Nondestructive readout in thick magnetic film devices

To study the nondestructive readout properties of thick magnetic film devices, sense voltages are computed for triangular interrogate word pulses. Eddy currents are assumed to be the exclusive damping mechanism. For a given pulse rise time one can obtain a maximum signal peak value with a given optimum film thickness, and the read signal of an optimized film can be approximated by simple analytical functions. In an attempt to describe the nondestructive readout stability, a relation is developed between pulse widths, angle of rotation of the magnetization, and reversibility threshold.     

Narrow track magnetic head fabricated by ion-etching method

Manufacturing narrow track magnetic heads is difficult as track widths become very narrow. A new flying-head manufacturing method is established by applying ion-etching. Optimum conditions for ion-etching of the ferrite material were determined. More than 10 μm ion-etched depth and even less than 5 μm track width were obtained. Through experimental and theoretical evaluation, it is proven that ion-etched heads have equivalent or better read/write characteristics than mechanically manufactured heads.     

Narrow track MR head technology

Narrow track technologies for shielded MR heads are discussed in terms of off-track performance and increased head output. Permanent magnet bias heads are increasingly superior to exchange coupling bias heads as tracks become narrower, because of the excellent off-track performance of the permanent magnet type. Increasing the sense current is one way to increase head output. However, the current increase raises the temperature of the MR element and degrades performance. An analysis of the thermal behavior of the shielded MR head shows that increasing the thermal conduction of the shield and gap layers, which are the main sources of heat radiation, and improving the crystalline properties of the Ni-Fe film effectively suppress the rise in temperature of the MR element     

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%.     

Magnetization switching in a garnet thin film

An improved technique for observing subnanosecond magnetization changes in thin films has been used to study the underdamped oscillatory behavior in a (Y2.5Gd0.5) (Fe4Ga1)O12sample. The measurement setup uses a version of the alternate sampling technique in which the sensitivity has been upgraded by the use of a lock-in amplifier. By using a shorted stripline of appropriate length to apply to the film two properly timed magnetic field steps, the precession of the magnetization can be quelled, and the magnetization can be switched between initial and final directions in a half-cycle of the resonant frequency.     

Epitaxial iron garnet films with submicron diameter magnetic bubbles

Mixed rare earth iron garnet compositions have been developed in the form of epitaxial films which support submicron diameter magnetic bubbles. Sm_0.75Yb_2.25Fe₅O₁₂, Eu_0.9Yb_2.1Fe₅O₁₂ and Eu_1.2Lu_1.8Fe₅O₁₂ films on (111) Gd₃Ga₅O₁₂ as well as Sm_1.5Yb_1.5Fe₅O₁₂ and Eu_1.7Yb_1.3Fe₅O₁₂ films on (111) Sm₃Ga₅O₁₂ can be grown by an isothermal liquid phase epitaxy method. The effects of growth conditions on film composition and the lattice mismatch between a film and its substrate are discussed.     

Lithographically patterned magnetic calorimeter X-ray detectors with integrated SQUID readout

We describe the design, fabrication and performance of a fully lithographically patterned magnetic microcalorimeter X-ray detector. The detector is fabricated on the same chip as a low-noise SQUID that measures the change in the magnetic sensor film's magnetization as the film is heated by absorbed X-rays. Our proof-of-principle detectors use a 100 μm×100 μm–2 μm paramagnetic Au:Er film coupled to a low-noise on-chip SQUID via a meandering superconducting pickup loop that also provides the magnetic field bias to the film. Absorption of 6 keV X-rays in the film causes heating on the order of 1 mK with a decay time of 1 ms or less, the fastest reported using a magnetic calorimeter. However, the resolution is currently poor due to poor Au:Er film properties and non-optimized coupling to the SQUID. We describe the design and fabrication of this device and present measurements of the heat capacity, decay time constant and effective thermal conductance of the microcalorimeter as a function of temperature. Because the SQUID and calorimeter are lithographically patterned on the same substrate, this technology can be readily applied to the fabrication of arrays of multiplexed magnetic microcalorimeter detectors.     

Reproducible garnet film preparation using fast growth rates

Large numbers of uniform defect free magnetic garnet films having essentially identical properties can be prepared from the same melt using conventional LPE dipping techniques when growth rates of 2–4 μm/min are employed. Such fast growth rates suppress compositional drift in successive films and avoid second-phase precipitation. Data is given for 40 highly perfect films (> 1 cm², 4πM_S = ～ 200 gauss) grown sequentially from the same melt. Melt composition and growth conditions necessary to achieve fast growth are discussed.     

New annealing method and CaGe garnet film coercivity origin

A new annealing method which drastically reduces the coercivity of GaGe-garnet films grown by LPE has been developed. The as-grown films of (YSmLuCa)₃(FeGe)₅O₁₂ and (YEuTmCa)₃(FeGe)₅ O₁₂ are sealed with Y₂O₃ and metal Ca in an evacuated tube having three chambers. When heated, the tube is filled with Ca and O₂ gases which are supplied from the metal Ca and Y₂O₃, respectively. Ca and oxygen are believed to be incorporated into the film occupying the dodecahedral cation vacancies and oxygen vacancies, respectively. By this annealing, coercivities higher than 5.0 Oe are drastically reduced to less than 0.5 Oe. At the same time, the 4πMs and lattice constant af are decreased, while the Curie temperature T_c is increased. A point defect structure model containing c-site vacancies, oxygen vacancies and a-site Ge²⁺ ions is proposed. The origin of the coercivity is believed to be the oxygen vacancies.     

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.     

Control of the properties of magnetic garnet films

Large numbers of magnetic garnet films having essentially identical properties can be prepared from the same melt using the conventional LPE dipping technique, in which substrate rotation rate is adjusted to compensate for the difference between the growth temperature and the objective, thereby the growth rate can be completely controlled. Melt compositional drift has been suppressed by the periodic addition of garnet oxides and flux to the melt.     

Feasibility of lorentz readout of a high-density fast magnetic memory

A random-access magnetic-film memory with a desired bit density ρLof 10⁸/cm²is postulated with writing and reading each to be performed with a deflectable electron beam within 1 μs. Curie point writing is suggested, while detection of the deflection of electrons due to the Lorentz force is proposed for readout. The transmission, reflection, mirror, and impact scanning methods of Lorentz microscopy are each explored as readout methods. Because of the absence of a heat-absorbing substrate, the transmission method is completely impractical, while the desired value ofrhoLis unattainable for the mirror and impact scanning methods because of the fields from other bits (ρLmust be reduced to4 times 10^{6}/cm²for these methods). Moreover, the small attainable Lorentz deflection angles and the low brightness of conventional electron emitters necessitate further reduction of the bit density to values much less than ρLfor the reflection and mirror methods, unless a field-emission source is used. Excessive power dissipation forces a reduction of density below ρLfor the impact scanning method. The reflection method may be feasible if a sufficiently high fraction of elastically scattered electrons can be demonstrated, if a reliable field emission source can be employed, and if the sophisticated electron optics involved can be built and installed in ultrahigh vacuum.     

Comparison of readout characteristics between magnetooptic transferheads and magnetic heads

The readout characteristics of a magnetooptic transfer (MOT) head were compared with those of a magnetic head. Magnetic recording/readout was done on a CrO₂ flexible disk by using a head with a track width of 5 μm. A Bi-substituted garnet film with a domain width of 1.2 μm and an He-Ne laser spot focused down to 3 μm were used as the MOT head. Readout waveforms from both heads were surprisingly similar. The maximum carrier-to-noise ratio obtained was 50 dB (bandwidth: 30 kHz) for both heads. Experimental data for off-track and crosstalk characteristics demonstrated that the MOT head was suitable for use as a high-track-density readout head. The potential advantages of multitrack readout using MOT heads are described     

Faraday-effect optical current sensor with a garnet film/ring core in a transverse configuration

A new scheme of the Faraday effectcurrent sensor of a magnetic ring-core type is reported. With this scheme a uniaxial magnetic garnet film is obliquely inserted into a very narrow gap in the ring core and a probe light beam is passed through the garnet film in the transverse direction to the core. It is experimentally demonstrated that this current sensor shows good sensitivity as well as good isolation from surrounding currents, particularly because of the very narrow core gap allowed. This transverse scheme is much simpler and less expensive than the known longitudinal scheme and can offer a practical optical current sensor of a ring-core type.     

Perpendicular magnetic recording with a composite anisotropy film

For a recently proposed perpendicular recording system, a composite anisotropy medium has been developed to improve the recording sensitivity of the perpendicular recording head. The medium is composed of a Fe-Ni soft magnetic film and a Co-Cr perpendicular anisotropy film, which are successively deposited on a base by an r. f. sputtering. By using the new double layer medium, an extremely high recording sensitivity could be obtained, compared with the single layer Co-Cr medium. The recording current needed to saturate the double layer film decreased to one-tenth of that for the single layer Co-Cr film. Although the Fe-Ni layer was soft magnetic material, neither deterioration of the frequency response nor peak shift was observed for the double layer film. The reproduction with a perpendicular head was also investigated, and a high output voltage and a high signal-to-noise ratio were obtained.     

Helical anisotropic magnetic film

The magnetic coupling between the magnetization in two nonmagnetostrictive Ni-Fe layers separated by a SiO layer has been investigated by means of a transverse susceptibility measurement. The main results are that 1) the coupling energy Ec per Unit area of the multilayered film has a form ofE_{c}= -A cos (phi_{1}-phi_{2}), wherephi_{1}-phi_{2}is the angle between the magnetization vectors in the two Ni-Fe layers, and 2) the dependence of the coupling constant on the thickness b of the intermediate SiO layer can be interpreted quantitatively by the combination of the coupling energy due to Néel's topography model and that due to the magnetostatic interaction between the magnetic free poles appearing at the edges of the two Ni-Fe layers. The former coupling energy is given byE_{c1} = -frac{p}{2sqrt{2}}omega^{2}M^{2} exp(-sqrt{2}pb) cos (phi_{1}-phi_{2})wherep=2pi/LandLandware the wavelength and the amplitude of the undulation of the interface between Ni-Fe and SiO layers, respectively. The latter is given byE_{c2} = frac{2M^{2}D^{2}}{R} ln (frac{R}{D+b}) cos (phi_{1}-phi_{2})whereDis the thickness of each Ni-Fe layer, andRis the radius of the film.     

Calcium germanium substituted iron garnet films for magnetic bubble applications

Mixed rare earth iron garnet films containing calcium and germanium have been grown on (111) oriented gadolinium gallium garnet (GGG) substrates. The effect of growth conditions on film composition and properties is discussed. The 5 μm diameter bubble films have good magnetic properties; however, temperature control during film growth is very critical.     

The effect of substrate defects on epitaxial magnetic garnet films

The stress patterns associated with various defects in Czochralski-grown gadolinium gallium garnet (GGG) crystals have been observed using a polarizing microscope. The effect of these defects on both the surface topography and the magnetic behaviour of epitaxial magnetic garnet films grown on GGG substrates is reported. In particular, iridium inclusions and defects of a filamentary nature affect the surface topography. Other types of defect influence the magnetic behaviour of the films in a manner attributable to a change in the lattice parameter.     

Analysis of tracks in the stacked film track detector

A method to analyze the experimental data of a plastic track detector is presented. The charge distribution obtained for the cosmic ray heavy charged particles is shown, and it is used to criticize theories which have been proposed until now to explain the rate of chemical etching along ion tracks. The primary ionization J and the restricted energy loss REL are evaluated and compared at the same etching velocity for various charges.REL fits the etching rate well only for light elements Z < 15, and J is suitable either for Z < 15 or Z > 15, while overall fitting is not possible for either theory. The fine structures of the curves, REL, J, and $\text{(}\text{Z}{}^1\text{β}\text{)}{}^2$ vs Z, serve as a measure to estimate the real mean atomic mass of each element in cosmic rays.     

The magnetic domain structure in low anisotropy epitaxial garnet films

Single-crystal films of (YCa)₃(FeGe)₅O₁₂ garnets were prepared by the liquid phase epitaxial method on the (111) plane of Gd₃Ga₅O₁₂ substrates. These magnetic bubble domain films have comparable uniaxial anisotropy energy, cubic anisotropy energy and demagnetizing energy. A photographic technique was developed to measure the angles of inclination of the domain magnetization vectors with respect to the normal to the sample surface. Without an applied magnetic field these inclination angles were 58° and 122°. A simple stripe domain model was used to provide an explanation of the experimental results. This model can easily be extended for other types of mixed anisotropy cases.