Control of the anisotropy field of successively grown garnet films

The anisotropy field of magnetic bubble garnet films grown by LPE is an important property to be controlled as well as the bubble collapse field and the strip width. The anisotropy field is comparatively likely to drift with run number even if the characteristic length can be kept constant and directly affects the device operation. In order to suppress the drift of the anisotropy field, the melt compositional drift must be suppressed by the periodic addition of garnet oxides and flux to the melt.     

The characterization of magnetic bubble-domain materials with X-ray topography

The use of X-ray topography to characterize magnetic bubble materials is discussed. By mapping the crystal strains, the technique reveals defects which can influence the bubble domains and which are not easily observed by other methods. Examples of topographs are given for flux grown iron garnets, Czochralski grown Gd3Ga5O12substrate crystals, and LPE iron garnet layers. The influence of substrate defects on the magnetic film is demonstrated. Transmission topographs of substrate-film combinations have not revealed an intermediate layer of misfit dislocations. The application of X-ray topography to a study of the growth induced anisotropy in magnetic garnets is also described. The topographs reveal a correlation between regions having a particular growth band symmetry and those having a particular noncubic anisotropy.     

Magnetic inhomogeneities in (YSmCa)3(GeFe)5O12 and their elimination by improved growth procedures

We have found that inhomogeneities in the growth direction are a very common occurrence in LPE films of (YSmCa)₃(GeFe)₅O₁₂. These inhomogeneities, which occur over a wide range of solution compositions and growth conditions, are not detected by the usual measurements of bubble parameters. However, they can be observed in the presence of an in-plane field using a technique which we describe. Experiments using this technique indicate that there is a thin (≤ 1 μm) layer at the film/substrate interface which has magnetic properties significantly different from the rest of the film. Special growth procedures which will avoid the formation of this “interface layer” are described in detail. Comparisons between films with and without this layer show, surprisingly, that there is no direct correlation between the coercivity and the presence of this layer. However, a correlation between coercivity and samarium concentration is observed when all other growth parameters are kept as constant as possible.     

Growth reproducibility of the bubble properties of (YSmLuCa)3 (FeGe)5 O12 films

Reproducibility of CaGe garnet films grown by LPE is discussed in comparison with that of Ga garnet films. The influence of growth temperature deviation on the properties of CaGe garnet films is not compensated by the adjustment of the substrate rotation rate, although it is effective for Ga garnet film growth. On the other hand, the bubble collapse field of CaGe garnet films is less sensitive to the growth temperature than that of Ga garnet films. Therefore by determining the growth time accurately the bubble collapse field of as-grown films has been controlled within ±1%, even if the growth temperature deviates from the desired value by ±1°C.     

Determination of the composition of thin garnet films by use of radioactive tracer techniques

An isotope tracer technique has been developed for the composition analysis of thin films of magnetic garnets on substrates. The method has been extended to four elements labelled at the same time. The major advantages of the technique are that it is non-destructive and that it is independent of the presence of common ions in the substrate and the film. Using the analysis results, the segregation coefficients of Y, Gd, Fe and Ga have been determined as a function of the film preparation temperature in a PbO-B₂O₃ flux for a bubble domain type material produced by the LPE technique. This method of analysis can also be applied to the CVD method of film preparation.     

Substrate facet replication by epitaxial magnetic garnet films

Epitaxial ferrimagnetic garnet films were deposited on Czochralski grown single crystal gadolinium gallium garnet substrates containing faceted regions. Films grown by both chemical vapor deposition (CVD) and liquid phase epitaxy (LPE) were studied. Lattice parameter distributions were determined by the method of X-ray double crystal topography with rocking curve analysis. The demagnetized domain strip width, magnetization, and characteristic length were measured in a CVD film in regions inside and outside the substrate facets. It was determined that replication of the substrate facets by the epitaxial film is accomplished by a difference in film stress. This stress difference arises from the lattice parameter difference between the faceted and unfaceted regions of the substrate. These results lead to the establishment of a criterion for allowable lattice parameter variation in substrates to be used for magnetic bubble domain films with stress induced anisotropy.     

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.     

The propagation of magnetic bubbles on permalloy disks

A magnetic bubble generator consisting of a Permalloy disk and a current conductor loop has been used recently in a mass memory design utilizing magnetic bubble technology. The bias field range in which the disk can hold the seed bubble is measured in this report as a function of of the rotating field frequency. Above a critical frequency fc, the bias field margins begin to decrease. The dependence of fcon disk size is obtained for disks with diameters from 16 μm up to 43 μm at rotating fields of 20 and 30 Oe. The separation between Permalloy disks and the garnet film is kept at 0.8 μm or 1.6 μm. Results show that at a fixed rotating field, a smaller disk is preferable at higher frequency for a magnetic bubble material with a given mobility. The critical frequency fcobtained is in good agreement with a theoretical calculation using the viscous damping model by Rossol et al. For frequencies below fc, the bias field margin on the disk is equal to that of the propagating channel and circuit failure due to the loss of the generator seed bubble can be eliminated.     

Transient bubble domain configuration in garnet materials observed using high speed photography

The dynamic domain configurations of bubbles in garnet materials have been studied using a sampling optical microscope capable of single exposure photographs with a 10 nsec exposure time. The microscope is an integral part of a sampling system so that the transient shape of the bubble is recorded at various times after a field pulse or, for bubbles in field access devices, during a clock cycle. A triggerable flowing nitrogen gas laser pumping a low Q Rodamine 6G Dye laser is used as an illumination source giving light pulses of ∼1.5 KW for 10 nsec. This light is sufficient to expose Kodak 4 × 16 mm movie film. Standard pulse generators (HP 214A) are used to make free bubble radial velocity measurements. A modified generator allows free bubble collapse measurement to be made. For bubbles propagating at operating frequency within field access devices, a standard bubble exerciser is used, synchronized to the sampling system. In this case, special samples with an internal mirror and epi-mode illumination are used. Illustrative results of bubble domain shapes in a chevron propagating structure and a 90° chevron expander detector are included.     

Frequency response of a thin cobalt film magnetooptic sensor

The magnetooptic effect is due to a change in the polarization of the light when it is reflected or passes through a magnetized material. The rotation of the polarization plane is proportional to the magnetic field. The great advantage of using a magnetooptic sensor to measure intensity or magnetic fields is its wide bandwidth. This fact is widely known; however, no effective measurements have been taken. In this paper, we present the frequency response of a cobalt thin film used as magnetooptic material. It was first excited by several sinusoidal magnetic fields at different frequencies. The range of frequencies studied in the first experiment reached 179 Hz, which is suitable for measuring power line intensity or magnetic fields. Because the coil that creates the magnetic field has a great impedance at higher frequencies, an alternative method based on magnetic impulses has been designed to obtain high-frequency data. With the latest experiments we have been able to measure frequencies as high as 2 MHz, obtaining a flat frequency response.     

Applications of amorphous magnetic-layers in surface-acoustic-wave devices

This paper reviews the theory and potential applications of magnetically variable delay lines and oscillators which employ a magnetostrictive film on a piezoelectric surface-acoustic-wave (SAW) substrate. Cases analyzed in detail indicate that the delay change arises mainly from a rotation of the magnetic moment from the films' easy axes toward the applied field direction; thus the interaction is essentially nondispersive. Use of amorphous metallic-glass overlays is particularly attractive because their high magnetostriction and low magnetic anisotropy makes a relatively large delay variation possible with a small change in bias field. Since the SAW velocity can be changed only bysim .1% or less with present film technology, applications are restricted to those where only a small frequency or delay adjustment is required. Two prototype examples are considered in detail--a variable delay line for steering an adaptive array antenna and a tunable resonator oscillator capable of tracking high speed Doppler targets. In both of these examples, the magnetic film/SAW substrate geometry is seen to be an attractive alternative to competitive approaches.     

薄膜高频磁阻抗效应及其应用

本文介绍了Ｆｅ－Ｎｉ软磁薄膜高频磁阻抗效应及在磁传感技术中的应用，分析了讨论了获得高灵敏度磁传感的途径，磁性薄膜厚度的一般控制在４００ｎｍ以上抑制Ｎｅｅｌ壁的出现，为了提高电压输出变化灵敏度，需要沿被测场方向加向Ｏｅ的偏磁场并使高频磁场形成闭合回路，感生各向异性通过在溅射薄膜过程中加几百Ｏｅ平行于膜的直流磁场获得，与各向异性磁阻效应和巨磁阻效应相比，高频磁阻抗效应传感技术灵敏度高，无巴克豪森噪声，     

Growth and temperature characteristics of small bubble YSmLuCaFeGe garnets

Several hundred YSmLuCaFeGe garnet films supporting bubbles 1–3 μm in diameter were grown by LPE, and their static bubble properties were measured from ?50 to 100°C and compared with those of similar films supporting 5 μm bubbles. Bubble mobilities in the range ～ 300–1000 cm/sec-0e and q values as high as 4.7 were achieved, along with saturation magnetizations from 330 to 550 G and collapse-field temperature coefficients in the range ?0.20 to ?0.27%/°C. Static coercivities were generally less than 0.3 0e at 25°C and 1 0e at ?25°C. The growth-induced component of the uniaxial anisotropy field, when scaled by bubble diameter, was found to vary in proportion to the relative concentrations of Sm³⁺ and Y³⁺ in the film, i.e., H_K/d ∝ Sm/Y.     

Gallium concentration effects on magnetic bubble films of EuTm2(Fe,Ga)5O112 garnet grown by liquid phase epitaxy

By varying the Ga concentrations x of EuTm₂Fe_5?xGa_xO₁₂ from x = 0 to about 0.8, thin magnetic films supporting stable bubbles with diameters from 0.5 to about 8μm, respectively, are deposited epitaxially onto gadolinium gallium garnet substrates oriented (111). For low values of x, the Ga segregation coefficient is about 2 and it decreases slightly as x increases. Thus, films contain roughly twice as much Ga as the LPE fluxed melts in which they grow. Both the characteristic length and magnetic bubble stability factor (Q) increase rapidly with x and, to a lesser degree, so does the uniaxial anisotropy field H_A while both 4πM and the uniaxial anisotropy K_u decrease.     

Stress-induced anisotropy in LPE grown Ni(Fe,Al)2O4 films

Preliminary results are reported about the growth of single crystal Ni(Fe,Al)₂O₄ films, grown by means of liquid phase epitaxy on (111)MgO and on (111)ZnGa₂O₄ substrates using a PbO-B₂O₃-Fe₂O₃ solvent. While films grown upon MgO show stress relief at the growth temperature, films grown upon ZnGa₂O₄ possess a tensile strain due to elastic deformation. Since λ₁₁₁ for NiFe₂O₄ is strongly negative a stress-induced uniaxial anisotropy is present in the films. Stripe domains can be observed with the Bitter technique and when a magnetic field is applied perpendicular to the plane of the film, magnetic bubbles with a diameter of ～2 μm appear. A bubble stability factor q exceeding unity is obtained. For the first time magnetic bubbles are found in LPE grown spinel ferrites.     

A Low Power Photoemission Source for Electrons on Liquid Helium

Electrons on the surface of liquid helium are a widely studied system that may also provide a promising method to implement a quantum computer. One experimental challenge in these studies is to generate electrons on the helium surface in a reliable manner without heating the cryo-system. An electron source relying on photoemission from a zinc film has been previously described using a high power continuous light source that heated the low temperature system. This work has been reproduced more compactly by using a low power pulsed lamp that avoids any heating. About 5×10³ electrons are collected on 1 cm² of helium surface for every pulse of light. A time-resolved experiment suggests that electrons are either emitted over or tunnel through the 1 eV barrier formed by the thin superfluid helium film on the zinc surface. No evidence of trapping or bubble formation is seen.     

The effects of spacing between garnet film and Permalloy overlay circuit in magnetic bubble devices

An experimental test set for magnetic bubble devices has been constructed in which the spacing between the garnet film and the Permalloy overlay is variable. The experimental uncertainty in spacing is approximatelypm.15mum, and spacings as small as.5mum have been attained. Bias margin data are presented which were taken at 1 Hz on a 20 micron period chevron circuit as a function of spacing. The collapse and strip-out fields begin to be affected when the spacing is comparable to the garnet film thickness, increasing as the spacing decreases. At larger spacing the high-bias failure mode changes from collapse to uncorrelated bubble motion. A theoretical model which accounts for some aspects of the spacing dependence of the strip-out and collapse fields is described. This model approximates the circuit by a continuous Permalloy sheet. At the low spacing required for efficient use of the rotating field, the model indicates that ±10% nonuniformity in a 2 micron spacing over the device area results in a degradation of the bias field operating range by about 12%.     

Thermodynamic simulation of magnetic-field-modulation methods for pulsed laser irradiation in magneto-optical disks

Direct overwrite magnetic-field modulation recording with dc and pulsed laser irradiation in a magneto-optical disk was simulated thermodynamically. Because the laser light heats the magnetic film, which allows the magnetization to be reversed, the waveform of the laser light strongly affects the shape of the magnetic domain. The dynamic temperature changes in magnetic thin films are simulated for dc and pulsed laser irradiations. With pulsed irradiation the heat is constrained within a limited area, which results in jitter-free magnetic domains, high optical efficiency, and high tracks-per-inch and bits-per-inch recordings.     

Temperature dependence of the magnetic properties of amorphous Co-Gd-Mo thin films

RF sputter deposited amorphous Co-Gd-Mo thin films have previously been shown to be potential materials for bubble device applications. However, the operation of a bubble device over a realistic temperature range requires materials with magnetic properties which are relatively temperature insensitive. An assessment of the temperature dependence of the magnetic properties of amorphous Co-Gd-Mo films is reported. The results of a mean field analysis of the magnetization data of Co-Gd-Mo thin films over wide composition and temperature ranges allowed optimization of the film composition for obtaining most suitable temperature dependences of magnetic properties. It has been found that even with the optimum compositions, the degree of temperature insensitivity improves with decreasing bubble diameters and is satisfactory only for sub-half-micron bubble materials. These results and analysis suggest that similar qualitative behavior may also be expected for other transition metal-rare earth alloys.     

Growth and characterization of Al-substituted iron garnets for submicron diameter bubble films

Al-substituted rare-earth iron garnet films have been grown that support submicron diameter bubbles. In developing small diameter bubble films, reduction of the saturation induction of a film, 4πMS, is very important because power consumption in drive coils and chip heating increase drastically with higher magnetization. Aluminum ions have been found to be one of the most suitable substitution ions for this purpose. Moreover, owing to the small ionic radius of Al ions, it is easy to meet the lattice matching requirement between films and GGG substrates without lattice adjusting ions such as lutetium. Submicron diameter bubble films are easily grown by the conventional LPE technique in the garnet system (YSmGd)₃ (FeAl)₅ O₁₂. They show good bubble properties, well suited to practical use.