetic skyrmions are topological swirling spin textures objects that can be manipulated and employed as information carriers. This is accomplished based either on their ground-state properties or their thermodynamic properties. Landauer’s principle establishes an irreversible conversion from information to physics. The inverse mechanism, the inverse mechanism is proposed for magnetic topological defects forming in magnetic nanostructures that are regarded as closed thermodynamic systems confirming Szilard’s and Brillouin’s hypotheses. This mechanism consists of the creation of bits of information using a thermodynamic source having a form of negentropy. In this perspective article, the following are proved for magnetic skyrmions: 1) Landauer’s principle expressed in terms of negentropy and 2) the generalized second principle of thermodynamics based on Brillouin’s negentropy principle of information. The thermodynamic entropy is converted into information entropy at the expense of negentropy, “negative entropy” corresponding to the loss of thermodynamic entropy from the magnetic skyrmion itself. A recently proposed practical device enables the verification of points 1) and 2) and allows a full understanding of the interchange between thermodynamics and information and vice versa regarding skyrmions as information units and showing, in perspective, the considerable advantages offered by this type of storing and coding information.

Spin-glass transition in the spin\u2013orbit-entangled Jeff\u2009=\u20090 Mott insulating double-perovskite ruthenate

We have successfully synthesized new Ru4+ double perovskite oxides SrLaInRuO6 and SrLaGaRuO6, which are expected to be a spin–orbit coupled Jeff = 0 Mott insulating ground state. Their magnetic susceptibility is much significant than that expected for a single Ru4+ ion for which exchange coupling with other ions is negligible. Their isothermal magnetization process suggests that there are about 20 percent isolated spins. These origins would be the Ru3+/Ru5+ magnetic defects, while the regular Ru4+ sites remain nonmagnetic. Moreover, SrLaGaRuO6 shows a spin-glass-like magnetic transition at low temperatures, probably caused by isolated spins. The observed spin-glass can be interpreted by the analogy of a dilute magnetic alloy, which can be seen as a precursor to the mobile Jeff = 1 exciton as a dispersive mode as predicted.

Formation Mechanism and Improvement of Magnetic Particle Inspection Defects in Cr5 Backup Roller Forged Ingot

Industrial tests and thermodynamic calculations were utilized to investigate the source and formation of magnetic particle inspection defects identified on the near-surface of the Cr5 back-up roll forged ingot, which was used in large cold rolling mills. The results showed that the linear aggregating SiO2-MnO-Al2O3 liquid inclusions up to 3 mm led to the flaw detection failure. SiO2-MnO-Al2O3 liquid inclusions were firstly formed in the inductive furnace. Due to its low contact angle, a huge amount of SiO2-MnO-Al2O3 liquid inclusions were inherited into the forged ingot. The formation of SiO2-MnO-Al2O3 liquid inclusions was attributed to the over-oxidation and relatively low aluminum content in the molten steel, as calculated by Factsage 8.1. Controlling the amount of aluminum in molten steel during the smelting process could modify the formation of SiO2-MnO-Al2O3 and CaO-SiO2-Al2O3 liquid oxide into solid Al2O3 type inclusions that were easily removed. Besides, the CaO-SiO2-Al2O3 liquid oxide could be transformed from CaO-Al2O3 type oxide by the significant loss of aluminum content during the VD process or slag entrapment. Certain content of aluminum in the molten steel could improve the flaw detection caused by the aggregating SiO2-MnO-Al2O3 inclusions effectively.

Mixed Fault Classification of Sensorless PMSM Drive in Dynamic Operations Based on External Stray Flux Sensors.

This paper aims to classify local demagnetisation and inter-turn short-circuit (ITSC) on position sensorless permanent magnet synchronous motors (PMSM) in transient states based on external stray flux and learning classifier. Within the framework, four supervised machine learning tools were tested: ensemble decision tree (EDT), k-nearest neighbours (KNN), support vector machine (SVM), and feedforward neural network (FNN). All algorithms are trained on datasets from one operational profile but tested on other different operation profiles. Their input features or spectrograms are computed from resampled time-series data based on the estimated position of the rotor from one stray flux sensor through an optimisation problem. This eliminates the need for the position sensors, allowing for the fault classification of sensorless PMSM drives using only two external stray flux sensors alone. Both SVM and FNN algorithms could identify a single fault of the magnet defect with an accuracy higher than 95% in transient states. For mixed faults, the FNN-based algorithm could identify ITSC in parallel-strands stator winding and local partial demagnetisation with an accuracy of 87.1%.