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.     

Relationship of bias field setting procedure to field stability against external field perturbations for magnetic bubble memory bias field structures

In magnetic bubble memory packages having a bias field HBsupplied by a Ba-ferrite permanent magnet structure, the magnitude of HBmay be appreciably altered after setting by transient exposure to externally applied magnetic fields Hextas much as 10 times smaller than the field used in setting HB. We examine this effect for a particular magnet design having a permalloy yoke, a gap of 0.270 inches, and a saturation fieldH_{sat} simeq 240Oe. We find that the magnitude of the effect depends upon the ratioH_{B}/H_{sat}and upon the procedure used in setting HB. After setting toH_{B} = 200Oe from saturation with a demagnetizing fieldH_{ext} = -1400Oe, a remagnetizing fieldH_{ext} simeq 1800Oe is required to increase HBby 1%. On the other hand, after setting toH_{B} = 100Oe withH_{ext} = -2300Oe, a remagnetizing fieldH_{ext} simeq 400Oe is sufficient to increase HBby 1%. Setting by demagnetizing from saturation yields superior stability to setting by magnetizing from the demagnetized state, and stability of the set magnet may be further improved by demagnetizing with a ringing (alternating) field. This behavior is explained with a simple model and its importance for magnet design is discussed.     

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.     

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.     

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.     

Domain models of eddy current loss due to normal magnetization in slightly misoriented

In Goss-oriented SiFe laminations, regions exist with a significant normal magnetization component arising from dip misorientation of the crystallites. It has recently been suggested by Imamura and co-workers that this normal magnetization can make a major contributionP_{perp}to the eddy current losses incurred during cyclic magnetization along the texture axis.P_{perp}is calculated for three distinct domain models of the magnetization process in laminar crystallites with modest dip misorientation. Contrary to the Imamura work,P_{perp}is found to be practically negligible for a simple slab domain structure without closure domains. This is because the normal demagnetizing field restricts normal induction to the neighborhood of the domain walls and ensures that its volume average is practically zero. In a second model monotonic reversal of normal magnetization in closure domain structures is also shown to give rise to rather low normal lossP_{perp}. A third model concerns how closure domains may interact with the major boundaries between slab domains and be swept along with them. Quite substantial lossesP_{perp}are predicted if all closure domains in a zone extending a distance rather greater than the sheet thickness either side of the major domain wall are displaced in this way. However, even this mechanism can explain only a small part of the marked increase in eddy loss with dip misorientation reported, and it is concluded that normal flux is probably not primarily responsible.     

Laser annealing of gallium-substituted garnet epitaxial films in pressurized oxygen

The effects of laser annealing Ga-substituted EuYIG epitaxial films in still, controlled ambients of air and one to five atmospheres of pure oxygen have been studied. An approximately logarithmic increase in saturation magnetization (4pi M_{s}) and decrease in film magnetic Q factor were observed with increasing oxygen pressure in the annealing ambient. Compared with the most successful previous experiments, annealing at five atmospheres of oxygen displaced nearly twice the fraction of Ga from tetrahedral to octahedral lattice sites. As evidenced by the lack of film damage, the correlation of lattice parameter with the location of the Ga ions and the lack of broadening in the ferromagnetic resonance (FMR) lines, effective annealing temperatures above 1850K were achieved with no significant loss of oxygen from the films. The use of FMR to deduce the extent of film annealing was affirmed through the agreement of independent measurements of4piM_{s}by bubble statics and SQUID magnetometry in films indicated to be homogeneous by FMR.     

A new permanent magnet material: Modified Ca ferrite

An experiment was carried out to investigate the preparation condition of anisotropic Ca system ferrite magnets with optimum magnetic and physical properties. Compositions were chosen according to the formula (CaO.nFe2O3)100-x(La2O3)x, wherexwas varied between 0 and 4.0, and n between 5.5 and 6.25. The optimum condition of making magnets and some properties of a typical specimen are as follows. The preparation condition: composition (CaO.6Fe2O3)97(La2O3)3, semisintering condition 1250°C × 1 h in O2gas, sintering condition 1275°C × 0.5 h in O2gas. Magnetic properties:4piI_{10}k = 4200G, 4πIr = 4100 G,IHC= 2100 Oe,BHC= 2050 Oe,(BH)_{max} = 3.5MG . Oe, Ku = 3.34 × 10⁶erg/cc, HA= 20.5 kOe,sigma_{s} = 64.8emu/g, Tc = 446°C.     

Influence of an in-plane magnetic field on the domain-wall velocity in Ga: YIG films

Results are reported of an investigation on the velocity of a single straight magnetic domain wall in a Ga:YIG film as a function of the drive field and of a static magnetic field applied perpendicular to the wall in the plane of the film. At all drive fields a substantial increase of the wall velocity was observed when the in-plane field was applied. At an in-plane field of about 400 Oe and at a rather low drive field (2.4 Oe above the coercive field) a maximum value in wall velocity of 270 ms^-1was observed. At higher drive fields the wall velocity decreased to a constant value of 110 ms^-1, independent of the drive field. This behavior can be explained by extending Slonczewski's theory of domain wall motion to the present case. From the observed wall mobility parameter we have calculated the reduced Landau-Lifshitz damping constantlambda/gamma^{2}(3.7 times 10^{-9}Oe²s). This value is near to the value obtained by Spencer and LeCraw from linewidth measurements in FMR on Ga:YIG spheres (5 times 10^{-10}Oe²s).     

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.     

Properties of amorphous alloy films mainly composed of Co--Nb

Amorphous alloy films, mainly composed of Co-Nb, have been studied by radio frequency (RF) sputtering. These amorphous films having small magnetostriction show excellent soft magnetic properties after the magnetic annealing we have developed. One of them with the composition of (Co83.5Fe2)92/85.5Nb8was found to have very high saturation magnetizationB_{s} = 14300(gauss). The effects of additives such as Zr to the alloys have been investigated and amorphous films having high crystallization temperature as well as high saturation magnetization were successfully obtained. These amorphous films show fairly good wear and corrosion resistance as well as excellent thermal stability of the magnetic properties.     

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.     

Direct Modeling of Motional Eddy Currents in Highly Saturated Solid Conductors by the Magnetic Equivalent Circuit Method

Motional eddy currents are induced in conductors due to a localized $vec{v}timesvec{B}$ electric field. Direct modeling of this term by finite-element-based analysis may lead to nonphysical oscillations. In this paper, two modified magnetic equivalent circuit (MEC) methods to calculate motional eddy currents in highly saturated moving solid conductors are presented. The comparison between the results of the proposed methods with the finite-element method (FEM) proves high accuracy of the proposed methods in a wide velocity range for highly saturated solid conductors. The mesh sensitivity analysis shows the higher stability of the calculations of the proposed methods, compared with FEM, as no numerical oscillations occur.      

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.     

Magnetoresistance and Magnetic Anisotropy Properties of Strain-Induced Co/Ag Multilayer Films

We report on the magnetoresistance (MR) properties of [Co(tCo)/Ag 1.5 nm]$_{20}$ multilayer and alloy films grown with the pulse electrochemical deposition on a polyamide substrate (1 cm$^{2}$ ). The induced uniaxial magnetic anisotropy was observed due to the effect of strain in all the multilayer films. The multilayer [Co 1.5 nm/Ag 1.5 nm]$_{20}$ showed a minimum hysteresis loss. The maximum MR ratio for Co/Ag was 9.2% at 1 kOe. A remarkable difference of magnetic field dependence of the magnetoresistance ratio was observed, corresponding to the orientation of magnetization curves.      

Surface impedance of superconducting Nb3Sn

Microwave cavities with a resonant frequency of 8 GHz are coated with Nb3Sn by the vapour deposition technique. The surface resistance and the change of the penetration depth were determinded by measuring the quality factor and the shift of the resonant frequency of the cavity in the temperature range from 2 K to 20K. The temperature dependence of the surface resistance can be described well by the BCS-theory in the temperature rangeT < 0.5 T_{c}, however, the value of the reduced energy gapDelta_{0}/kT_{c}has to be increased from 1.76 to 2.15. The temperature dependence of the penetration depth shows significant deviations from the predictions of the BCS-theory for temperatureT < 0.5 T_{c}. The increase of the reduced energy gap is not sufficient to fit the data but one has to treat the effects of strong electron-phonon coupling consistently. Therefore, we calculated the surface impedance for strong coupling superconductors using an Einstein model for the phonon density of states. The absolute value and the temperature dependence of the surface impedance in the whole temperature rangeT < T_{c}are discussed and a comparison with the experimental data is given.     

Effect of Interface Roughness on Magnetoresistance and Magnetization Switching in Double-Barrier Magnetic Tunnel Junction

The three kinds of double-barrier magnetic tunnel junction (DMTJ) with or without amorphous ferromagnetic Co$_{70.5}$ Fe$_{4.5}$ Si$_{15}$ B$_{10}$ (in at. %) free-layer were investigated to understand the effect of the free-layer on the bias voltage dependence of tunneling magnetoresistance (TMR) ratio. The DMTJ structure consisted of Ta 45/Ru 9.5/IrMn 10/CoFe 7/AlO$_{rm x}$/free-layer 7/AlO$_{rm x}$/CoFe 7/IrMn 10/Ru 60 (thickness in nm). Various free layers, such as CoFe 7, CoFeSiB 7, and CoFe 1.5/CoFeSiB 4/CoFe 1.5 were prepared and compared. The roughness values of the interface between free-layer and tunnel barrier were confirmed by using the techniques of X-ray reflectivity and transmission electron microscopy. As a result, the amorphous free-layer made the interface roughness of DMTJ smoother, reducing the interlayer coupling field and suppressing the bias voltage dependence of TMR ratio at a given voltage.      

Rayleigh hysteresis--A new look at an old law

Experimental evidence is cited that, for very small hysteresis loops, the remanent magnetic flux density Bris not proportional to the square of the amplitude of the magnetic field excursion H1(as given by the Rayleigh relation), but rather to the first power. It is shown that (subject to one very general assumption), were this dependence exact, it would lead to negative hysteresis loss. It is also shown that (subject to the same assumption) any power-series dependence ofBonHwill always giveB_{r} propto Hmin{1}max{2}. An approximate first-power dependence is found to follow naturally from the spring model. A new Rayleigh relation is proposed which contains a logarithmic term which may not be expanded in a power series about zero argument.     

Induced in-plane magnetic anisotropy in YIG films

We have studied the symmetry properties of the in-plane induced magnetic anisotropy in {100} and {110} disks of single crystal YIG epitaxially grown on gadolinium gallium garnet (GGG) substrates. The ferromagnetic resonance (FMR) technique was used to determine the values of the cubic and induced magnetic anisotropies. The induced anisotropy is interpreted in terms of a magnetostrictive magnetic potential energy. We find that the effective field induced in the plane of the film is isotropic for {100} disks but for {110} disks the induced anisotropy is uniaxial within the plane.     

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.