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 recent development of permanent magnet materials containing rare earth metals

The prospect that permanent magnets with previously unattainable coercivities and energy products might be made from cobalt-rare earth alloys has caused intense research efforts in the last three years. Alternative ways of preparing magnets from powders and by casting were demonstrated in several laboratories.(BH)_{max} approx 20MG.Oe andMH_{c} > 25 000Oe have been achieved with SmCo5, and the development of manufacturing processes for magnets made from this alloy has begun. This paper reviews the basic concepts, properties of the alloys of interest, and the physical factors influencing the coercive force. Approaches to alloy, powder, and magnet fabrication are discussed, with their merits and drawbacks; also problems incurred in the materials development and their possible solutions. Application areas are reviewed and some economic factors considered. It is concluded that theRCo5magnets are indeed beginning to live up to their promise, but that more materials research, process development, and circuit redesign are needed if their potential is to be fully utilized.     

Investigations of high-coercivity alnico alloys

For Alnico alloys containing 32-42 percent Co, 0-9 percent Ti, 6-8 percent Al, 14 percent Ni, 3 percent Cu, 0-0.5 percent S, 0-2 percent Nb, remainder Fe, the liquidus and solidus temperatures and the upper limit of the (α + γ) phase field were determined. The greatest influence is exerted by titanium in that all temperatures decrease as the Ti content increases. The influence of cobalt on temperature is considerably smaller, while sulfur has no influence at all. The addition of 2 percent Nb resulted only in the upper limit of the (α + γ) phase field being shifted to temperatures 20 to 30°C lower. Alnico alloys containing about 39 percent Co and 7-8 percent Ti were brought to columnar crystallization by controlled cooling, using a seed crystal. The seed contained 6 percent Ti and the lowering speed was between 0.50 and 3.75 cm/h. At a lowering speed of 0.50 cm/h and a temperature gradient of 5 to 10°C/mm, it was possible to obtain columnar crystallization without using a seed. The following magnetic values were obtained:(BH)_{max} = 11.0-11.7MG.Oe,BH_{c} = 1780-2100Oe,B_{r} = 8920-10 050gauss.     

Preparation of thin films in the system γFe2O3- Fe3O4for recording media by spray pyrolysis of organometallic solutions using an ultrasonic pump

Thin films of magnetic spinel iron oxides were prepared by pyrolysis of an ultrasonically generated aerosol of organometallic compounds. The bases of the method and experimental procedure are described. Polycrystalline films of good adherence and homogeneity were obtained. Lattice parameters were observed to be modified by film substrate strain interaction and to depend strongly on the hydrogen content. The oxidation state of iron was discussed in regard to the experimental growing conditions. Corresponding magnetic properties were investigated. Pure magnetite Fe3O4films withH_{c} = 400Oe andM_{s} = 400EMU/cm³and gamma ferric oxides films withH_{c} = 400Oe andM_{s} = 250EMU/cm³have been obtained without post deposition heat treatment. Annealing magnetite films in air resulted in a considerable increase of their coercivity (up to 800 Oe).     

New magnetic materials derived from chromium dioxide

A new oxidative method of preparation of the chromium dioxide from Cr2O3is described. For the improvement of magnetic properties, in particular coercive field, of such a ferromagnetic oxide a substitution by rhodium +IV leads to the new interesting material Cr1-xRhxO2with needlike particles (H_{c} simeq 500Oe).     

A new magnetic alloy of high constant permeability

Several conventional alloys of constant permeability used previously were of lower values of permeability, or they would change obviously in different intensities of magnetic field and thus could not satisfy some specific requirements of electrical application. The alloys consisted of Ni 60-70 wt%, Fe 29-39wt% and Mn 0-1 wt% were investigated. The best of those alloys for certain use is 1J66 alloy in China. It is a new alloy of high constant permeability with the chemical composition as Fe 34 wt%, Ni 65 wt% and Mn 1 wt%. Its AC induction permeabilitymu_{L}= 3400G/Oe (f=60Hz). Within the range of Bm=20-6800 G, the stability of AC induction permeability (f=60Hz)alpha_{sim} = 4.3%. The α is defined asalpha = mumax-mumin/mumin,alphamaxandmuminare the maximum and the minimum of permeability respectively in the range of given magnetic field. The temperature stability αTof μLwithin the range of temperature from -60°C to +90°C is 2.9%. The saturation magnetic induction of 1J66 alloy equals 13500 G. The remanenceB_{r} < 150G. The coercive forceH_{c} = 0.05Oe. The oxygen content of 0.015- 0.04% in the alloy is considered necessary for obtaining a good stability of permeability.     

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     

Investigation of the inverted state of nuclear magnetic levels in Co59films

Experiments that yield inversion of the Co⁵⁹thin film nuclear system were carried out. For nanosecond magnetization reversal by a field with a growth rate greater than5 cdot 10^{11}Oe/s, 85% inversion was obtained. The investigation of the inverted state at frequencies corresponding to hexagonal (α) and f.c.c. (β) phases revealed that Co⁵⁹-β switches 1.3 times faster than Co⁵⁹-α. This is explained by the large contribution of rotational processes for Co⁵⁹in β phase. Longitudinal relaxation time T1is measured by the method of pulse inversion.     

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.     

Filament-size dependent critical current of multifilamentary Nb3Sn conductors

The critical current densityjmin{c}max{ov}of multifilamentary Nb3Sn conductors depends on the diameter of the filaments. At low fields (less than about 12 T)jmin{c}max{ov}increases with decreasing filament diameter because of the finer grain size. At high fieldsjmin{c}max{ov}increases with increasing filament diameter. Measurements of the critical current under externally applied tensile stress revealed that changes of the internal compression of the Nb3Sn within the composite account for this effect. The larger the filament diameter, the smaller the compressive strain, the higher the upper critical field and thereforejmin{c}max{ov}at high fields. The decrease of compressive strain with increasing filament size results from a decrease in the yield stress of the bronze which is caused by an increase in grain size of the bronze between the filaments.     

Electron microscopy on high-coercive-force Co-Cr composite films

Electron microscopy was used to analyze the crystallographic and magnetic structures of high-coercive-force Co-Cr composite films. The chromium sublayer appears to give high coercive forces by causing the growth of cobalt in the hexagonal phase with a relatively narrow distribution of grain sizes. Magnetization reversal was examined by Lorentz microscopy of films with coercive fields up toH_{c} = 900Oe. It proceeds by nucleation and extension of magnetostatically coupled domains. The progressive limitation of these extensions as the coercive force increases illustrates the connection between the magnetostatic coupling, the coercive force, and the squareness of the hysteresis loop. Lorentz microscopy was also used to investigate a recorded transition between NRZ 1 and 0 states and showed a saw-toothed structure, characteristic of a magnetization in the plane of the film.     

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.     

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.     

Temperature Stability and Flux Losses Over Time in Sintered Nd–Fe–B Permanent Magnets

Demagnetization occurring with time in a constant temperature is often assumed to be negligible, provided that the operating point of the magnet is above the knee of the $Bhbox{–}H$ curve. However, because it is important to ensure that no permanent flux losses occur in permanent magnets during their use in industrial applications, there is a clear need to quantify these time-dependent losses. We measured losses over time for four commercial sintered Nd–Fe–B magnet grades at five different temperatures (23$,^{circ}$ C–150$,^{circ}$ C). We tested samples with three different permeance coefficient (Pc) values for each material. The time-dependent losses measured fitted the logarithmic law of magnetic viscosity well. We demonstrated that the total flux loss in a lifetime of 30 years can be estimated according to the temperature, coercivity of the material, and the permeance coefficient of the magnet. With proper selection of the magnet material, in accordance with the designed Pc of the application, the total flux loss in 30 years can be minimized almost to zero even at 150$,^{circ}$ C.      

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.     

Magnetization of Ni-Fe-Mo Electrolytic films at low temperatures

H_{c), J_{s}, and Jrwere measured on Ni-Fe-Mo films at low temperatures. As the temperature dropped from 300 to 80°K, the Hccoercive field of Ni-Fe-Mo films with 0 percent content of Mo decreases from 1800 to 680 A/m. On the contrary, for the films with 5 and 10 percent Mo, the Hcincreases. The magnetization Jsvaries with temperature in the same sense as Hc. From the curvesJ = f(H/T)atT_{1} = 300degK andT_{2} = 80degK, it is found that the Ni-Fe-Mo electrolytic films with a concentration of over 10 percent Mo present properties characteristic of the superparamagnetic state.     

0.25 × 106bit/in2NDRO coupled film memory elements

The fabrication and operation of high-density (0.25 × 10⁶bit/in²) nondestructive readout (NDRO) memory elements are described. The high density is made possible by coupled films and Permalloy keeper. The NDRO is made possible by multiple-pulse WRITE or hard-direction bias field. Typical performance parameters areI_{w} = 60mA,I_{b} = 30mA, andV_{s} = 150μV/3 ns. The small signal is detectable by multiple-pulse READ. When such memory elements are to be fabricated with peripheral circuits on the same Si chip, a self-contained chip will be obtained. Such chips could enjoy the same advantages as semiconductor memory chips such as few leads, modularity, amenability to bit organization, and possibility for error-correction, but would be capable of higher storage density due to simpler planar configuration.     

The development of <100> texture in Fe-Cr-Co-Mo permanentmagnet alloys

The cold-rolled and recrystallization textures of Fe-Cr-Co-Mo permanent magnet alloys are described. The studied composition is Fe-30%Cr-15%Co-3%Mo (in wt.%). The cold-rolled texture can be considered to be {111}<110>, {111}<112>, {100}<110>, and {211}<110>, while the recrystallization texture can be considered to be {111}<100>, {110}<112>, {211}<110>, and {110}<110>. The secondary recrystallization is caused by heat-treating the alloys in the sequence of α, α+γ, α+γ+σ, α phase region. This results in a favorable texture of {110}<110> and <100> direction, aligning along the transverse direction (TD) of the strips. The best magnetic properties obtained in this study were 1.2 T (12.0 kG), iH _c=82.0 kAm^-1 (1025 Oe), and (BH)max= 60.8 kJm^-3 (7.6 MGOe) with TD alloys     

A study of narrow linewidth Li-ferrite

Using conventional ceramic procedure and cheap raw materials, we have succeeded in preparing narrow linewidth polycrystalline Li-ferrites with the chemical formula: Li0.5(1-x+y)ZnxTiyFe2.5-0.5(x+3y)-zMnzO4in the region of0.15 < x leq 0.30,0.85 < y < 1.10,0.03 leq z leq 0.05and observed the growth condition of the grains. The performances obtained are:4piMs=200-460Gs,DeltaH=33-23Oe, Tc=104-147°C,tg delta e < 7 times 10^{-4}. The effect of the preparation technology and the composition was discussed. The results show that the growth procedure of the grains is closely related with the distribution of the particle sizes of the powder. The experimental results show that this ferrite is suitable for applications in some microwave devices, such as low power circulators, due to its good performances and low cost.     

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.