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.     

Dielectric behaviour of a ferrofluid subjected to a uniform magnetic field

The electric susceptibility of samples of ferrofluids subjected to a uniform magnetic fieldHwas measured. The electric susceptibilitychiis dependent on the magnitude of the magnetic field and on the relative direction between the electric fieldEand the magnetic fieldH. 1) WhenEis perpendicular toH, frac{partialchi_{perp}}{partialH} < 02) WhenEis parallel toH, frac{partialchi_{parellel}}{partialH} > 0These results have been interpreted as a magneto-electric directive effect. A model is proposed, based on the assumption that the magnetic particles are roughly ellipsoidal and conducting grains.     

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.     

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.     

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.     

A model of magnetostriction interaction in thin double-layer magnetic films of Ni-Fe alloy

Under the assumption that in a double-layer film both layers are equally deformed, the following formula for the magnetostriction energyE = K_{c} sin^{2} alphahas been derived whereK_{c} = (9/40)(2U'U"/U'_{0} + U_{0}")is the coupling constant and α is the angle between the magnetization vectors. This formula is valid if we can neglect interaction with the substrate. The sign of the coupling constant is discussed. It turns out that where the composition of the two layers are different, the value of Kccan be, in some special cases, negative.     

Ultimate performance limitations of high-power ferrite circulators and phase shifters

The power-handling capability of circulators and phase shifters in rectangular waveguide is analyzed. It is shown that an appropriate measure of the suitability of microwave ferrites for high-power applications is given by the "high-power figure of merit"F_{hp}* = 4piMgamma^{2}h_{crit}/omega^{2}mu'. Here γ is the gyromagnetic ratio, hcritthe critical RF field, ω the (angular) frequency,mu"the imaginary part of the diagonal component of the permeability tensor. In applications to nonlatching devices,Mis the saturation magnetization; in applications to latching devices it is the remanent magnetization. The figures of merit of various rare-earth substituted garnets are reported. The figures of merit obtained to date are approximately 2. Theoretical analysis indicates that significantly larger figures of merit can be Obtained only at the cost of reducing the saturation magnetization. It is concluded that for circulators using theH-plane configuration and having an insertion loss of 0.5 dB the maximum attainable peak power level is of the order of 50 MW. For latching twin slab phase shifters (E-plane configuration) having an insertion loss of 1 dB the maximum attainable peak power level is estimated to be 80 kW.     

Coupling between flat films

The magnetization distribution in a rectangular film, magnetized parallel to one side, is approximated by assuming that the magnetizationoverrightarrow{M}has the same direction everywhere in the film and that its magnitude rises from zero at the edge to the saturation magnetization Msat a distancecfrom the edge, maintaining this magnitude throughout the remainder of the film area. The function representing the magnitude ofoverrightarrow{M}in the edge region is chosen in such a way that the resulting field has no singularities,cis determined by the coercive force, which is set equal to the sum of the maximum demagnetizing field and an applied reversing field. This model lends itself better to the discussion of demagnetizing effects in single or coupled films than either the ellipsoid or the line-charge model. It takes account of the fact that the width of the edge region, where curling and domain walls occur, varies with the applied field, and, in this respect, it agrees fairly well with experiment. It, furthermore, permits the definition of a disturb sensitivity for memory elements consisting of single or coupled films.     

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.     

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.     

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.     

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.     

EBSD Investigation on Oriented Nucleation in IF Steels

The mechanism responsible for the formation of recrystallization texture in cold-rolled Ti bearing interstitial free （IF） steel sheets was investigated using electron back-scatter diffraction （EBSD）. In addition, the origin of nuclei with specific orientations was studied. The formation of recrystallization texture was explained by oriented nucleation. Most nuclei have a high misorientation angle of 25-55° with the surrounding deformed matrices, but no specific orientation of misorientation axis between the nucleus and the surrounding deformed matrix is observed. The stored energy of deformed grains is in the decreasing order of the {111}〈112〉,{111}〈110〉, {112}〈110〉 and {001}〈110〉 orientations. New {111}〈110〉 grains are nucleated within deformed {111}〈112〉 grains and new {111}〈112〉 grains originate in the deformed {111}〈110〉 grains.     

Monte Carlo and series expansion investigations of magnetic surfaces

The presence of a free surface leads to an inhomogeneous distribution of the local magnetization and other quantities describing the magnetic state of the system. After a brief outline of a general thermodynamic theory, the mean field treatment is discussed and shown to be inconsistent with a more general scaling theory, even if one uses a temperature-dependent extrapolation length. More reliable information is obtained by numerical techniques such as high temperature series expansions and Monte Carlo calculations, which are applied to Ising and Heisenberg models. These numerical results are shown to be consistent with the scaling theory, but it is also shown that the magnetization of the surface layer m1depends strongly on the local conditions (e.g., exchange constant Jsdifferent fromJin the bulk). Therefore the observability of the universal asymptotic exponents may be restricted to a very narrow region around the (bulk) critical temperature Tcb. The case where the surface layer orders at a temperatureT_{cs} > T_{cb}is also considered. The consequences of the theory for thin magnetic films and small particles are outlined and experimental applications are briefly discussed.     

Stripe-domain dynamics in bubble-domain circuits

It is widely known that bubble domains can exist when the bias field is between the elliptic instability (runout) field Heand the bubble collapse field Hbc. Values for Heand Hbcwere calculated by Thiele. It is not widely recognized that long stripe domains can also exist in the bottom 20% of this range. Stripes are stable up to the "stripe contraction" field, Hsc, which is about 0.02 M/μ0above Hefor thickness over intrinsic length,h/l, values from 4 to 10. (Hbcis about 0.10 M/μ0above He.) Values for Hscwere calculated by Kooy and Enz of Philips Research Labs. in 1960, although the importance of their result to bubble-domain devices was not apparent at the time. The velocity of the domain tip during stripout and contraction such as occurs during domain detection or transfer is important for calculating maximum circuit speeds. This is given byV = cG (H-H_{sc}), whereHis the field (bias field plus local field from currents or Permalloy elements),Gis the wall mobility, andcis a constant approximately equal to 0.5.     

Bias susceptibility measurements in thin permalloy films

Susceptibility measurements at 5000 Hz have been performed in the easy and hard directions on two 80-20 Permalloy films (1000 Å, 1 cm in diameter) cut from large samples. The experimental results agree with the predictions of the Hoffmann theory. The susceptibility parallel to the average direction ofMis found to be proportional to1/(H - H_{k})^{7/4}or1/(H_{k} - H)^{7/4}when the net field is large enough to prevent blocking.     

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.      

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.      

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.