On spontaneous nucleation in field-accessed bubble devices

The dependence ot the in-plane drive field at which bubble domains spontaneously nucleate in field-accessed bubble devices has been investigated as a function ofH_{k} - 4piM_{s}and of spacer thickness between the bubble film and permalloy propagation elements. The experiments were carried out on amorphous GdCoMo bubble films with T-bar and Y-bar structures. For a given structure and spacer thickness the nucleation field increases linearly withH_{k} - 4piM. Larger spacer thicknesses also lead to increased nucleation fields. A model based on the Stoner-Wohlfarth astroid is compared to these data and found to be useful in explaining the qualitative trends, but to be in poor quantitative agreement. It is concluded that since the drive field required in a device is proportional to4piM_{s}, Q - 1 = (H_{k} - 4piM_{s})/4piM_{s}must be greater than some minimum value for a given device structure and spacer thickness to permit reliable device operation.     

The effect of DC in plane field on the operation of field access bubble memory devices

The effect of a small dc in-plane field on the start-stop operation of field access bubble devices has been studied. Experimental results show that the bias margin in this mode is very sensitive to the magnitude of the field and its orientation relative to the start-stop direction of the drive field. In a T-I circuit a complete margin loss was observed for an in-plane field of 3 Oe oriented antiparallel to the start-stop direction. For parallel orientations of the in-plane field the start-stop margin improved and approached that of the continuous propagation margin at an in-plane field of approximately 6 Oe. Dependence of the start-stop margin on the orientation of the start-stop direction relative to the pattern was also observed. Measurements of the bubble collapse field at various points in the pattern show a very strong dependence on the in-plane field and the permalloy geometry. The collapse-field results and magnetostatic energy considerations which take into account local field variations and bubble-bubble interactions provide a basis for understanding the experimentally observed start-stop margins. These results show that a small tilt (2 to 3°) should be introduced in the bias field to overcome normal alignment tolerances and ensure that a favorable in-plane field is always present. This assures reliable start-stop operation.     

The magnetooptic bubble display

The various factors that affect the properties of magneto-optic (M/O) bubble displays which use bismuth substituted iron garnets are discussed. A composite bias magnet-ferrite drive coil assembly is described which allows optical access to the display chip. The drive coils required 300 mW to produce an in-plane field of 30 Oe when operated at 10 kHz, and the bias field was uniform to ± 1.5% over an area of radius equal to 1/10 that of the bias magnet. Also discussed is the brightness and heating of the display. A screen brightness of 160 candelas/m²is possible using a X20 projection lens, and the temperature rise of the display chip could be less than 1°C if adequate heat sinking were used. Finally a prototype hand-held display is described which contains a 1.3 k bit capacity display chip. Examples of characters written into that chip are given.     

Stripe and bubble domains in CoCr films

Using the colloid-scanning-electron-microscope (SEM) method, an investigation was conducted of the domain structure of CoCr films (400-1740 nm thick) at different points of the hysteresis loop. In some cases, an AC field exceeding the value of the coercive force was applied in addition to the DC field. For low-coercivity samples (H_c/H_k~0.02) a bubble structure, which is energetically more favorable above a certain critical field, was observed. From the dependence of the bubble density on the DC field the value of the anhysteretic collapse field was estimated and compared with the theory. Attempts were made to observe the field dependence of the domain structure of high-coercivity samples (H_c/ H_k~0.08) with different thicknesses. For one sample the results were compared with those determined using the Kerr effect. For thicker films the colloid-SEM method permits the simultaneous observation of the magnetic domains and the columnar structure at the surface of the sample     

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).     

New longitudinal recording media CoxNiyCrzfrom high rate static magnetron sputtering system

Three new magnetic alloys A:Co64.2Ni27.4Cr8.4. B:Co74.1Ni15.9Cr10.0, and C:Co82Ni18were used as targets to sputter thin films with coercivities up to 1015 Oe, 926 Oe, and 825 Oe respectively. Chromium films were used as the base layer. With a base pressure below 2 × 10^-7torr and chromium thickness of 2500Å, the following magnetic characteristics of alloys A and B were obtained: Hc>900 Oe, Brδ>530 G-µm, S>78%, S* > 93%, S/N (at 5MHz) > 45dB and D50> 17KBPI. X-ray diffraction measurements indicate the (1011) texture of CoNiCr films is the reason for the high coercivity and squareness ratio. This orientation may result because the in-plane atom density for Co alloys HCP(1011) and Cr BCC     

Magnetic and magnetooptical properties of sputtered Fe5Si3films

Polycrystalline films of Fe5Si3have been prepared by RF sputtering from a sintered target composed of 62.5 at% Fe and 37.5 at% Si. The Faraday rotation and optical absorption were measured for these films in the visible and near infrared. At a wavelength of 6328Å, room-temperature absorption (corrected for reflection) and specific Faraday rotation are 3.7 × 10⁵cm^-1and 1.3 × 10⁵deg/cm, respectively. The rotation decreases with rising temperature and vanishes around 120°C, the Curie temperature of the films. From polar and in-plane hysteresis loop measurements the spontaneous magnetization is found to lie in the film plane. Square hysteresis loops are observed when the external field is applied in the plane of the film, and the in-plane coercive force decreases monotonically with temperature at an average rate of -2 Oe/°C as Tcis approached.     

Magnetometer Design for Measuring Ultralow DC Magnetic Fields Using YBa2Cu3O7−x Superconductors

In the present work we examined a special sensor design using a YBa₂Cu₃O_7?x superconductor as a sensing element for the detection of ultralow DC magnetic fields. The experimental results have shown that the sensor signal, which is the second harmonic signal generated by co-application of AC and DC fields to high-T _c superconductors, varies linearly with the applied DC field even in the ?0.5 Oe to 0.5 Oe range. The critical factors affecting the strength of the second harmonic signal, such as the frequency and amplitude of the AC field, the intergrain critical field H _c1J , and the minimum field required for penetration through the entire specimen H ^?, were optimized. In this way we could detect DC magnetic fields as low as 1 nT.     

New method for routine measurement of coercivity in magnetic bubble films

We describe a new technique for measuring coercivity in magnetic bubble films which consists of placing the film in a weak field gradient (∼1 Oe./μm) in order to obtain a set of finger-like domains. The unconstrained ends of these domains are caused to move back and forth in response to an oscillatory field, and the coercivity is obtained from an extrapolation of the linear portion of the response vs. drive field curve. We present a comparison between coercivity values in materials with 3μm and 1.7μm stripe-widths obtained using the new technique and bubble translation. Good correlation is observed for both types of material, the values obtained with the new technique being somewhat higher than the bubble translation values. The difference is ascribed to material non-uniformities.     

Characterization of magnetic bubble generators

Two types of magnetic bubble generators suitable for a field-access bubble memory have been tested at 1.00 kHz bit rate at in-plane rotating fields above 15 Oe. The bias field margins of the generators at 30 Oe rotating field are equal to or greater than those of loop propagation. Both designs are based on the principle of stretching and cutting seed bubbles circulating around a Permalloy disk. Functions of stretching, cutting and transferring in the generator sequence are accomplished either with Permalloy elements or pulsed current conductors. The operating conditions of the generators in terms of current pulse amplitudes, widths and phase angles are presented. Among the two designs, namely Permalloy-stretch and conductor-stretch generators, the latter has a wider phase-angle margin.     

High-frequency propagation and failure of asymmetric half-disk field access magnetic bubble device elements

High-frequency propagation characteristics and failure modes in 14-μm period, 1.8-μm gap, asymmetric half-disk field-access device were studied using a high-speed optical sampling technique. Propagation elements as well as normal and hand gun corners and chevron structures were included. The operating bias margin at 1MHz, for a structure that had 1.2 MHz as highest possible frequency, was about half of the margin for frequencies of 200 kHz and below. The phase lag between the bubble leading wall and the instantaneous rotating field direction was nearly 90° as the bubble moved through the center of the element where the lag was the greatest. The peak velocity of the leading wall of 55 m/s and the trailing wall of 46 m/s is attributed to bubble interaction with the Permalloy structure creating a ∼125 Oe in-plane field that greatly increases the free bubble "saturation" velocity.     

Some characteristics of ion-implanted bubble chips

The relations between the position of charged walls and the bubble motion around propagation circuits are discussed. Long walls which extend between adjacent propagation loops are revealed by the Bitter technique. The examination of the domain structure in the implanted layer shows the existence of a magnetic gradient which is a function of the distance from the propagation circuits. The switching of magnetization in particular directions of the in-plane field is reported and correlated with the bubble movement. An additional easy axis is observed along the circuits due to shape anisotropy. Propagation margins are very similar to those obtained with permalloy circuits. Fabrication technology as well as design of 16 μm period circuits is discussed. Nucleation and transfer have been achieved with currents in the range of 50 mA to 200 mA. Phase margins of about a quarter of a period are found, and bias field margins fall between 10 and 15 Oe.     

Submicron bubble garnets and their characterizations

New Submicron (0.5-1.0 mum diam) bubble garnets have been developed. In order to reduce saturation induction4piMsas low as possible, suitable choice of amounts of nonmagnetic ions substituting for ferric ions in both tetrahedral and octahedral sites were studied in (YSm)3(FeAl)5O12, (YSmLu)3- (FeGaSc)5O12, (YSmLu)3(FeAlSc)5O12and (LaLuSm)3- (FeGa)5O12garnet systems. For example, in (YSmLu)3- (FeAlSc)5O12a film with the following properties was grown; strip widthw = 0.7mum, film thicknessh= 0.7mum, quality factorq=2.8, Curie temperatureTc= 140degC and4piMs=770G. The temperature properties of those films could be improved drastically by doping with a small amount of Gd ion. Wall mobilities μw of those films are in the region from 200 to 500cm/s/Oe. Preceding these material studies, several film characterization methods have been investigated. Film thickness was measured by Fluorescent X-ray method, strip width by using a highly sensitivity TV camera, and bubble collapse field by FMR resonance technique respectively.     

A magnetic bubble passive time-slot interchanging gate

The design and operation of a magnetic bubble logic gate, able to perform the basic retardation operations for a magnetic bubble PCM time-slot interchanger,are reported. With this design no external current pulses are needed to perform those functions. With a 32 µ circuit periodicity and using (SmY)3(GaFe)5O12, bias field marginsashigh as 11.5% for the passive logic function are reported in a 25 Oe rotating field. Because no precautions are taken againsthard bubbles the unsuspicious range of frequency is limited to 20 kHz. Nearly no difference is seen in operating margins between low andhigh frequencies. Design rules are given that canlead to other bubble-to-bubble logic circuits with high operating margins.     

Ultra-low DC Magnetic Field Detection Using Ceramic Superconductors

In the present study, the linear behavior of the second harmonic signal generated by application of very low DC magnetic fields to the polycrystalline bulk high-T _c superconductors has been examined. The amplitude of AC field, H _ac , intergranular first critical field, H _1cJ , and minimum field required for the magnetization of specimen as a whole, H ^?, were determined and a special coil design has been presented. The results reveal that the second harmonic signal varies linearly with the applied DC field in the ?0.1 Oe to 0.1 Oe range. Additionally, detection of ultra-low DC fields as small as 5 nT has been achieved. Repeatability and reproducibility tests were performed and the resultant data have been found to be consistent. In the light of our study, a commercial magnetometer can be produced to be utilized in successful detection of very low magnetic fields.     

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.     

Magnetic properties, microstructures, and corrosion resistance ofhigh-saturation FeMoN and FeRhN films for recording heads

We have investigated high-saturation FeMoN and FeRhN films, deposited by radio frequency-diode reactive sputtering on alumina-TiC substrates, for inductive head applications. A minimum coercivity of ~1.2 Oe is obtained in (Fe_97.8Mo_2.2)N films at a N ₂/Ar flow ratio of ~6.2%. A minimum coercivity of ~1.6 Oe is obtained in (Fe_96.9Rh_3.1)N films at a N₂/Ar flow ratio of ~4.6%. The films mainly consist of α-Fe phase and γ'-Fe₄N phase; The magnetic properties of these films are stable under easy axis field annealing up to 350°C. Addition of Rh or Mo to FeN has resulted in a significant improvement in corrosion resistance over that of FeN. The localized corrosion resistance of FeRhN and FeMoN can be comparable to that of Permalloy. In contrast, their intrinsic corrosion resistance is inferior to that of Permalloy, but it can be adjusted and controlled by pH level     

Oblique-field annealing effect for in-plane magnetic anisotropy ofsoft magnetic Co-Nb-Zr thin films

The effect of annealing in a magnetic field applied obliquely to the surface of soft magnetic thin films has been investigated. This annealing method was found to be extremely effective to control in-plane magnetic anisotropy without a change of annealing temperature and to suppress local anisotropy dispersion. For sputtered amorphous Co_85.5Nb_8.9Zr_5.6 thin films, it has been found that the in-plane uniaxial anisotropy energy was varied from 250 J/m³ to near zero with the coercive force H_c less than 6.5 A/m by changing the oblique-field annealing angle α. Experimental values of in-plane anisotropy energy agreed well with calculated ones predicted from α and intrinsic anisotropy induced by quasi-directional ordering. These films showed good high frequency characteristics for applying to miniaturized inductive devices     

68 kbit capacity 16 µm-period magnetic bubble memory chip design with 2 µm minimum features

A bubble propagating structure that operates well on a 14 μm to 18 μm propagate period with a nominal 2 μm minimum feature size has been designed. The structure consists of only 1 discrete permalloy feature per circuit period. Sixty-eight kbit-capacity memory chips based on such structures have been designed, built, characterized, packaged and the packages have been characterized. The chip is organized as a set of minor (storage) loops with separate write and read major lines. The bubble manipulating functions, of which the replicate and transfer gates are the most critical, have also been designed with 2 μm minimum features. The design is adequate to provide a 14 Oe bias field margin range with drive fields of about 35 Oe, using a bubble garnet material with approximately 170 Oe free bubble collapse field. Sixty-eight kbit single loop shift register type chips designed using similar propagating structures, however, provide over 20 Oe bias field margin ranges with drive fields of about 35 Oe.     

Eight-micron period bubble devices

Magnetic bubble shift register devices of 68 121- and 266 473-bit capacity have been fabricated and tested. The epitaxial garnet bubble films were nominally 1.7 μm thick, supported nominally 1.7-μm diameter bubbles, and had collapse fields of about 260 Oe. The storage area per bit was 64 μm², which was realized with a minimum coded feature dimension of 1 μm and contact photolithography using EBES chrome masters. Initial yields obtained in two experimental batches each of the two chip capacities are discussed. Parametric test results are presented for generator current, transfer current and phase, and rotating field intensity. Nominal values have been established to be 130-mA generate current, 21-mA transfer current, and 60-Oe drive. The detector signals were about half as large as normally obtained from 3.3-μm bubble devices with comparable resistance and conventional design.