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     

The temperature and magnetic field dependence of superconducting critical current densities of multifilamentary Nb3Sn and NbTi composite wires

Commercial NbTi and Nb3Sn multifilamentary superconducting wire is becoming increasing important for use in research and commercial magnet systems. In both materials the temperature dependence of Jcplays a major role in the determination of magnet system operating parameters and design stability margins. We report here critical current density measurements as a function of temperature from 4.2 to 19 K and of applied magnetic field upto 8 T for multifilamentary Nb3Sn wire and for 2 alloys of NbTi superconducting wire. From this data [partial J_{c}(H_{a})/partialT] and[partialH_{c2}/partialT]T=T_{c}can be obtained and stability criteria and other superconducting parameters of the wires may be extracted.     

Models of elastic and inelastic flux switching

The totaldot{phi}(t)waveform of a square-loop magnetic core switched by MMFF(t)from negative remanencephi = -phi_{r}is composed of elasticdot{phi}_{epsilon}(t), decaying inelasticdot{phi}_{i}(t), and bell-shaped main inelasticdot{phi}_{ma}(t). The worst delta noise in a coincident-current memory core pair is essentiallydot{phi}_{i}(t). The sources of these components and the staticphi(F)curve are explained qualitatively by means of the random variations of the energy gradient vs. domain-wall position: elastic wall displacements and elastic rotation of magnetization inducedot{phi}_{epsilon}(t); minor inelastic wall displacements of essentially constant wall areas inducedot{phi}_{i}(t); and major inelastic wall displacements (involving domain collisions) of varying wall areas inducedot{phi}_{ma}(t). Semiempirical models for thedot{phi}components are based on the characteristics of these displacements. A satisfactory agreement is obtained between these models anddot{phi}(t)oscillograms of a thin ferrite core.     

Effect of Factors on Coercivity in Sr–La–Co Sintered Ferrite Magnets

We varied the composition and sintering temperature of Sr–La–Co ferrite magnets to analyze the effects of various important factors on coercivity $(H_{rm cJ})$. We examined the effects of crystal grain size and distribution, the mechanism of magnetization reversal, the degree of crystal grain orientation (OD), and the anisotropy field $(H_{rm A})$ on $H_{rm cJ}$. We proposed an equation based on the experimental results that expresses the measured $H_{rm cJ}$ and considers these effects as $H_{rm cJ} = C_{rm t}(0.4/R_{rm h})$ OD $(H_{rm A} - H_{rm d} - H_{rm in})$, where $C_{rm t}, R_{rm h}, H_{rm d}$, and $H_{rm in}$ are the crystal grain size effects on $H_{rm cJ}$ of sintered magnet, rotational hysteresis integral corresponding to the mechanism of magnetization reversal, demagnetizing field of shape anisotropy, and interaction field between crystal grains, respectively. We found that apart from the volume ratio for single-domain crystal grains and $H_{rm A}$, the mechanism of magnetization reversal had significant effects on $H_{rm cJ}$ for Sr–La–Co sintered ferrite magnets.      

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.     

On spontaneous nucleation in field-accessed bubble devices

The dependence ot the in-plane drive field at which bubble domains spontaneously nucleate in field-accessed bubble devices has been investigated as a function ofH_{k} - 4piM_{s}and of spacer thickness between the bubble film and permalloy propagation elements. The experiments were carried out on amorphous GdCoMo bubble films with T-bar and Y-bar structures. For a given structure and spacer thickness the nucleation field increases linearly withH_{k} - 4piM. Larger spacer thicknesses also lead to increased nucleation fields. A model based on the Stoner-Wohlfarth astroid is compared to these data and found to be useful in explaining the qualitative trends, but to be in poor quantitative agreement. It is concluded that since the drive field required in a device is proportional to4piM_{s}, Q - 1 = (H_{k} - 4piM_{s})/4piM_{s}must be greater than some minimum value for a given device structure and spacer thickness to permit reliable device operation.     

The measurement of the anisotropy field of cylindrical thin magnetic films

The described method for the anisotropy field measurement uses two fields HLand HT. The deviation of magnetization from the easy direction, and hence also sense signals, are proportional to these fields. The ratio of two integrated sense signals atH_{L} = 0andH_{L} neq 0reaches a certain value (∼ 2.5) with fieldH_{T} = H_{k}. In this way, Hkmay be measured by a device designed for the coercive force measurement.     

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.     

Helical anisotropic magnetic film

The magnetic coupling between the magnetization in two nonmagnetostrictive Ni-Fe layers separated by a SiO layer has been investigated by means of a transverse susceptibility measurement. The main results are that 1) the coupling energy Ec per Unit area of the multilayered film has a form ofE_{c}= -A cos (phi_{1}-phi_{2}), wherephi_{1}-phi_{2}is the angle between the magnetization vectors in the two Ni-Fe layers, and 2) the dependence of the coupling constant on the thickness b of the intermediate SiO layer can be interpreted quantitatively by the combination of the coupling energy due to Néel's topography model and that due to the magnetostatic interaction between the magnetic free poles appearing at the edges of the two Ni-Fe layers. The former coupling energy is given byE_{c1} = -frac{p}{2sqrt{2}}omega^{2}M^{2} exp(-sqrt{2}pb) cos (phi_{1}-phi_{2})wherep=2pi/LandLandware the wavelength and the amplitude of the undulation of the interface between Ni-Fe and SiO layers, respectively. The latter is given byE_{c2} = frac{2M^{2}D^{2}}{R} ln (frac{R}{D+b}) cos (phi_{1}-phi_{2})whereDis the thickness of each Ni-Fe layer, andRis the radius of the film.     

Static wall coercive force in ferromagnetic thin films

The static wall coercive force of thin ferromagnetic films has been calculated from a model of conservative wall energy. The spatial dependence of the wall energy γwis assumed to be given by the mean value of the local anisotropy energy, averaged over a coupling volume of the wall. The result of the calculation shows thatH_{w} propto frac{S}{M_{s}} (frac{D}{W})^{1/2} frac{1}{L}where the structure constantSincludes the local anisotropy,Dis the mean diameter of a region with constant anisotropy (for instance crystallites),Wis the wall width; andLis the coupling length parallel to the wall.     

Dependence of coercivity on driving field in multilayer films of nickel

The variation in the coercivity of magnetization loops of multilayer films of nickel was investigated as function of the rate of riserin the applied magnetic field. The films were prepared by deposition of nickel and copper alternately in a vacuum of2 times 10^{-6}mm of Hg on to substrates made of thin aluminium foils. The thickness of the nickel layersLvaried in the different films from 8 to 1000 angstroms. The loops were cycled with sinusoidal or triangular waveform driving fields, with a variety of amplitudes from 1 kOe to 4 kOe, with frequencies from 0.01 to 2 c/s. The measurements were performed from room down to liquid hydrogen temperatures. It was found that the coercive force could be expressed byH_{c} = H_{o} + Q(ln r - ln r_{o})/T^{1/2}for values ofrchanging from 1 to 5 Oe/ms, whereH_{o}, Aand rovaried slightly with temperatureT. The coercive force was a very sensitive function ofLand of the thickness δ of the copper layers, having the formH_{c} = A(L + delta) exp - BLwithBnearly constant in films deposited on a substrate at room temperature. The dependence of the coercivity onLwas displayed by a nonmonotonical function showing two peaks in Hccorresponding to values ofLof about 35 and 400 angstroms. The observed dependence of Hcon the rate of rise in the applied field, as well as on the thicknesses of the layers, can be discussed on the assumption of nonuniform magnetization within the thin layers as a result of their superparamagnetic properties and of the magneto-statical coupling between neighboring layers.     

Magnetic materials for integrated cores

Magnetic materials for an integrated magnetic core memory have been developed. The materials are for a storage layer about 1 μm thick and a low reluctance keeper layer. Materials investigated were Co-Ni-Fe for the storage layer and Cr-Ni-Fe for the keeper. Measurements were made ofH_{c}, B_{s}, B_{r}, lambda_{s}, S_{w}, H_{0}, and Hdisturb. Layer stress and adhesion were also measured. Special apparatus, which is described, has been developed to perform some of the measurements. Variation of properties with substrate surface, substrate temperature, and oxygen pressure during evaporation are reported.     

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).     

Thermal Decomposition of Molecular Materials {\{{\rm N}(n{-}{\rm C}_{4}{\rm H}_{9})_{4}[{\rm M}^{\rm II} {\rm Fe}^{\rm III}({\rm C}_{2} {\rm O}_{4})_{3}]\}_{\infty},\,{\rm M}^{\rm II} = {\rm Zn},\, {\rm Co},\, {\rm Fe}}

Thermal decomposition of oxalate-based molecular precursors, namely ${\{{\rm N}(n{-} {\rm C}_{4} {\rm H}_{9})_{4}[{\rm Zn}^{\rm II}{\rm Fe}^{\rm III}({\rm C}_{2} {\rm O}_{4})_{3}]\}_{\infty}, \{{\rm N}(n{-}{\rm C}_{4}{\rm H}_{9})_{4}[{\rm Co}^{\rm II}{\rm Fe}^{\rm III}({\rm C}_{2}{\rm O}_{4})_{3}]\}_{\infty}}$ , and ${\{{\rm N}(n{-}{\rm C}_{4} {\rm H}_{9})_{4}[{\rm Fe}^{\rm II}{\rm Fe}^{\rm III}({\rm C}_{2}{\rm O}_{4})_{3}]\}_{\infty}}$ , abbreviated as BuZnFe, BuCoFe, and BuFeFe, respectively, are studied using thermogravimetry (TG) in the temperature range from ~300?K to ~675?K at multiple heating rates. This study also deals with how the thermal decomposition of the complexes proceed stepwise through a series of intermediate reactions. The effect of the divalent metal M^II on the nature of thermal decomposition of the complexes, reflected in their TG profiles in terms of number of steps involved, is reported in this study. The temperature range of thermal decomposition steps for BuZnFe, BuCoFe, and BuFeFe with the same heating rates are studied systematically. Two different isoconversional methods, namely an improved iterative method and a model-free method are employed to calculate the kinetic parameters, and thus the most probable reaction mechanism of thermal decomposition is determined. Based on kinetic parameters, the important thermodynamic parameters such as the changes of entropy, enthalpy, and Gibbs free energy are estimated for the activated complex formation from the precursors. Considering the mass loss during the different thermal decomposition steps of BuZnFe, BuCoFe, and BuFeFe, observed in the thermogravimetry profiles, the overall reactions of the thermal decompositions are demonstrated.     

CMOS Microelectrode Array for Electrochemical Lab-on-a-Chip Applications

Microelectrode arrays (MEAs) offer numerous benefits over macroelectrodes due to their smaller sample size requirement, small form factor, low-power consumption, and higher sensitivity due to increased rates of mass transport. These features make MEAs well suited for microfluidic lab-on-a-chip applications. This paper presents two implementations of MEAs with and without an on chip potentiostat. We first describe an 8$,times,$ 8 array of 6 $mu{rm m}$ circular microelectrodes with center to center 37 $mu{rm m}$ spacing fabricated on silicon using conventional microfabrication techniques. Pads are provided for external connections to a potentiostat for electrochemical analysis. The second implementation is an individually addressable 32$,times,$32 array of 7 $mu{rm m}$ square microelectrodes with 37 $mu{rm m}$ center to center spacing on a CMOS chip with built-in very-large-scale integration potentiostat for electrochemical analysis. The integrated CMOS MEA is post processed at the die level to coat the exposed Al layers with Au. To verify microelectrode array behavior with individual addressability, cyclic voltammetry was performed using a potassium ferricyanide $({rm K} _{3}{rm Fe}({rm CN})_{6})$) solution.      

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.     

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.     

Magnetic properties of amorphous (fe, ni)80B20, (fe, ni)80B19si1, and (fe, ni)80P14B6alloys

Magnetic measurements were performed on amorphous (Fe, Ni)80B20, (Fe, Ni)80B19Si1, and (Fe, Ni)80P14B6alloy ribbons to yield the values of the magnetic moment per transition metal atom at 0 K and the Curie temperature. A close fit to the moment data, obtained by allowing not only the moments on Ni and Fe atoms to vary with Fe concentrationx, as revealed by neutron diffraction in crystalline alloys, but also the number of electrons per atom npand nbthat P and B atoms, respectively, donate to the transition metaldbands as described by the relationsn_{p} = 1.74 + 0.75 (1 - x)andn_{b} = 0.58 + 1.20 (1 - x), suggests a moment of 0 andsim0.13 mu_{B}per Ni atom in amorphous Ni80P14B6and Ni80B20alloys, respectively. The functional dependence of the Curie temperature onx, when followed closely for low Fe concentrations, gives the critical concentration xcabove which ferromagnetism appears in amorphous FexNi80-xB20and FexNi80-xP14B6alloy series asx_{c} cong 5and 8 at % Fe, respectively. The results of a theory based on coherent-potential approximation have been fitted to the measured values of Curie temperature in order to arrive at the Fe concentration dependence of the exchange interaction (JNiNi) between Ni-Ni pairs. The physical significance of the variation of JNiNiwithxobtained thereby has been discussed.     

Limit laws for the maxima of stationary chi-processes under random index

Let \(\{\chi _{k}(t), t\ge 0\}\) be a stationary \(\chi \) -process with \(k\) degrees of freedom. In this paper, we consider the maxima \(M_{k}(T)= \max \{\chi _{k}(t), \forall t\in [0,T]\}\) with random index \(\mathcal {T}_{T}\) , where \(\mathcal {T}_{T}/T\) converges to a non-degenerate distribution or to a positive random variable in probability, and show that the limit distribution of \(M_{k}(\mathcal {T}_{T})\) exists under some additional conditions.     

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.