Static and anhysteretic magnetic properties of tapes

The anhysteretic remanencebar{M}_{ar}(H_{o},T)of solidified suspensions of magnetic particles with predominant shape anisotropy is calculated from first principles for small dc fields Hoand arbitrary temperatureT < T_{B}(blocking temperature), describing the particle interactions by a mean field and assuming constant decrement of the ac field,2H_{d}per cycle. ForH_{d}< 2H_{o}, the anhysteretic distribution of particle magnetizations is found to be subject to the condition that the net internal dc fieldbar{H}_{i}is a minimum, and, for small Ho, to the condition,bar{H}_{i} = 0. The theory yieldsbar{M}_{ar}(H_{o},T)as a unique function of independently measurable static magnetic material properties, i.e., it contains no adjustable parameters and is hence quantitatively related to experimental data. Further, according to theory, ifbar{M}_{ar}(H_{o},T,T_{m})denotesbar{M}_{ar}as acquired in Hoat T and measured atT_{m}, bar{M}_{ar}(H_{o},T,T_{m} = T)is independent ofTforH_{d} ll 2H_{o}, andbar{M}_{ar}(H_{o},T,T_{m} neq T) = [M_{s}(T_{m})/M_{s}(T)] cdot bar{M}_{ar}(H_{o},T,T_{m} = T). The thermoremanent magnetization acquired in Hoand measured at a temperatureT_{m} ll T_{B},bar{M}_{thr}(H_{o},T_{m}), is related tobar{M}_{ar}(H_{o},T = T_{m}, T_{m})bybar{M}_{thr}(H_{o},T_{m}) = [M_{s}(T_{m})/M_{s}(T_{B})]bar{M}_{ar}(H_{o},T=T_{m},T_{m}), where TBis the blocking temperature below whichbar{M}_{thr}becomes thermally stable. Up to a constant factor of about 2, the theoretical results agree quantitatively with the experimental data on all materials that correspond to the premises of the theory, i.e., solidified suspensions, tapes in particular, of particles having predominant shape anisotropy.     

Wide PRIME-current range for all-magnetic shift registers

In an all-magnetic resistance-type shift register, a PRIME current pulsei_{p}(t), of amplitude Ip, is applied to Npand Nbturns through the minor and major apertures, respectively. For given operation frequencyf, the ratioR = I_{p}^{max}/I_{p}^{min}, whereI_{p}^{min} < I_{p} < I_{p}^{max}is the PRIME range of bistable operation, is maximized by matchingN_{p}/N_{b}so thatI_{p}^{max}values determined by spurious ZERO buildup and ONE dropout are the same. For a rectangular (or dc)i_{p}(t), the matchedN_{p}/N_{b}is fixed by the core properties, and R^maxis limited (e.g., <7). However, ifi_{p}(t)rises gradually, the matchedN_{p}/N_{b}depends also on the rise time Trofi_{p}(t). The lowerfis, with corresponding larger Tr, the smaller is the matchedN_{p}/N_{b}, and the larger is R^max. Calculation ofRis carried for ramp and half-sinusoidali_{p}(t)waveforms. The latter, for instance, atT = 25degC yields R^maxvalues of 14.0 and 22.0 forfof 1.0 and 0.5 kc/s respectively. Such wide PRIME ranges permit reliable register operation in a wide temperature range without resorting to temperature compensation of Ip. Experimental results are in agreement with the calculation.     

Combinatorial Screening of the BiDyYb Iron Garnet Material System for High Kerr Rotation Composition

We investigated the $(hbox{Bi}_{rm x}hbox{Dy}_{rm y}hbox{Yb}_{3-{rm x}-{rm y}})hbox{Fe}_{5}hbox{O}_{12}$ ternary combinatorial composition spread with the goal of finding new compositions with a large magneto-optical effect for possible use as magneto-optical storage materials. High-throughput magneto-optical characterization of the spread showed that the $hbox{Dy}_{0.6}hbox{Yb}_{0.5}hbox{Bi}_{1.9}hbox{Fe}_{5}hbox{O}_{12}$ composition has the largest Kerr effect in this ternary system. After annealing at 690 $^{circ}hbox{C}$ for 1 h, a scaled-up thin-film sample of this composition has a remanent magnetization as high as 90% of its saturation magnetization, indicating a good storage application potential.      

Modeling and Simulation of a Single Tin Dioxide Nanobelt FET for Chemical Sensors

This paper presents the modeling and simulation of a tin dioxide (${rm SnO}_{2}$) field-effect transistor (FET)-based nanobelt gas sensor. The model results are compared to numerical simulations and experimental data obtained from published results describing the fabrication of single crystal nanobelts grown through thermal evaporation techniques. The fabricated sensor shows good response when exposed to oxygen (${rm O} _{2}$) and hydrogen (${rm H} _{2}$) at room temperature. Gas adsorption causes changes in the electrical contacts due to oxygen vacancies in the bulk. As a result, the ${rm I}$ -${rm V}$ characteristics are very different when the device is exposed to (${rm O} _{2}$) versus (${rm H} _{2}$ ). In the presence of ${rm H} _{2}$, the behavior of the contacts is ohmic and saturation is caused by pinch-off of the channel at the drain contact. However, in the presence of ${rm O} _{2}$ , the behavior of the contacts is Schottky, and device saturation occurs at the source end of the device. Our model is based on a depletion mode MOSFET and it accounts for both ohmic and Schottky contacts when the device is exposed to oxygen or hydrogen. It also provides a possible explanation for the gate bias dependence of the saturation current seen in some published characterization data.      

A new principle for fast DC-sensitive potential-independent current comparators

A new principle of operation makes feasible small, cheap potential-independent current comparators whose performance reaches or exceeds the best properties of bulky magnetic amplifiers of the second-harmonic type. Connection of a ferrite-core coil in parallel with a suitable negative resistance generates relaxation oscillationsg(t). Inherently, at large oscillations the coil (i,phi)- characteristic is perfectly symmetric about the origin:phi(-i) = -phi(i). Provided that the negative-resistance (i,u)- characteristic has the same symmetry:i(-u) = -i(u), the oscillationsg(t)are perfectly symmetrical in the sense thatg(t+T/2) = -g(t). Heregrepresents either the coil flux φ, the currentior the voltageu. The time istandTthe oscillation period. External magnetomotive force shifts the coil (i,phi)-characteristics, invalidating (1) and therefore (3). Consequently, the deviation from 50% duty-ratio of the oscillatingq(t)is a sensitive and extremely stable measure of this MMF. Neither core temperature, pressure nor magnetic creep cause any zero drift. Orders of magnitude reached experimentally are: Short-time zero instability and noise:leq 10^{-5}ampere-turn, time resolution:leq 10^{-4}s, zero drift from -70 to +100°C:< 10^{-4}ampere-turn.     

Permanent magnet utilization criteria in a DC motor

In a dc motor with permanent magnet stator, the mean no-load working point of the magnet material can, after stabilization by stall current, be represented by a pointB_{m},H_{m}on a recoil line inside the demagnetization curve. The point results from the application during stall of an effective mean field Hadue to armature reaction in addition to the self-demagnetizing field due to circuit reluctance. It is shown that the motor specification and sizes lead to a specific value forH_{m}/H_{a}. The limiting values of this ratio (Hmzero or Hazero) imply that the corresponding optimum magnet designs for minimum magnet volume should be based either on maximum recoil energy or on(BH)_{max}. In practice, the best extreme working point during stall should lie between the points for these two criteria, dependent on the actualH_{m}/H_{a}. In some existing motorsH_{m}/H_{a}has been found to be between 0.5 and 1.5. For such values ofH_{m}/H_{a}, the variation of Bmand ofB_{m},H_{m}with working point is illustrated for high coercivity ferrite and for two grades of cast alnico alloy. A note on design methods is appended     

Finite piecewise polynomial parametrization of plane rational algebraic curves

We present an algorithm with the following characteristics: given a real non-polynomial rational parametrization of a plane curve and a tolerance , is decomposed as union of finitely many intervals, and for each interval I of the partition, with the exception of some isolating intervals, the algorithm generates a polynomial parametrization . Moreover, as an option, one may also input a natural number N and then the algorithm returns polynomial parametrizations with degrees smaller or equal to N. In addition, we present an error analysis where we prove that the curve piece is in the offset region of at distance at most , and conversely. Authors partially supported by the Spanish “Ministerio de Educación y Ciencia” under the Project MTM2005-08690-C02-01, and by the “Dirección General de Universidades de la Consejería de Educación de la CAM y la Universidad de Alcalá” under the project CAM-UAH2005/053.     

Computer simulation of the operation of a magnetic bubble domain propagation circuit modeled after a current-access device

The operation of a bubble-domain straight-line propagation circuit has been simulated successfully. This simulation has been achieved by our approximating the motion of an s = 0 frozen-azimuth bubble placed under a drive fieldH_{Z}(X, Y, T)= -H_{p} cdot cos [2pi(X/R_{X} - n(T)/4)] cdot exp [-(Y/R_{Y})^{2}]. The simulation has been generated from a previously developed numerical scheme to simulate the motion of a bubble, whose domain shape and magnetization structure along its domain wall were variable. The drive field has been modeled after a dual conductor-sheet, current-access propagation structure, which has a bit period RXand a transverse width on the order of2R_{Y}. The entire field contour has been advanced stepwise in the positiveXdirection by an increase of the integern(T), which represents the drive-phase number. The bubble motion has been observed during the first six drive phases to produce operating margin diagrams for drive frequencies of 250 KHz, 796 KHz and 1 MHz. The method of calculation and the results of the simulation are given.     

Measurements of superconducting Nb3Sn cavities in the GHz range

Superconducting Nb3Sn Cavities have potential advantages over rf cavities with Nb surfaces To test possible applications and to improve the understanding of Nb3Sn coatings on Nb, rf cavities have been measured between 1.5 and 8K and between 0.1 and 7GHz. The temperature dependence of the surface resistance R(T) indicates weak superconducting spots with transition temperaturesTmin{c}max{ast} < 1K andTmin{c}max{ast} simeq 2.5K. The normal conducting spotsTmin{c}max{ast} lsim 1K cause the large rf residual lossesR'_{res} propto f^{2}observed up to date. The spots withTmin_{c}max_{ast} simeq 2.5K cause temperature dependences ofR'(T)between 2 and 6K, where RBCS(Nb3Sn) is still negligible. In line withR_{res} propto f^{2}, the lowest rf lossesR_{res} < 2.10^{-9}Omegaand the highest field strengthB_{crit} = 83 m^{T}(wedgeE_{peak} = 29have been observed at the lowest frequency 0.1GHz measured. Surface resistance and penetration depth measurements have shown that grain boundaries or hydrogen clusters do not cause the weak spots observed withTmin{c}max{ast} < 2.5K. The origin and the chemistry of the weak spots withTmin{c}max{ast} lsim 1K, which cause the largeR_{res} propto f^{2}and the lowB_{crit} (T) simeq const, are still not clear. They seem related to the Nb3Sn surface. The weak spots withTmin{c}max{ast} simeq 2.5K consist most likely of Nb6Sn5, which in cooling below 950°C precipitates due to the excess Sn present in Nb3Sn coatings grown in Sn vapor.     

Small bubble garnet films with collapse-field temperature coefficients ranging from -0.3 to about 0%/°C

It is confirmed that collapse-field temperature coefficientsDeltaH_{o}(T)of conventional YSmLuCaGeIG (SL) films varied in the range of -0.1 ∼ -0.3 %/°C in proportion to the molar ratio of Lu2O3/Sm2O3in 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 withDeltaH_{o}(T)of less than -0.2 %/°C, Er and Gd substituted garnet films were studied. YSmErGdLuCaGeIG films withDeltaH_{o}(T)of -0.10 %/°C and an YSmErCaGeIG films of -0.06 %/°C were obtained. From the resulting ofDeltaH_{o}(T)and saturation magnetization temperature coefficientDeltaM_{s}(T), it was derived thatDeltaM_{s}(T)accounts for most of theDeltaH_{o}(T).     

Determination of anisotropy parameters in magnetically uniaxial specimens by torque derivatives

Formulae are given for determination, from torque curve slopes, of the first- and second-order anisotropy parameters in magnetically uniaxial specimens. The customary situation in which torque is plotted versus applied magnetic field orientation angle is examined. Corrections to measured torque slope arise from the off-set of the magnetization vectoroverrightarrow{M}_{s}, from the applied magnetic fieldoverrightarrow{H}direction.     

Bit Storage by 360 $^{circ}$ Domain Walls in Ferromagnetic Nanorings

We propose a theoretical design for a magnetic memory cell, based on thin-film ferromagnetic nanorings, that can efficiently store, record, and read out information. An information bit is represented by the polarity of a stable 360$^{circ}$ domain wall introduced into the ring. Switching between the two magnetization states is done by a current applied to a wire passing through the ring, whereby the 360$^{circ}$ domain wall splits into two charged 180 $^{circ}$ walls, which then move to the opposite extreme of the ring to recombine into a 360 $^{circ}$ wall of the opposite polarity.      

Microstructure, Electrical, and Magnetic Properties of ZrO$_{2}$ Added MnZn Ferrites

We investigated the influence of ZrO$_{2}$ on the microstructure and electromagnetic properties of MnZn ferrites by characterizing fracture surface micrographs, magnetic properties, and dc resistivity. Powders of Mn $_{0.68}$Zn $_{0.25}$Fe $_{2.07}$O $_{4}$ composition were prepared by the conventional ceramic technique. Toroidal cores were sintered at 1350 $^{circ}$C for 4 h in N$_{2}$/O$_{2}$ atmosphere with 4% oxygen. The results show that the lattice constant and average grain size increase with ZrO$_{2}$ concentration, but excessive ZrO $_{2}$ concentration will result in exaggerated grain growth and porosity increase. The dc resistivity, activation energy, saturation magnetic flux density, and initial magnetic permeability increase monotonically when the ZrO$_{2}$ concentration is not more than 0.04 wt% and then decrease with further increase of ZrO$_{2}$ concentration. On the other hand, the porosity, drift mobility, resonance frequency, and core loss decrease initially and then increase with the increase of ZrO$_{2}$ concentration.      

Novel Carbon Dioxide Microsensor Based on Tin Oxide Nanomaterial Doped With Copper Oxide

Carbon dioxide $({rm CO}_{2})$ is one of the major indicators of fire and therefore its measurement is very important for low-false-alarm fire detection and emissions monitoring. However, only a limited number of ${rm CO}_{2}$ sensing materials exist due to the high chemical stability of ${rm CO}_{2}$. In this work, a novel ${rm CO}_{2}$ microsensor based on nanocrystalline tin oxide $({rm SnO}_{2})$ doped with copper oxide (CuO) has been successfully demonstrated. The ${rm CuO}hbox{-}{rm SnO}_{2}$ based ${rm CO}_{2}$ microsensors are fabricated by means of microelectromechanical systems technology and sol-gel nanomaterial-synthesis processes. At a doping level of ${rm CuO}:{rm SnO}_{2} =1:8$ (molar ratio), the resistance of the sensor has a linear response to ${rm CO}_{2}$ concentrations for the range of 1% to 4% ${rm CO}_{2}$ in air at 450$^{circ}{rm C}$. This approach has demonstrated the use of ${rm SnO}_{2}$, typically used for the detection of reducing gases, in the detection of an oxidizing gas.      

The effects of low temperature fatigue on the RRR and strength of pure aluminum

Low temperature fatigue effects on residual resistivity ratio (RRR = rho_{273 K}/rho_{4.2K}) and strength of 300 and 1000 RRR aluminum are reported. The objective of this investigation is to select the best initial purity for the stabilizer aluminum used in energy storage magnets. Monolythic centimeter diameter specimens were fatigued at 4.2 K to strains (ε) reaching 0.3 percent. The resistivity ratio rapidly decreases during the first 100 cycles and approaches saturation (RRRf) after about 1000 cycles for all strains tested. The RRRfvalues are different for different initial resistivity ratio (RRRi) values, but all tend to come together at 0.3% strain independent of RRRi. The maximum specimen stress (sigma_{max}) is reached after about 1000 cycles also, and approaches a common value (sigma_{max} = εE/2, where ε is the strain range and E the elastic modulus) independent of RRRi. Thus high purity aluminum becomes "fully hard" at equilibrium and behaves elastically. The impact of fatigue damage on conductor design and choice of stabilizer purity is considered.     

Magnetic Normal Modes in Squared Antidot Array With Circular Holes: A Combined Brillouin Light Scattering and Broadband Ferromagnetic Resonance Study

We present a combined experimental investigation of magnetic normal modes in an antidot lattice using both Brillouin light scattering and broadband ferromagnetic resonance. It was fabricated on a silicon substrate using optical ultraviolet lithography. The sample consisted of a 30-nm-thick ${rm Ni}_{80}{rm Fe}_{20}$ squared antidot array with circular holes whose diameter and edge-to-edge spacing are 250 and 150 nm, respectively. Experiments were performed as a function of the applied magnetic field $mu_{0}{rm H}_{rm ext}$ in the range from $-$100 to 100 mT, with ${rm H}_{rm ext}$ applied along both the square lattice axis and its diagonal. Several peaks were observed in both the Brillouin light scattering and ferromagnetic resonance spectra, and their evolution with the intensity and the direction of the applied field ${rm H}_{rm ext}$ was measured. Micromagnetic simulations enabled us to identify the modes in terms of their symmetry obtaining a good quantitative agreement with the measured frequencies. In addition, we show how the inhomogeneity of the internal field affected the properties of the magnetic eigenmodes and their localization in different regions of the antidot lattice.      

Limits of validity of the one-dimensional wall theory for bubble materials

It is shown that the one-dimensional studies of the wall surrounding a bubble domain do not violate some necessary self-consistency requirements. Moreover, it is shown that the ratio of the magnetostatic self energy (which is neglected in these studies) to the total one-dimensional wall energy isT/Q, whereTis of the order of 1 for typical film thickness of a typical bubble material. This justifies the use of the one-dimensional wall for these materials, as long as the quality factorQ = K/(2piMmin{s}max{2})is large.     

The influence of applied tensile stress on power loss in Co-rich amorphous Co-Fe-Si-B ribbons with induced magnetic anisotropy

The influence on power loss PTof applied tensile stress σ in amorphous (Co0.89Fe0.11)72Mo3Si15B10(lambda_{s} > 0) and Co73Mo2Si15B10(lambda_{s} < 0) ribbons with different induced magnetic anisotropy Kuis reported. The losses are measured under sinusoidal flux conditions atf = 50Hz,J_{max} = 0.57T and atf = 400Hz,J_{max} = 0.10T. Measurements are carried out on samples in a stress-relieved state and with magnetic anisotropies induced by stress or field annealing. Atf = 50Hz, a minimum m Ptversus σ is observed. The σ-value (sigma_{min}) corresponding to the minimum PTincreases with increasing |Ku|. Atf = 400Hz, a minimum in PTversus σ is observed in the samples with induced magnetic anisotropy, whereassigma_{min} = 0in the stress-relieved samiales. However, no correlation between andsigma_{min}and Kuis possible from the present data.     

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.     

New stable state of bubbles containing Bloch lines

A new kind of bubble having two stable states for a bias field HBhas been found in thin garnet films. The bubble becomes smaller with increasing HBand disappears abruptly at some critical fieldH_{C1}. However, it does not collapse atH_{C1}. When HBis lowered, it comes into sight suddenly at another critical fieldH_{C2}. This means that for HBbetweenH_{C1}andH_{C2}the bubble has two stable states, one for a large bubble and the other for an unobservably small bubble. This has been well explained in terms of the stability of bubbles containing a definite number of Bloch lines.