CA1056040A - Image pickup element and system utilizing magnetic bubbles - Google Patents

Abstract

IMAGE PICKUP ELEMENT AND SYSTEM UTILIZING
MAGNETIC BUBBLES

ABSTRACT OF THE DISCLOSURE
A photomagnetic image pickup element and system, the element comprising a thin film of magnetic material capable of having magnetic bubbles formed therein where the intensity of the magnetic-bubble collapse field varies with temperature; a first conductor pattern disposed on one side of the thin film; and a second conductor pattern disposed either on the one side or on the other side of the thin film, the first and second conductor patterns being so disposed with respect to one another as to form a lattice shape on the thin film.

Description

lOS6040 BACKGROUND OF THE INVENTION
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Field of the Invention The present invention relates to an image pickup element and system for converting an optical image of character or the like into a magnetic-bubble pattern.
Heretofore, image pickup tubes using photoelectric transducers have been principally employed. In this type of pickup tube, however, there exist certain disadvantages such as susceptibility to mechanical shock, a complicated structure, and difficulty in attaining a compact structure. Further, the -`
output signal obtained is serial.
Recently, attempts have been made to eliminate such disadvantages, and, in particular, an image pickup -~
element of photomagnetic-effect substance using magnetic-bubbles has been proposed. However, in this image pickup element a bubble propagation element for propagating the bubbles from a sequential bubble generator through a photo-magnetic substance such as silicon-added yttrium iron-garnet is required. Therefore, the above element is disadvantageous in that it involves intricate processes such as the setting of the propagation circuit with the photomagnetic-effect sub-stance and the setting of the bubble generator. Further, the' bubble propagation method is limited only to the utilization of a rotating magnetic field.
SUMMARY OF THE IN~7ENTION
In accordance with one aspect of this invention there is provided a photomagnetic image pickup element com-pri~sing a thin film of magnetic material capable of having magnetic-bubbles formed therein where the intensity of the magnetic-bubble collapse field varies with temperature; a first conductor set disposed on said one side of said thin film; and a

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second conductor set disposed either on said one side or on the other side of said thin film; said first and second con-ductor sets being orthogonal with respect to one another;
each of said sets c~mprising a plurality of parallel conductor elements, the pitch between successive elements of each set being Pl and P2 where P2 is greater than Pl.
In accordance with another aspect of this invention there is provided an image pickup system comprising an image pickup element including a thin film of magnetic material capable of having magnetic-bubbles formed therein where the intensity of the magnetic-bubble collapse field varies with temperature; first conductor pattern disposed on one side of :
said thin film; and a second conductor pattern disposed either on said one side or on the other side of said thin film, said first and second conductor patterns being so dis-posed with respect to one another as to form a lattice shape ~:
on sald thin film; said first and second conductor patterns being orthogonal with respect to one another and each com- ~:
prising a plurality of parallel conductor elements disposed :~.
in a first direction, the pitch between succèssive elements alternately being Pl and P2 where pitch Pl is substantially greater than pitch P2; means for respectlvely applying first and second current pulses to said first and second conductor patterns to establish insular magnetic domains having a pre-determined direction of magnetization only where said P2 pitch portions of said first and second.conductor patterns overlap;
means for applying a bias magnetic field to said thin film, the direction of said bias magnetic field being opposite to said magnetization of said insular magnetic domains to thereby create a magnetic-bubble lattice in said thin film and (HCo)Kl~HB>(~co)K2 where ~ is the intensity of said bias 1056C~40 magnetic field, (HCo)Kl is the field intensity required to extinguish magnetic-bubbles in said thin film at temperature Kl and (HCo)K2 is the field intensity required to extinguish said magnetic-bubbles at temperature K2 where K~>Kl; and means for projecting an image of light and dark on one face of said thin film to thereby selectively raise the temperature at the light irradiated portions from Kl toward K2 and thus selectively annihilate the light irradiated bubbles to form a magnetic-bubble pattern in the thin film corresponding to said image.
By way of added explanation, the image pickup element of this invention utilizes magnetic bubbles to convert an optical image into a magnetic-bubble pattern. In particular, a magnetic thin film is employed, the intensity of the film's magnetic-bubble collapse field changing in accordance with temperature rise resulting from the absorption of irradiated light. The film is covered with a minute, lattice-shaped conductor pattern for forming a magnetic domain lattice therein, the domains being formed by the magnetic fields corresponding to the current passing through the conductor pattern, A magnetic-bubble lattice is then created by the application of a bias magnetic field. An optical image may be then projected onto the magnetic~bubble lattice to select-ively annihilate the magnetic bubbles and thus produce a 1 magnetic-bubble pattern corresponding to the optical image.
l This invention will be more apparent from a reading of the following specification and claims taken with the drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a plane view of a conductor pattern in accordance with the invention.

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~056040 Figure 2 is a sectional view of an image pickup element in accordance with the invention.
Figure 3 is a magnetic field distribution in a magnetic thin film produced by an application of current pulse applied to conductor patterns.
Figure 4 is a plane view of a magnetic image pickup element, showing a setting of seed bubbles.
Figure 5 is a plane view of a magnetic image pickup element, showing a production of strip domains from the seed bubbles in Fig. 4.
Figure 6 is a plane view of a magnetic image pickup element illustrating three area groups for explanation.
Figure 7 is a plane view of a magnetic image pickup element, showing insular domains produced from the strip -domains by di~iding the latter.
Figure 8 shows a resultant magnetic bubble lattioe.
Figure 9 illustrates the timing of application of current pulses to the conductor patterns and of biasing current.
~igure 10 is a plane view of a magnetic bubble pattern propa~ation circuit, Figure 11 is an example of the conductor pattern driving methods.
DETAILED DESCRIPTION OF THE PREF~RRED EMBODIMENT
7 In this specification and the following claims, a magnetic bubble denotes cylindrical magnetism existing under a bias magnetic field exerted in the direction perpendicular to the surface of a thin film of suitable magnetic material ~ -such as a rare earth orthoferrite, plumbite or rare earth iron-garnet. Various properties of magnetic bubbles are discussed in "Properties and Dev1ce Applications of Magnetic , . . . . . .

~a356040 Domains In Orthoferrites" by A.H. Bobeck, The Bell System Technical Journal, Vol. XLVI, No. 8, October, 1967, pp. 1901 -1925 and "Propagation of Cylindrical Magnetic Domains in Orthoferrites" by Anthony J~ Perneski, IEEE Transactions on Magnetics, Vol. Mag -5, No. 3, September, 1969. The magnetic bubble diameter changes with film thickness, bias magnetic field intensity or temperature, and the magnetic field in-tensity for annihilating the magnetlc bubbles changes with film thickness or temperature.
As an example of the magnetic materials usable in the present invention, samarium-terbium mixed orthoferrite ~$m0 55 Tbo 45 EeO3) is considered. In the magnetic bubble device, it is known on BSTJ. Dec., 1969, pages 3287 to 3335 that the properties of bubble can be represented, in the standardized form, by the characteristic material length 1, the thickness h and the saturation magnetism 4~Ms and these properties do not change even when the magnetic material is chan~ed, For SmO 55 Tbo 45 FeO3~ the bubble diameter d and the collapse magnetic field Hco are changed with temperature in which Hco is abruptly decreased with increase of temp-erature in a normal temperature range (290K to 350K).
See "Temperature Dependence of Rare-Earth Orthoferrite Properties Relevant to Propagating Domain Device Application"
by Rossol, IEEE Transaction on Mag., vol. MAG-5, No. 3, ~ -~
September, 1969. In this paper, it is shown that at the normal temperature (300K), the diameter of bubble having characteristic material length of 4 jU under a bias magnetic field H bias being 58 oe is about 30 ~ and the bubble collapse magnetic field Hco and the strip-out magnetic field Hs are 64 oe and 50 oe, respectively. The collapse field Hco at 10S6~40 320K decreases to 55 oe. This means that when the bias field H bias at a normal temperature (300K) is 58 oe, the bubble existing in the magnetic thin film is disappeared upon in-crease of temperature of the film to 320X. Further, at the normal tempera~ure, the bias field H bias is reduced to a value lower than 50 oe due to same external magnetic field, the bubble is stripped-out, resulting in a strip domain the thickness is about 40 ,u ! in the above case.
When a pair of bubbles are coexisting with a small distance therebetween, an expelling force acts on the both bubbles. The distance by which the bubbles are not given a mobility force is larger than 3d where d is the diameter of the bubble. Therefore, by setting the bias field H bias to 58 oe so that the diameter d becomes 30 ~ and by seiecting the distance between the adjacent bubbles forming the bubble lattice as 3d, a sum of the narrow pitch portion P and the wide pitch portion of P2 of the pattern 10, or 12 becomes 3d, This will be described in detail with Pl and P2 being 30,u and 60 ~, respectively.
In this case, the width of the conductor is determined as 15 ~ according to Goldstein et al. "Bubble Forces in Cylindrical Magnetic Domain Systems", J. Appln.
Phys. vol. 44, No~ 11, November, 1973, In this case, the thickness of the conductor is several microns.
In the image pickup element according to the present -~
inVention, a conductor pattern 10 such as shown in Figure 1 may be proviqed on both sides of a magnetic thin film in an orthogonal relationship with each other as shown in Figure 2 to form a lattice structure. As will be explained in furthér 3a detail hereinafter, when a current is applied to the ortho-gonal conductor patterns, a magnetic field configuration . .

corresponding to the patterns is produced. Each conductorpattern comprises a plurality of parallel conductor elements lOa, lOb, lOc, lOd, lOe, lOf, lOg, lOh,...lOx, the pitch between successive elements alternately being Pl and P2 where pitch P2 is substantially greater than pitch Pl. Although conductor patterns 101 and 12 are each shown as single con-ductors, the conductor elements lOa - lOx may each be driven by separate current sources if desired. Since each conductor ~-pattern is cyclical having a narrow-pitch portion Pl and a wide-pitch portion P2, a striped means magnetic field con-figuration, which is produced by one conductor pattern and is perpendicular to the surface of the magnetic thin film, intersects a striped mean magnetic field configuration in the opposite direction caused by the other conductor pattern, thereby producing insular magnetic fields in the magnetic thin film, As will be explained in detail hereinafter, the fields are acted upon by a bias field to establish the magnetic-bubble lattice, In Fig. 1, the conductor pattern 10, is to form an 11 x 11 bubble lattice and the member of 20 the parallel conductor elements lOa to lOx is twenty-four. ~ ~-This~conductor pattern is formed on a substrate of such as glass by using the etching technique. The resultant pattern is put on an upper surface of the magnetic thin film in an intimate contact therewith. Thereafter, when an electric current is supplied, a magnetic field is produced in the -magnetic thin film.
Fig, 3 shows a magnetic field distribution in the -film. The vertical axis is the magnetic field component Hz in the normal direction to the film surface, averaged with 30 respect to the film thickness. As shown in Fig. 3, the `
direction of the bias magnetic field is denoted as the plus lOS6040 direction. The peak values of the magnetic fields produced in the respective narrow and wide pitch portions Pl and P2 are ~ 73 oe and - 41 oe, respectively, when a current of one ampere is flown through the conductor pattern. That is, the value of the magnetic field produced in the narrow pitch portion Pl is about 1.8 times that produced in the wide pitch portion P2.
The directional relation between the external magnetic field including the bias magnetic field and the field `~
produced by the current flowing through the conductor pattern and the magnetization in the magnetic thin film is in just reverse to that between the bias magnetic field and the magnetization of the cyllndrical magnetic domain (bubble).
The bias magnetic field range within which the bubble is stabilized is from the strip-out magnetic field + 50 oe to the bubble collapse magnetic field + 64 oe. In this range the bubble diameter is reduced with increase of the bias magnetic field. The direction of the bias magnetic field H bias is shown as coming in through the paper sheet, in Fig. 4 and, in this case, the upper portion and the lower portion or the bubble in the magnetic film become S and N poles respectively.
The method of producing the bubble lattice will be described with reference to the drawings. Firstly, a biasing magnetic field + 58 oe which is within the bubble stabilizing range is applied to the surface of the magnetic thin film in the direction normal to the surface by a Helmholty coil to set up seed bubbles in an end area of the wide pitch portions P2 of the conductor pattern 10, as shown in Fig. 4. The sètting up may be performed by applying the bias magnetic _ g _ field (+ 58 oe) to form bubbles and guiding them to the seed bubble setting positions respectively by means of magnetic needles of polarity (N pole) capable of attracting the bubbles. The guiding may be performed visually by the use of polarization microscope, in the simplest case, or by the utilization of magneto-optical effect. As to the stabiliza-tion of the seed bubbles,`it is advisable to provide thin film (5000~ ~) of high permeable magnetic material on the magnetic bubble film by etching to provide bubble stabilizing position on the magnetic bubble film. This is shown as four PERMALOY* dots in "Theory of Single - Current D~main Propagation Circuits" by Copeland, IEEE Traus. on Mag., Letters, June U972~ pages 241 to 243. It is also described in ~'~pplication of Orthoferrites to Domain-Wall Devices" by Bobeck et al, IEEE Traus. on Mag., MAG-5, No. 3, Sept., ~
1969, that with a presence of a magnetic bubble in the~ -stabilizinq position, the bubble collapse magnetic field becomes larger than that required to collapse free bubble by several Oersteds.
~fter the seed bubbles are set up, a current of -about 0.5 ampereS is flown to the conductor pattern 10, in the arrow direction in Fig. 5 by closing a switch Sw of a ! ~' ' pulse current source 22. Due to the current, the magnetic field is produced in the magnetic thin film in the distribution ` shown in ~ig. 3~ In this case, the magnetic field produced in the wide pitch portion P2 of the conduction pattern 10, has a direction reverse to that of the bias magnetic field (+ 58 Oersteds~. Therefore the bias magnetic field in this portion becomes 38 Oersteds because the field in the pitch portion P2 is - 20 Oersteds. Since the strip out * trade mark for a nickel-iron alloy containing more than 30% nickel lOS6040 magnetic field Hs is 50 Oersteds, the bias magnetic field is smaller enough than the strip out field, and therefore, the seed bub~les are stripped out to positions in the magnetic thin film corresponding to the wide pitch portions P2 of the conductor pattern 10, as shown in Fig. 5. The distance of extension of the stripped-out domaln depends upon the wave height of electric current pulse flowing through the conductor pattern 10, and the width of the pulse. The distance required for the 11 x 11 bubble lattice is about 1 mm where the wave height is 0.5 ampere and the width is 10 3~ 10 5 seconds.
After the strip domains are aligned as shown in Fig. 5, a current is flown through the conductor pattern 12 in the direction as shown in Fig. 6 by closing a switch Sw of a pulse source 24. In the magnetic bubble device, a production of new bubbles are made by dividing the seed bubbles as is well known. For samarium-terbium mixed ortho-ferrite, it is shown in the article of Bobeck et al that the bubble dividing magnetic field is 37.5 Oersteds. The dis-tribution of the magnetic field produced in the magnetic film by the current flowing through the conductor pattern 12 is shown in Fig. 3. Since the patterns 101 and 12 are orthogonal, the distribution of the composite magnetic field becomes complicated. For explanatory purpose, overlapping portions of the two patterns are shown by portions A, B and C. The portion A shows a portion where the wide pitch portion P2 of the pattern 101 and the narrow pitch portion P
of the pattern 12 are overlapped, the portion B shows a portion where the wide pitch portions P~ of the patterns 101 -and 12 are overlapped and the portion C is a portion where the narrow pitch portion Pl of the pattern 101 and the wide pitch portion P2 of the pattern 12 are overlapped. The ' - 11 - .~ :. , ~: .

magnetic fields produced in the magnetic film portions corresponding to these overlapped portions by the currents flowing through the patterns 101 and 12 are denoted by HA, HB and Hc. When a current of 0.9 amperes flows through the pattern 102, HA is a subtraction of 20 Oersteds (the field produced in the wide pitch portion of the pattern 101) from 66 Oersteds (the field produced in the wide pitch portion of the pattern 102) and, therefore, becomes 44 Oersteds. The latter is larger than the bubble dividing magnetic field, so that each of the aligned strip domains are cut at the respective portions A causing insular domains as shown in Fig. 7. The reason for that there is no bubble produced even when the bias magnetic field is applied is that the field in the portion B when the land domain exist becomes - 57 Oersteds which substantially cancels the biasing magnetic field therein. A magnetic field HC in the portion C is sub-stantially zero and, therefore, there is the bias magnetic field H bias as it is. The width of the current pulse flow-` ing through the pattern 10~ is 10 3 - 10 6 seconds. After the production of the insular domains in this manner, the currentS flowing through the patterns 101, and 12 respect-ively are cutout, resulting in a bukble lattice shown in Fig.

The amount of image radiation light is about 50mJ/-mm2 and the wave length is within the absorption range of the magnetic thin film (equal to or shorter than about 6000A).
It is easy to increase the number of lattice points, i.e., bubbles, in the bubble lattice. For example, if the number of the conductive elements of the conduction pattern is increased to 66, the active area of the lattice becomes .: . ~ . .
about 3 mm X 3 mm and the bubble lattice is 32 X 32. ~he ~
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bias field may be applied via a loop or the like, the plane of which is parallel to the thin film. For example, the techniques disclosed in the articles cited hereinbefore may be employed or any conventional technique may be used.
The timing of the current pulses applied to the upper and lower conductor patterns lOl and 12 is adjustable although a preferred effect is achieved, as shown in Figure 3, by delaying application of a current pulse to either con-ductor pattern (for example, 101) and increasing its magnitude.
In Figure lO, Il is a current pulse applied to upper con-ductor pattern 10l, I2 is a current pulse applied to lower conductor pattern 12 and IB is a current applied to a loop or the like to generate a bias magnetic field. The duration of IB is indefinite and depends on the length of time the bubbles are to be maintained. -Although the above description refers to an embodi-ment where upper and lower conductor patterns lOl and 12 sandwich the top and bottom surfaces 2Oa and 2Ob of magnetic thin film 20, it is of course possible to attain the above results by a pair of orthogonal conductor patterns lOl and 12 disposed on only one surface of magnetic thin film 20.
The magnetic thin film~of the present invention is made of a material such as samarium-terbium-mixed ortho-~errite in which the magnetic field, intensity needed to annihilate magnetic bubbles changes sha~rply with temperature ~luctuation. Assuming the magnetic bubble extinction field intensitY at temperature Kl is (HCo)Kl and the magnetic bubble extlnctlon field intensity at temperature K2 is ~HCo~K2, the bias magnetic field intensity HB is so set as to satisfy the following relationship assuming K1 is lower than X2; ~
. .

i.. . . . .. . ~ . ,. . . . : :,. . :

(Hco)Kl ~ HB (HCo)K2 , ................. (1) By selective raising of the temperature from Kl to K2, the magnetic bubbles are selectively annihilated in the portions where the temperature has been raised.
Therefore, after magnetic bubbles are generated as aforesaid on the above-stated image pickup element, an optical image of a character, figure or the like is focused by projector 28 on the surface of the magnetic thin film as shown in Fig. 11. By the selective temperature rise resulting from the selective absorption of irradiated light, magnetic bubble collapse field in the light-irradiated portion is lowered in intensity compared to the bias magnetic field.
Hence, in accordance with Equation ~1) annihilation of the light-irradiated magnetic bubbles is effected and the optical image ls converted into a magnetic-bubble pattern. -The magnetic-bubble pattern thus formed can be propagated in any direction desired through a propagation circuit 30 shown in Figure 11 and is usable also as an input to a device utilizing magnetic bubbles. Furthermore, the magnetic bubble pattern can be converted into an electrical signal by the Hall effect or the magnetic reluctance effect.
Also easy conversion into a serial or p~arallel signal is ~chieved, The aforementioned articles disclose various devices for propagating and utilizing magnetic bubbles.
When the number of the conductive elements is in- -creased and the length of the conductive element is increased, the resistance of the conductor is increased accordingly.
However, by dividing the conductor pattern into a plurality of conductor segments and drlving these segments in parallel, ` 3Q the increase of electric resistance may be avoided.
The resolution of the image pickup element using magnetic bubble depends upon the bubble diameter and there-fore, in order to increase the resolution it is usual to make the bubble diameter as small as possible and to use a bubble material having properties that the bubble collapse magnetic field is reduced with temperature increase. As an example, a mixture garnet represented by Eul 7 Erl 3Alo 7-GaO 8Fe3 5 12 is suitable as the bubble material. The characteristic material length of the mixture garnet is 0.75 at 25C and the bubble diameter is about 6 ~. The bubble collapse magnetic field is about 52 Qersteds at 300K and about 37 Oersteds at 313~K, showing a large dependency on temperature. ~-It becomes possible, when this material is used, to form a 56 X 56 bubble lattice in a magnetic thin film having area of 1 mm2, as described in "The temperature dependence of the Auisotropy field and Coercivity in epit- -axial films of mixed rare-earth iron garnets", by Shumate, Jr. et al, J. Appl. Phys., vol. 44, No. 1, January, 1973.
By virtue of the above-described structure, the , 20 present invention requires no sequential generation of magnetic bubbles by a magnetic bubble generator, the botch processing is possible to form a magnetic-bubble lattice by the application of current pulses to the conductor pattern 10 thereby attaining the advantages peculiax to the present .
' invention including the reduction of processing time and less - -'~ limitation on the method for transferring of the magnetic-bubble patterr.

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Claims (3)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A photomagnetic image pickup element comprising a thin film of magnetic material capable of having magnetic-bubbles formed therein where the intensity of the magnetic-bubble collapse field varies with temperature;
a first conductor set disposed on said one side of said thin film; and a second conductor set disposed either on said one side or on the other side of said thin film;
said first and second conductor sets being ortho-gonal with respect to one another;
each of said sets comprising a plurality of parallel conductor elements, the pitch between successive elements of each set being P1 and P2 where P2 is greater than P1.

2. A pickup element as in Claim 1 where each said first and second conductor pattern comprises a single con-ductor composed of said conductor elements.

3. An image pickup system comprising an image pickup element including a thin film of magnetic material capable of having magnetic-bubbles formed therein where the intensity of the magnetic-bubble collapse field varies with temperature;
first conductor pattern disposed on one side of said thin film; and a second conductor pattern disposed either on said one side or on the other side of said thin film, said first and second conductor patterns being so disposed with respect to one another as to form a lattice shape on said thin film;
said first and second conductor patterns being orthogonal with respect to one another and each compris-ing a plurality of parallel conductor elements disposed in a first direction, the pitch between successive elements alternately being P1 and P2 where pitch P1 is substantially greater than pitch P2;
means for respectively applying first and second current pulses to said first and second conductor patterns to establish insular magnetic domains having a predetermined direction of magnetization only where said P2 pitch portions of said first and second conductor patterns overlap;
means for applying a bias magnetic field to said thin film, the direction of said bias magnetic field being opposite to said magnetization of said insular magnetic domains to thereby create a magnetic-bubble lattice in said thin film and (HCO)K1> HB > (HCO)K2 where HB is the intensity of said bias magnetic field, (HCO)K1 is the field intensity required to extinguish magnetic-bubbles in said thin film at temperature K1 and (HCO)K2 is the field intensity required to extinguish said magnetic-bubbles at temperature K2 where K2>K1; and means for projecting an image of light and dark on one face of said thin film to thereby selectively raise the temperature at the light irradiated portions from K1 toward K2 and thus selectively annihilate the light irradiated bubbles to form a magnetic-bubble pattern in the thin film corresponding to said image.