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1 Mar 1965

Volume 36, Issue 3, pp. 675-1270

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Smooth Controlled Wall Motion in Thin Ni∕Fe∕Co Films

Robert J. Spain and Harvey I. Jauvtis

J. Appl. Phys. 36, 1101 (1965); http://dx.doi.org/10.1063/1.1714119 (2 pages) | Cited 1 time

Online Publication Date: 14 July 2004

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Smooth, controlled motion of domain walls in thin Ni∕Fe∕Co films, without spontaneous nucleation of extraneous domains, has been obtained in a planar thin‐film geometry in which the easy anisotropy axis is parallel to the narrow film dimension and displacement of the walls is produced in a perpendicular direction. It is this case for which the stray field from the film edges presents the greatest difficulty to achieving the desired wall motion properties. The film edges are shown to have a pronounced effect upon the shape of a domain wall put into motion by an applied field as a result of the stray field from the edges and an increase in coercive force within the tapered regions. Tapered film edges, in producing a local ``pinning'' of the wall, are shown to provide means for monitoring the presence or location of a domain wall without causing the wall to undergo any net displacement.

Some New Thin‐Film Shift Register Designs

Robert J. Spain, Harvey I. Jauvtis, and Harrison W. Fuller

J. Appl. Phys. 36, 1103 (1965); http://dx.doi.org/10.1063/1.1714120 (2 pages)

Online Publication Date: 14 July 2004

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Thin‐film shift registers have been designed which utilize smooth controlled domain wall motion that occurs in tapered‐edge magnetic films. Films used are long and narrow with an easy axis parallel to the short dimension. Information in the form of a sequence of domains of reversed magnetization is introduced serially at one end of an initially saturated film and propagated along the film by sidewise motion of the domain boundaries. Two shift registers are described which differ in the method of bringing the domain walls to predetermined positions during the shifting sequence. One method, denoted as graded coercive force, utilizes a pattern of local, rapid variations in the wall motion coercive force and a time sequence of uniformly applied drive fields to correctly position the domain walls at specific locations. The other method uses the nonuniform drive fields produced by a set of specially shaped conductors to step domains along a film of uniform wall motion coercive force. At present, the operating principles of graded coercive force devices have been verified and working models of shift registers utilizing nonuniform drive fields have been operated.

Cavity Sensing of Cryoelectric Memory Planes

L. L. Burns and A. R. Sass

J. Appl. Phys. 36, 1105 (1965); http://dx.doi.org/10.1063/1.1714121 (5 pages)

Online Publication Date: 14 July 2004

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Continuous film cryoelectric memory planes can be sensed by means of a superconducting line or a normally conducting sense plane located beneath the memory film. These types of sensing, known as line sensing and cavity sensing, respectively, have certain attributes and limitations which are discussed in detail. An expression for the sense voltage output is derived based on the change of the thermodynamic free energy of the memory during the read cycle. With this expression the sense outputs for line sensing and cavity sensing are calculated and compared with experiment. The signal outputs from cavities are compared with simple line sensing. Structured cavities, i.e., arrangements wherein the sense plane is multiply connected are discussed.
It is shown that at cold temperatures (T≈3.0°K) and thick memory films (a>5000 Å), useful sense signal outputs can be obtained for cavities large enough to be used in practical computer memories. However, for large sense signals (Vs>1 mV) some form of line sensing or structured cavity will be necessary.

Magneto‐Optical Variable Memory Based Upon the Properties of a Transparent Ferrimagnetic Garnet at Its Compensation Temperature

J. T. Chang, J. F. Dillon, and U. F. Gianola

J. Appl. Phys. 36, 1110 (1965); http://dx.doi.org/10.1063/1.1714122 (2 pages) | Cited 26 times

Online Publication Date: 14 July 2004

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The requirements to be met by a changeable memory medium for flying spot variable memory system are discussed. Although simple reversible photochromic materials satisfy most of these requirements, their lack of a writing threshold limits their applicability. The application of a transparent ferrimagnetic garnet near its compensation temperature is discussed. At the compensation temperature the net magnetization of the garnet is zero so that the remanent magnetization of the sublattices normal to the surface of a thin crystal is not subject to demagnetization fields. The magnetization of the iron sublattice of the garnet produces a large magneto‐optical rotation of the polarization of a transmitted light beam in opposite senses for the two directions of the remanent magnetization, which can, therefore, be interrogated non‐destructively. In passing through the compensation temperature, the coercive field for a magnetization reversal passes through a sharp maximum. Thus, the direction of remanent magnetization can be changed by applying a magnetic field in coincidence with a high‐intensity pulsed light beam which produces a transient temperature excursion of ∼3°C from the compensation temperature. This provides a writing‐threshold making it possible to change the remanent magnetization of a selected crystal in an array without disturbing the magnetization of the remainder. Practical aspects of this approach are discussed.

Thickness Dependence of Creep Switching in Magnetic Films

Gilbert P. Gagnon and Thomas S. Crowther

J. Appl. Phys. 36, 1112 (1965); http://dx.doi.org/10.1063/1.1714123 (2 pages) | Cited 2 times

Online Publication Date: 14 July 2004

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The dependence of creep switching on film thickness has been measured in Permalloy magnetic films 200 to 2000 Å thick. No creep was observed in films thinner than 400 Å. With a 1‐Oe (0.6 Hw) dc easy‐axis field present, the transverse threshold field for many‐pulse disturbing decreases an order of magnitude between 400 and 600 Å. Between 600 and 2000 Å, this creep threshold remains essentially constant. The practical significance in limiting the word line density in magnetic film memories and providing a writing mode for NDRO or band‐switched memories are discussed.

Efficiency of High‐Frequency Mixing in a Permalloy Toroid

Ronald J. Webber and Claude P. Battarel

J. Appl. Phys. 36, 1114 (1965); http://dx.doi.org/10.1063/1.1714124 (2 pages)

Online Publication Date: 14 July 2004

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A theoretical harmonic analysis is made of the output voltage from a small, etched Permalloy sheet core which is energized by the sum of two low‐amplitude currents of different frequencies. The magnitudes of the two fundamentals, the sum and the difference frequencies, are predicted by the analysis. The two‐frequency analysis of a hysteretic device is complicated by the multivaluedness of the input—output relation. The small signal behavior of a Permalloy core is further complicated by the lack of a model for high‐frequency drive (∼1.0 Mc∕sec). The analysis of mixing begins, therefore, with an experimentally measured high‐frequency minor loop of a saturated core. The amplitude and frequency of the applied current is selected to insure nonswitching behavior. Once the flux—current relation is established, multiple Fourier analysis is used to calculate the output voltage waveform. The theoretical amplitudes of the frequency components are compared with measured values from a small Permalloy core (o.d.=23 mil, i.d.=17 mil, thickness=0.25 mil).

Exchange Coupling of Uniaxial Magnetic Thin Films

William T. Siegle

J. Appl. Phys. 36, 1116 (1965); http://dx.doi.org/10.1063/1.1714125 (2 pages) | Cited 9 times

Online Publication Date: 14 July 2004

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The properties of two uniaxial magnetic films in intimate contact have been investigated. It is shown that when the uniaxial easy axes of the two films are orthogonal, and the magnitude of the uniaxial anisotropy the same in each film, uniaxial anisotropy in the composite film vanishes, to be replaced by a field‐dependent biaxial anisotropy. The theoretical analysis relates this biaxiality to exchange coupling between the two halves of the composite film. Experimental measurements on several films are compared with the theory and imply for A, the exchange coupling constant, an average value of 0.4×10−6 erg∕cm in a 66–34 Ni‐Fe alloy film.

Precise Long‐Range Linear Transducer Using Magnetic Techniques

H. W. Trechsel, J. E. Moe, and C. E. Johnson

J. Appl. Phys. 36, 1118 (1965); http://dx.doi.org/10.1063/1.1714126 (2 pages)

Online Publication Date: 14 July 2004

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By precisely recording a magnetic thread on a magnetic bar such as Vicalloy, a linear standard of high precision can be fabricated. Accuracies of ±0.00001 in. are possible over several feet. Since there is no wear, recalibration is never necessary. Readout is done by a rotating, noncontacting nut, with an interspersed coil, of the same pitch as the magnetic thread. A phase detector compares this signal to the angular position of the rotating bar. This transducer can be used not only for precision linear measurement, but also as an element in a servo‐control system.

Longitudinal Kerr Effect Using a Very Thin Fe Film

D. O. Smith

J. Appl. Phys. 36, 1120 (1965); http://dx.doi.org/10.1063/1.1714127 (2 pages) | Cited 6 times

Online Publication Date: 14 July 2004

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Using known materials, the optimum magneto‐optical light switch utilizing the longitudinal Kerr effect would operate at a wavelength of ∼1 μ and be made from a very thin [D = 2π(thickness/wavelength) ≤ math] Fe film placed on the surface of a mirror (tangential E=0) in an optical standing wave. A multilayer dielectric structure would be used to couple the incident light to the magnetic film and the maximum possible magnitude of the Kerr coefficient k is calculated to be ∣k∣max∼3.4×10−2. This is about 10 times greater than for a thick Permalloy film operating at visible wavelengths.

Investigation of Eddy‐Current‐Induced Nondestructive Readout in Thin‐Film Memories

M. S. Maltz

J. Appl. Phys. 36, 1121 (1965); http://dx.doi.org/10.1063/1.1714128 (2 pages) | Cited 1 time

Online Publication Date: 14 July 2004

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When a magnetic thin‐film bit is switched, eddy currents arise, creating fields which tend to remagnetize the bit in its original direction. A structure in which these fields are strong enough to produce NDRO was given a preliminary investigation. The bits were nickel‐iron 17–83 Permalloy, 50×50 mil by 500 Å, evaporated onto a glass substrate. The structure layers were: bit line, word line, planar sense line, bit, glass, and word line. If the word drive pulse was 100 nsec long or less, the structure with the most unfavorable geometry gave stable NDRO at drive currents up to 10 times as large as the normal DRO value. Scale factors are displayed and applied to show that NDRO is possible with structures much smaller than the one tested. The eddy fields could even remagnetize partially demagnetized bits.
When the eddy field was measured as a function of time and geometry, the time constant and magnitude of the decay was a sensitive function of the word linewidth and separation, varying between 76 and 191 nsec. Experimental and theoretical evidence was found for a rapid transient decay mode which precedes the slower decay mode observed in the experiments.

Wall Motion Reversal in Easy‐Axis‐Coupled Film Strips

J. M. Daughton and H. Chang

J. Appl. Phys. 36, 1123 (1965); http://dx.doi.org/10.1063/1.1714129 (3 pages) | Cited 2 times

Online Publication Date: 14 July 2004

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Wall motion reversal of the magnetization in coupled film strips is examined theoretically and experimentally. The structure is a conductor strip sandwiched between two Permalloy strips having their easy directions across the strips. If a train of current pulses of sufficient amplitude passes through the conductor strip, the Permalloy layers may be switched around the conductor in either of two sense depending on the polarity of the current. Criteria are presented for coupling (that is, that the two layers remain essentially oppositely magnetized) as a function of width of the strip, thickness of the Permalloy, coercivities of the films, and thickness of the intermediate conductor. It is found that coupling may break down either if the strip is too narrow or too wide. For strips which are coupled and have relatively thin enclosed conductor layers, the multiple‐pulse field necessary to switch the film strip is about the average of the coercivities of the two Permalloy layers. For thicker conductor layers, this field is reduced by a geometric factor accurately predicted by a model using only magnetostatic interactions between the two Permalloy layers. Data are presented for 2000‐Å layers of Permalloy in 0.125‐ to 4‐mm strips with separations between the Permalloy strips between 0.6 and 8.6 μ. The coercivities of the films range between 0.6 and 6 Oe.

Low‐Field Hard‐Direction Resonance in Uniaxial Permalloy Films

D. T. Ngo

J. Appl. Phys. 36, 1125 (1965); http://dx.doi.org/10.1063/1.1714130 (2 pages) | Cited 3 times

Online Publication Date: 14 July 2004

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The resonance experiment for hard‐direction bias fields below Hk has been hindered by the change in coupling between rf field M and pick‐up loop as the bias field varies. To solve the problem, a new measurement technique is used. For each value of hard‐direction bias field (HT), M is rotated to an angle θ with respect to the easy axis. A bias field Hθ is then applied parallel to M. For the above biasing conditions, the following resonance relation is derived: (1∕4πM) (ω∕γ)2=Hk cos2θ+Hθ which, for a fixed θ, is the equation of a straight line whose ω2 axis intercept corresponds to the solution of the standard resonance equation at the corresponding value of HT less than Hk.
An experimental apparatus is designed so that the coupling between rf field M and pick‐up loop is constant throughout the experiment. For each value of HT, the orientation of M can be located precisely with a newly developed technique. Using the above techniques, the theoretical hard‐direction resonance curve is experimentally verified for the first time.
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