Thomas S. Crowther

Angular and Magnitude Dispersion of the Anisotropy in Magnetic Films

A physical model for the angular and magnitude dispersion of the anisotropy of magnetic films has been investigated. It is based on the effect of anisotropic strain on a microscopic scale acting on magnetostrictive material. If the strain‐induced anisotropy component Hks is less than the intrinsic unstrained value Hko the following results are obtained: (1) Regions with highest and lowest local Hk values are unskewed. Regions with maximum skew have approximately average Hk. (2) Although local skew α and the resultant anisotropy Hk are not uniquely related since each depends on both direction and magnitude of applied stress, their maximum values for a composite film having isotropic strain on a macroscopic scale are related by; αmax=(1/2) arcsin (Hk max−Hko)/Hko.The following measurements have been taken which tend to support the theory. (1) The angular distribution function has been found to be Gaussian using a measurement technique for which the angle can be read with ±0.1°. (2) The broadening of the Sto...

Magnetic Film Memory Design

Thin magnetic films of permalloy have characteristics ideal for high-speed digital storage. A simple rotational model modified to include the effects of wall switching and dispersion of the preferred direction of magnetization provides a basis for describing properties of engineering interest. A selection system has been chosen which allows great latitude in film uniformity. Production of films with magnetic properties uniform to within ± 10 per cent is readily achieved. Specifications for operation in a destructive mode can easily be met by existing film arrays; the nondestructive mode is considerably more stringent unless very small signals can be tolerated. The first film memory has been in reliable operation since the summer of 1959. It has 32 ten-bit words and has been operated with a minimum cycle time of 0.4 , μsec. Higher speed and larger capacities will require higher bit densities and improved techniques to minimize undesirable coupling between drive and sense lines. The use of 10 × 60 mil rectangles, balanced sense windings, and longer words will hopefully permit memories of about 200,000 bits with cycle time under 0.2 μsec.

Magnetic lung measurements in relation to occupational exposure in asbestos miners and millers of Quebec

Abstract Fe3O4 particles (ferrimagnetic) are usually attached to asbestos fibers (nonferrimagnetic) in the chrysotile asbestos mining and milling industries; therefore, a magnetic measurement of Fe3O4 in the lungs of workers in these industries could help determine the amount of asbestos which has been inhaled and retained in their lungs. As a first assessment of this method, magnetic measurements were made of Fe3O4 in the lungs of 115 miners and millers in Quebec. These measurements at an industrial site were found to be feasible and practical; however, the amount of Fe3O4 seen in the lungs of those with welding exposure was large enough to mask the Fe3O4 contributed by asbestos, and this subgroup was considered separately. For the remainder (nonwelders), the amount of Fe3O4 was plotted against a total dust exposure index (asbestos and other dust) estimated for each worker. Although the correlation between these quantities was not high, it was statistically significant at the 1% level. Because retained asbestos is likely to increase with increasing exposure to total dust, this correlation suggests that a magnetic lung measurement of a chrysotile miner or a miller does reflect, to some extent, the amount of asbestos in his lung. There was considerable scatter in the data, partly due to individual variations in deposition and clearance, to which this method is sensitive. When the data of only the nonsmokers were plotted, the amount of Fe3O4 was greater than for the total group of nonwelders. This is consistent with previous findings that less dust is deeply deposited in the lungs of smokers, due to constriction of small airways.

Magnetoelastic Sensitivities in Evaporated and Electrodeposited Permalloy Films

When a magnetostrictive Permalloy film is stressed in the direction of its preferred axis of magnetic orientation, an effective anisotropy is induced in the film causing a change in measured Hk. Magnetoelastic measurements were made on evaporated and plated films 100–4000 A thick and varying in composition from 70% Ni, 30% Fe to 85% Ni, 15% Fe. Composition was determined by x‐ray fluorescence. It was found that the magnetoelastic coupling constant B = (2/M) (dHk/de) was significantly less for plated than for evaporated films of the same composition. There is evidence of both intercrystallite or internal crystallite slip and film‐to‐substrate slip, but neither is large enough to account for the observed differences.

Specifications and yields of composite magnetic films for a high-density memory

General design equations have been derived which relate magnetic film parameters to memory operating currents and bit geometry. The application of these equations is demonstrated in determining the magnetic specifications of a memory having 12 500 bit/in2. These characteristics were most easily realized in a composite magnetic film vacuum deposited sequentially from melts of 50-percent Co, 47-percent Ni, and 3-percent Fe and 83-percent Ni and 17-percent Fe. This is an excellent way of achieving characteristics which lie between those of any two layers taken separately as long as the total thickness is less than the exchange length, i.e., <2000A. Before etching and final processing, yields to magnetic specifications of 80 percent were achieved using these composite films.

Parallel Oblique‐Incidence Anisotropy in NiFe Films

Magnetic films exhibiting an easy axis normal to the incidence plane (perpendicular anisotropy) when deposited at oblique incidence are well known. Anisotropic crystallite chains created by self‐shadowing were postulated to explain this effect. One of the physical measurements supporting this model was the observation of optical dichroism. In the present work, an easy axis parallel to the incidence plane (parallel anisotropy) has been found in nonmagnetostrictive films deposited at low (<45°) incidence angles for certain substrate temperatures and film thicknesses. The value of Hk and direction of the easy axis in oblique incidence films was very sensitive to changes in incidence angle, substrate temperature, film thickness, and deposition rate. The magnitude of the magnetic anisotropy Hk was found to be linearly related to the magnitude of the dichroism Δγ by the same constant, dHk/dΔγ = 5.5 × 105 A·Oe, for oblique incidence films having either parallel or perpendicular anisotropy. This result furnishes ...

A progress report on large capacity magnetic film memory development

In 1964 we proposed an approach to magnetic film memory development aimed at providing large, high-speed, low-cost random-access memories. Almost without exception, all early attempts at film memory design emphasized speed with little consideration for the potential of batch-fabrication to reduce costs. Based on our earlier work in building the first film memory in 1959, and a 1,000 word, 400 nsec model for the TX-2 computer in 1962, we had reached some fundamental conclusions about the compatibility of high speed and low cost for destructive-readout film memories.

A Proposal for an Associative Memory Using Magnetic Films

Abstract : The proposed memory consists of a normal word line over a strip of magnetic film and a digit line split into three parallel conductors. Writing is accomplished using the standard method of coincidence of transverse and digit fields (all three digit conductors are excited equally). The result is a region at the intersection which is magnetized either to the right for a ONE or to the left for a ZERO. To interrogate the memory in all bits simultaneously, the middle digit conductors alone are excited with a field to the left, if searching for a ZERO, and with a field to the right, if searching for a ONE. For either match condition, there will be essentially no change in the magnetization of the interrogated cell. For a mismatch, the magnetization will rotate noncoherently to the multidomain pattern which occurs just below nucleation. It is proposed that the magnetoresistive effect be used to sense the mismatch condition. Experimentally, it is found that a resistance change corresponding to approximately 10 per cent of that for full transverse switching is obtained.

Thickness Dependence of Creep Switching in Magnetic Films

The dependence of creep switching on film thickness has been measured in Permalloy magnetic films 200 to 2000 A thick. No creep was observed in films thinner than 400 A. 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 A. Between 600 and 2000 A, 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.

Saturable Shielding‐A Technique for Nonlinear Coupling in Magnetic Circuits

If a magnetic film (the shield) with transverse saturation demagnetizing field Hd and anisotropy field Hk is placed in a transverse field below saturation strength (Hd+Hk), it experiences a net field less than Hk. A second film (the storage film), when placed on top of the first, will experience the same net field. If the anisotropy field of the storage film is much greater than that of the shield, the magnetization of the storage film undergoes a negligible rotation for all applied fields below Hd+Hk. Once the shield has saturated, any further increase in field is applied directly to storage film. The resulting plot of the net field at the storage film versus applied field shows a symmetrical offset threshold of approximate magnitude Hd. Control of the shield geometry provides a means for adjusting this threshold.To investigate the shielding effect, two experiments were performed. The first used a 0.5‐in.‐wide Supermalloy tape to approximate an ideal shield and measured net field at its center line as a ...