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.     

Changes in recording tape magnetization produced by stress

When mechanical forces are applied to single domain particles that are used in magnetic recording tapes, the resultant stress anisotropy energy alters the magnetized state, producing reversible and irreversible changes. We use four different experimental techniques to measure these changes and analyze the results in terms of the values for the saturation magnetostriction constants (λs) and the applied stress (σx). The reversible changes in magnetization (Mx) are roughly proportional tolambda_{s}sigma_{x}; the greatest irreversible or loss component occurs when the product of σxand λsalong the magnetizing axis (x) is negative, increasing when major easy axes make large angles withx.     

Electric and magnetic properties of Eu1-xGdxS films and GdS-EuS film junctions

Polycrystalline Eu1-xGdxS films (0 < x < 0.35 and x = 1; thickness is 40 to 600 nm) were prepared by electron-beam and flash evaporation onto heated substrates. The electrical and magnetic properties of these films were investigated through resistivity and hysteresis measurements at temperatures between 4 and 300 K, with a magnetic field, B, of 0.4 Tesla. After a short discussion of intrinsic differences between the electric and magnetic behavior of films and single crystals, the first results on I-V characteristics of isomorphic GdS-EuS-GdS film junctions are presented. With increasing x and/or lattice-defect concentration, the Curie temperature, TC(x), increases to about 150 K, while the magnetization Js(x), strongly decreases (extrapolating to Js=0 at x=0.5). Typical properties of the GdS-EuS-GdS junctions are the N-type I-V characteristics and the attributed current oscillations at T ≪ TC, the change to Ohmic I-V characteristics for T approaching TC, and the large negative magnetoresistance, ranging up to ΔR/R/ΔB = -10 T^-1, at small B and T > TC, where ρ(T) passes a broad maximum.     

Performances of metal pigment tapes with 600-700 Oe coercivity

Audio or video recording performances of various magnetic tapes including Co-modified γ-Fe2O3, metal and Fe4N tapes having their coercivity range of 600-700 Oe were investigated. The tapes were prepared by the conventional method using the above magnetic pigments which were prepared to give their coercivity range of 600-800 Oe. The electromagnetic characteristics of the tapes were discussed in comparison with the commercial reference tape. The metal tapes were superior to the other two tapes on Sensitivity (S), Frequency Properties (F), Maximum Output Level (MOL) and Dynamic Range (D.R.) The Fe4N tape has an excellent property on Distortion (THD).     

High field properties of ternary metal-molybdenum-sulfides

Measurements of the upper critical fields are presented for ternary metal-molybdenum-sulfides as a function of temperature, T, with dc magnetic fields up to ∼215kG and pulsed fields to ∼500kG. A nearly linear variation of HC2versus T is observed up to the highest dc fields for all the compounds. The materials (nominal composition), Tc(measured);(dH_{C2}/dT)_{T=T_{c}}(measured), and HC2(4.2 K) (calculated) for a dirty type II superconductor assuming no paramagnetic limiting are respectively: (a) Pb0.9Mo5.1S6: 11.7 K, 60 kG/K, 390 kG; (b) SnAl0.5Mo4.5S6: 14.2 K, 32 kG/K; 275 kG; (c) SnMo5S6:13.4K, 37kG/K; 290 kG; (d) PbMo5.1S6: 14.4, 60kG/K, 515kG. Pulsed field measurements yield HC2(4.2 K) = 390 kG for (a), 275 kG for (b), 290 kG for (c), and ≃510kG for (d). The PbMo5.1S6has the highest value of HC2measured to date.     

Effects of different rare earths on the temperature dependence of rare earth cobalt magnets

In many applications, it is necessary to have permanent magnets whose characteristics vary little around a temperature Tr. We have developed a calculation to describe the thermal variation of the magnetization of the rare earth (RE) in such magnets including the effects of crystalline field. The exchange field of cobalt has been deduced at all temperatures from magnetization measurements on a single crystal of YCo5. From the calculation we can deduce the concentration x0, which gives dM/dT = 0 around room temperature, the resulting magnetization and the correspondingd^{2}M/dT^{2}.     

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.     

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.     

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.     

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).     

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.     

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.     

Preparation of coated iron powders by chemical plating

Small iron particles can be produced by reduction of γ-Fe2O3and α-FeOOH with hydrogen; they show outstanding magnetic properties and can be used for recording media. The main difficulty concerns the tendency of these particles to oxidize. We have studied a preparation process in two steps: a) reduction of iron oxides or oxy-hydroxides; b) chemical plating in the same furnace. The magnetic properties of the iron powders chiefly depend on the reduction step. The water content in the gas leaving the furnace has been shown to be the main parameter determining the obtainment of powders with high magnetic properties. The iron powders, which were pyrophoric, have been quenched in chemical plating bath of suitable formulation and coated with cobalt or copper. Typical magnetic properties are: saturation magnetization σsat= 130-165 emu/g, intrinsic coercivityjHc= 350-450 Oe; best magnetic properties: σsat=155 emu/g,jHc=700 Oe. The coated powders have been submitted to heat treatment in air saturated with water, showing a good resistance to oxidation.     

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 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.     

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.     

Magnetoresistance,Irreversibility Fields,and Critical Currents of Superconducting 2G Tape

The magnetoresistance, irreversibility fields, and critical current density were studied for a commercial 2G tape at the two relative orientations of magnetic field and superconductor plane. The critical temperatures of this tape of T _{c50 %} = 91.5 K and T _c0 = 90 K and the width of superconducting transition of ΔT = 1.5 K were obtained. The widths of the transition from the normal to the superconducting state do not increase at the applied magnetic field up to 90 kOe and do not depend on the orientation of the magnetic field with respect to the tape plane. The irreversibility field values were obtained and successfully fitted as a function of temperature using the formula: \(H_{\text {irr}} =H_{\text {irr0}} \left ({1-\frac {T}{T_{\text {c0}} }} \right )^{n}\). The irreversibility fields show an anisotropy, and at the liquid nitrogen temperature, they reach H _irr = 430 kOe and H _irr = 106 kOe for the parallel and perpendicular directions, respectively. The anisotropy ratio amounts to γ = 4 at 77 K and is small in comparison with other high-temperature superconducting materials. The critical current density of this tape was found to be of the order of 10⁶ A cm^?2 at 77 K in the self-magnetic field.     

Magnetic interactions in pseudobinary rare earth-3d intermetallics

Magnetic measurements on pseudobinary intermetallic compounds of the series R(MnxFe1-x)2(R = Y, Gd, Er) R6(MnxFe1-x)23(R = Y, Gd, Tb, Dy, Ho, Er, Tm) R2(FexCo1-x)17(R = Y, Gd, Dy) R2(FexNi1-x)17(R = Y, Gd) are reported. Magnetization and susceptibility measurements have been performed at temperatures from 4 K to 1500 K. The magnetic interactions between Mn and Fe are discussed in terms of a localized-moment model. In the case of the R2(FexM1-x)17(M = Co, Ni) compounds, a band model seems to be more appropriate. Mössbauer Fe⁵⁷spectra obtained on the R2(Fe,Co)17series can be explained by a superposition of four different six-line patterns corresponding to the four crystallographically nonequivalent 3d sublattices.     

Permanent magnetic materials with low reversible temperature coefficient

Permanent magnets made from SmCo5exhibit negative reversible change in magnetization with increasing temperature, typically of the order of 0.04% per deg C between -100 and +200°C. For certain special applications in precision instruments such as gyros and accelerometers, it is highly desirable to improve this property. Earlier studies have shown that ternaries of the composition RxSm1-xCo5(where R = Gd, Ho, Er, or Dy and x is nearly 0.4) exhibit improved temperature compensation. A systematic investigation of the temperature coefficients of magnetization of a number of quaternaries in the temperature range 100-400 K has been undertaken. The effect of variation of the cobalt concentration on the temperature compensation of one typical system has also been examined. The results indicate that good temperature compensated magnets can be synthesized with composition Sm0.6Gd0.3DY0.1Co5(α = 0.0056 at 200-300 K; 0.002 at 300-350 K and ∼0 at 350-400 K). Slight variation of cobalt concentration does not have significant effects on the temperature compensation. Lattice constants, saturation magnetization, and the theoretical energy products of a number of quaternaries are also reported.     

A magnetic recording tape based on iron particles

Tapes based on acicular iron particles give an outstanding recording performance compared to γ-Fe2O3and CrO2tapes. Especially in the short wavelength region the signal to noise ratio is, respectively, 12 and 7 dB better, whereas the bias noise level is somewhat lower. The iron tapes have a high coercivity, between 80 and90 times 10^{3}A/m, and hence the required bias is about 9 dB higher than for γ-Fe2O3. The print through properties are excellent.