NiFe films deposited by sputter gun and applications to bubble devices

The sputter gun system has a unique feature which is that the substrate holder is electrically isolated from and physically outside the cathode and anode gun assembly. As a result the substrate surface temperature can be maintained at a lower level than that for the conventional diode sputtering system. This feature is particularly useful for metallization by the lift-off process. NiFe films with excellent physical and magnetic properties have been obtained. The application to overlay patterns for single-level masking bubble devices is presented.     

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.     

The preparation of large area films for bubble domain devices

The growth of LPE films 3.8 cm in diameter has been achieved under conditions which lead to a high yield of defect free material suitable for bubble domain circuits. Temperature control to ±0.1°C, larger melt volumes, more precise timing of runs and an increase in saturation temperature of the melt to 970°C all contribute to the improved quality. The change to 970°C also sharply reduces platinum attack. Films having a collapse field controlled to ±1 Oe can be grown in yields exceeding 35%.     

Template-grown NiFe/Cu/NiFe nanowires for spin transfer devices

We have developed a new reliable method combining template synthesis and nanolithography-based contacting technique to elaborate current perpendicular-to-plane giant magnetoresistance spin valve nanowires, which are very promising for the exploration of electrical spin transfer phenomena. The method allows the electrical connection of one single nanowire in a large assembly of wires embedded in anodic porous alumina supported on Si substrate with diameters and periodicities to be controllable to a large extent. Both magnetic excitations and switching phenomena driven by a spin-polarized current were clearly demonstrated in our electrodeposited NiFe/Cu/ NiFe trilayer nanowires. This novel approach promises to be of strong interest for subsequent fabrication of phase-locked arrays of spin transfer nano-oscillators with increased output power for microwave applications.     

Operation of 3µm bubble serial loop devices designed on single level masking

An all-permalloy single mask level design for a serial loop circuit has been developed and successfully operated using 3 μm bubbles. The design of individual elements for a single level circuit such as generator, replicator and annihilator which were compatible with half-disk propagation patterns was successfully investigated and optimization of permalloy and SiO2spacer film thickness was achieved. The small capacity test chips fabricated provided 13-16 Oe "window margins" with circular drive field ranges of 40 to 50 Oe at 100 kHz. The critical stretching pulse phase margin for replication, which was the minimum phase margin for all functions was found to be 10 degrees. Details of design and characteristics are also discussed including operating margin dependence on frequency and temperature.     

Planar processing for magnetic bubble devices

A two-mask level, conductor first thin film process is described for fabrication of magnetic bubble devices. The process permits a stepless permalloy level over a conductor that may be two to three times as thick as conventional processing The planar process is attained by anodizing a thick aluminum alloy film in all regions where no conductor is needed. The process described solves problems in conventional processing caused by thin conductor metalization and permalloy step coverage. Replication, nucleation, and annihilation devices made with this process promise superior performance.     

High coercivity in nanostructured Co-ferrite thin films

Three methods including sol-gel, rf sputtering and pulsed laser deposition (PLD) have been used for the fabrication of high coercivity Co-ferrite thin films with a nanocrystalline structure. The PLD method is demonstrated to be a possible tool to achieve Co-ferrite films with high coercivity and small grain size at low deposition temperature. High coercivity, over 10 kOe, has been successfully achieved in Co-ferrite films with a thickness of ∼ 100 nm deposited using PLD with a substrate temperature at 550°C. The Co-ferrite films prepared by PLD at over 300°C on different substrates including amorphous glass, quartz and silicon exhibits an obvious (111) textured structure and possesses perpendicular anisotropy. Our study has also shown that the high coercivity is related with a large residual strain, which may induce an additional magnetic anisotropy (stress anisotropy) and at the same time serve as pinning centres, which can restrict the domain wall movement and therefore, increase the coercivity.     

Logic functions for magnetic bubble devices

An analysis has been made of the number and types of logical functions which can be performed using the interaction of circular magnetic domains in rare earth iron oxides. Multiple logic functions are found to be produced simultaneously at any logical area. These conjunctive output sets have been categorized. Several conjunctive logic gates have been designed, fabricated, and tested successfully in Sm0.55Tb0.45FeO3using Permalloy overlays. Utilizing a circuit in which AND/OR logic gates are coupled to a dynamic memory bank, the total correlation of two data streams has been performed. It is shown how this multiply accessed dynamic memory serves to establish the correlation threshold.     

Applications of bubble devices

The applications of magnetic bubbles that are currently seen as most attractive and the current status of development of such devices are considered. A major application is thought to occur in solid-state mass memories. An attractive form of organization is possible whereby relatively short access times can be achieved while using only a small number of read and write circuits. A repertory dialer memory, which has been chosen as a test vehicle, has been developed. The design and performance of the propagating circuits, the self-latching magnetic gates and the generator, which comprise the memory, are discussed. A novel area of application may be in pulse code modulation (PCM) switching networks. It is shown how a network operating as a PCM crossbar switch can be designed using bubbles. A circuit is described which can eliminate the delay associated with bubble propagation in such networks.     

Bias magnet design for bubble memory devices

A study has been made of a system consisting of a pair of magnets with thin plates of permeable material on their inner faces. The effect of the plates on the profile of the field between the magnets is predicted as a function of the geometry and the permeability and the way in which the uniformity can be improved is demonstrated.The effect of the permeability is very limited.     

Optimum design considerations for dual conductor bubble devices

A design for dual conductor, current-access bubble devices with 8-μm periods has been optimized with a numerical calculation method for bubble motion in a propagating magnetic field, generated around hole patterns in conductor layers. Magnetic bias field distributions are calculated for an oval hole chain in the conductor layers. Bubble motion equations are obtained with analytical field distribution functions approximating the calculated field distributions. Minimum drive current density Jminfor normal bubble propagation is determined by a solution to the equations. The hole shape has been optimized by the minimization of the drive power Pmin, the product of Jminand conductor resistance, which is calculated from current distributions around the hole pattern. Optimum layer thickness have also been obtained for 8-μm period bubble devices. Both registration tolerance between the two conductor layers and bubble skew effects have been studied semiquantitatively on the basis of the equations of motion. The numerical calculation method developed here is found to be a highly effective means to optimize pattern design for smaller period devices.     

Deposition rate dependence of coercivity in vaccum evaporated CoCr films

Deposition rate dependence of perpendicular coercivity Hcl in Cocr evaporated films at the deposition rate ranging from 0.4nm/s to 400nm/s is studied. Hcl increases with decreasing deposition rate as well as with increasing substrate temperature. A High Hcl over 500 Oe, which has ever been obtained only at a substrate temperature over 250°C when deposition rate is high(400nm/s), is obtained at a substrate temperature of 150°C under deposition rate of 2nm/s. "Shoulder" in the hysteresis loop disappears as Hcl exceeds 400 Oe. Therefore to decrease the deposition rate and to increase the substrate temperature have the same effect on Hcl. Saturation magnetization of the films with the same Cr concentration increases with decreasing deposition rate, and it is suggested that the degree of Cr segregation increases with decreasing deposition rate. Hcl is independent of the background pressure when the relative pressure, the quotient of (background pressure/deposition rate), is between 2×10^-8and 5×10^-6torr/(nm/s). From the results above, it is clarified that the difference of deposition rate between vacuum evaporation and sputtering is one of the major reasons for the difference of Hcl between the two.     

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.     

Aging effect on coercivity of iron oxide thin films

Changes in magnetic properties for Fe_3-x□_x O₄ (x: oxidation degree) thin films made by reactive sputtering and subsequent heat treatments have been examined under room temperature aging and constant temperature annealing. Aging causes variations in coercivity of insufficiently oxidized films which have a specific resistance of less than 1×10¹ Ω·cm, while the coercivity of γ-Fe₂O₃ (x=1/3) did not change. This phenomenon did not depend on additive elements or preparation method. Other magnetic properties such as saturation magnetization, residual magnetization, squareness ratio and coercive squareness, were not affected by aging for any Fe_3-x □_xO₄ composition. The activation energy for a coercivity change is 0.72-0.95 eV near room temperature for films with a specific resistance below 1×10¹ Ω·cm. It was confirmed that only the coercivity varied at 20°C, while both coercivity and the degree of oxidation changed with annealing at 100°C     

Characteristics of YLaTm-garnet films for magnetic bubble applications

Films of Y_1.6La_0.3Tm_1.1(FeGa)₅O₁₂ have been grown by liquidphase epitaxy on Gd₃Ga₅O₁₂ substrates. This is a new film composition whose high mobility (～1000 cm/sec/Oe) and low temperature coefficient of bubble diameter (～0.1%/°C) make it attractive for bubble domain devices. Films supporting 3 to 8μm-diameter bubbles have been prepared with typical defect densities less than 5/cm². Data are presented to show that this material can be grown reproducibly despite a rather large La segregation coefficient. Measurements of the uniaxial anisotropy as a function of lattice mismatch are presented along with the results of ion implantation studies which show that a dosage of 3 × 10¹³ cm^?2 of Ne is sufficient to suppress hard bubbles in this material. Also, bubble velocity measurements are discussed in detail as they reveal a number of interesting effects. These include the existence of a perpendicular velocity component which can be virtually eliminated by applying an in-plane field and the absence of extreme dynamic conversion effects in high gradient fields.     

An analytical design theory for field-access bubble domain devices

This paper describes the derivation and applications of an analytical model for magnetic bubble domain propagation. A fleld-access bubble device is treated as a magnetic circuit, emphasizing fluxes rather than fields as in most previous treatments. The energy changes are calculated within the bubble domain rather than in the propagation pattern, making possible a simple analytical-graphical process for predicting device operating margins. Agreement with a large number of device experiments is shown to be good. Closed-form expressions are derived which relate the essential features of the operating margin to device geometry and bubble material magnetic properties. The application of this model to device diagnosis, optimization, and scale down is discussed.     

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.     

Influence of triboelectric treatment on the coercivity of magnetic films

A study has been made to determine how the properties of magnetic films are influenced by the triboelectric fields accompanying the polishing of articles with fur and results are presented. For insulating films of bismuth-containing iron garnet an increase in the coercivity was observed, evidently caused by the electromagnetic effect, in spite of reduced defects in the crystal structure. Pis’ma Zh. Tekh. Fiz. 24, 73–78 (January 12, 1998)