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.     

Uniaxial and cubic anisotropy constants in ion-implanted Sm,Tm,Ga:YIG bubble thin films

The effects of ion implantation into (SmTmY)3(GaFe)5O12garnet thin films have been studied by ferromagnetic resonance. He+ ions were used for implantation with doses ranging from 3 to 4 × 10¹⁵He+/ cm²and implantation energy ranging from 150 to 175 Kev. The uniaxial and cubic anisotropy constants have been studied as a function of temperature from 0°C to 100°C for both as-grown and ion-implanted films. The implantation has effectively changed an easy-axis anisotropy to that of an easy plane in the implanted layers of the films. A cubic anisotropy constant K1with values varying from 4 × 10³to 6 × 10³erg/cm³at room temperature has been observed in these films. The results also indicated that to the first order, the implanted region in the film was essentially magnetically uncoupled from the bulk of the film.     

Directional solidification of Co-Cu-R permanent-magnet alloys

Co-Fe-Cu-Ce permanent-magnet alloys have been prepared by directional solidification using a modified Bridgman technique. Samples melted at low superheat temperatures (DeltaT sim 20degC above the melting point of about 1100°C) and solidified at moderate rates (∼2.3 cm/h) resulted in a reasonably homogeneous columnar grain structure with a preferred crystallographic orientation. Thecaxis is generally aligned within 15° of the growth axis. Increasing the speed of solidification led to a fine-grained structure with no texture, while decreasing the speed led to coarse columnar grains with erratic orientation. A large superheat temperature (DeltaT sim 300-400degC resulted in a reaction of the liquid with the alumina crucible wall and led to the formation of face-centered cubic Co-rich dendrites. With the modified Bridgman technique, oriented samples 8 cm long and 2.54 cm in diameter have been prepared with good magnetic properties. After annealing at 1000°C followed by aging at 400°C, a Co3.5Fe0.5CuCe alloy exhibited values ofiHc= 7000 Oe, Br= 6100 G, and(BH)_{max} = 9.2MG.Oe. A Co3.6Fe0.5Cu0.9Ce alloy exhibited values ofiHc= 6000 Oe,B_{r} = 6250G, aud(BH)_{max} = 9.5MG.Oe after similar treatment.     

The anisotropy field Hkof thin ferromagnetic films, measured by magnetoresistive methods

Several methods are described based on the magnetoresistance and the planar Hall effect by which the anisotropy field strength Hkmay be determined. The Hkvalues deduced from the different methods are discussed especially with respect to their dependence upon the magnetization ripple and on distinctH_{c}/H_{k}ratios. Although some of the reviewed methods yield Hkvalues not influenced by the ripple hysteresis, only the planar Hall effect method and the modified Kobelev method enable rapid measurements or continuous recording of Hk. These methods are, therefore, found to be especially suitable for measurements of the Hkrelaxation during annealing experiments.     

Difference between temperature dependences of both initial permeability and maximum permeability of sendust alloys

The temperature dependences (-10-50°) of the initial permeability (μ5), the maximum permeability (μmax), and the coercive force(Hc) of Sendust alloys (Fe-9 to 10 wt% Si-5 to 7 wt% Al) continuously cooled to room temperature, were investigated. Most of the studied alloys gave at least one peak value for both μ5and μmax, in the studied temperature range. The temperature giving the peak μ5agreed well with that of μmax(an effective permeability at an alternating field): the peak temperature of μeffseems to correspond to the temperature giving a zero magnetocrystalline anisotropy constant (K1= 0). The relationship between temperature and μ5was not always the same as that between temperature and μmax. From the compositional dependence of μmaxat various temperatures (10-30°C), the zero polycrystalline magnetostriction constant (λs = 0) line of the ternary Sendust alloys at each temperature could be evaluated. It is clarified that μ5of the alloys with K1< 0 is determined by both K1and λs, whereas μmaxis primarily determined by λs. When K1changes with temperature from a negative to a positive value, both μ5and μmaxof the alloys markedly decrease, independently of λs.     

Epitaxial growth and annealing control of FMR properties of thick homogeneous Ga substituted yttrium iron garnet films

Homogeneous, 30 μm to 70 μm thick Ga substituted yttrium iron garnet films have been grown on Y or Al substituted gadolinium gallium garnet substrates having lattice parameters matched to that of the films. Resonance field and FMR linewidth measurements as a function of frequency and annealing experiments revealed that the magnetization and cubic anisotropy of the films are identical to data from flux grown bulk single crystals, the FMR losses of the films are only slightly higher. For films grown with supercooling ΔT < 50 °C a negative, growth induced, uniaxial anisotropy was found which could be removed by annealing in air at 1100 °C. A compensation of the temperature drift of the FMR frequency can be adjusted in the Ga substituted films by changing the frozen-in Ga-Fe cation distribution by annealing and quenching from different temperatures > 800 °C.     

A comparison of frequency limited bubble device performance and material characteristics

We have observed the propagation margins as a function of frequency and temperature for a simple half-disk type test circuit fabricated on an (YSmLuCa)3(FeGe)5O12film. We have also measured the temperature dependence of the stripe width, anisotropy field, collapse field, mobility and dynamic coercivity. The frequency dependence of the margin demonstrates that in this circuit, the material is velocity limited above 300 kHz. In the range -20°C to +60°C, the upper and lower margin limits at a given frequency track the collapse field. Over this same temperature range, the mobility varies from 500 to 850 cm/sec-Oe and we conclude that the mobility is not the parameter which limits the device performance. The calculated saturation velocity is independent of temperature in this region and we propose that it is responsible for the failure of the circuit at the highest frequency. In the range from -20°C to +60°C, the coercivity becomes appreciable and the circuit performance deteriorates due to a decrease in the effective drive field.     

Rotatable magnetic anisotropy in epitaxial ferrite layers

Magnetic domain structure and magnetic anisotropy were studied in monocrystalline epilayers of Mg0.9Mn0.3Fe1.8O4ferrite. The layers, several micrometers thick, were obtained by a CVD method on monocrystalline MgO substrates. Domain observations were performed by the Bitter's method. Magnetic anisotropy measurements were performed by torque and FMR methods. In the demagnetized state, a typical stripe structure of 2.0 to 2.8 μm period was observed. From the domains behavior in the in-plane magnetic fields it was found that in these epilayers the rotatable anisotropy was present. The existence of this anisotropy was confirmed by torque measurements in small in-plane fields. The magnetic parameters characterizing these layers are: 4ΠM = 3500 Gs, K1= - 2.2 × 10⁴ergs/cc, KN= 2.3 × 10⁵ergs/cc.     

Temperature dependence of the effective permeability of heat treated Sendust alloys

The temperature dependence (-10°C-70°C) of the effective permeability (μeff) of Sendust alloys (Fe-9-10 wt% Si-5-7 wt% Al) quenched at a room temperature from the various temperatures (400°C-700°C) was investigated at every 2°C in the temperature range of -10°C to 70°C. Most of the studied alloys gave a peak of μeffin the studied temperature range. The peak temperature (Tp) giving the peak μeffvaried with different alloy composition. The alloys of Tp = 20°C are supposed to correspond to the alloys of the zero magnetocrystalline anisotropy constant (K1= 0) at 20°C. The deduced K1= 0 lines at various temperatures (-10°C-50°C) were obtained for an Fe-Si-Al ternary system. The values of peak μeffat the same Tp were different, depending on alloy composition, This difference is due to the difference in the polycrystalline magnetostriction constant (λ s) of the alloys. The peak temperatures of the studied alloys vary with the different heat treatments. This variation is due to the variation of K1, induced by the change in microstructures of the alloys. The variation of the values of peak μeffwith heat treatments was small compared to the difference of those in the alloys of different composition. It seems that the variation of the λ s with heat treatments would be small. Both the compositional dependence of μeffat 20°C and the temperature sensitivity (Δμeff/ΔT) were obtained for both the alloys cooled continuously to room temperature and those quenched at room temperature from 400°C.     

Magnetostatic interactions in a duplex ferromagnetic alloy

The chill-cast alloy, Fe75Si15B10, consists of submicron rods of Fe2B, a magnetically "hard' phase with high Tc, in a matrix of Fe3Si, a "soft" phase with low Tc. Magnetic properties change markedly on cooling through 590°C, the Tcof Fe3Si, and 245°C, where the magnetocrystalline anisotropy of Fe2B changes from easy-axis to easy-plane. The unusual magnetic and thermomagnetic behavior observed is discussed in terms of the short-range and long-range magnetostatic interactions between the two phases.     

High-field magnetism in non-polar γ-Fe2O3recording particles

Fine γ-Fe2O3particles produced by a process which involves hydrothermal conversion of iron hydroxides to α-Fe2O3have been investigated. Such particles appear to lack pores or dentrites which cause internal magnetic poles, and exhibit superior properties for magnetic recording. Mössbauer spectroscopy, x-ray diffraction, and transmission electron-microscopy were used. The particles were of length 250-500 nm and width 35-50 nm; some samples had cobalt adsorbed onto the surface. Mössbauer spectra were collected at temperatures from 4.2 to 300 K, and in zero and 5 T applied magnetic fields. In general the patterns are typical of bulk γ-Fe2O3. The 300 K spectra of Co-doped samples show sub-patterns of weak intensity associated with at least two distinct additional components with reduced splitting. One of these has Bhf= 45.2(2) T; its origin is suggested to be iron-atoms lying in or close to the cobalt-modified surface. In a 5 T field, the iron-atom moments deviate from the applied field direction by average angles in the range 13-15°. Because the angle for conventional similarly sized γ-Fe2O3issim 13deg, it is concluded that the improved particle morphology does not lead to an improvement in the high-field alignment.     

The effects of low temperature fatigue on the RRR and strength of pure aluminum

Low temperature fatigue effects on residual resistivity ratio (RRR = rho_{273 K}/rho_{4.2K}) and strength of 300 and 1000 RRR aluminum are reported. The objective of this investigation is to select the best initial purity for the stabilizer aluminum used in energy storage magnets. Monolythic centimeter diameter specimens were fatigued at 4.2 K to strains (ε) reaching 0.3 percent. The resistivity ratio rapidly decreases during the first 100 cycles and approaches saturation (RRRf) after about 1000 cycles for all strains tested. The RRRfvalues are different for different initial resistivity ratio (RRRi) values, but all tend to come together at 0.3% strain independent of RRRi. The maximum specimen stress (sigma_{max}) is reached after about 1000 cycles also, and approaches a common value (sigma_{max} = εE/2, where ε is the strain range and E the elastic modulus) independent of RRRi. Thus high purity aluminum becomes "fully hard" at equilibrium and behaves elastically. The impact of fatigue damage on conductor design and choice of stabilizer purity is considered.     

Influence of R.F. sputter parameters on the magnetic orientation of Co-Cr layers

Co-Cr layers for the perpendicular recording mode were deposited by means of RF-sputtering. The most important sputter parameters, i.e. the RF sputter high voltage VRF, the argon pressure Parand the substrate holder temperature Tsh, gave an optimum value for perpendicular orientation of the magnetization. The crystal structure is always hcp within the ranges of varied parameters and no other magnetic phases were observed. If the sputter parameters do not have optimum values an additional hcp compound with in-plane orientation of the c-axis is observed. This orientation causes an increase of the in-plane remanenceS//= (M_{r}/M_{s}). Measurements of the substrate temperature Tsas an function of the various sputter parameters lead to the conclusion that an exclusive perpendicular c-axis orientation is only obtained atT_{s} sim 15thetadegc. At other Tsestablished either directly by changing Tshor indirectly by changing the sputter conditions an additional hcp compound with in-plane c-axis orientation appears. We concluded that Tsis the dominant parameter for sputtering CoCr layers.     

Influence of annealing temperature and time on hysteresis parameters and microstructure of an Fe/Co/V alloy

The variations in coercivity, remanence, and microstructure for2frac{1}{2}hour aging anneals between 300°C and 1000°C on an Fe/Co/3% V alloy have been determined. Additionally, the effects of varying aging times for temperatures between 885°C and 925°C have been analyzed. Hysteresis parameters and optical microstructures were characterized on wire samples which had been either cold worked by stamping or remained undeformed prior to the aging anneals. Results show that the alloy exhibits maximum magnetic hardness in its initial cold worked or undeformed conditions. Aging anneals produce a general decrease in magnetic properties with increasing temperatures to 700°C. However, the deformed material exhibits a secondary maximum for aging temperatures between 590°C and 610°C. For anneals above 800°C magnetic parameters again increase as a two-phased, duplex BCC (α1+ α2) structure is developed. By varying the aging time for anneals that produce this duplex structure, a level of coercivity comparable to that achieved for a 600°C anneal was attainable. However, a comparable level of remanence and, thus, a similar squareness ratio, could not be produced with the duplex structures developed by aging near 900°C.     

Magnetostriction in some magnetic oxide compacted powders

Magnetostriction measurements have been made on a series of compacted powder samples of oxide pigments used in magnetic recording, to provide information for the evaluation of the role of magnetostriction in the recording process. Two composition series were measured at 294 K: one from γFe2O3through a range of partial reductions to Fe3O4, and another from 0 to 4 at. % Co in γFe2O3; also samples of CrO2. In addition, a 2 at. % Co-doped Fe3O4powder was measured as a function of field and temperature from 77 to 300 K. Magnetostriction was measured using a semiconductor strain gage on a rotating sample, in fields up to 9 kOe.     

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.     

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.     

Mechanically induced anisotropy and its effect on magnetic permeability in single crystal ferrites

Magnetic anisotropy and initial permeability in worked Mn-Zn and Mn-Fe ferrite single crystals are investigated in connection with the average magnetostriction constant λs. These data are compared with those obtained for etched specimens. Induced anisotropy is observed for all of the worked specimens and is proportional to λs, i.e., its origin should be the magnetoelastic energy λσ induced by stress σ remaining in the worked specimens. The ratio of initial permeability, μi(as-worked)/μi(etched), at room temperature, is found to increase with increasing λ111in a low frequency range and to be greater than unity when |λ111/λ100|> 3. The second peak in μi- T curves shifts with lapping to lower temperatures for the specimens with |λ111/λ100|<3, while to higher temperatures where |λ111/λ100|<3. Such phenomena are discussed in terms of domain wall energy affected by lapping-induced anisotropy.     

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 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.