Ferromagnetic Surface Research Articles

Novel austenitic steel ageing classification method using eddy current testing and a support vector machine

This paper describes the development of an original eddy current method for the characterization and classification of different aging states of heat resistant austenitic steel tubes, commonly used in petrochemical industry to produce oil derivatives. These tubes are exposed to high temperatures causing microstructural transformations. They are also under oxidizing environments leading on the formation of an external surface with ferromagnetic behavior. An eddy current testing (with a Hall sensor) was used in order to observe magnetic changes in the specimen. The amplitude and phase-shift of the eddy current signals are calculated and used as features for the samples characterization. An electromagnet was implemented in order to overpass the ferromagnetic external surface and measure the base metal response. A finite element simulation was also developed in order to estimate the skin depth of the eddy currents in samples with different aging states. A machine learning algorithm has been used to classify the test specimen based on the extracted features. Results suggest that the proposed method is a potential non-destructive technique for the characterization and classification of heat-resistant austenitic steel tubes with different aging states.

Separation of enantiomers by their enantiospecific interaction with achiral magnetic substrates.

It is commonly assumed that recognition and discrimination of chirality, both in nature and in artificial systems, depend solely on spatial effects. However, recent studies have suggested that charge redistribution in chiral molecules manifests an enantiospecific preference in electron spin orientation. We therefore reasoned that the induced spin polarization may affect enantiorecognition through exchange interactions. Here we show experimentally that the interaction of chiral molecules with a perpendicularly magnetized substrate is enantiospecific. Thus, one enantiomer adsorbs preferentially when the magnetic dipole is pointing up, whereas the other adsorbs faster for the opposite alignment of the magnetization. The interaction is not controlled by the magnetic field per se, but rather by the electron spin orientations, and opens prospects for a distinct approach to enantiomeric separations.

Vanadium poisoning of FCC catalysts: A quantitative analysis of impregnated and real equilibrium catalysts

Amongst the contaminant transition metals Fe, Ni, V that deposits onto the fluid catalytic cracking (FCC) catalyst and promote unwanted secondary reactions, vanadium is the most deleterious one because not only acts as undesired dehydrogenation site but also migrates deeper inside the catalyst particles under the FCC regeneration conditions, stabilizing in different phases that attack the crystalline structure of the catalyst, thus resulting in its irreversible loss of activity. The mechanisms through which vanadium affect the catalyst during its use in the FCC process remain under debate and the accurate determination of the chemical species involved and of their concentration is of paramount importance for the investigations in this field. In this work, a quantification methodology for contaminant vanadium in FCC catalysts was developed based on X-ray fluorescence (XRF) and electron paramagnetic resonance (EPR). A commercial FCC catalyst was artificially contaminated with vanadium at different loads by pore volume impregnation with aqueous vanadyl sulfate solution, in the presence or not of a complexing agent, followed by steam deactivation at 788 °C/5 h, and the samples were compared to spent FCC catalysts from different Brazilian refineries. It has been shown that EPR is a very sensitive and reliable technique to quantify vanadyl ions (V4+) in FCC catalysts while lower valence states such as V3+ have not been detected in any of the samples studied. Vanadyl ions (VO2+) deposited onto the catalyst are mostly oxidized to higher valence state species and the extent of this oxidation in equilibrium catalysts from FCC units was calculated and related to the combustion mode at which the FCC regenerators operate. The artificial vanadium contamination and deactivation protocol adopted in this work resulted in a proportion of V4+/Vtotal of nearly 10%, similar to that of equilibrium catalysts sampled from FCC units with full combustion regeneration. The present study brings a new interpretation for the EPR signal detected as a broad unstructured line, with evidences of some vanadium existing as a ferri-/ferromagnetic surface phase in the catalyst particles, likely formed by induced oxygen vacancies during reaction (reducing atmosphere) and quite resistant to the regeneration step.

An initial new approach for magnetorheological finishing of ferromagnetic internal cylindrical surfaces

The requirement of surface characteristics on the internal cylindrical ferromagnetic surface without any defects is an important consideration in different manufacturing components like injection barrels, hydraulic cylinders, and bearings. These components are being finished by conventional finishing processes because of their easy availability and lesser manufacturing cost. But the main problem arises during finishing with this solid abrasive tool is the uncontrollable cutting forces during actual finishing cycle. These uncontrollable cutting forces provide various surface defects which result in the reduction in operating life. The formation of high-quality surface finish without any defects is still an important challenged for today’s industry. To counteract these types of circumstances, a new finishing process is developed based on magnetorheological polishing fluid. In the present tool design, the position of magnetic core has been designed to move inside the cylindrical workpiece to finish its internal surface. The inside position of the electromagnetic tool causes maximum magnetic flux gradient at the outer tool surface than the internal cylindrical surface (made of ferromagnetic materials). Thus, the polishing fluid is retained and stiffened magnetically over the surface of the tool only during finishing of the internal surface of the ferromagnetic as well as non-ferromagnetic cylindrical workpieces. From the preliminary experimental results, the surface roughness Ra, Rq, and Rz values are reduced to 60.25, 58.30, and 53.98%, respectively, after 120 min of finishing. This clearly demonstrated that the developed process is capable to improve the surface characteristics of the internal ferromagnetic cylindrical surface by reducing the surface roughness values and removing the various surface defects after the grinding operation.

MODELING OF CLAMPING MAGNETS INTERACTION WITH FERROMAGNETIC SURFACE FOR WHEEL MOBILE ROBOTS

The mathematical models of clamping permanent and electro- magnets for clamping force determination by means of field theory (method of secondary sources and features of integrated forces calculation) are offered for magnetically operated movers of mobile robots (MR) for moving on inclined or vertical ferromagnetic surfaces. The results of experimental investigations of magnetic induction and clamping force measurements in different spatial positions of clamping magnet relative to the ferromagnetic surface are given. The developed simulation models based on field theory and finite element method allow to validate the obtained functional dependencies with respect to gaps between magnet and working surface, caused by excrescences of shells, and ferromagnetic surface thickness changes due to its technological features, as well as help at the stage of MR’s design. Authors propose control and measuring system of clamping force that can be successfully used for new and existing MRs’ constructions for their reliable operating on complex surfaces taking into account the nature of their uncertainties (ferromagnetic or non-ferromagnetic).

MODELS OF ROBOT’S WHEEL-MOVER BEHAVIOR ON FERROMAGNETIC SURFACES

The mobile robots which can move on complicated working surfaces play a significant role in the automation of various technological processes, in particular, ship repair, fire fighting, inspection of welding quality, rescue operations, etc. This work is a continuation of the authors’ investigation of the mobile robot’s moving on inclined and vertical ferromagnetic surfaces based on a magnetically operated wheel-mover. Special attention is paid to constructing magnetically operated wheel-mover with twelve legs and modeling of the robot’s wheel-mover behavior in different working modes including investigations of the wheel-mover center trajectory, behavior of control signals, etc. Geometrical dependences between a number of wheel-mover legs and deviation of the wheel center path from horizontal line are described. In the present article the modeling results for movement of the wheel-mover on both plain and non-plain surfaces are discussed. For this purpose, the mathematical model of the wheel-mover was created and analyzed and the results were verified using a simulation approach.

Spin- and Angle-Resolved Photoemission Study of the Alq3/Co Interface

Controlling the interaction between organic semiconductors and ferromagnetic surfaces is one of the key issues for designing metal–organic hybrid interfaces for spintronic applications. The strong chemical interaction across such hybrid interfaces results in the formation of new spin-polarized hybrid interface states which determine all device-relevant properties. Here, we revisit the hybrid interface formed between the prototypical molecule Alq3 and the Co surface using spin- and angle-resolved photoemission. We reveal a significant change of the spectroscopic lineshape of the cobalt 3d bands by the adsorption of Alq3. The hole-like minority and the electron-like majority bands of the bare Co surface are replaced by an energetically very broad band with neglectable band dispersion along the Γ–X direction. Moreover, the magnitude and shape of the spin polarization of the Alq3/Co valence band structure are also significantly modified by the adsorption of Alq3 and become completely independent of the mome...

Spontaneous growth of 2D coordination polymers on functionalized ferromagnetic surfaces.

The spontaneous growth of lamellar metal-alkanethiolates (LMAs) on reactive ferromagnetic surfaces as a result of surface oxidation has been observed. When alkanethiol self-assembled monolayers (SAMs) grown under an inert atmosphere over cobalt or permalloy (Ni80Fe20) are exposed to air, oxygen diffuses through the molecular layer. This induces an oxidation of metal atoms at the metal surface and a release of the resulting metal cations that migrate coordinated by the alkanethiol molecules to form lamellar structures over the SAMs. This process has been imaged in real-time, under ambient conditions, by means of different microscopy techniques. The influence of the alkyl chain length, the nature of the ferromagnet, the temperature and the atmospheric moisture on the number, area and height of the resulting features has been systematically evaluated. Remarkably, the possibility to follow the migration in real-time makes it a promising model system for the study of surface/molecule interface processes. Most importantly, the composition and crystallinity of these LMAs have been studied, evidencing that real 2D coordination polymers are formed on the surface. Hence, one could envision this strategy as a new method for the assembly of more complex low-dimensional (2D) magnetic materials based on coordination polymers.

Controlling Oxygen-Based Electrochemical Reactions through Spin Orientation

The role of spin orientation on the reactivity of oxygen reduction reaction (ORR) intermediates (O, OH) on a ferromagnetic electrode surface is studied using constrained density functional theory formalism. We show that the strength of the binding of these reaction intermediates depend on their relative spin orientations with respect to the magnetization of the electrode. This suggests that oxygen-based electrochemical reactions on ferromagnetic catalyst surfaces can be controlled through the applied magnetic field. In the present study, we demonstrate such a possibility through the study of an oxygen reduction reaction on a PdFe (001) surface by introducing a new concept: spin orientation dependent overpotential. Also, we have explained the origin of lower dissociation barrier for the O2 molecule on ferromagnetic surfaces when its spin moment is antiparallel to the surface magnetization as reported in the recent experiments.

The structure and magnetic properties of pure single phase BiFeO3 nanoparticles by microwave-assisted sol-gel method

The pure single phase BiFeO3 nanoparticles have been prepared by microwave-assisted sol-gel process and its structure and magnetic properties have been studied. X-ray diffractometer, infrared and transmission electron microscope results show that the synthesized BiFeO3 nanoparticles at the calcining temperature of 450 °C exhibits a rhombohedrally distorted perovskite structure without secondary phase and the size is about 40 nm. Meanwhile, the pure single phase BiFeO3 particles exhibit typical ferromagnetic properties at room temperature and the obvious large exchange bias phenomenon at 60 K. A exchange bias field (HE) of 302 Oe at 60 K for pure single phase BiFeO3 nanoparticles at calcining temperature 450 °C after field cooling at 20 KOe has been observed. The MT curve at H = 100 Oe shows that the FC and ZFC magnetization curves start to differ in the temperature range from 60 K to 300 K revealing spin glass behavior of BiFeO3 nanoparticles at calcining temperature 450 °C, 500 °C and 550 °C. The ferromagnetic behavior is attributed to size of effects, which partially destroy the long-wavelength cycloid spin structure expected in bulk BiFeO3. The exchange bias effect in single crystalline BiFeO3 nanoparticles arises from co-existence of different magnetic phases of an antiferromagnetic core and a ferromagnetic surface. The ferromagnetic, exchange bias effect and spin glass behavior of BiFeO3 nanoparticles are assigned to the uncompensated or canted spins at the ferromagnetic surface and an antiferromagnetic core of BiFeO3 nanoparticles.

MgO monolayer epitaxy on Ni (100)

The growth of two-dimensional oxide films with accurate control of their structural and electronic properties is considered challenging for engineering nanotechnological applications. We address here the particular case of MgO ultrathin films grown on Ni (100), a system for which neither crystallization nor extended surface ordering has been established previously in the monolayer range. Using Scanning Tunneling Microscopy and Auger Electron Spectroscopy, we report on experiments showing MgO monolayer (ML) epitaxy on a ferromagnetic nickel surface, down to the limit of atomic thickness. Alternate steps of Mg ML deposition, O2 gas exposure, and ultrahigh vacuum thermal treatment enable the production of a textured film of ordered MgO nano-domains. This study could open interesting prospects for controlled epitaxy of ultrathin oxide films with a high magneto-resistance ratio on ferromagnetic substrates, enabling improvement in high-efficiency spintronics and magnetic tunnel junction devices.

The magnetic properties and hysteresis behaviors of the mixed spin-(1/2,1) Ferrimagnetic nanowire

In this work, the mixed spin Ising nanowire model consisting of a spin-1 ferromagnetic core, which is surrounded by a spin-1/2 ferromagnetic surface shell is studied in the presence of magnetic and crystal fields using of the Monte Carlo (MC) Simulations based on the heat bath algorithm. We assume that the exchange interaction between two nearest-neighbor spins at the surface shell and the core is antiferromagnetic. We have examined the effects of the surface and the crystal field on the critical and compensation temperatures. The thermodynamic properties, the hysteresis behaviors are also studied. For appropriate values of the system parameters, the compensation point and multi-loops are found.

Hysteresis loop behaviors of a decorated double-walled cubic nanotube

The effect of surface shell parameters on the hysteresis loop behaviors of a decorated Ising cubic nanotube, consisting of a ferromagnetic spin-12 core which is interacting ferrimagnetically with a ferromagnetic spin-1 surface shell, is investigated, in the present work, within the effective-field theory with correlations based on the probability distribution technique. We have found that these parameters have a strong effect on the shape and the number of hysteresis loops and also on the coercive field and remanent magnetization behaviors. Indeed, triple, quintuple, septuple and nonuple hysteresis loop patterns have also been observed.

Atomic Mechanism of Hybridization-Dependent Surface Reconstruction with Tailored Functionality in Hexagonal Multiferroics.

The broken symmetry along with anomalous defect structures and charging conditions at multiferroics surface can alter both crystal structures and electronic configurations, bringing in emergent physical properties. Extraordinary surface states are induced into original mutually coupled order parameters in such strongly correlated oxides, which flourish in diverse properties but remain less explored. Here, we report the peculiar surface ferroelectric states and reconfigurable functionalities driven by the relaxation of surface and consequent changes in O 2p and Y 4d orbital (p-d) hybridization within a representative hexagonal multiferroics, YMnO3. An unprecedented surface reconstruction is achieved by tailored p-d hybridization coupling with in-plane oxygen vacancies, which is atomically revealed on the basis of the advantages of state-of-the-art aberration-corrected (scanning) transmission electron microscopy. Further ab initio density functional theory calculations verify the key roles of in-plane oxygen vacancies in modulating polarization properties and electronic structure, which should be regarded as the atomic multiferroic element. This surface configuration is found to induce tunable functionalities, such as surface ferromagnetism and conductivity. Meanwhile, the controversial origin of improper ferroelectricity that is unexpectedly free from critical size has also been atomically unraveled. Our findings provide new insights into the design and implementation of surface chemistry devices by simply controlling the oxygen stoichiometry, greatly advance our understandings of surface science in strongly correlated oxides, and enable exciting innovations and new technological functionality paradigms.

The thermal behaviors and phase diagrams of the Ising-type endohedral fullerene with magnetic core and diluted magnetic shell (Core@Shell20)

We have carried out theoretical studies on Ising-type endohedral fullerene (EF) structure with a dopant magnetic atom encaged within the diluted magnetic spherical cage to examine the evolution in magnetic behaviors. We show how the thermal behaviors and phase diagrams of Ising-type EF are affected by diluted surface, crystal field and exchange couplings. We have used to investigate theoretically the effect of Hamiltonian parameters the effective field calculations within Ising model framework. The model Hamiltonian includes nearest neighbor ferromagnetic and antiferromagnetic center-surface (C-S) interaction as well as ferromagnetic surface interaction. We have shown that the system exhibits the first and second order phase transitions as well as tricritical point. In particular, the conditions for the occurrence of these reentrant and double reentrant behaviors are given explicitly.

Vacuum Control in the Surface Attaching Devices of the Mobile Robots during their Movement under the Aerodynamic Attachment Force

During the movement of the horizon mobile robots over different complex ferromagnetic and non-ferromagnetic surfaces, like walls, ceilings or slopes of various angles, their contacts with the surfaces can be realized by the attaching devices, which ensure an aerodynamic attachment force due to creation of vacuum. In industrial robotics such devices are designed for grasping objects with manipulators on the assembly lines. In the mobile robotics such devices are suitable for creation of an attachment force for the robots. The control system provides an aerodynamic attachment force due to control of the pressure level inside the vacuum area. This ensures adaptation for the mobile robots moving over the surfaces with different quality parameters. This paper describes creation of vacuum with a centrifugal pump and a turbine. It also presents an attachment device based on a centrifugal pump or a turbine, which can create an aerodynamic attachment force for movement of a mobile robot over the sloped surfaces. The air pressure inside the turbine depends on the shape of the blades, the air density and the circumferential speed of the blades. The air pressure created by the centrifugal pump or the turbine is proportional to the blades' rotary speed squared. The dependence of the air pressure on the air flow is obtained. This ensures calculation of the attachment force for the control system of a mobile robot. Simultaneous solving of the equations presented in this paper for the air pressure and the air flow provides an opportunity to obtain the operating modes for different types of the vacuum devices based on a centrifugal pump or a turbine. A possibility of the aerodynamic attachment force control by varying of the gap between the mobile robot and the surface is analyzed. Various parameters of air, such as density, average velocity and flow rate, are discussed. Creation of vacuum inside the vacuum area is considered. The experimental results concerning the pressure control, the attachment force and vacuum creation are presented.

Development of magnetorheological fluid-based process for finishing of ferromagnetic cylindrical workpiece

ABSTRACTA magnetorheological fluid-based process is developed for the internal surface finishing of ferromagnetic cylindrical workpiece. The existing finishing processes based on magnetorheological fluid are not equipped to finish the internal ferromagnetic cylindrical surface significantly as it obtained higher magnetic flux density than the MR polishing fluid. At present, magnetorheological fluid-based finishing tools are designed to ensure the maximum magnetic flux density always present on the outer finishing tool core surface as compared to internal surface of ferromagnetic cylindrical workpiece surface. To validate this present principal idea, the magnetostatic finite element analysis has been performed on the newly designed finishing tools. The preliminary experiments have also been conducted to evaluate the finishing performance with the two newly designed finishing tools. The percentage reduction in surface roughness (Ra) values with I-shaped tool core is found as 65–78% after 150 min of finishing, whereas, with rectangular shaped tool core is found as 78–81% after 90 min of finishing. The results clearly revealed that the present finishing tool with rectangular shaped core is more suitable for uniform significant finishing of ferromagnetic cylindrical internal workpiece than the I-shaped core. The developed process can be useful in finishing of cylindrical mold and dies, hydraulic cylinder, barrel for injection molding, etc.

Hysteresis in magnetoresistance and formation of spin glass like structure in PVA capped Fe3O4

Here, we report the temperature dependent hysteresis in the magneto transport in PVA capped Fe3O4 nanoparticles. The synthesis of the nanoparticles of size 10–15 nm were carried out via co-precipitation method followed by heat treatment at different temperatures. Structural studies confirm the formation of polycrystalline Fd $$\bar 3{m}$$ phase of Fe3O4 when the samples were sintered up to 600 °C. FTIR studies show that PVA and magnetic grains were attached through hydrogen bonding between hydroxyl group of PVA and protonated surface of the oxide. Spin glass type transition was observed in the ZFC–FC plots at 125 K which is also the Verwey transition temperature TV as measured from the transport studies. A hysteresis in the MR plot was found which was explained in the realm of tunnel transport through ferromagnetic grain and antiferromagnetic surface of the ferrite particles. Tunnelling through the dielectric spacer layer of PVA has an overall impact to increase the resistance of the samples. The temperature dependence of the hysteresis gives a major hint to the origin of the spin glass type behaviour of the samples.

An ab initio study of the adsorption of Eu3+, Pu3+, Am3+, and Cm3+ hydroxide complexes on hematite (001) surface: Role of magnetism on adsorption

Inner-sphere hydroxide complexes of Eu3+, Pu3+, Am3+, and Cm3+ adsorbed to antiferromagnetic and ferromagnetic hematite (001) surfaces were modeled by using DFT (GGA+U) calculations in order to compare the electronic properties, atomic structures, and adsorption energies between the trivalent actinide analog Eu3+ and the corresponding actinides An3+. Eu3+ forms a tridentate–binuclear surface complex on the hematite surface with an Eu–Fe distance of 3.4–3.8Å. Eu3+ is partially reduced upon adsorption, as indicated by the increase of original Eu3+ spin value from 6 to ∼7. Eu3+ adsorption on the ferromagnetic surface is energetically more favorable than on the antiferromagnetic surface, while the opposite is the case for all actinides An3+. When retaining a similar adsorption configuration (e.g., coordination number=6), An3+ cations are adsorbed ∼0.2Å closer to the surface compared to Eu3+. Partial density of states analyses indicate ionic bonding between O ions on the hematite surface and the Eu3+/An3+ adsorbates. An increasing number of bands above the Fermi energy were found when Eu3+ and An3+ were adsorbed to the ferromagnetic surface but not on the antiferromagnetic surface, indicating the ferromagnetic substrate becomes more semiconducting upon cation adsorption. These results show that the spin configurations of the hematite (001) surface have a weak but distinct influence on the cation adsorption energy. Although the antiferromagnetic spin configuration of hematite is dominant in nature, metastable ferromagnetic domains on nanohematite grains may influence the energetics of certain cation adsorption reactions.

Very efficient spin polarization analysis (VESPA): new exchange scattering-based setup for spin-resolved ARPES at APE-NFFA beamline at Elettra.

Complete photoemission experiments, enabling measurement of the full quantum set of the photoelectron final state, are in high demand for studying materials and nanostructures whose properties are determined by strong electron and spin correlations. Here the implementation of the new spin polarimeter VESPA (Very Efficient Spin Polarization Analysis) at the APE-NFFA beamline at Elettra is reported, which is based on the exchange coupling between the photoelectron spin and a ferromagnetic surface in a reflectometry setup. The system was designed to be integrated with a dedicated Scienta-Omicron DA30 electron energy analyzer allowing for two simultaneous reflectometry measurements, along perpendicular axes, that, after magnetization switching of the two targets, allow the three-dimensional vectorial reconstruction of the spin polarization to be performed while operating the DA30 in high-resolution mode. VESPA represents the very first installation for spin-resolved ARPES (SPARPES) at the Elettra synchrotron in Trieste, and is being heavily exploited by SPARPES users since autumn 2015.