IEEE Transactions on Magnetics

Published by Institute of Electrical and Electronics Engineers
Online ISSN: 0018-9464
Publications
Article
We report a magneto-nanosensor biochip for fungal detection. The chip is made of arrays of giant magnetoresistive (GMR) spin-valve sensors, and is able to detect protein biomarkers at low concentrations in solutions. As a demonstration, a standard curve for fungal pathogen Asp f 1 was obtained by measuring signals from various concentrations of Asp f 1 spiked in PBS solutions, indicating a detection limit of ~100 pg/ml. Five positive and negative Asp f 1 solution samples were discriminated correctly in blind experiments. Our data suggest that the magneto-nanosensor biochips are very promising as sensitive diagnostic devices for fungal pathogens. Given the generality of the detection scheme used in the magneto-nanosensor, we anticipate that the platform will be very useful for the detection of many types of biomarkers.
 
Article
Biomedical nanomagnetics is a multidisciplinary area of research in science, engineering and medicine with broad applications in imaging, diagnostics and therapy. Recent developments offer exciting possibilities in personalized medicine provided a truly integrated approach, combining chemistry, materials science, physics, engineering, biology and medicine, is implemented. Emphasizing this perspective, here we address important issues for the rapid development of the field, i.e., magnetic behavior at the nanoscale with emphasis on the relaxation dynamics, synthesis and surface functionalization of nanoparticles and core-shell structures, biocompatibility and toxicity studies, biological constraints and opportunities, and in vivo and in vitro applications. Specifically, we discuss targeted drug delivery and triggered release, novel contrast agents for magnetic resonance imaging, cancer therapy using magnetic fluid hyperthermia, in vitro diagnostics and the emerging magnetic particle imaging technique, that is quantitative and sensitive enough to compete with established imaging methods. In addition, the physics of self-assembly, which is fundamental to both biology and the future development of nanoscience, is illustrated with magnetic nanoparticles. It is shown that various competing energies associated with self-assembly converge on the nanometer length scale and different assemblies can be tailored by varying particle size and size distribution. Throughout this paper, while we discuss our recent research in the broad context of the multidisciplinary literature, we hope to bridge the gap between related work in physics/chemistry/engineering and biology/medicine and, at the same time, present the essential concepts in the individual disciplines. This approach is essential as biomedical nanomagnetics moves into the next phase of innovative translational research with emphasis on development of quantitative in vivo imaging, targeted and triggered drug release, and image guided therapy including validation of delivery and therapy response.
 
Article
The availability of thorough system simulations for detailed and accurate performance prediction and optimization of existing and future designs for a new modality such as magnetic particle imaging (MPI) are very important. Our framework aims to simulate a complete MPI system by providing a description of all (drive and receive) coils, permanent magnet configurations, magnetic nanoparticle (MNP) distributions, and characteristics of the signal processing chain. The simulation is performed on a user defined spatial and temporal discrete grid. The magnetization of the MNP is modelled by either the Langevin theory or as ideal particles with infinite steepness and ideal saturation. The magnetic fields are approximated in first order by calculating the Biot-Savart integral. Additionally the coupling constants between the excitation coils (e.g. drive field coils) and the receive coils can be determined. All coils can be described by an XML description language based on primitive geometric shapes. First simulations of a modelled μMPI system are shown. In this regard μMPI refers to a small one dimensional system for samples of a size of a few tens of a cubic millimeter and a spatial resolution of about 200 μm.
 
Article
In nowadays Magnetic Particle Imaging (MPI) signal detection and excitation happens at the same time. This concept, however, leads to strong coupling of the drive (excitation) field (DF) with the receive chain. As the induced DF signal is several orders of magnitude higher, special measures have to be taken to suppress this signal portion within the receive signal to keep the required dynamic range of the subsequent analog to digital conversion in a technically feasible range. For "frequency space MPI" high-order band-stop-filters have been successfully used to remove the DF signals, which unfortunately as well removes the fundamental harmonic components of the signal of the magnetic nanoparticles (MNP). According to the Langevin theory the fundamental harmonic component has a large signal contribution and is important for direct reconstruction of the particle concentration. In order to separate the fundamental harmonic component of the MNP from the induced DF signal, different concepts have been proposed using signal cancelation based on additional DF signals, also in combination with additional filtering. In this paper, we propose a field-cancelation (FC) concept in which a receive coil (RC) consists of a series connection of a primary coil in combination with an additional cancelation coil. The geometry of the primary coil was chosen to be sensitive for the MNP signal while the cancelation coil was chosen to minimize the overall inductive coupling of the FC-RC with the DF. Sensitivity plots and mutual coupling coefficients were calculated using a thin-wire approximation. A prototype FC-RC was manufactured and effectiveness of the reduction of the mutual inductive coupling (d) was tested in an existing mouse MPI scanner. The difference between simulations (d s =70 dB) and the measurements (d ms =55 dB) indicated the feasibility as well as the need for further investigations.
 
Article
The inclusion of magnetic nanoparticles into block copolymer micelles was studied towards the development of a targeted, magnetically triggered drug delivery system for cancer therapy. Herein, we report the synthesis of magnetic nanoparticles and poly(ethylene glycol-b-caprolactone) block copolymers, and experimental verification of magnetic heating of the nanoparticles, self-assembly of the block copolymers to form magnetic micelles, and thermally-enhanced drug release. The semicrystalline core of the micelles melted at temperatures just above physiological conditions, indicating that they could be used to release a chemotherapy agent from a thermo-responsive polymer system. The magnetic nanoparticles were shown to heat effectively in high frequency magnetic fields ranging from 30-70 kA/m. Magnetic micelles also showed heating properties, that when combined with a chemotherapeutic agent and a targeting ligand could be developed for localized, triggered drug delivery. During the magnetic heating experiments, a time lag was observed in the temperature profile for magnetic micelles, likely due to the heat of fusion of melting of polycaprolactone micelle cores before bulk solution temperatures increased. Doxorubicin, incorporated into the micelles, released faster when the micelles were heated above the core melting point.
 
Hydrodynamic size data of SPIONs dispersed in DI water (solid) and its evolution in 10% FBS–RPMI cell culture medium over time. SPIONs were coated with (a) PMAO-5kPEG (20), (b) PMAO-5kPEG (30), and (c) PMAO-20kPEG (30).  
MPS measurements showing mass normalized m (H ) plots of SPIONs coated with (a) PMAO-5kPEG (20), (b) PMAO-5kPEG (30), and (c) PMAO- 20kPEG (30), dispersed in 10% FBS–RPMI cell culture medium. Dashed lines indicate the m (H ) plot as the MPS drive field reverses direction. Inset: intensity normalized plots for clearer comparison of peak widths.  
Blood half-life (t 1/2 ) of SPIONs with different hydrodynamic diameters in female CD-1 mice. Sample A (d H = 86 nm): R = 0.17 min −1 , t 1/2 = 4 min. Sample B (d H = 42 nm): R = 0.036 min −1 , t 1/2 = 19 min.  
Article
Surface coatings are important components of Magnetic Particle Imaging (MPI) tracers - they preserve their key properties responsible for optimum tracer performance in physiological environments. In vivo, surface coatings form a physical barrier between the hydrophobic SPION cores and the physiological environment, and their design dictates the blood half-life and biodistribution of MPI tracers. Here we show the effect of tuning poly(ethylene glycol) (PEG)-based surface coatings on both in vitro and in vivo (mouse model) MPI performance of SPIONs. Our results showed that varying PEG molecular weight had a profound impact on colloidal stability, characterized using Dynamic Light Scattering (DLS), and the m'(H) response of SPIONs, measured in a 25 kHz/20 mTμ0 (-1) max Magnetic Particle Spectrometer (MPS). Increasing PEG molecular weight from 5 kDa to 20 kDa preserved colloidal stability and m'(H) response of ~25 nm SPIONs - the optimum core diameter for MPI - in serum-rich cell culture medium for up to 24 hours. Furthermore, we compared the in vivo circulation time of SPIONs as a function of hydrodynamic diameter and showed that clustered SPIONs can adversely affect blood half-life; critically, SPIONs with clusters had 5 times shorter blood half-life than individually coated SPIONs. We anticipate that the development of MPI SPION tracers with long blood half-lives have potential not only in vascular imaging applications, but also enable opportunities in molecular targeting and imaging - a critical step towards early cancer detection using the new MPI modality.
 
Article
In this work, we focus on the methods for controlling cell function with ferromagnetic disk-shaped particles. We will first review the history of magnetically assisted modulation of cell behavior and applications of magnetic particles for studying physical properties of a cell. Then, we consider the biological applications of the microdisks such as the method for induction of cancer cell apoptosis, controlled drug release, hyperthermia and MRI imaging.
 
Article
In our experiments with NCI-H1650 lung cancer cell lines labeled with magnetic nanoparticles via the Epithelial Cell Adhesion Molecule (EpCAM) antigen, we demonstrate capture efficiencies above 90% even at sample flow rates of 5 ml/h through our microfabricated magnetic sifter. We also improve the elution efficiencies from between 50% and 60% to close to 90% via optimization of the permanent magnet size and position used to magnetize the sifter. We then explain our observations via the use of finite element software for magnetic field and field gradient distributions, and a particle tracing algorithm, illustrating the impact of magnetic field gradients on the performance of the magnetic sifter. The high capture and elution efficiencies observed here is especially significant for magnetic separation of biologically interesting but rare moieties such as cancer stem cells for downstream analysis.
 
Article
First Page of the Article
 
Article
A biorthogonal wavelet-based MoM is presented. Specifically, the MoM based on compactly supported biorthogonal spline wavelets on the interval [0,1] are implemented and applied to solving 2-dimensional electromagnetic scattering problems. The numerical results show the effectiveness and usefulness of the proposed approach
 
Article
Magnetic properties of Fe<sub>49-x</sub>Co<sub>x</sub>Pt<sub>51</sub> (x=0.0,0.7,1.3, and 2.2) thin films are investigated. Cobalt is found to enhance the magnetization and energy product of the films. A maximum energy product of 18.4 MGOe is obtained at x=1.3. This value is 87% larger than that of a binary FePt film. High-saturation magnetization of M<sub>s</sub>=1180 emu/cm<sup>3</sup> and remanent magnetization of M<sub>r</sub>=862emu/cm<sup>3</sup> are obtained as the sample with x=1.3 was annealed at 500°C. The addition of cobalt increases the ordering temperature of the film, which retains more disordered phase in the film and thus produces higher magnetization than the binary sample. Exchange coupling is found to exist between ordered and disordered FePt phases. The intensive intergranular exchange coupling also makes the switching of magnetic moments in the thin film more cooperative. We consider that both the magnetization enhancement and the cooperated switching behavior in the thin film are responsible for the increased energy product.
 
Article
Two meander planar thin film transformers were fabricated using microlithography on a flexible polyamide film surface. One transformer was fabricated on the same polyamide surface (single sided transformer). The other transformer had coils which were placed on either side of the polyamide surface (double sided transformer). The output characteristics of these thin film transformers were measured using a spectrum analyzer in the frequency range from 0.01 GHz to 2.6 GHz and compared to the output characteristics obtained by simulating the transformer using a distributed parameter circuit theory. The simulated results for a single sided transformer agreed well with the measured results. On the other hand, the simulated results obtained using a double sided transformer did not fit the measured results for frequency above 1 GHz
 
Article
Soft magnet devices have a wide variety of applications in aerospace, military, and industrial sectors. Generally, devices used in industrial applications are required to operate satisfactorily under ambient temperatures of +10 degC to +50 degC under normal pressure, whereas those used in aerospace and military applications will face a wider range of temperatures from -60 degC to +80 degC and generally high pressure. In most of the soft magnetic materials, the use of a temperature compensating and stress-sensitive alloy will introduce changes in their normal performance. Therefore, an attempt is made to devise a method that ensures inherent temperature compensation and stress proof. Recent work in characterizing the performance of multielement high-entropy magnetic nanocomposite systems will be described. Specifically, we have developed soft magnetic alloys that exhibit tolerance of high stress and large temperatures changing above current operating ranges for four-mode ring laser gyro (RLG) devices
 
Article
The Kerr effect was used to observe and photograph surface domain patterns in the 0.014-inch 50-percent Ni-Fe strip after high temperature vacuum annealing treatments on commercial cold rolled material. The type of domain pattern observed depended on the recrystallization texture and also on the duration of the annealing treatment. The domain structure in primary recrystallization textures, (100)[001], consisted of coarsely spaced antiparallel domains aligned in the rolling direction. Some 90° domains appeared near texture imperfections and the sample edges. Three interrelated domain configurations were observed in fully secondary recrystallized grains of {112} and {120} planar textures; the domain structure in each grain was determined mainly by its crystallographic orientation in the strip. After a brief anneal at 2150°F, all grains had a finely spaced transverse domain structure which is interpreted as a surface closure structure. After prolonged annealing, the transverse structure degenerated into a maze and coarse maze pattern. It is concluded that the change in domain pattern from transverse to maze is associated with a decrease in solute sulfur induced stress anisotropy.
 
Article
Cast alloys and melt-spun ribbons with nominal compositions of Sm(Co<sub>bal</sub>Fe<sub>0.31</sub>Zr<sub>0.05</sub>Cu<sub>0.04</sub>B<sub>x</sub>)<sub>z</sub> (x=0.02-0.04,z=7.5-12) have been synthesized and characterized in a temperature range of 10-1273 K and at fields up to 5T. The main phase in the as-cast alloys exhibited a Th<sub>2</sub>Ni<sub>17</sub> type structure, with a strong uniaxial anisotropy. Minor phases with a TbCu<sub>7</sub> and/or CaCu<sub>5</sub> structure emerged as z (3d/R) decreased. As a result, the anisotropy field (H<sub>A</sub>) increased from 67 to 120 kOe, while 4πM<sub>s</sub> fell from 12.8 to 10.5 kG at 300 K when z was decreased from 12 to 7.5. For melt-spun ribbons, they are nano-structured in nature and magnetically hard, even in the as-spun state. By lowering the value of z(3d/R) and raising the B content, a finer microstructure and a higher H<sub>ci</sub> were obtained. Hard magnetic properties of H<sub>ci</sub>=4.9-12 kOe, 4πM<sub>s</sub>=9.0-12.0 kG at 300 K have been obtained from ribbon samples. Among them, Sm(Co<sub>bal</sub>Fe<sub>0.31</sub>Zr<sub>0.05</sub>Cu<sub>0.04</sub>B<sub>0.02</sub>)<sub>10</sub> ribbon showed the highest (BH)<sub>max</sub> of 10.8 MGOe at 300 K. A Henkel plot analysis suggested the existence of exchange-coupling interaction between the magnetically hard and soft phases in the ribbon materials.
 
Article
Four types of excess noise have been identified in dc SQUIDs operated in the temperature range 0.022 to 4.2 K. At temperatures between about 2 and 4 K, the spectral density of the low frequency flux noise of a wide variety of thin-film dc SQUIDs scales as 1/f<sup>m</sup>where m = 1.0 ± 0.1. In SQUIDs with Nb loops the noise originates as an "apparent flux noise", whereas in those with Pb or PbIn loops the noise is substantially lower and originates in critical current fluctuations. When any of these devices is cooled to temperatures below about 0.5 K, the spectral density of the excess flux noise scales as 1/f<sup>m</sup>, with m = 0.66 ± 0.08 in most cases, and the noise always originates as an apparent flux noise. At the lowest temperatures, the white noise saturates at an effective temperature of about 150 mK; this excess noise probably arises from self-heating in the resistance shunting each tunnel junction.
 
Article
We have investigated the structural, magnetic and electrical properties of Co-doped La<sub>0.7</sub>Ca<sub>0.3</sub>Mn<sub>1-x</sub>Co<sub>x</sub>O<sub>3</sub> (LCMCO) system using X-ray diffraction (XRD), thermal gravimetric analysis (TGA), scanning electron microscopy, field cooled dc magnetization and magnetoresistance (MR) measurements. Polycrystalline bulk samples of La<sub>0.7</sub>Ca<sub>0.3</sub>Mn<sub>1-x</sub>Co<sub>x</sub>O<sub>3</sub> (0.0 les x les 0.05) were prepared by conventional solid state reaction method. XRD pattern infers that all samples show single phase polycrystalline behavior. The field cooled magnetization measured as a function of temperature indicates that all samples exhibit ferromagnetic (FM) to paramagnetic (PM) transition with decrease in the temperature. It is observed that FM to PM transition temperature (T<sub>c</sub>) decreases with increase concentration of Co ions. The decrease in the T<sub>c</sub> indicates that doping Co ions affect the magnetic ordering due to the decrease in Mn<sup>3+</sup>. The resistance measured as a function temperature demonstrates that all the samples display metal to semiconductor transition. The MR was measured in the presence of 7 T field. The MR value shifted to low temperature with the increment of Co concentration and then we can found that the high Co concentration is helpful to stabilize the MR ratio at low temperature.
 
Article
The magnetic properties of amorphous alloys (Ni 1-c Co c ) 0.78 P 0.14 B 0.08 have been investigated. The samples were prepared by the splat-cooling method. The Curie temperatures have been determined and the magnetization measurements performed for 1.7°K leq T leq 270deg K and fields up to 70 kOe. Ni 0.78 P 0.14 B 0.08 is paramagnetic, whereas Co 0.78 P 0.14 B 0.08 is ferromagnetic until the crystallization temperature (678°K). The average moment per cobalt atom is 1.15 mu_{B} . In (Ni 1-c Co c ) 0.78 P 0.14 B 0.08 the critical concentration for the paramagnetic-ferromagnetic transition is c simeq 0.15 , this transition occurs in an inhomogeneous way. The saturation magnetization in the whole concentration range can be interpreted (as for some crystallized alloys and compounds) by a local environment model, when a reasonable short range order is assumed. In such a model the magnetic moment per cobalt atom is related merely to the number of its Co first neighbors n Co . For n Co =0 and 1 the cobalt atom is not magnetic, for n Co = 2 and 3 it carries a small moment mu_{1} = 0.50 mu_{B} and for n Co > 3 it is magnetic with mu_{2} = 1.15 mu_{B} as in Co 0.78 P 0.14 B 0.08 ; the nickel atoms do not carry a substantial moment in the entire concentration range. These features are comparable to those obtained in some crystalline alloys.
 
Article
The influence of magnetic annealing on iron loss has been investigated in 0.1-mm-thick grain-oriented high-silicon steels. To prepare such thin high-silicon steels, 0.3-mm-thick commercial electrical steels were mechanically ground to a specific thickness and then annealed at 1200°C under a high vacuum atmosphere. Due to much faster evaporation of Fe than Si, strip thickness decreased and silicon content increased. The 0.23-mm-thick 3% Si samples typically were thinned to a thickness of 0.10 mm, and silicon content increased to 6.5% after annealing for approximately 50 h. The high-silicon steels were further magnetically annealed at 770°C in a dc magnetic field, and an excellent iron loss of around 0.65 W/kg (at 1.5 T/60 Hz) was obtained. The iron loss behavior with both siliconizing and magnetic annealing processes is discussed in the light of microstructural evolution of B2 ordering phases and domain boundary drag effect of surface subsidiary domains.
 
Article
In some recorder applications there is a demand for both high data rates and high capacity. Radio astronomy is an example of such an application, and the longitudinal recorder has been developed for an array of radio telescopes known as the Very Long Baseline Array. The advantages, limitations, and future potential of this system are discussed in the light of new technologies for large arrays of heads, and extremely thin magnetic tapes
 
Article
A micro magnetic analysis, has been performed for ultra dense magnetoresistive memory cells employing giant magnetoresistive materials that have array densities from 2×10<sup>8</sup> to 10<sup>9</sup> elements per square cm. The analysis shows that as elements are made smaller, it is necessary to increase the effective anisotropy constant by increasing the demagnetizing factor in the long dimension of the element (or by finding GMR materials with large anisotropy fields). With diminishing cell size, exchange torque becomes progressively more important, and depending in detail on the thickness of the magnetic and separation layers, the layers ultimately act semi-coherently. Cell widths are eventually limited to values between 0.05 and 0.1 microns by thermal considerations because the smaller cells require progressively larger word and sense fields. The analysis assumed GMR elements made from magnetic layers 50 to 60 Angstroms thick of Ni, Fe, Co alloys with Cu, Ag, or Au separation layers of 15 to 30 Angstroms and with a magnetoresistive coefficient of 6 to 9%
 
Article
A split-ring resonator of beta_{0} simeq 0.1 designed for use in the Stony Brook heavy-ion booster has been developed and tested. The resonator, operating at 150 MHz and with an effective length of 21 cm was constructed of OFHC copper and electroplated with lead. Losses of 8.5 W at an accelerating field of 3.0 MV/m and 11 W at 3.25 MV/m were obtained; the resonator could be operated continuously up to at least 3.5 MV/m without breakdown or thermal run-away. Vibration-induced frequency excursions were found to be small (∼ 20 Hz pp). Under these conditions the split-ring resonator was electronically stabilized to an accuracy of 0.005 radian at 2.5 MV/m.
 
TGA curve of the La synthetic dynamic atmosphere. 
XRD patterns of the compound La Sr MnO (a) after cooprecipitation, (b) heated at 610 C, and (c) heated at 950 C (the Rietveld plot at the end of the refinement).
Temperature dependence of dc magnetization of the compound La Sr MnO ( x = 0.1; 0.2 and 0.3). 
Article
Synthesis of La<sub>1-x</sub>Sr<sub>x</sub>MnO<sub>3</sub> (x=0.1, 0.2 and 0.3) by homogenous coprecipitation method using urea as precipitant agent is reported. The particles are smaller than 200 nm after heating at 950°C. Temperature dependence of the electrical resistivity was found to be similar to the reported value for single crystals of these manganites.
 
Article
The magnetization behaviors have been investigated for La<sub>1-x</sub>Pb<sub>x</sub>MnO<sub>3</sub> (x=0.1,0.2,0.3) alloys. The Curie temperature increased from 195 to 352 K with increasing Pb concentration. A large magnetic entropy change (ΔS<sub>M</sub>), which is calculated from H versus M curves associated with the ferromagnetic-paramagnetic transitions, has been observed. The maximum ΔS<sub>M</sub> of La<sub>0.8</sub>Pb<sub>0.2</sub>MnO<sub>3</sub> was 1.22 J/kg K at 294 K for an applied field of 1.5 T. Adiabatic temperature change (ΔT<sub>ad</sub>) was measured directly by a special cryostat. The maximum ΔT<sub>ad</sub> of La<sub>0.7</sub>Pb<sub>0.3</sub>MnO<sub>3</sub> was 1.00 K at 352 K for an applied field of 2 T.
 
Article
A niobium split ring resonator has been designed with an optimum particle velocity β = v/c = 0.16. This type of resonator will be used to extend the Argonne superconducting heavy-ion linac. The peak surface electric field in the resonator is 3.9 times the effective accelerating field, a value 20% lower than for previously developed split-ring resonators. A prototype niobium resonator has been completed. Results of performance tests are discussed.
 
Article
Regular grain oriented 3% silicon steels were produced by conventional techniques to final gages ranging from 0.13 to 0.34 mm thick. The steels were evaluated at frequencies from 20 to 60 Hz and at inductions from 1.0 to 1.7T. Core loss minima were observed at about 0.2 mm; scribing at 5 mm intervals transverse to the rolling direction removed the minima and resulted in monotonic reduction in loss to as low as 1.32 W/kg/60Hz/1.7T at 0.135 mm thickness. Hysteresis loss, coercive force and domain wall spacing increased as thickness was decreased below 0.20 mm, and overcame the reduction in classical eddy-current losses, resulting in the observed minima. Scribing reduced the domain wall spacing and consequently the synchronous and asynchronous eddy-current losses. Electropolished samples with no forsterite and very smooth surfaces had larger domain wall spacings than the above samples, but the hysteresis losses and the synchronous and asynchronous eddy-current losses were reduced so that core losses were as low as 1.1W/kg/60Hz/1.7T and, with the sample under 10.3 MPa uniaxial tension, were as low as 1.00 W/kg/60Hz/1.7T at 0.10 mm thickness. With improved texture (lower hysteresis loss) and domain refinement (lower eddy-current losses), at 0.10 mm thick, core losses below 0.76 W/kg/60Hz/1.7T (0.34 W/#/60Hz/1.7T) are achievable.
 
Article
Regular grain oriented (RGO) silicon steels (0.18, 0.23, and 0.28 mm thick) with forsterite coating were formed into 37.2 and 63.6 kg uncut toroids, gapped toroids, and gapped, pressed rectangular cores. During the core (or toroid) manufacturing process, samples of the steel were obtained for Epstein testing at the front and back ends of each core or toroid. The cores/toroids and Epsteins were stress relief annealed at 800° for 4 hours in an 85% nitrogen - 15% hydrogen atmosphere. Additionally, gapped rectangular cores were made from forsterite-coated and stress-coated 0.18, 0.23, and 0.28 mm RGO silicon steels and 0.23 mm stress-coated high-permeability oriented (HGO) silicon steels. Process Factors (PF) were calculated by the equation: PF (%) = frac{W(core) - W(Epst)}{W(Epst)} times 100 % For pressed rectangular cores made from RGO steel the process factors improve (i.e. become more negative) with decreasing steel thickness at a given test induction. Comparing gapped rectangular cores, gapped toroids, and uncut toroids, the most negative process factors were obtained in the uncut toroid, reaching as low as about -10% for the 0.18 mm and 0.23 mm RGO steels. For pressed rectangular cores, the high permeability oriented (HGO) silicon steels experienced more positive process factors than the regular grain oriented (RGO) silicon steels. At 1.7T the 29.0Kg cores made from 0.23 mm stress-coated HGO steel had an average process factor of +8.2% while the cores made from stress-coated RGO steel had an average process factor of -1.1%. The differences in core loss seen between Epstein tests of regular grain oriented (RGO) steels and high-permeability grain oriented (HGO) steels are not as large in transformer cores. For transformer manufacturers designing to high loss evaluations (i.e., lower design inductions), RGO steels may be the steels of choice, particularly 0.18 mm thick steel at inductions below 1.5T.
 
Article
We have considered the ferromagnetic domain wall pinning caused by a planar type defect characterized by a change in the magnitude of the exchange constant and the uniaxial magnetic anisotropy and in the angular deviation of the magnetic anisotropy axis. Our model is appropriate for bulk material when the magnitude of the radius of curvature of the defect is not large compared to the width of the domain wall. Expressions are obtained and graphed for the coercive force due to such pinning. It is shown that the angular mismatch between the defect and the host material breaks the degeneracy between clockwise and counterclockwise rotating Bloch walls. Further, the results are applied to the Japanese commercial TDK material, Sm 2 Co 10 Cu 1.48 Fe 3.16 Zr 0.194 , which shows a fine cellular structure of approximately 500 A scale.
 
Article
Fine ceramic powders of Sr<sub>0.9</sub>Ca<sub>0.1</sub>Zn<sub>2 </sub>Fe<sub>16</sub>O<sub>27</sub> (Sr<sub>0.9</sub>Ca<sub>0.1</sub>Zn <sub>2</sub>-W) ferrite, of almost single domain grain size ~1 μm, have been synthesized successfully by sintering for a few hours a mixture of SrCO<sub>3</sub>, CaCO<sub>3</sub>, ZnO, and α-Fe<sub>2 </sub>O<sub>3</sub> in stoichiometric amounts at an effectively low temperature (T<sub>s</sub>) of ~1100°C. Carbon dioxide evolves in the reaction Sr(Ca)CO<sub>3</sub>+2ZnO+8 α-Fe<sub>2</sub>D<sub>3 </sub>&lrarr2;Sr(Ca)Zn<sub>2</sub>Fe<sub>16</sub>O<sub>2</sub>+C<sub>2 </sub>, making fine pores in the sample, which prevent large grain growth of the material and result in a very loose powder. The small &les;10% substitution of Sr by Ca activates the reaction, but preserves the crystallization of the small-sized grains. This yields magnetic properties useful for most permanent magnet applications, with a promisingly high coercivity H <sub>c</sub>~3650 Oe and saturation magnetization M <sub>s</sub>~65 emu/g. A considerably better saturation magnetization M <sub>s</sub>~85 emu/g can be obtained by sintering the samples at higher T <sub>s</sub>~1300°C, but that results in a rather very low coercivity H <sub>c</sub>~105 Oe
 
Article
Magnetic domain expansion readout under the condition of no alternative magnetic field are studied. The reproduced waveform of a 0.2 μm domain has a very sharp shape and large amplitude of 70% to the saturated signal level. It is found that the linearity of the readout signal including domain expansion phenomenon is maintained as well as a conventional readout, therefore relatively clear reproduced signal of the 0.2 μm packed domain can be detected. In order to improve it, the sharper readout waveform must be reproduced by using a gating layer between the expansion and the recording layers, and the readout signal of the 0.2 μm packed domain is enlarged two times relative to that without a gating layer
 
Article
The magnetocrystalline anisotropy, the magnetostriction and the structure-sensitive properties in the magnetization process were studied in the ordered and disordered states of specimens between 70.1 wt% Ni and 79.7 wt% Ni. The single crystals were cooled at the rate of 1 K/day to obtain the fully ordered state. Magnetization curves and hysteresis loops were measured using picture frame specimens. The magnetocrystalline anisotropy and the magnetostriction were also measured in disc specimens. New experimental results obtained in the order state are considerably different from those obtained by previous investigators. The difference is attributed to the degree of order and the magnetic properties of the previous investigators are those of a partially ordered state
 
Article
This paper assesses the scaling-down possibility, and determines feasible geometries for miniaturizing the perforated-sheet devices, which offer high speed and simple packaging, but must be improved in density and functional versatilities (replication, swapping, editing, etc.). The use of separate control conductors can provide design flexibility, and over-sized gates and permalloy keepers can relax scaling-down constraints (i.e. electromigration and on-chip power dissipation). Miniaturized devices are implemented with thin propagation sheets, and thick but over-sized control conductors plus keepers. Moreover, 0.2μ bubble devices with functional versatilities appear achievable if suitable bubble medium, lithography, and thin films could be provided.
 
Article
Memory elements 0.2 microns wide made from multilayer GMR material have been studied experimentally and analytically. When etched into narrow elements, the 12 to 15% material exhibits a memory mode with large outputs of ±8%. Analysis shows that the mode is suitable for multi-megabit die with high performance if yield can be achieved
 
Article
The phase transitions in the manganite perovskite compounds La<sub>1-x</sub>Ca<sub>x</sub>MnO<sub>3</sub> (x=0.2, 0.3, 0.4) have been studied using magnetization, resistivity, and photoacoustic measurements. The phase transition temperatures determined from the anomaly of the photoacoustic signal are consistent with those determined from the magnetization measurements. For x=0.3, the paramagnetic to ferromagnetic phase transition can be regarded as a second order phase transition with no latent heat at the phase transition temperature. The relative value of the multiplication of specific heat and thermal conductivity can be obtained from the photoacoustic signal using Rosencwaig and Gersho's theory,
 
Article
To probe magnetic ordering in single crystals of Pr<sub>1-x</sub>Sr<sub>x</sub>MnO<sub>3</sub> ( x = 0.22, 0.24, 0.26), the X -band electron magnetic resonance (EMR) measurements were carried out in the temperature range 5 K les T les 600 K. It appears that two additional (as compared to well-known La<sub>1-x</sub>Ca<sub>x</sub>MnO<sub>3</sub> system) factors influence on such ordering and on magnetic correlations in paramagnetic state. They are: low temperature ferromagnetic like order of Pr-subsystem and strong charge ordered state (CO). In particular, the competition of CO and ferromagnetic coupling results in suppression of EMR susceptibility at low temperatures, as well as in its notable deviation from the standard Curie-Weiss behavior in paramagnetic state. It is shown that the character of paramagnetic spin dynamics changes notably with Sr-doping, i.e., the higher doping-the stronger contribution of electron-impurity spin relaxation mechanism.
 
Article
We have developed a 0.23 mm thick high permeability grain oriented silicon steel (Hi-B) using AlN and MnS as grain growth inhibitors through the one stage cold rolling process. When the sheet thickness decreases, the secondary recrystallization tends to become unstable. We have successfully stabilized the secondary recrystallization of the thinner material by adding such alloying elements as tin and others. Magnetic properties of the newly-developed 0.23 mm thick Hi-B were superior to those of any other commercial grain oriented silicon steels. The core loss at 50 Hz of 0.23 mm Hi-B was about 0.66 W/kg at 1.5T and about 0.92 W/kg at 1.7T. Model transformer tests showed 0.23 mm thick Hi-B had excellent performance characteristics, especially when its surface was irradiated in cross rolling direction by laser. The relative economic value of 0.23 mm thick Hi-B as the material for transformers was estimated to be very high.
 
Article
SmFeTi, (Sm<sub>0.75</sub>Tm<sub>0.25</sub>)FeTi films and Fe/SmFeTi bilayers are prepared by magnetron sputtering on Mo substrates. Their magnetic properties and the induced anisotropy on the Fe layers are studied by conversion electron Moessbauer spectroscopy and DC magnetization measurements
 
Article
The fabrication and operation of high-density (0.25 × 10<sup>6</sup>bit/in<sup>2</sup>) nondestructive readout (NDRO) memory elements are described. The high density is made possible by coupled films and Permalloy keeper. The NDRO is made possible by multiple-pulse WRITE or hard-direction bias field. Typical performance parameters are I_{w} = 60 mA, I_{b} = 30 mA, and V_{s} = 150 μV/3 ns. The small signal is detectable by multiple-pulse READ. When such memory elements are to be fabricated with peripheral circuits on the same Si chip, a self-contained chip will be obtained. Such chips could enjoy the same advantages as semiconductor memory chips such as few leads, modularity, amenability to bit organization, and possibility for error-correction, but would be capable of higher storage density due to simpler planar configuration.
 
Article
A number of partially populated NDRO film arrays employing 2-layer films were constructed and tested to demonstrate that high-performance low-power NDRO film memories can be made with read and write requirements which are compatible with integrated circuits. Test arrays were made in which two or four turns of fine wire were used for word lines spaced on 0.010-inch centers. The read word current for the arrays with 2-turn word lines was nominally 75 mA and the write word current was 180 mA. Digit current for all arrays was approximately 35 mA. The read word current for the array with 4-turn word lines was nominally 30 mA and the write word current was 85 mA.
 
Article
The experiments were done using a powder mixture of Nb, Al and Ge with a particle diameter of less than 50 μm which was filled into Ta- or Nb-tubes. These composites were cold-worked and heated from 400°C (10°C/min) up to 850 or 900°C and annealed 20 min on this temperature. By this heat treatment in many areas the intermetallic compound Nb(Al,Ge) 3 is formed within the powder mixture. After a second cold deformation the composites were annealed at 1000, 1200 or 1300°C with different annealing times. Microprobe analysis was used to investigate the phase distribution. The formation of the A15-phase was also investigated by transition temperature measurements, which were done by the inductive method. The highest transition temperature was found after 1300°C furnace annealing. Maximum critical current was achieved by short-time resistive annealing at 1200°C of 0.5 mm θ wire. The reason for the high current carrying capacity of the short-time annealed samples can be seen from the T c -measurements which show that a A15-phase with high transition temperature is formed even after a short annealing time. Possible improvements of superconductors made from powder mixtures are discussed.
 
Article
It is confirmed that collapse-field temperature coefficients DeltaH_{o}(T) of conventional YSmLuCaGeIG (SL) films varied in the range of -0.1 ∼ -0.3 %/°C in proportion to the molar ratio of Lu 2 O 3 /Sm 2 O 3 in the melt composition. DeltaH_{o}(T) of the SL system was found to be limited in the range of -0.21 ∼ -0.3 %/°C when considering the lattice parameter matching between GGG substrates and films. To obtain garnet films with DeltaH_{o}(T) of less than -0.2 %/°C, Er and Gd substituted garnet films were studied. YSmErGdLuCaGeIG films with DeltaH_{o}(T) of -0.10 %/°C and an YSmErCaGeIG films of -0.06 %/°C were obtained. From the resulting of DeltaH_{o}(T) and saturation magnetization temperature coefficient DeltaM_{s}(T) , it was derived that DeltaM_{s}(T) accounts for most of the DeltaH_{o}(T) .
 
Article
In this paper, we present an analysis of the performance of a 0.3-Tb/in<sup>2</sup> ultralow-power magnetic-force-microscopy-based scanning-probe storage device actuated by microelectromechanical systems technology. The device is currently under development at Carnegie Mellon University, Pittsburgh, PA. The analysis shows that, with an optimized commercial single-layered Co-based perpendicular medium with an optimized tip trajectory, a signal-to-noise ratio of 20-25 dB is achievable. The analysis includes general design considerations as well as various aspects of performance such as recording dynamics, PW<sub>50</sub>, intersymbol-interference limit, detection sensitivity, thermal degradation, intertrack interference, off-track errors, process variations, and surface fluctuation effect. Design/performance standards for the new device are suggested.
 
Article
The (Bi<sub>0.7</sub>Pb<sub>0.3</sub>)<sub>1-y</sub>Sr<sub>1</sub>Ca<sub>1 </sub>Cu<sub>1.8</sub> superconductor sintered above 855°C is shown to have a microstructure consisting of superconducting platelike grains embedded in a matrix formed by semiconducting rodlike grains due to the melting of Bi<sub>2</sub>O<sub>3</sub>, whereas it has a microstructure consisting of a small number of semiconducting rodlike grains embedded in the superconducting matrix when it is sintered at 845°C. The amount of high- T <sub>c</sub> phase increases and that of low T <sub>c</sub> phase decreases as the sintering period increases, making it possible to fabricate superconductors with T <sub>c</sub> above 100 K by sintering at 845°C in air for 240 h. The sintering time can be shortened by sintering first below 850°C to obtain a microstructure consisting of the platelike grain matrix and resintering at higher temperature to enhance the growth of the high- T <sub>c</sub> phase. Superconductors with a T <sub>c</sub> of 106 K have been fabricated by sintering at 845°C for 48 h followed by sintering at 855°C for 24 h
 
Article
A systematic study of the effect of substitution of Cu in MM[Co 1-x Cu x ] 5 0 leq x leq 1 and Fe in[MM Co 1-x Fe x ] 5 0 leq x leq 0.3 were made. The Cu substitution for Co gives stable CaCu 5 type phase in the entire composition range. The lattice parameters underline{a} and underline{c} of the 1:5 phase increase linearly with increasing Cu content. Fe has only a limited solid solubility in the 1:5 phase. For MM[Co 1-x Cu x ] 5 alloys the 4piM_{s} decreased from 95 e.m.u. g<sup>-1</sup>at x=0 to about 3 e.m.u. g<sup>-1</sup>at x=1 while for MM[Co 1-x Fe x ] 5 alloys it increased to 101 e.m.u. g<sup>-1</sup>at x=0.1 . A detailed phase composition of the alloys at 900°C and 700°C and the effect of Cu on the T c of the 1:5 phase are also reported.
 
Article
The magnetooptical recording characteristics of a 0.2-0.3-μm-length domain were studied using a magnetic field modulation method. The ultimate goal of the investigation, performed on an extremely high-density magnetooptical disk, was to realize high-speed rotating 5.25-in disks. Through optical observation, it was found that domains of 0.3-μm length could be recorded separately under the condition of 15.1 m/s linear velocity and 25.2 MHz. However, the domain shape became more irregular with increasing recording power and linear velocity. This could be explained by calculated results showing that the domain wall temperature when the magnetic field is reversed becomes higher as the power or the velocity increases. It was observed that the domain shape of a quadri-layer structure disk with a reflective layer having a higher thermal conductivity, such as Al film, is more irregular. This is explained by the difference in temperature profiles obtained from simulation
 
Article
Measurements of the structure, magnetic properties, and magnetostriction were made on arc-melted polycrystalline (Tb<sub>0.7 </sub>Dy<sub>0.3</sub>)<sub>0.7</sub>Pr<sub>0.3</sub>(Fe<sub>1-x</sub>Co <sub>x</sub>)<sub>1.85</sub> (0&les;×&les;0.6) alloys by X-ray diffraction, initial susceptibility, vibrating sample magnetometry, and standard strain gauge techniques. The measurements showed that the matrix of (Tb<sub>0.7</sub>Dy<sub>0.3</sub>)<sub>0.7</sub>Pr<sub>0.3 </sub>(Fe<sub>1-x</sub>Co<sub>x</sub>)<sub>1.85</sub> predominantly consists of the MgCu<sub>2</sub>-type cubic Laves phase, with a small amount of PuNi<sub>3</sub>-type and rare-earth rich phases. The amount of (Tb,Dy,Pr)(Fe,Co)<sub>2</sub> in (Tb<sub>0.7</sub>Dy<sub>0.3</sub>) <sub>0.7</sub>Pr<sub>0.3</sub>(Fe<sub>1-x</sub>Co<sub>x</sub>)<sub>1.85 </sub> increases with increasing Co concentration and becomes almost the only phase present when x&ges;0.4. The lattice parameter decreases approximately linearly with increasing x from 0.7357 nm when x=0 to 0.7300 nm when x=0.6. The Curie temperature increases with increasing x from 640 K when x=0 to 677 K when x=0.3, and then decreases to 572 K when x=0.6. The saturation magnetization M<sub>s</sub> exhibits a minimum near x=0.3. The polycrystalline magnetostriction of (Tb<sub>0.7 </sub>Dy<sub>0.3</sub>)<sub>0.7</sub>Pr<sub>0.3</sub>(Fe<sub>1-x</sub>Co <sub>x</sub>)<sub>1.85</sub> decreases monotonically with increasing x. There is no evidence of an anomalously large λ<sub>100</sub> value as is the case for other R(Fe<sub>1-x</sub>Co<sub>x</sub>)<sub>2 </sub> compounds
 
Temperature dependence of magnetizations for LSM and LBM measured at 100 Oe.  
Temperature dependence of the dc resistance for the two compounds studied.  
1H versus T ;(0) is a fit curve according to a function of 1H (T) = 1H + (A=T)exp(0E =k T ).
Temperature dependence of the EPR intensity for the LSM and LBM sample.  
Article
In this paper, we investigate in detail influences of A-site substitution on the EPR parameters for La<sub>0.7</sub>Sr<sub>0.3</sub>MnO<sub>3</sub> (LSM) and La<sub>0.7</sub>Ba<sub>0.3</sub>MnO<sub>3</sub> (LBM) compounds by means of electron paramagnetic resonance (EPR) study. Magnetization and resistivity measurements were also performed. The Curie temperature T<sub>C</sub> is 365 and 322 K for LSM and LBM, respectively. In the whole temperature range studied, metallic behavior was observed. The minimum linewidth ΔH<sub>min</sub> is 402 and 378 Oe for LSM and LBM, respectively. Temperature dependences of the EPR linewidth ΔH<sub>pp</sub>(T) for the samples were analyzed by using the model of adiabatic hopping motion of small polarons, ΔH<sub>pp</sub>(T)=ΔH<sub>0</sub>+(A/T)exp(-E<sub>a</sub>/k<sub>B</sub>T). The activation energies E<sub>a</sub><sup>(1)</sup> and E<sub>a</sub><sup>(2)</sup>, obtained from ΔH<sub>pp</sub>(T) and EPR intensity I(T) data, are different. At temperatures T>T<sub>min</sub>, the Lande factor of both samples is the same (g≈2), indicating that spin-spin interactions among electrons of Mn<sup>3+</sup> and Mn<sup>4+</sup> ions play a main role.
 
Article
The application of small cryocoolers to cooling a superconducting magnet at 12 K has important advantages, especially for small and medium-size magnets. Simple construction and a helium-free magnet system were obtained. The demonstration magnet developed is a six-coil system with a volume of 75 L and can be regarded as a 1:3 scale MRI magnet. With a current of 100 A, a 1-T central field is generated with a maximum of 1.9 T in the windings. The magnet consists of six coil formers and five aluminum spacing rings, providing easy service and disassembly. The superconductor, a 0.6-mm-diameter Nb<sub>3</sub>Sn wire, is wound on the thin-walled stainless steel coil formers, after which the coil is heat treated and vacuum impregnated. Afterwards, the coil system is assembled and the electrical and thermal connections are made. At an operating temperature of the demonstration magnet of 12 K the test coil reached its nominal current after a few training quenches
 
Article
The coercivity of the melt-spun pseudobinary (NdFe<sub>10</sub>Cr <sub>2</sub>)<sub>1-x</sub> (Nd<sub>2</sub>B)<sub>x</sub> alloys, in which Nd<sub>2</sub>B represents a composition of Nd<sub>0.67</sub>B<sub>0.33</sub>, was systematically studied. It was found that significant coercivities are possible with optimal additions of the Nd-B composition at x =0.5-0.7. The as-spun (substrate velocity=10 m/s) coercivity increased from 0.1 kOe at x =0.0 to maxima of 7.0 and 7.5 kOe at x =0.5 and 0.7, due to the Nd<sub>2 </sub>Fe<sub>14</sub>B and a new Fe-Nd phase, respectively, as evidenced from thermomagnetic analysis and X-ray diffraction. Annealing the overquenched amorphous x =0.7 alloy led to the crystallization of the Nd<sub>2</sub>Fe<sub>14</sub>B phase, and a coercivity of 8.5 kOe was obtained
 
Article
We have studied electron-spin resonance (ESR) spectra of a La<sub>0.7</sub>Sr<sub>0.3</sub>MnO<sub>3</sub> slab. Experimental results reveal that ESR spectra in the ferromagnetic (FM) region are followed with a broadly resonant mode as θ = 90<sup>°</sup>, but several modes as θ = 0<sup>°</sup>. Their spectral parameters depend strongly on θ. When keeping θ = 0<sup>°</sup> and measuring the temperature dependence of ESR spectra, it is found that several resonant modes in the FM region become a single mode in the Dysonian line-shape at temperatures T >; T <sub>min</sub> ≈ 1.1 TC . At T <sub>min</sub>, the spectral linewidth reaches to a minimum value of Δ H <sub>min</sub>= 283 Oe. Temperature dependences of the EPR intensity above T <sub>min</sub> obey a function I ( T ) = Io exp( Ea / kB T ). Having based on the relation of I ( T ) ∝ χ( T ) = 1/( T -Θ) , we obtained the Curie-Weiss temperature Θ to be 376 K. If using a simple relation of g = h ν/ μ B Hr ( Hr is the resonant position, and μ B is the Bohr magneton), we have found that the effective Lande factor g increases from 1.91 (around T <sub>min</sub>) to ~ 1.98 (at 473 K) close to the value of g = 2.0 for spin-spin interactions of Mn ions. This could be due to the additional presence of magnetic interactions, besides spin-spin interactions, such as interactions between FM clusters and/or spin-orbital coupling. Interestingly, despite the sample in the PM regime, the spectral parameters depend on the angle θ. This is assigned to an anisotropic field related to Mn <sup>3+</sup>-Mn <sup>4 +</sup> FM clusters persisting above T <sub>min</sub>. At 470 K, the anisotropy constant obtained is Ku ≈ 57 erg/cm <sup>3</sup>, much sm- - aller than that in the FM region.
 
Top-cited authors
Z.Q. Zhu
  • The University of Sheffield
Isaak D. Mayergoyz
  • University of Maryland, College Park
Dieter Weller
  • HGST, A Western Digital Company
S.L. Ho
  • The Hong Kong Polytechnic University
Gerard Meunier
  • French National Centre for Scientific Research