Quantification of the Interaction Field in Arrays of Magnetic Nanowires from the Remanence Curves

A method is presented that allows quantifying the average value of the interaction field in arrays of magnetic nanowires from the field difference between the isothermal remanence (IRM) and the DC demagnetizing (DCD) remanence curves when the normalized magnetization is equal to one third. Arrays of magnetic nanowires of different diameters and packing fractions are used to experimentally test the method. The results have been compared with those obtained using the method based on the difference between the remanence coercivity fields and with a mean-field expression for the interaction field, providing a very good agreement and thus validating the method. Additionally, it is shown that both the position (m₀) and the shift along the magnetization axis of the intersection between the remanence curves with respect to the value of one third (δm = m₀ ??1/3) provide qualitative information about the interaction field. The former indicates the type of interaction depending if the intersection is above (m₀ >?1/3) or below (m₀ <?1/3), which corresponds to a ferro or anti-ferro magnetic interaction, respectively. While for the latter, it is shown that the maximum deviation of the Delta-M plot from zero (ΔM_max) corresponds to three times the shift (ΔM_max =?3δm).     

Tunable,Flexible Composite Magnets for Marine Monitoring Applications

This paper presents flexible NdFeB‐PDMS composite magnets with tunable magnetic and mechanical properties optimized for applications in corrosive marine environments. The magnetic and mechanical properties are studied for different NdFeB powder concentrations and the performance of the magnetic composites for different exposure times to sea water investigated systematically. The remanence and saturation magnetizations could be tailored by the powder concentration, whereby up to 70 wt% concentration could be employed without compromising the integrity of the magnets. The elastic modulus of the composite magnets is about 10⁵ times lower than the one of a bulk permanent magnet. This ensures a high bending flexibility, which allows the magnets to be attached to curved surfaces as illustrated for a giant clam, crab, and turtle. At the same time, the weight of the composite magnets is reduced by a factor of about 10, which poses less burden to animals’ natural free movement. Without a protective layer, the composite magnets lose more than 50% of their magnetization after 51 days in seawater. However, the durability of the composite magnets has been improved considerably by using polymer coatings. Parylene C is the most effective for this, providing corrosion resistance, flexibility, and enhanced biocompatibility. Parylene C films of 2 and 4 μm thicknesses provided excellent protection of the magnetic composite in corrosive aqueous environments for 65 and 82 days, respectively. By combining the composite magnets with tunnel magnetoresistance sensors, a magnetic animal monitoring system is established that is used to track the behavior of giant clam, crab, and turtle.

     

Permanent magnetic thick films from remanence optimized NdFeB-inks

NdFeB films with thickness between 10 and 50 m are prepared by screen printing of standard inks containing 70–80 wt % of NdFeB powder (MQP-B). The coarse MQ-powder is fine-milled to give a mean particle size of about 10 m in order to obtain well-printable inks and homogeneous films. Typically, remanent inductions of 200–320 mT are obtained. The maximum loading of magnetic powder in the ink is increased to 85–90 wt % by optimizing the powder-particle packing. Mixtures of MQ-powders of different grain size were prepared in order to maximize the powders tap density and reduce the interparticle volume. Inks prepared from such powder mixtures display an increased remanence of up to 500 mT. Screen-printed NdFeB thick films are important components for miniature actuators or micro-electromechanical systems. In miniature systems the magnetic component has to provide a strong field without having a large size. Therefore, the reported enhancement of the remanence of NdFeB inks for screen printing of magnetic thick films represents dramatic progress compared to standard NdFeB inks.     

Uniaxial anisotropy by "radiomagnetic" treatment; controlling factors in a new process

A new process-an electron-"radiomagnetic" treatment-for obtaining high-remanence, low-coercive-force loops in magnetic alloys was recently announced. As an example, 2-MeV electron irradiation of 6-mil-thick ring laminations of polycrystalline 5-80 Mo Permalloy with 10¹⁷e/cm²in an applied circumferential magnetic field of 0.2 Oe atsim100degC produced record highs in remanence (∼6700 G) for this material. Additional studies of this process have been made to determine some of the controlling factors and the range of application. In particular, the effects of the dose (number of e/cm²) and of the preirradiation magnetic properties were examined. The results show that: 1) for a given dose of1.1 times 10^{17}2-MeV e/cm², the relative change in remanence (DeltaB_{r}/B_{r}) is always positive, ranging from 10 to 50 percent, but varies inversely with the preirradiation value of remanence (Br); 2) for the same dose, the relative change in coercive force (DeltaH_{c}/H_{c}) also depends upon the preirradiation value of remanence, but in a different way. ForB_{r} < 5000G,DeltaH_{c}/H_{c}is either negative or zero. ForB_{r} > 5000G,DeltaH_{c}/H_{c}is positive, ranging from 20 to 150 percent, and increases linearly withB_{r}; 3) if the dose is reduced tosim0.8 times 10^{17}e/cm², thenDeltaH_{c}/H_{c}is reduced to a tolerable level (∼10 percent) with no significant sacrifice in the positive gain in remanence and rectangularity. Hence, there are optimum dose ranges in the "radio-magnetic" treatments of alloys, where significant gains in remanence may be obtained without appreciable increases in coercive force.     

Magnetic Hysteresis Loop Technique as a NDE Tool for the Evaluation of Microstructure and Mechanical Properties of 2.25Cr–1Mo Steel

This study intends to understand the effect of short-term overheating on microstructure modifications, and variation in mechanical and magnetic properties of boiler tubes. It is based on the hypothesis that short-term overheating on boiler tubes leads to microstructural changes degrading their mechanical properties, thus resulting in their failure. As part of this study, fresh 2.25Cr–1Mo boiler tube samples were heat treated in the range of 700–950 \(^{\circ }\hbox {C}\). Magnetic hysteresis loop (MHL) measurements were carried for the non-destructive evaluation (NDE) of the mechanical properties that get altered due to microstructural modifications. For comparison, MHL was also carried out on a service-exposed boiler tube, which had failed due to short-term overheating. The magnetic parameters were correlated with the change in microstructure and micro-hardness of the samples. A decrease in coercivity, remanence, maximum induction and micro hardness were found at the lower soaking temperatures, due to: easy magnetic domain wall and dislocation motions for the stress relaxation; annihilation and recovery of dislocations; increase in inter carbide distance; and the decrease in the number density of carbides for the coarsening of carbides. A subsequent increase in coercivity, remanence and maximum inductions along with hardness are due to the nucleation and growth of fresh bainitic phase, obstructing the magnetic domain wall and dislocation motion. A drastic decrease in coercivity for the service-exposed tube is due to the transformation of MX type carbides to \(\hbox {M}_{23}\hbox {C}_{6}\) type and accumulation of such carbides at the grain boundaries along with the decrease in number density of the carbides. The presence of scale has less effect on the coercivity, but its demagnetizing effect largely decreases the remanence and maximum induction. The results of the study show that the MHL can be a suitable NDE technique for the evaluation of change in microstructure and degradation of mechanical properties in power plant steels.     

Exploring the Nickel–Graphene Nanocomposite Coatings for Superior Corrosion Resistance: Manipulating the Effect of Deposition Current Density on its Morphology,Mechanical Properties,and Erosion‐Corrosion Performance

The use of graphene‐based composite as anti‐corrosion and protective coatings for metallic materials is still a provocative topic worthy of debate. Nickel–graphene nanocomposite coatings have been successfully fabricated onto the mild steel by electrochemical co‐deposition technique. This research demonstrates the properties of nickel–graphene composite coatings influenced by different electrodeposition current densities. The effect of deposition current density on the; surface morphologies, composition, microstructures, grain sizes, mechanical, and electrochemical properties of the composite coatings are executed. The coarseness of deposited coatings increases with the increasing of deposition current density. The carbon content in the composite coatings increases first and then decreases by further increasing of current density. The improved mechanical properties and superior anti‐corrosion performance of composite coatings are obtained at the peak value of current density of 9 A dm^?2. The incorporation of graphene sheets into nickel metal matrix lead to enhance the micro hardness, surface roughness, and adhesion strength of produced composite coatings. Furthermore, the presence of graphene in composite coating exhibits the reduced grain sizes and the enhanced erosion–corrosion resistance properties.

     

Organic Functional Molecules towards Information Processing and High‐Density Information Storage

In this Progress Report, we discuss our recent achievements in design and synthesis of new functional molecules towards information processing at the molecular level and high‐density information storage. These include: 1) new molecular switches, logic gates, and combinational logic circuits based on molecules and ensembles with photochromic spiropyran units that undergo reversible structural transformation among multistates, in response to external inputs such as light, protons, and metal ions; 2) high‐density information storage, mainly focusing on nanometer‐scale electrical recording based on the conductance transition of organic molecules, and multimode data storage on multiresponsive molecules. Relevant progress and an outlook in this area are also discussed.

     

Effect of niobium on the directional solidification and properties of Alnico alloys

The effect of niobium on the extent of columnar growth of grains and on the magnetic properties in Alnico alloys has been studied. Alloys containing 1 % Nb show maximum growth of columnar crystals. In columnar alloys, coercivity increases slowly with increasing niobium content up to 1 %. With further increase in niobium, coercivity and remanence decrease. The maximum energy product (55 kJ m^–3) has been obtained at 1 % Nb. Niobium addition has also been found to suppress the precipitation of the undesirable phase.     

Preparation of magnetic coatings from strontium hexaferrite on tin and cardboard by cold rolling

A finely dispersed powder of strontium hexaferrite doped with aluminum of the composition SrFe_12?x Al _x O₁₉ with an aluminum content x = 0.6 ± 0.1 is prepared through crystallization of oxide glasses. The powder is characterized by a saturation magnetization of 60.2 A m²/kg and a coercive force of 550 kA/m. The hexaferrite particles predominantly have the shape of thick hexagonal platelets with a diameter ranging from 300 to 500 nm and a thickness-to-diameter ratio varying from 0.3 to 0.5. Magnetic coatings on tin and cardboard substrates are produced by cold rolling of strontium hexaferrite powders. It is shown that hexaferrite particles in the magnetic coatings have the preferred orientation of the well-developed facets along the rolling plane, which manifests itself in anisotropy of the magnetic properties of the coatings. The degree of texturing in the strontium hexaferrite coatings on cardboard and tin substrates is equal to 44 and 66%, respectively.     

Magnetic and mössbauer studies on ductile Fe-Cr-Co permanent magnet alloys

Iron-chromium-cobalt alloys possess attractive magnetic properties combined with good formability and hence are identified as technologically important magnetic materials. Alloys with compositions Fe-28·9 Cr-15·6 Co and Fe-28·4 Cr-20·1Co (weight percent) have been studied. Heat-treatment parameters during thermomagnetic treatment viz temperature, time and external magnetic field were optimized with reference to magnetic properties. The fully treated anisotropic alloys develop remanence=11·5–12·0 kilo Gauss, coercivity=600–650 Oersted and energy product=4–4·5 million Gauss Oersted. Electron microscopic and Mössbauer spectroscopic techniques were used to identify the original and transformed phases. During the various stages of the development of the alloy, the changes in mechanical hardness were correlated with magnetic hardness.     

Topographical characterization and microstructural interface analysis of vacuum-plasma-sprayed titanium and hydroxyapatite coatings on carbon fibre-reinforced poly(etheretherketone)

In the present study, topographical characterization and microstructural interface analysis of vacuum-plasma-sprayed titanium and hydroxyapatite (HA) coatings on carbon fibre-reinforced polyetheretherketone (CF/PEEK) was performed. VPS-Ti coatings with high roughness values (Ra=28.29±3.07 m, Rz=145.35±9.88 m) were obtained. On this titanium, intermediate layer HA coatings of various thicknesses were produced. With increasing coating thickness, roughness values of the HA coatings decreased. A high increase of profile length ratio, Lr, of the VPS-Ti coatings (Lr=1.45) compared to the grit-blasted CF/PEEK substrate (Lr=1.08) was observed. Increasing the HA coating thickness resulted in a reduction of the Lr values similar to the roughness values. Fractal analysis of the obtained roughness profiles revealed that the VPS-Ti coatings showed the highest fractal dimension of D=1.34±0.02. Fractal dimension dropped to a value of 1.23–1.25 for all HA coatings. No physical deterioration of the CF/PEEK substrate was observed, indicating that substrate drying and the used VPS process parameter led to the desired coatings on the composite material. Cross-section analysis revealed a good interlocking between the titanium intermediate layer and the PEEK substrate. It is therefore assumed that this interlocking results in suitable mechanical adhesive strength. From the results obtained in this study it is concluded that VPS is a suitable method for manufacturing HA coatings on carbon fibre-reinforced PEEK implant materials.     

Evaluation of Power Heat Losses in Multidomain Iron Particles Under the Influence of AC Magnetic Field in RF Range

The magnetic properties and hyperthermia effect were studied in a magnetorheological fluid (MRF) containing iron particles of $1 \upmu \mathrm{m}\, \text{ to}\, 5 \,\upmu \mathrm{m}$ in diameter. The measurements showed that the magnetization in the saturation state reaches a value of 171 $\text{ A}\cdot \text{ m}^{2}\cdot \mathrm{kg}^{-1}$ with very small values of coercivity and remanence. They also showed the ferromagnetic behavior in the system together with a value of the magnetic susceptibility of 1.7. Theoretical and experimental results of the calorimetric effect investigation under a changeable magnetic field of high frequency ( $f = 504$ kHz) in an MRF will be presented in the article. The sample was subjected to an alternating magnetic field of different strengths ( $H = 0$ to 4 $\text{ kA}\cdot \text{ m}^{-1})$ . It results from a theoretical analysis that the heat power density (released in the MRF sample) referenced to the eddy current is proportional to the square of frequency, the magnetic field amplitude, and the iron grain diameter. Experimental results indicate that there are some reasons for the released heat energy such as: energy losses from magnetic hysteresis and eddy currents induced in the iron grains. If the magnetic field intensity amplitude grows, the participation of losses connected with magnetic hysteresis is increased. From the calorimetric measurements, the conclusion is as follows: for a magnetic field $H<1946\,\text{ A}\cdot \mathrm{m}^{-1}$ , the eddy current processes dominate in the heat generation mechanism, whereas hysteresis processes for the total release of thermal energy dominate for higher magnetic fields. Both mechanisms take equal parts in heating the tested sample at a magnetic field intensity amplitude $H= 1946\,\text{ A}\cdot \mathrm{m}^{-1}$ . The specific absorption rate referenced to the mass unit of the MRF sample at the amplitude of the magnetic field strength 4 $\text{ kA}\cdot \mathrm{m}^{-1}$ equals 24.94 $\text{ W} \cdot \mathrm{kg}^{-1}$ at a frequency $f$ = 504 kHz.     

Self-assembled ferromagnetic cobalt/yttria-stabilized zirconia nanocomposites for ultrahigh density storage applications

We report on a low-cost, innovative approach for synthesizing prepatterned, magnetic nanostructures, the shapes and dimensions of which can be easily tuned to meet requirements for next-generation data storage technology. The magnetic nanostructures consist of self-assembled Co nanodots and nanowires embedded in yttria-stabilized zirconia (YSZ) matrices. The controllable size and aspect ratio of the nanostructures allows the selection of morphologies ranging from nanodots to nanowires. Co nanowires show strong shape anisotropy and large remanence at 300 K. In contrast, Co nanodots display minimal effects of magnetocrystalline anisotropy and superparamagnetic relaxation above the blocking temperature. These prepatterned magnetic nanostructures are very promising candidates for data storage technology with an ultrahigh density of 1 terabit in(-2) or higher.     

Development and Characterization of Zr‐Based Multi‐Component Nanocomposite Coatings Prepared Using Single Alloying Target

Nanocomposite coatings comprising multiple phases on the nanoscale are increasingly used for automobile engine systems as they offer high thermal stability, high hardness, and low coefficient of friction. Since it is not easy to fabricate alloying targets, multiple targets have been used to prepare conventional nanocomposite coatings, resulting in the need for complex equipment and processes. Therefore, single alloying targets with homogeneous compositions and high toughness will simplify mass production of nanocomposite coatings. In this study, amorphous alloys are introduced to prepare the alloying target because of their homogeneous composition, good mechanical properties, and design rules that are similar to those for nanocomposites. Multi‐component alloying targets comprising Zr, Al, Mo, and Cu are successfully prepared by atomization and spark plasma sintering, both of which are powder metallurgical processes. The targets with amorphous structure are subsequently heat‐treated in a vacuum at ≈800 °C to crystallize the microstructure and improve the mechanical properties for sputtering. This study is the first report on modification of the fracture toughness of Zr‐based intermetallic compounds by changing the grain size. Improved fracture toughness of these compounds with a specific range of grain size is obtained by heat treatment of the amorphous target.

     

Multifunctional FeCo/TiN Multilayer Thin Films with Combined Magnetic and Protective Properties

Coatings with thicknesses ranging from a few nanometer up to several micrometer produced by physical vapor deposition (PVD) processes have been established in engineering technologies since the early 1980s. In particular, magnetron sputtered wear resistance coatings are industrially established and capable to enhance tool lifetimes significantly. However, in cases where optical inspection of a coating in use is not possible, an intrinsic sensor function of the film would be beneficial. Therefore, the development of wear resistant coatings with an integrated sensor functionality based on the insertion of a magnetoelastic ferromagnetic phase is suggested. In combination with appropriate read‐out electronics such a film system would be ready for online monitoring of the coatings' actual state (e.g., strain, temperature, volume loss). This paper focuses on the development of wear resistance coatings which simultaneously supply beneficial mechanical properties as well as ferromagnetic properties optimized for online non‐contact read‐out applications. Multilayer coatings obtained through alternate stacking of magnetron sputtered TiN and FeCo layers with a nominal total thickness of 1000 nm were produced as a model system meeting the above conditions. The bilayer period was varied down to 2.6 nm while the individual layer thickness ratio t_TiN/t_FeCo was determined by the deposition rates and maintained constant at a value of about 3/1. The films were vacuum annealed ex situ in a static magnetic field subsequent to the deposition. The constitution of the as‐deposited and annealed coatings as well as their mechanical (nanohardness, Young's modulus) and magnetic properties (magnetization hysteresis, frequency‐dependent permeability) are described. Finally, the suitability of the coatings for the use in remote‐interrogable wear sensor applications is briefly discussed.     

Effect of rheology and processing parameters on the EPD coatings of basic sol-gel particulate sol

Homogeneous thick silica coatings were prepared onto metals substrates by Electrophoretic Deposition (EPD) using particulate sol-gel sols. The synthesis of the sols was performed by mixing tetraethoxysilane and methyltriethoxysilane under basic catalysis. The synthesis, storage and processing conditions were optimised, correlating the rheological properties of the sols with EPD performance and reliability. Homogeneous and crack-free coatings are obtained when both, the synthesis and the deposition, are performed under tight conditions avoiding the contact with room humidity. The maximum crack-free thickness of the sintered coatings was 12 m, three times more than those obtained by dipping.     

High‐Performance Microwave‐Derived Multi‐Principal Element Alloy Coatings for Tribological Application

The authors report the development of Al_xCoCrFeNi (x = 0.1 to 3) high entropy alloy (HEA) coatings using a simple and straightforward microwave technique. The microstructure of the developed coatings is composed of a cellular structure and diffused interface with the substrate. The microstructure of the HEA coatings varies as a direct function of Al content. An increase in Al fraction shows structural transformation from FCC to BCC along with the evolution of σ and B₂ as the major secondary phases. The diffusion of Mo from the substrate enhances the mixing entropy and promotes σ‐phase formation. The HEA coatings show significantly high hardness compared to SS316L substrate steel (227 HV) with a maximum value of 726 HV observed for three‐molar composition. The fracture toughness exhibits an inverse correlation with the Al fraction with the highest value of around 49 MPa m^1/2 observed for Al_0.1CoCrFeNi coating. The equimolar coating composition shows lowest erosion rates among all the tested samples due to optimum combination of the mechanical properties. The erosion resistance of the equimolar coating is 2 to 5 times higher than steel substrate and around 1.5 times higher than the non‐equimolar counterparts depending upon the impingement angles.

     

Nanostructured Materials for Electrochemical Energy Conversion and Storage Devices

One of the greatest challenges for our society is providing powerful electrochemical energy conversion and storage devices. Rechargeable lithium‐ion batteries and fuel cells are amongst the most promising candidates in terms of energy densities and power densities. Nanostructured materials are currently of interest for such devices because of their high surface area, novel size effects, significantly enhanced kinetics, and so on. This Progress Report describes some recent developments in nanostructured anode and cathode materials for lithium‐ion batteries, addressing the benefits of nanometer‐size effects, the disadvantages of ‘nano’, and strategies to solve these issues such as nano/micro hierarchical structures and surface coatings, as well as developments in the discovery of nanostructured Pt‐based electrocatalysts for direct methanol fuel cells (DMFCs). Approaches to lowering the cost of Pt catalysts include the use of i) novel nanostructures of Pt, ii)new cost‐effective synthesis routes, iii) binary or multiple catalysts, and iv) new catalyst supports.

     

Template‐Assisted Electroforming of Fully Semi‐Hard‐Magnetic Helical Microactuators

The authors report on the fabrication of semi‐hard‐magnetic microhelices using template‐assisted electroforming. The method consists of electrodepositing a material on a sacrificial mandrel on which a pattern has been previously written. To electroform the helical microswimmers, a helical template on a polymer‐coated metallic mandrel is created using a laser, which precisely ablates the polymer coating and exposes the mandrel surface. Subsequently, the semi‐hard‐magnetic material is electrodeposited in the trenches produced by the laser. In this investigation, the helical structures are obtained from an electrolyte, which enables the production of hard‐magnetic CoPt alloys. The authors also show that electroformed semi‐hard‐magnetic helical microswimmers can propel in viscous environments such as silicon oil in three dimensions and against gravity. Their manufacturing approach can be used for the fabrication of more complex architectures for a wide range of applications and can be potentially extended to any electroplatable material.

     

Deformation and Fracture of Micron‐Sized Metal‐Coated Polymer Spheres: An In Situ Study

In situ imaging and analysis of the mechanical behavior of micron‐sized metal‐coated polymer particles under compression is reported. A nanoindentation set‐up mounted in a scanning electron microscope is used to observe the deformation and fracture of 10 μm polymer spheres with Ni, Ni/Au, Au, and Ag coatings. The spheres fracture in one of two metallization‐dependent modes, brittle, and ductile, depending only on the presence of a nickel layer. The metal coating always fractures parallel to the direction of compression. The mechanical properties up to the point of coating fracture are rate‐dependent due to the viscoelastic polymer core. Metal‐coated polymer spheres are an important composite material in electronics packaging, and this study demonstrates a novel method of evaluating the mechanical properties of particles to tailor them for electronic materials.