US3065105A

US3065105A - Process and apparatus for producing magnetic material and resulting article

Info

Publication number: US3065105A
Application number: US741696A
Authority: US
Inventors: Arthur V Pohm
Original assignee: Sperry Rand Corp
Current assignee: Sperry Corp
Priority date: 1958-06-12
Filing date: 1958-06-12
Publication date: 1962-11-20
Anticipated expiration: 1979-11-20
Also published as: GB872070A

Description

Nov. 20, 1962 A. v. POHM 3,065,105

PROCESS AND APPARATUS FOR PRODUCING MAGNETIC MATERIAL AND RESULTING ARTICLE 2 Sheets-Sheet 1 Filed June 12, 1958 INVENTOR ARTHUR V. POHM BY 2 Z Z a a ,ZTTORNEYS Nov. 20, 1962 A. v. POHM 3,065,105

PROCESS AND APPARATUS FOR PRODUCING MAGNETIC MATERIAL AND RESULTING ARTICLE 2 Sheets-Sheet 2 Filed June 12, 1958 INVENTOR ARTHUR V. POHM T'roRNEYg nit tates am.

3,@55,lfi Patented Nov. 20, 1962 3,065,105 PRUCESS AND APPARATUS FOR PRODUCING MAGNETIC MATERIAL AND RESULTING ARTILE Arthur V. Pohrn, White Bear Lake, Minn, assignor to Sperry Rand Corporation, New York, N .Y., a corporation of Delaware Filed June 12, 1958, Ser. No. 741,696 26 Claims. (Cl. 11793) This invention relates generally to magnetic material and especially to thin magnetic films such as ones for use as memory or switching elements in electronic computers, and more specifically to a process and apparatus for making bistable ferro-magnetic material particularly of the film type so as to enhance the magnetic properties thereof and to the resulting article.

Magnetic films, produced in accordance with the teachings of an application by Sidney M. Rubens, Serial No. 599,100, filed July 20, 1956, now Patent No. 2,900,282, by the deposition of material by condensation methods under high vacuum in the presence of an orienting magnetic field, have been developed for use in data processing equipment. The direction in which the orienting field is applied becomes the preferred or easy direction or axis of magnetization while the direction in the plane of the film, orthogonal to the easy direction, becomes the socalled transverse or difficult direction or axis of magnetizaiton.

It has been found that thin films produced in the presence of an orienting field by vacuum deposition, electrochemical deposition or thermal decomposition exhibit substantial magnetic anisotropy in the plane of the film. It has also been found that if this anisotropy energy, i.e., the energy required to rotate the magnetization from the easy to the difficult direction can be reduced, the resulting films have improved switching characteristics; this improvement consisting of a lower drive field requirement for rotational switching.

The concept of so-called rotational switching is fully discussed in another copending application of Sidney M. Rubens et al., Serial No. 626,945, filed December 7, 1956. This concept in brief, contemplates the use of both longitudinal and transverse switching field components to produce rotation of the magnetic domains by applying, in effect, a torque action thereto. If H is defined as the magnitude of the longitudinal switching field, and H is defined as the magnitude of the transverse field used during the switching process, as the value of H is increased, the longitudinal field H necessary to cause rotational switching is correspondingly decreased. There appears to be a threshold at which this rotational switching occurs determined by the magnetic characteristics of the film and the applied fields. Below the threshold, switching takes place mainly by the somewhat slower so called 180 wall motion switching process. The threshold at which rotational reversal of magnetization in a thin ferromagnetic film specimen occurs is linearly determined by the magnitude of the anisotropy field 1-1,; which is equal to the saturation magnetization divided by the initial susceptibility in the difficult direction of magnetization. By reducing H less drive field is required to change or switch the remanent state of the magnetic element.

The present invention contemplates reducing the anisotropy field of the resulting deposited magnetic material by subjecting the material as it is being formed, either by a vacuum deposition process, a thermal decomposition process, or an electrochemical deposition process, to a magnetic field which changes direction at a non-uniform speed. Such a field may be the resultant field of a constant field component in conjunction with a super-imposed alternating field, preferably of the same value as the constant field, applied preferably orthogonal to the constant field, both fields being in the plane of the magnetic material. Another Way of producing the resultant field is to apply two angulated (e.g., orthogonal) alternating fields difiering in magnitude, preferably by a 2:1 ratio, and differing in phase preferably by The invention includes a novel method for reducing the switching time of magnetic material particularly of the film type, novel apparatus for accomplishing this end, and the novel resulting article.

It is accordingly an object of the present invention to provide an improved method and an improved apparatus whereby the anisotropy field of deposited magnetic mate rial is less than heretofore achieved during a deposition process, and to provide a novel piece of magnetic material so produced.

Another object is conformance with the preceding object is the altering of the time required to change the resulting magnetic material from one bistable state to another, such altering being effected by the novel method and apparatus.

Another object of the invention is the provision of apparatus and a method respectively for causing and subjecting magnetic material as it is being formed by a deposition process to a magnetic field which changes direction at a non-uniform speed.

A further object in connection with the last preceding object is the provision of two magnetic field components oriented in the plane of the magnetic material and causing a resultant field therein which changes direction at a nonuniform speed.

Still another object of the invention in conjunction with the last preceding object is to provide and subject the magnetic material as it is being formed to a primary constant field and a secondary alternating field oriented orthogonal to the constant field.

Another object of the invention in conjunction with the second preceding object is to provide and subject the magnetic material as it is being formed to two orthogonal alternating fields differing in magnitude and phase displacement.

Still another object of the present invention is to provide a piece of magnetic material made in accordance with any of the preceding objects.

Other objects of this invention will become apparent to those of ordinary skill in the art by reading the following detailed description of the exemplary embodiments of the invention and the appended claims. The various features of the exemplary embodiments according to the invention may best be understood with reference to the accompanying drawings, wherein:

FIGURE 1 illustrates exemplary apparatus used in an illustrative vacuum deposition process according to the invention;

FIGURE 2 shows the vector relationship existing when an alternating field and a constant field are applied orthogonally during a deposition process;

FIGURE 3 is a calculated curve of the variation of the anisotropy field as a function of the ratio of the AC. to DC. orienting field;

FIGURE 4 is a schematic plan view of the apparatus of FIGURE 1;

FIGURE 5 is a vector diagram of the conditions existing when two alternating fields differing in phase by 90 and in magnitude by 2:1 are applied orthogonally during a deposition process;

FIGURE 6 is a calculated curve based on Equation 17 and showing the variation of anisotropy energy with the ratio of applied field strength;

FIGURE 7 illustrates conventional apparatus for electrochemically depositing magnetic material, and

FIGURE 8 is a partial view of apparatus like FIG- URE 7 but with the schematic addition of exemplary coils in accordance with this invention.

The apparatus shown in FlGURE 1 may be used during a vacuum deposition process basically in the manner explained in the foregoing Rubens Patent. Within bell jar 10 is a smooth glass substrate 12. The substrate is preferably previously cleaned in a detergent, thoroughly rinsed in distilled water, then placed in an ion glow discharge which removes all dust and other foreign matter thereby yielding an extremely clean surface. This surface is then placed in a mask (not shown) clamped to a substrate heater (not shown). The mask is preferably of such a shape as to create one or more circular film speciments having, for example, a diameter of 1 centimeter. Approximately 24 grams of a melt perferably consisting of approximately 82.5% nickel and 17.5% iron is then placed in a crucible 14 within work coil 16 of an induction type heater (not shown). The bell jar is then evacuated to a pressure of 10- millimeters (more or less as desired) of mercury by a suitable vacuum pump (not shown). Preferably the substrate is heated to approximately 300 C. and the melt to about 1600 C. while a constant magnetic field created by a permanent or electromagnetic yoke structure as described in the Rubens Patent above mentioned, or by direct current coils 18 is maintained at about 30 oersteds for example. A gate (not shown) over the opening of the mask is held closed for a feW minutes to avoid the effects of greater fractional distillation as evaporation first starts. The gate is then opened for approximately 1 minute with evaporation then occurring preferably at the rate of approximately 2400 Angstrom units per minute, the condensate adhering to substrate 12 and forming magnetic film 20 on the bottom of the substrate. The substrate and film are then allowed to cool.

The resulting film has a preferred or easy direction of magnetization as indicated by vector 22 which is in the direction in which the DC. field is applied. Tests made on speciments prepared in the foregoing manner indicate that the transverse or anisotropy field required to rotate the magnetization from the easy to the difficult direction is approximately 3.0 oersteds.

As an aid in understanding the inventive concept of the improved and novel deposition method in accordance with this invention, consider a large number, say 20 unsupported films, each 100 Angstrom units thick, stacked one on top of the other with their easy axes of magnetization difiering slightly from one another. As a result of the orientation differences, the average anisotropy energy of the stack is reduced yielding a 2000 Angstrom unit film with reduced anisotropy.

From this conceptual model, one would expect the anisotropy to be reduced from that obtained with a DO field alone if the orienting magnetic field is made to vary in direction during the deposition. Rather than actually stacking individual films, this invention contemplates in one embodiment the use of a second field indicated by vector 24 angulated to the constant field indicated by vector 22 preferably orthogonally as shown. This second field is produced by means of an alternating current flowing through coils 26 and is preferably equal at its maximum amplitude to the magnitude of the constant field.

Reference is now made to FIGURE 2 which show a vector diagram illustrating the field conditions existing during a deposition process in accordance with one embodiment of this invention. Vector 28 indicates in magnitude and direction the applied field caused by the direct current flowing in coils 18, while vector 30 indicate the instantaneous magnitude and the direction of the second field caused by an alternating current flowing in coils 26. Current in coils 26 oscillates preferably in a sinusoidal manner such that 1:1 sin wt. Consequently, the magnitude of the AC. field H increases to a maximum in one direction when wt equals collapses to a zero value at 0, and then increases to a maximum in the opposite direction at 90. The instantaneous resultant field acting on the film at a time when wt equals 60 during the deposition process as indicated by vector 32 when the preferred conditions of H maximum equals H As the AC. field varies in magnitude, this resultant field oscillates about the direction of the constant D.C. field as a reference and has a strength as indicated by the dash line locus 31 for the corresponding resultant angle e as referred to the constant field vector.

Prom FIGURE 2 it can be noted that the resultant field as represented by vector 32 changes its direction at a non-uniform speed. That is, as wt for H goes at a uni form speed from 0 to 90 for example, s changes from a comparatively fast speed upon leaving 0 to a relatively slow speed in approaching 45. For example as shown by the respective angular scales in FIGURE 2, instead of being only one-third (15) its maximum value when H is one-third maximum (30), 5 has already reached over one-half (approximately 27) its maximum value, and when H is two-thirds maximum (60), is over 0.9 maximum (about 41). Yet the changing speed of 75 slows down so as to reach its maximum (45) at the same instant that H becomes maximum at wt=90. From the foregoing it is apparent that vector 32 moves faster as it approaches, goes through, and leaves the vicinity of the constant field vector 28, for example between =:27, than at any other time. However, Since 1 is never more than i45 the resultant field always effects an overall easy axis of magnetization along a line coincident with the DC field vector 28, and the total anisotropy energy is substantially reduced (without being eliminated) from that resulting when only a constant field is employed.

Since the change of direction of vector 32 is occurring during the deposition process as the metallic atoms deposit on the substrate, it appears that there are formed incremental magnetic layers whose preferred or easy axes are respectively aligned with the instantaneous re sultant applied field with each incremental layer having an anisotropy energy characteristic of a film deposited in a static or constant field.

Using the same apparatus and following the illustrative examples of Rubens in his aforementioned patent as modified by the addition of the AC. field coils 26 producing a field of approximately 30 oersteds varying at a 60 c.p.s. rate, it has been possible to reduce the anisotropy energy from the original 3.0 oersteds obtained with a DC. field only to approximately 1.5 oersteds. This is an improvement of about 50%. It should be understood, however, that the above figures for field strength, frequency, etc., are illustrative and are not intended to limit the scope of the invention. For ex ample, different field strengths may be employed and any frequency can be used as long as the time for one cycle is short compared to the total deposition time. This insures that the different easy axes of the presumed numerous incremental layers will be averaged out to a large degree to effect the resulting desired easy axis along the constant field axis.

The following derivation clearly shows the manner in which the anisotropy energy is affected by variations in the applied A.C. field.

Consider the anisotropy energy E of an incremental layer a thickness dT to be described by the following expressions:

where K is the anisotropy constant of the material, M is the saturation magnetization, H is the anisotropy given by the following expression:

H AC -l (3) tan HDG Since for the interlayer distances involved the exchange forces would be expected to hold the magnetization into a single domain, the anisotropy of the film as a whole would be described by averaging the anisotropy contributions of the incremental layers taking into account the orientation differences introduced by the diifering field directions during deposition. The averaging process can be achieved by performing the following normalized integration.

L E dT 1 far where E;;' is the effective anisotropy energy of the entire film and K is the effective anisotropy constant resulting from the averaging process. If the deposition of magnetic material takes place uniformly or varies slowly in comparison with the periodicity of the alternating field, then for the calculation, the following expression can be used.

( dT=ecdt where dt is an increment of time and oc T/t where T is the total thickness of the film and t is the total deposition time. The expression for the average anisotropy,

substituting time as the variable of intgeration and renormalizing becomes:

Performing the averaging over one cycle of the alternating field, and assuming the field to be a sinusoidal one, the expression for the angle 5 between the constant field H and the total instantaneous field becomes:

H AC SlIl 27ft 7 HDC where H is the amplitude of the alternating field.

Substituting the expression (7) into the expression for the average anisotropy energy (6) gives:

( E :KY sin B-I-C 1 H sin 21.1)] 2 1 a d5 (9) j; K 811) [:6 tan G 1 9 HAC sin 27.1)] 2 {l-J; cos z|:6 tan DC tit and performing the integration results in, (11) .E "-;-1 COS 29 The effective anisotropy constant K measuring the energy difference between the states of the film when it is magnetized in the directions of minimum and maximum energy, i.e., the preferred and transverse directions respectively, is determined by the coefficient of the cos 26 term since a constant energy term merely changes the point of reference. Consequently, the manner in which the effective anisotropy K changes with an increase in the alternating field H can be determined by observing the manner in which the coefficient of the cos 20 term varies with a change in H Expressing the relationship in a convenient form, one obtains:

' E-- 2 1 H K I K1 W i H no where H is the effective anisotropy field resulting from the application of an alternating field during deposition and H is the anisotropy field obtained with a unidirectional field only.

FiGURE 3 is a calculated plot of HAG HK versus HDC and clearly shows the reduction in the anisotropy field with the application of an alternating transverse field during the deposition of a magnetic film.

Points

34 and 36 are actual measurements taken on a 2400 Angstrom unit thick, 1 centimeter in diameter magnetic film composed of 17.5% nickel and 82.5% iron. When an alternating field equal in maximum amplitude to the static field is applied, the effective anisotropy field is reduced to approximately one half of its original value as indicated by point 34. The correspondence between the measured and calculated point is seen to be well within the experimental accuracy of the experiment.

The above derivation is based on 'wobbling the field direction by applying an alternating field in a direction transverse to that of the constant field H It is also within the concept of this invention to reduce the anisotropy energy of a magnetic film by applying a rotating field which rotates more slowly in some directions than in others. Among the numerous ways to achieve this, one of the simplest is to use two sets of mutually perpendicular magnetic yokes or coils carrying currents of different magnitude in each and dififering in phase preferably by FIGURE 4 illustrates schematically a plan view of the apparatus shown in FIGURE 1. Coils 3 8-38 are energized by a current 1 cos wt, and coils 4040 are energized by a second current I sin wt. FIGURE 5 shows the locus 41 of the tip of the resulting field vector, and an instantaneous vector relationship controlling the magnetic orientation during the deposition of the magnetic material when H resulting from 1 is twice as large as H produced by I H and H differ in phase by 90 and H and H are angulated (oriented) orthogonal to each other. It can be seen from this figure that the instantaneous angular direction from the axis of one set of coils 3838 is given by the expression:

(13) tan tan uni where H and H are the maximum magnitudes of the two sinusoidal fields. Inspection of this equation and FIG- URE 5 shows that the resultant field vector 43 in this embodiment also changes direction at a non-uniform speed. That is, vector 43 moves relatively fast on leaving a position where wt and 4) are 0 but gradually slows down and approaches its 90 position at a comparatively slow speed. For example, by the time wt has reached 30 at a uniform speed, has already changed at a non-uniform but fast speed to 50, but 41 requires the remaining 60 of wt to change only 40". It becomes obvious therefore that vector 43 moves slowest while approaching, going through, and leavin the axis coincident with the H field axis, for example as between a of 45 to Since vector 43 remains longest in the general direction of the maximum field H the resulting easy axis of magnetization is the axis of the H field.

Substituting the phase factor (Equation 13) into the a sence expression for the effective anisotropy energy (Equation 6) and normalizing over a rotation of 1r radians, one obtains:

2 2 H +H and again expressing the results in a convenient form:

Figure 6 is a plot showing the reduction in the anisotropy field resulting from the us of two unequal sinusoidal varying fields difiering in phase by 90". It can be seen that the efiective anisotropy energy becomes zero when the magnitudes of the two fields become equal. The reason for this is that there is no longer a preferred or easy direction or axis of magnetization in the film since each incremental layer has its own preferred axis and these tend to average out or cancel one another. The important thing to note is that by properly adjusting the ratio of the two fields, the effective anisotropy energy can be fixed at any predetermined value.

This invention is also applicable to the production of bistable magnetic material as'formed by well known thermal decomposition or electrochemical deposition processes and apparatus. No limitation is intended to any basic decomposition process or apparatus. As a further example, FIGURE 7 shows conventional basic electroplating apparatus for depositing magnetic material such as thin magnetic films on a substrate-cathode 82 in accordance with the masking determined by mask 84 when battery 86 is connected between the anode 88 and the substrate-cathode 82 When disposed in a nickel-iron electrolyte within container 99.

FIGURE 8 shows the container 9% partially broken away for convenience in illustrating the addition of the two orthogonally disposed pairs of coils 92-9 2. and 94-94- in accordance with this invention. These pairs of coils respectively cause two fields in the plane of the films as as the films are being deposited. Again, the fields may be both alternating or one may be constant and the other alternating, all as above described.

It is to be understood that each of the different coils or other magnetic field producing means may be disposed within or without the

different containers

10 and 96, as desired.

It is therefore apparent that there is provided by this invention a process and apparatus whereby the anisotropy energy of bistable magnetic material, and especially of a thin magnetic film, may be reduced and controlled so as to improve the operating characteristics of the resultant novel magnetic material. Basically, it does not matter what the source of the magnetic particles is or what type of process (deposition or otherwise) is used to form the magnetic material, the only requirement being that the magnetic domains of the resulting material be oriented during the forming process in accordance with the resultant applied magnetic field before the domains freeze into a lattice structure.

Other modifications and other applications of this invention not described herein will become apparent to those of ordinary skill in the art after reading this disclosure. Therefore, it is intended that the material contained in the foregoing description and accompanying drawings be construed as illustrative and not limitativie, the scope of the invention being defined in the appended claims.

What is claimed is:

1. In the method of making bistable ferro-magnetic material the improvement comprising subjecting the material as it is being formed to a magnetic field which changes direction in a non-uniform speed wherein the sub ecting of the material to a magnetic field includes simultaneous subjection of the material to two angulated controlled magnetic field components at least one of each is altermating and wherein the other component is oriented orthogonal to the alternating field component.

2. A method as in claim 1 wherein said two field components have equal maximum values.

3. A method as in claim 1 wherein said other magnetic field component is alternating.

4. A method as in claim 3 wherein the maximum strength values of the two alternating field components have a ratio of 2:1.

5. A piece of magnetic material made in accordance with the method of claim 3 and comprising an alloy of between about 75% and nickel, balance iron, and having an anisotropy energy field of about 1.5 oersteds.

6. A method as in claim 1 wherein said other magnetic field component has a constant value.

7. A piece of magnetic material made in accordance with the method of claim 6 and comprising an alloy of between about 75% and 85% nickel, balance iron, and having an anisotropy energy field of about 1.5 oersteds.

8. In the method of making bistable magnetic material, the improvement comprising subjecting the material as it is being formed to a magnetic field which changes direction at a non-uniform speed so as 0 move slowest while approaching, going through and leaving a given axis whereby said axis becomes the easy axis of magnetization of the resulting magnetic material.

9. In the method of making a bistable magnetic film, the improvement comprising subjecting the film as it is being formed to two orthogonal magnetic fields which are oriented in the plane of the film, at least one of said fields being alternating.

10. A method as in claim 9 wherein the film is subjected simultaneously to a constant field and an alternating field which respectively are said two orthogonal magnetic fields and which effect a resultant field that oscillates about said constant field.

11. A method as in claim 10 wherein the value of the constant field is equal to the maximum value of said alternating field.

12. A method as in claim 10 wherein the instantaneous angle of said resultant field relative to said constant field is tan" AoU) wherein H U) is the instantaneous value of said alternating field and H is the value of said constant field.

13. A method as in claim 9 wherein the film is subjected simultaneously to two alternating fields which are respectively said two orthogonal fields and which difier in magnitude and phase relationship so as to effect a resultant field which rotates in varying magnitude at a non-uniform speed.

14. A method as in claim 13 wherein the instantaneous angle between said resultant field and one of said alternating fields is in tan- H 2 Hi tan wt) where H is the maximum magnitude of said one field and H is the maximum magnitude of the other alternaing field.

15. A method as in claim 13 wherein said two fields difier in phase by 16. A method as in claim 13 wherein the ratio of the strengths of said two alternating fields is at least approximately 2:1.

17. A magnetic film with a substantially reduced anisotropy field, made in accordance with the method of claim 16, and comprising an alloy having a composition of between about 75% and 85% nickel, balance iron, and having an anisotropy energy field of about 1.5 oersteds.

18. In the method of making at least one bistable magnetic film wherein the film is subjected as it is being formed to a constant magnetic field oriented along a given axis of the film, said axis resulting as the easy axis of magnetization while the axis transverse thereto is the difiicult axis of magnetization whereby the magnetization of the resulting film may be changed from one of its stable states to the other by a given amount of magnetic energy, the improvement comprising additionally subjecting the film as it is being formed to an alternating field oriented along said transverse axis and in the plane of said film, the method being such that the amount of energy required to rotate the magnetization as aforesaid is substantially less than said given amount.

19. In the method of making at least one bistable magnetic film whereby the resulting film may have its magnetization rotated from one stable state to another stable state by a given amount of energy, the improvement comprising subjecting the film as it is being formed to two orthognal alternating magnetic fields differing in magnitude and in phase by 90, both of said fields being oriented in the plane of the film, the method being such that the resultant film may have its magnetization changed from one stable state to the other by an amount of energy substantially less than said given amount.

20. In the method of making bistable ferro-magnetic material, the improvement comprising subjecting the material as it is being formed to a magnetic field which changes direction at a non-uniform speed wherein subjecting the material to a magnetic field includes simultaneous subjection of the material to two orthogonal magnetic field components which are alternating and of diflerent maximum strength.

21. Apparatus for making bistable magnetic material comprising means for forming a plane of said material and means for causing in the plane of said material a magnetic field which changes direction at a non-uniform speed while said forming means is operative.

22. Apparatus for making at least one bistable magnetic film comprising means for forming said films and means for causing the plane of the film as it is be- 10 ing formed two magnetic fields at least one of which is alternating.

23. Apparatus as in claim 22 wherein both said fields are alternating and phase displaced, said last mentioned means including means for causing one of the alternating fields to be in one direction and means for causing the other alternating field to be in a direction orthogonal to said one direction.

24. Apparatus as in claim 22 wherein said last mentioned means includes means for causing said alternating field in one direction and means for causing the other field in an orthogonal direction.

25. Apparatus as in claim 22 wherein said forming means includes means for making said film by a vacuum deposition process, the last mentioned means including a substrate for receiving the deposited film, said first men-- tioned means including two pairs of coils disposed at right angles to each other so as to provide said two magnetic fields.

26. Apparatus as in claim 22 wherein said forming means includes means for forming said magnetic film by an electrochemical deposition process, said last mentioned means including a substrate for receiving said film as it is formed, said first mentioned means including two pairs of coils disposed at right angles to each other so as to cause said two fields in the plane of said magnetic film.

References Cited in the file of this patent UNITED STATES PATENTS 1,658,872 Yaeger Feb. 14, 1928 2,694,167 Hadfield Nov. 9, 1954 2,765,161 Mungall Oct. 2, 1956 2,853,402 Blois Sept. 23, 1958 2,900,282 Rubens Aug. 18, 1959 FOREIGN PATENTS 5,060 Great Britain of 1882 328,057 Great Britain Apr. 24, 1930 572,409 Great Britain Oct. 8, 1945 670,993 Great Britain Apr. 30, 1952 1,134,869 France Dec. 10, 1956 OTHER REFERENCES Transactions of the Electrochemical Society, vol. 72 pp. 247-280 (1937).

Journal of Applied Physics, vol. 26, No. 8, August 1955, pp. 975-980.

Physical Review, October 15, 1955, vol. 100, 2nd Series, No. 2, pp. 746-747, October 15 ,1955.

Claims

1. IN THE METHOD OF MAKING BISTABLE FERRO-MAGNETIC MATERIAL THE IMPROVEMENT COMPRISING SUBJECTING THE MATERIAL AS IT IS BEING FORMED TO A MAGNETIC FIELD WHICH CHANGES DIRECTION IN A NON-UNIFORM SPEED WHEREIN THE SUBJECTING OF THE MATERIAL TO A MAGNETIC FIELD INCLUDES SIMULTANEOUS SUBJECTION OF THE MATERIAL TO TWO ANGULATED CONTROLLED MAGNETIC FIELD COMPONENTS AT LEAST ONE OF EACH IS ALTERNATING AND WHEREIN THE OTHER COMPONENT IS ORIENTED ORTHOGONAL TO THE ALTERING FIELD COMPONENT.