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.     

Movement of an individual Bloch wall in a single-crystal picture frame of silicon iron at very low velocities

The movement of an individual Bloch wall in a single-crystal picture frame of silicon iron was studied. By using an electronic apparatus the wall velocity was held constant during the magnetization reversal. It was possible to control the wall velocity down to 10^-2mm/s. Even at these low velocities the relationship between the wall velocityvand the driving magnetic fieldHis precisely linear:v = const . (H-H_{0}). This is to be expected under the given experimental conditiondB/dt = const.     

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.     

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.     

A general method for designing low-temperature drift, high-bandwidth, variable-reluctance position sensors

A method was developed to design high-bandwidth single-ended variable-reluctance position sensors with high accuracy and extremely low temperature drift for use in magnetic bearing control systems. The method was used to design a hermetic radial position sensor with a position sensitivity ({1/L_{0}}{dL/dx}) of 1 percent/µm over the range of 0-50 µm and a temperature drift ({1/L_{0}}{dL/dT}) of less than 0.02 percent/°C over the range of 0-100°C, i.e., less than a 2-µm position error for a temperature variation of 100°C.     

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.     

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.     

Rail and armature current distributions in electromagnetic launchers

The current and magnetic field distributions in the rails and armature of an electromagnetic launcher are obtained in closed form for the steady state. These solutions assume that the armature moves with a steady velocity and account fully for the two-dimensional skin effect caused by the relative motion between the rails and the armature. Both solid and laminated armatures are considered. It is found in the case of the laminated armature that the phenomenon can be described by a single dimensionless parameter,frac{ell}{w}frac{sigma_{o}}{sigma_{r}}sqrt{frac{uell}{pieta_{r}}}.     

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.     

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.     

Change of RF susceptibility and parametric oscillations of the magnetization of stripe films in magnetic fields normal to the surface

Multidomain ferromagnetic resonance (FMR) for Permalloy magnetic fills (thickness 2700 Å) 83 percent Ni with rotatable anisotropy was experimentally observed with an external magnetic field applied normal to the sample surface. The lower frequency branch of multidomain FMR was excited when the RF magnetic fieldhwas oriented along the film plane; forhoriented parallel to the fieldH_{perp}the higher frequency branch was excited. The dependence of the resonant fieldH_{perp}on the frequency of fieldhfor multidomain and single-domain FMR was determined. Increasing the RF fieldhoriented alongH_{perp}gave rise to oscillations of magnetization with the frequency(1/2)f, wherefis the pumping frequency. The dependence of the amplitude of these oscillations on the strength of the RF fieldhand on the magnetizing fieldH_{perp}was investigated.     

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.     

Factors affecting the sensitivity of gamma-level ring-core magnetometers

A theoretical and experimental analysis has been made of the geometrical and magnetic factors affecting the sensitivity of the ring-core magnetometer first introduced by Geyger in 1961. Using a drive-to-pickup transfer function derived from an assumed dynamic hysteresis loop and a given dc input signal, the second-harmonic output voltage was derived for both current-source and voltage-source driving conditions. Results of the analysis, which were confirmed by experiment, show the following. 1) When a single, diametrically wound (solenoid-type) pickup winding is used with a ring-core sensor, simultaneous minimization of both fundamental and second-harmonic feedthrough can be achieved by a simple rotational adjustment. This optimization is possible only with a circular core. 2) Sensitivity can be increased by increasing frequency, number of pickup turns, dynamic differential permeability, cross-sectional area, and effective sensor length, and by decreasing the dc initial permeability (μ0). 3) The dominant factor affecting the sensitivity is the demagnetizing termmu_{0}K = mu_{0}l^{-1.72} (4A/pi)^{0.86}, which determines the entrance attenuation of the dc signal to be measured. Highest sensitivity is obtained whenmu_{0}Kis small, which requires a low initial permeability, since a large dimensional ratiol/A^{1/2}is usually not attainable. When the demagnetizing term is not small, the sensitivity advantages of the other factors are negated by the input signal attenuation. Power considerations show that increasing sensitivity by raising area, length, or frequency results in increased power consumption. However, increasing sensitivity by using a pickup coil with a larger number of turns or by a sensor core with lower initial permeability can be achieved without additional power.     

Gap-tolerant half-disk bubble device margins

A simple model is presented which allows accurate prediction of bias margins of gap-tolerant half-disk propagation tracks for bubble domains. After this is verified by comparison with experimental margin data, an "isomargin" plot is derived to show how the margin varies as a function ofWandG, whereWis the minimum linewidth andGis the inter-bar gap. The bias margin is shown to decrease along a fairly straight line which goes to zero whenW + Gequals the runout diameter, i.e., whenW+G approx 1.5 W_{s}, where Wsis the bubble stripwidth or average bubble diameter. This agrees with experiment, and means that the minimum resolvable feature for half-disk type patterns must be less than0.75W_{s}, and probably will not be much larger than0.5W_{s}to0.6W_{s}. It is concluded that, if made with perfect Permalloy, T-bars and half-disks should propagate isolated bubbles equally well. The advantages of half-disks over T-bars are 1) the fatal bar-crossing problem of T-bars with multiple bubbles is avoided, 2) the minimum propagation field is lower than for T-bars, and 3) half-disks seem more tolerant of "bad" (e.g., high-coercivity) Permalloy. Also tabulated are the effects on margins of variations in the device parameters of a representative design, as might be encountered in a fabrication process with finite tolerances. A brief discussion of stop-start margins is given in conclusion.     

A statistical model of core noise in half-wave push-pull magnetic amplifiers

The core noise was measured for the half-wave push-pull magnetic amplifiers using various ferromagnetic materials as functions of the number of laminations of a stacked ring core and ac supply frequency. A statistical model of the core noise is proposed. It was deduced that the mean number of magnetic elementary regions associated with the core noise is proportional to values ranging fromftof^{3/5}, wherefis the ac frequency.     

Magnetic Properties of Interacting La0.67Sr0.33MnO3 Nanoparticles

Magnetic nanoparticles of La_0.67Sr_0.33MnO₃ (LSMO) with mean particle sizes of 13, 16, 18, and 21 nm were prepared by the sol?Cgel method. The samples were characterized by X-ray diffraction (XRD) using Rietveld refinement and transmission electron microscope (TEM). Fourier transform infrared (FTIR) transmission spectroscopy revealed that stretching and bending modes are influenced by annealing temperature. Dc magnetization versus magnetic field of the samples was carried out at room temperature. Magnetic dynamics of the samples was studied by the measurement of ac magnetic susceptibility versus temperature at different frequencies and ac magnetic fields. A frequency-dependent peak was observed in ac magnetic susceptibility versus temperature which is well described by Vogel?CFulcher and critical slowing down laws, and empirical $c_{1} = \frac{\Delta T_{f}}{T_{f}\Delta (\log _{10}f)}$ and $c_{2} = \frac{T_{f} -T_{0}}{T_{f}}$ parameters. By fitting the experimental data with Vogel?CFulcher magnetic anisotropy energy and an effective magnetic anisotropy constant have been estimated. The obtained values support the presence of strong interaction between magnetic nanoparticles of LSMO.     

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.     

Magnetic properties of evaporated CoCr films

Perpendicular magnetic CoCr films were prepared on glass substrates using electron beam evaporation. The magnetic properties depend strongly on the substrate temperature, Ts. The anisotropy field,H_{k}^{eff}, and the perpendicular coercivity, Hc(perp) show maximum values at Tsaround 250°C. In this case, the alignment of c-axis is optimal and the grain size is smallest. The lattice spacing of the c-planes increases wlth Tsuntil 300°C. When the films prepared below 200°C were annealed around 300°C in high vacuum, the saturation magnetization, Ms, increased. The value of Ms, however, decrease by annealing above 400°C. In spite of the decrease of Msdue to the annealing above 400°C,H_{k}^{eff}and Hc(perp) increase by annealing above 400°C. If the films were bombarded by argon ions during film growth, Msdecreased and the internal stress changed from tensile to compressive. For low substrate temperature (below 150°C),H_{k}^{eff}decreased due to ion bombardment.     

Optimum permanent-magnet dimensions for repulsion applications

The repulsive force between identical right-circular-cylindrical permanent magnets is calculated, and the ratio of this force to the weight of a magnet is maximized through optimization of magnet size and shape. It is assumed that the magnets are uniformly magnetized along their axes. For magnets of thicknesstand radiusR, separated by a distance2h, the optimum dimensions areR = 3.45handt = 2.7h. For these dimensions, the repulsive magnetic force has the value2.65 mu_{0}M^{2}h^{2}newtons, with the magnetizationMin At/m andhin meters.     

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