Magnetic Defects Research Articles

Quasibound states in short SNS junctions with point defects

Using the Green functions technique, we study the subgap spectrum of short three-dimensional superconductor--normal metal--superconductor junctions containing one or two point impurities in the normal layer. We find that a single nonmagnetic or magnetic defect induces two quasibound Shiba-like states. If the defect is located close to the junction edge, the energies of these states oscillate as functions of the distance between the impurity and the edge. In the case of two nonmagnetic impurities, there are generally four quasibound states (two per spin projection). Their energies oscillate as functions of the distance between the impurities, and reach their asymptotic values when this distance becomes much larger than the Fermi wavelength. The contributions of the impurities to the Josephson current, local density of states, and to the normal-state conductance of the junction are analyzed.

Phase control of spin waves based on a magnetic defect in a one-dimensional magnonic crystal

Magnonic crystals are interesting for spin-wave based data processing. We investigate one-dimensional magnonic crystals (1D MCs) consisting of bistable Co20Fe60B20 nanostripes separated by 75 nm wide air gaps. By adjusting the magnetic history, we program a single stripe of opposed magnetization in an otherwise saturated 1D MC. Its influence on propagating spin waves is studied via broadband microwave spectroscopy. Depending on an in-plane bias magnetic field, we observe spin wave phase shifts of up to almost π and field-controlled attenuation attributed to the reversed nanostripe. Our findings are of importance for magnetologics, where the control of spin wave phases is essential.

Impurity states in mesoscopic SNS junctions with a point defect

We study theoretically localized states in a clean three-dimensional SNS junction, containing one point defect (impurity) located in the normal layer. We find that the defect induces two quasibound states in a short junction, corresponding to different spin projections. The energies of these states coincide for a nonmagnetic defect and are generally different for a magnetic defect. The spatial structures of the impurity states resemble the structures of Shiba states in a bulk superconductor. We also consider the case when the impurity is located close to a fat edge of the junction. We find mesoscopic fluctuations of the energies of the quasibound states as a function of the distance between the impurity and the edge.

Magnetic Defects in Transitional Metal Di-Chalcogenide Semiconducting Layers

ABSTRACTIn this work, we report on the electron spin resonance (ESR) studies performed on few-layered nanocrystalline (NCs) MoS2, WS2, and TiS2 prepared using hydrothermal and vapor transport methods. From the temperature dependent ESR spectra collected from MoS2 NCs, we have identified adsorbed oxygen species, sulphur vacancies, thio- and oxo-Mo5+ related paramagnetic defect centers. WS2 NCs have exhibited W+3 and oxo-W+5 paramagnetic defect spin species. TiS2 NCs showed defects such as Fe3+ (unwanted), oxygen and sulfur vacancies. This work demonstrates the usage of spin-sensitive spectroscopy such as ESR in unravelling the defects which contain unpaired electron spin centers in layered NCs two-dimensional materials.

Токи Фуко, возникающие при работе внутритрубного магнитного дефектоскопа

Токи Фуко, возникающие при работе внутритрубного магнитного дефектоскопа

Polarizability of electrically induced magnetic vortex “atoms”

Electric field control of magnetic structures, particularly topological defects in magnetoelectric materials, draws a great attention, which has led to experimental success in creation and manipulation of single magnetic defects, such as skyrmions and domain walls. In this work we explore a scenario of electric field creation of another type of topological defects – magnetic vortices and antivortices. Because of interaction of magnetic and electric subsystems each magnetic vortex (antivortex) in magnetoelectric materials possesses quantized magnetic charge, responsible for interaction between vortices, and electric charge that couples them to electric field. This property of magnetic vortices makes possible their creation by electric fields. We show that the electric field, created by a cantilever tip, produces a “magnetic atom” with a localized spot of ordered vortices (“nucleus” of the atom) surrounded by antivortices (“electronic shells”). We analytically find the vortex density distribution profile and temperature dependence of polarizability of this structure and confirm it numerically by Monte Carlo simulation.

Depinning of the transverse domain wall trapped at magnetic impurities patterned in planar nanowires: Control of the wall motion using low-intensity and short-duration current pulses

Understanding and controlling of domain wall motion in magnetic nanowires is extremely important for the development and production of many spintronic devices. It is well known that notches are able to pin domain walls, but their pinning potential strength are too strong and it demands high-intensity current pulses to achieve wall depinning in magnetic nanowires. However, traps of pinning can be also originated from magnetic impurities, consisting of located variations of the nanowire’s magnetic properties, such as exchange stiffness constant, saturation magnetization, anisotropy constant, damping parameter, and so on. In this work, we have performed micromagnetic simulations to investigate the depinning mechanism of a transverse domain wall (TDW) trapped at an artificial magnetic defect using spin-polarized current pulses. In order to create pinning traps, a simplified magnetic impurity model, only based on a local reduction of the exchange stiffness constant, have been considered. In order to provide a background for experimental studies, we have varied the parameter related to the pinning potential strength of the magnetic impurity. By adjusting the pinning potential of magnetic impurities and choosing simultaneously a suitable current pulse, we have found that it is possible to obtain domain wall depinning by applying low-intensity and short-duration current pulses. Furthermore, it was considered a planar magnetic nanowire containing a linear distribution of equally-spaced magnetic impurities and we have demonstrated the position control of a single TDW by applying sequential current pulses; that means the wall movement from an impurity to another.

Analysis of the Effect of Different Initial States and Structural Defects on the Characteristics of the Nonequilibrium Critical Behavior of the 3D Ising Model

The effect of different initial values m0 of magnetization and structural defects on the nonequilibrium critical behavior of the 3D Ising model have been analyzed numerically using the Monte Carlo method. Analysis of the two-time dependences of the autocorrelation function and dynamic susceptibility has revealed a substantial influence of the initial states on the aging effects that are characterized by anomalous retardation of relaxation and correlation in the system upon an increase in the waiting time. We have studied the violations of the fluctuation–dissipation theorem and calculated the limiting fluctuation–dissipation ratio. It is shown that in the nonequilibrium critical behavior of the 3D Ising model, two universality subclasses corresponding to the evolution of the system from the high-temperature (with m0 = 0) and low-temperature (with m0 = 1) initial states with the values of the limiting fluctuation–dissipation ratio typical of these states can be singled out.

Giant magneto-optical Kerr rotation, quality factor and figure of merit in cobalt-ferrite magnetic nanoparticles doped in silica matrix as the only defect layer embedded in magnetophotonic crystals

In this work, we report on the theoretical study of one-dimensional magnetophotonic crystals (MPC) comprising of periodic dielectric structure Si/SiO and of silica matrix doped with cobalt-ferrite (CoFe2O4) magnetic nanoparticles as the only magnetic defect layer. Such structure can be prepared by sol-gel dip coating method that controls the thickness of each layer with nanometer level, hence, can overcome the problem of integration of the magneto-optical (MO) devices. We have studied the influence of the volume fraction (concentration of magnetic nanoparticles VF%) on the optical (reflectance, transmittance and absorption) and MO (Kerr rotation) responses in reflection-type one-dimensional MPCs. During investigation of the influence of magnetic nanoparticle’s concentration, we found that giant Kerr rotations (even ≈135° for VF = 39%) can be obtained accompanied by large reflectance and low amounts for transmittance and absorption. We report on the demonstration of large MO quality factor and figure of merit in cobalt-ferrite magnetic nanoparticles in the infrared regime. Given the large Kerr rotation, high reflectance accompanied by low absorption and nearly zero transmittance of the 1D MPC containing cobalt-ferrite magnetic nanoparticles, large MO Q factor and figure of merit are obtained.

Effective magnetic pinning schemes for enhanced superconducting property in high temperature superconductor YBa2Cu3O7−x: a review

Enhanced superconducting properties under magnetic field in high temperature superconductors are critical for their technological applications and can be enhanced by both defect pinning and magnetic pinning. Different from defect pinning introduced by nonmagnetic pinning centers, magnetic pinning has some advantages over defect pinning, as it pins the magnetic flux rather than the normal core vortices. Various magnetic materials and different designed architectures have been demonstrated to provide magnetic pinning effect. Four major pinning schemes including metal/YBCO, oxide/YBCO, nanocomposite/YBCO and nanoparticle embedded YBCO have been reviewed. Representative literatures for each magnetic pinning scheme are discussed in detail to explore the pinning enhancement for each scheme. In addition, combined magnetic pinning and defect pinning schemes are proposed to further improve superconducting properties.

Effect of Holmium substitution on the magnetic and magnetodielectric properties of multiferroic Bi2Fe4O9

In the quest for deriving new multiferroics from the existing ones, we have prepared and studied polycrystalline Bi2(1-x)Ho2xFe4O9 (0 ≤ x ≤ 0.02) ceramics. A substantial increase in the Néel temperature (TN) from 250 K (x = 0, BFO) to 266 K (x = 0.02, BHFO2) is observed for Ho-substituted samples. The magnetization measurements suggest that Ho3+ goes as isolated magnetic defects and interaction among them is reflected only when the temperature goes below 70 K. Interestingly, the Néel temperature is not clearly visible in the magnetization-temperature plot due to its masking by the high moment of isolated Ho3+ ions, but TN is very clearly reflected in the dielectric plot, thus indicating a plausible coupling between the magnetic and electric order parameters. Also, a dielectric crossover at T ∼ 200 K is observed for Ho3+ substituted samples and explained using a mean-field approximation model, thereby validating the presence of isolated defects arising due to Ho3+ substitution. At the same time, an enhanced magnetodielectric (MD) effect at 200 K i.e., ∼−1.6% (∼530 times) for BHFO2 is discerned as compared to BFO. Furthermore, confirmation to this coupling is drawn from MD% versus T plot and MD% versus H plot, where the latter is found to obey ∝(H)m behaviour.

Single vacancy defect in graphene: Insights into its magnetic properties from theoretical modeling

Magnetic properties of a single vacancy in graphene is a relevant and still unsolved problem. The experimental results point to a clearly detectable magnetic defect state at the Fermi energy, while several calculations based on density functional theory (DFT) yield widely varying results for the magnetic moment, in the range of $\mu=1.04-2.0$ $\mu_{B}$. We present a multi-tool \textit{ab initio} theoretical study of the same defect, using two simulation protocols for a defect in a crystal (cluster and periodic boundary conditions) and different DFT functionals - bare and hybrid DFT, mixing a fraction of exact Hartree-Fock exchange (XC). Our main conclusions are two-fold: First, we find that due to the $\pi$-character of the Fermi-energy states of graphene, inclusion of XC is crucial and for a single isolated vacancy we can predict an integer magnetic moment $\mu=2\mu_{B}$. Second, we find that due to the specific symmetry of the graphene lattice, periodic arrays of single vacancies may provide interesting diffuse spin-spin interactions.

Anomalous Hall Effect and Topological Defects in Antiferromagnetic Weyl Semimetals: Mn_{3}Sn/Ge.

We theoretically study the interplay between bulk Weyl electrons and magnetic topological defects, including magnetic domains, domain walls, and Z_{6} vortex lines, in the antiferromagnetic Weyl semimetals Mn_{3}Sn and Mn_{3}Ge with negative vector chirality. We argue that these materials possess a hierarchy of energy scales, which allows a description of the spin structure and spin dynamics using an XY model with Z_{6} anisotropy. We propose a dynamical equation of motion for the XY order parameter, which implies the presence of Z_{6} vortex lines, the double-domain pattern in the presence of magnetic fields, and the ability to control domains with current. We also introduce a minimal electronic model that allows efficient calculation of the electronic structure in the antiferromagnetic configuration, unveiling Fermi arcs at domain walls, and sharp quasibound states at Z_{6} vortices. Moreover, we have shown how these materials may allow electronic-based imaging of antiferromagnetic microstructure, and propose a possible device based on the domain-dependent anomalous Hall effect.

Micromagnetism in a planar system with a random magnetic anisotropy and two-dimensional magnetic correlations

The hysteresis loops and the micromagnetic structure of a ferromagnetic nanolayer with a randomly oriented local easy magnetization axis and two-dimensional magnetization correlations are studied using a micromagnetic simulation. The properties and the micromagnetic structure of the nanolayer are determined by the competition between the anisotropy and exchange energies and by the dipole–dipole interaction energy. The magnetic microstructure can be described as an ensemble of stochastic magnetic domains and topological magnetization defects. Dipole–dipole interaction suppresses the formation of topological magnetization defects. The topological defects in the magnetic microstructure can cause a sharper change in the coercive force with the crystallite size than that predicted by the random magnetic anisotropy model.

Highly-sensitive magneto-optical sensors based on thin transmission-type one-dimensional magnetophotonic crystals for the visualization of magnetic fields

We have performed a theoretical study on the case of thin magneto-optical (MO) sensors based on transmission-type one-dimensional magnetophotonic crystals (MPCs). We could achieve thin MPC structures having only two or three magnetic defect layers that lead to large Faraday rotations up to 30° at the wavelength 560 nm in the visible region. Since the sensitivity of a MO sensor depends on the thickness of the sensing film, these very thin MPCs can act as highly sensitive MO magnetic field sensors. We could achieve MPC-based sensors with sensitivities up to 0.057 °/G.

Polarizability of electrically induced magnetic vortex plasma

Electric field control of magnetic structures, particularly topological defects in magnetoelectric materials, draws a great attention in recent years, which has led to experimental success in creation and manipulation by electric field of single magnetic defects, such as domain walls and skyrmions. In this work we explore a scenario of electric field creation of another type of topological defects -- magnetic vortices and antivortices, which are characteristic for materials with easy plane (XY) symmetry. Each magnetic (anti)vortex in magnetoelectric materials (such as type-II multiferroics) possesses a quantized magnetic and an electric charge, where the former is responsible for interaction between vortices and the latter couples the vortices to electric field. This property of magnetic vortices opens a peculiar possibility of creation of magnetic vortex plasma by non-uniform electric fields. We show that the electric field, created by a cantilever tip, produces a "magnetic atom" with a localized spatially ordered spot of vortices ("nucleus" of the atom) surrounded by antivortices ("electronic shells"). We analytically find the vortex density distribution profile and temperature dependence of polarizability of this structure and confirm it numerically. We show that electric polarizability of the "magnetic atom" depends on temperature as $\alpha \sim 1/T^{1-\eta}$ ($\eta>0$), which is consistent with Euclidean random matrix theory prediction.

Adsorption Study of a Water Molecule on Vacancy-Defected Nonpolar CdS Surfaces.

A detailed understanding of the water–semiconductor interface is of major importance for elucidating the molecular interactions at the photocatalyst’s surface. Here, we studied the effect of vacancy defects on the adsorption of a water molecule on the (101̅0) and (112̅0) CdS surfaces, using spin-polarized density functional theory. We observed that the local spin polarization did not persist for most of the cationic vacancies on the surfaces, unlike in bulk, owing to surface reconstructions caused by displaced S atoms. This result suggests that cationic vacancies on these surfaces may not be the leading cause of the experimentally observed magnetism in CdS nanostructures. The surface vacancies are predominantly nonmagnetic except for one case, where a magnetic cationic vacancy is relatively stable due to constraints posed by the (101̅0) surface geometry. At this particular magnetic defect site, we found a very strong interaction with the H2O molecule leading to a case of chemisorption, where the local spin polarization vanishes concurrently. At the same defect site, adsorption of an O2 molecule was also simulated, and the results were found to be consistent with experimental electron paramagnetic resonance findings for powdered CdS. The anion vacancies on these surfaces were always found to be nonmagnetic and did not affect the water adsorption at these surfaces.

Analysis of Relationships Between Three-Dimensional Magnetic Memory Signals and Artificial Defects Under Tensile Load

Abstract This paper mainly discusses the relationships between three-dimensional magnetic memory signals and artificial defects under different load levels. The correlations between the defect signal characteristic parameters and applied stress were analyzed. The magnetic signals at the surface of FV520B steels were measured by a 3D magnetic sensor. It has been concluded that three-dimensional magnetic signals had different detection sensitivity to the defects in different directions, and the defects can be accurately recognized based on the distinctive signal features. Additionally, the peak value of the tangential component and the peak-to-peak value of the normal component (i.e., Bxp and Bzp-p) presented different characteristics in elastic and plastic deformation stages with the increase of applied stress. The amplitude changes of the tangential component and maximum gradient value of the normal component (i.e., ΔBx and KG) were capable of estimating the mechanical damage degree of ferromagnetic materials.

Comprehensive Analysis of Magnet Defect Fault Monitoring Through Leakage Flux

This paper presents a detailed magnet defect fault detection analysis through fluxgate sensors by monitoring the leakage flux around permanent magnet synchronous motors. The flux spectra of electric machines contain direct and most critical information to monitor and characterize magnet defect faults and their progressions. In the mainstream diagnosis techniques based on phase current and back-EMF analysis, the fault corresponding signature characteristics may vary and cause misleading results depending on motor topology, winding configuration, number and location of defective magnets, and controller parameters. In this paper, it is shown that leakage flux analysis provides some superior results for magnet defect diagnosis. For this purpose, the fault patterns in the leakage flux spectrum are exhaustively analyzed at different torque/speed profiles. Simulation and experimental results show that the deployment of a direction sensitive fluxgate sensor in magnet defect fault detection yields very promising results both in time and frequency domain analyses.

Experimental and first-principle studies of ferromagnetism in Na-doped SnO2 nanoparticles

Sn1−xNaxO2 nanoparticles (about 5 nm) have been successfully synthesized by a template-free hydrothermal growth method. XRD and XPS analyses reveal that the incorporated Na atoms substitute for Sn atoms in SnO2 lattice. The absorption spectra data show the band-gap decreases at low doping concentration (x ≤ 4%). With further doping, a transformation of Na atoms from substitutional sites to interstitial sites occurs. Magnetic measurements at room temperature showed a weak ferromagnetic behavior characterized by an open hysteresis loop. Their saturation magnetization MS increases initially with increasing Na concentrations and the largest saturation magnetization of 1.1 memu/g appears in Sn0.96Na0.04O2; however for x > 0.04, MS decreases. With the combination of defect analysis based on experimental results and first-principle calculations of the possible magnetic defect centers in Na-doped SnO2, the effect of defects on the nature and origin of ferromagnetism was investigated. The results suggest in Na-doped SnO2 nanoparticles the holes created by Na substituted incorporation may be the origin of the ferromagnetism in Na-doped SnO2 nanoparticles.