[1]United States Patent Office 314409626 3,440,626 MAGNETIC AIEMORY EMPLOYING TWO THIN FILMS egeli, Ralph F. Penoyer, Poughkeepsie, N.Y., and Otto Vo West Laf-,tyette, Ind., assignors to International Busi ness Machines Corporation, Arnionk, N.Y., a corporation of New York Filed June 30, 1965, Ser. No. 468,314 Int. Cl. Gllb 510-0 U.S. Cl. 340-174 4 Claims ABSTRACT OF THE DISCLOSURE The ma.-netic thin film storage element includes two thin films havin.- uniaxial anisotopy mounted one above the other with their easy axes parallel. When storing binary information the films are ma@netized alon- their easy axis in opposit,- directions. One film has a hi2her uniaxial anisotropy field than the other and the Alms are tightly coupied so that their fields interact. Writin@ and nondestructive reading is accomplished usin.@ wor@ and bit conductors arranged completely external to the stora-e element formed by the two films. In both writing and nondestructive reading swi,chin.- is by the high speed rotational switching. The present invention rela'es to magnetic memory and more specifically to improved magnelic slora,@e devices formed ot coupled thin films of ma.-netic material. Thou.-h most of the memories presently in commercial tise employ ferrite cores as the primary storage meditim, the increasing demands for higher memory speeds h,,ive focused attention on the development of storage devicesfabricated of magnetic thin films. Thin film s'@ora,-e devices have been i-abricated usin.- a sin.-le f@lm having tiniaxial aniso,ropy, that is, an easy axis of magnetization parallel to which the magnetic moments in the film are oriented in the absencp, of a magnetic field. Si-ich films siore information by bein.- caused to assume either a first stable state wiih the moments ori@@nted in o@-ic direction alon.- the easy axis, or a second stable sla!e wiih the monients orien'ted in the opposite direc@io,i aon- th-, easy axis. These fil.-ns can be operated at @igh, speeds, since magn,-"ization changes in th.- films can be effected by rota'@ional switchin.-, which is iruch fasler than the predominate domain wall swifchiii.- employed in othcr magnetic devices. However, due t o the fact that single film storage devices are open flux path s'ructures, many difficulties are encountered in the successflil applicaL;oTi of this type of slorage devices. For exampl.-, the magnetization in the illm in one direction prodtices a self demagnetizing field which tends to chan_ge tho niagnetization slates. Further, stray fields are p-@-oduced by the magnetization in the film which can affect olher film s'@oragp- devi-,es in the immediate vicinity. Thcre is also a tendency for a t)henomenon termed "creep" to ocetir wh-2n thin film siora,-e devices are repeatedly inlerrogated in a nondesructive mode. As a result of th:s "creep," repeated interrogations produce svccessive irreversible chan.-cs in the magnetization until the information siored is lost. Iii order to reduce the seriousness of these diffictilties and the related consequences, coupled film storage devices have been develor)ed in which a second film is placed adjacent 'Lo the first film to provide an essentially closed flux path typ-- of structure. Examples of coup@'ed film s,ruc,iires are described in U.S. Patents Nos. 3,015,807, issued Jan. 2, 1962, to A. V. Pohm et a].; 3,188,613, isst@ed June 8, 1965 to 0. A. Fedde, and in aii article entitied "Coincide-tit CtirPatented Apr. 22, 1969 2 rent Nondestruc' ' ive Readout from Thin Magnetic Films," by L. J. Oakland and T. D. Rosen, which appeared in the Journal of Applied Physics, vol. 30, No. 4, stipp., pages 54S to 55S, April 1959. The coupled film storage devices of the type to which 5 the subject invention relates provide distinct advantages o - ver previously d.-veloped devices in that the magnetizat,.On can be switched for either writing or nondestructive reading by rotational switching and, ftirther, rotational 10 reading and writing operations can be accomplished using conduclors which are external to the coupled film storage devices. This latter type of geometry allows the use of drive and se@-ise conduclors which are relatively thick and, therefore, exhibit relatively low D.C. resistance. Coupled 15 film s,ruc,ures previously developed either employed dorpain wall switchin.- or required that there be at least onc condlictor placed between the coupled films. The use of a thick conductor between the film increases the space between the films and thereby the magnetic opera20 tion suffers. If a thinner conductor is used, the impendarce problems are encountered. As illustrated in the embodiments discussed herein, theadvantages of the present invention are realized by using a pair of mangetic thin films arranged one 25 above the other, with the easy axis of the films parallel to each other. The coupled films have two sforage states, in one of which the magnetization in the first film is in a first direction and that in the second film in the opposite direction. In the second storage state, the magnetization 30 in both films is reversed. The films are coupled by an interac'ion field so that each film applies to the other film, when in either storage state, a field which is in the same direction as the magnetization in the other film. One of the i9lms is fabricaed to have a greater uniaxial 35 anisotropy field than the other film, the uniaxial anisotropy field being defined as the minimum field necessary to be appli@,d in th-. hard direction perpendicular to the easy axis to orient the magnetization entirely in the hard direction. As a result of this difference in the anisotropy 40 fields for the films, a field applied to the films solely in a direction perpendictilar to the easy axis produces a rolation of the moments which is greater in one film than in the other. As a restilt, the coupling between the 'films changes during this rotation causing a field to be 45 produced external to the films which can be sensed by a conductor external to the films. Upon termination of the applied field, the films reasstime their original direction of magnetizaiio-i, again by rotational switching. Writing is accomplished in these films by applying a digit field 5o in the desired direction parallel to the easy axis jointly with a field perpendicular to the easy axis. As these fields are te-rminated, the magnetic moments in th-- film having the higher uniaxial anisotropy field rotate in th@- direction of the applied digit fi-.1d. However, even in the presence 55 of the di@-it field, the film having the lower uniaxial anisotropy is controlled by the field from the other film so that the moments therein are rotated to assume a stable state in a direction opposite to that of the applied digit field. All of the above operations can be accomplished 60 using conductors external to the films, though it is possible, of course, to use a conventional sensing conductor arranged between the two films. Thus, a primary object of the present invention is to provide an improved high speed magnetic storage device 65 formed of coupled magnetic films. Still another object of this invention is to provide an improved coupled film storage device which can be nondestructively interrogated at high speeds using rotational switching. 7o A further object of this invention is to provide a magnetic coupled film storage device in which writing operations ca-@i be accomplished using rotational switching.
[2]32440,626 3 Still it is another object of this invention to provide a coupled film stora-e device of this type which may be, operated in either a word mode or a digit mode. A further and significant object of this invention is to provide an improved coupled film storage device which can be nondestructively interrogated by rotational switch'5 ing using conductors external to the coupled films. The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of 10 the invention, as illustrated in the accompanyin.- drawings. In the drawings: FIG. I is a schematic representation of a thin film of magnetic material. FIG. 2 is a plot depicting the rotational switching char- 15 acteristics of the film of FIG. 1. FIG. 3 is a front view, partly schematic, of an embodiment of a coupled film storage device fabricated in accordance with the principles of the present invention. FIG. 4 is a plot depicting the rotational switchin@ char- @go acteristics of the stora-e devices of FIG. 3. FIG. 5 is a plot depicting the manner in which the magnetization is rotated and the fields of the two films interact when the stora-e device of FIG. 3 is operated in a nondestructive mode. 25 FIG. 6 shows in more detail an embodiment of a coupled film stora,-e device together with the associated circuitry necessary for its operation. FIG. 6A is a cross section of the structure shown in FIG. 6. 30 FIG. 7 is a cross sectional view showing another embodiment of the invention wherein the coupled films form a closed flux path. FIG. 8 illustrates a further embodinient of a coupled film stora,-e device with drive conductors arranged ex- 35 ternal to the films and a sense conductor between the films. FIG. 8A is a cross sectional view of the sti-ucture of FIG. 8. FIG. 9 is a schematic representation of a memory 40 array of storage devices constr-ucted in accordance with the principles of the present invention. FIG. 9A is a cross sectional view of a storage device used in the memory array of FIG. 9. Referring now to FIG. 1, there is shown in schematic - form a circularly shaped magnetic thin film element F. 4' This element is made of a magnetic material such as Permalloy and is approxiniately 1000 angstroms thick. A film of this type is capable of acting essentially as a sin-le domain wherein magnetization changes are effected by rotation of the magnetic moments rather than by domain 50 wall switchin,-. The element F is fabricated to have anisotropic ma,@netic properties, by which is meant it exhibits an easy axis and the magnetic moments in the plane of the film are oriented parallel to this axis in the absence - of an applied field. The easy axis of the film element F 00 is in the horizontal direction indicated by the arrows 12. The direction perpendicular to this direction is termed the hard axis of the film and is indicated by the arrows 14. Though the ma,-netic moments in such a film can be 60 oriented along the hard axis, that is vertically in FIG. 1, by the application of an appropriate field, upon termination of the applied field, the moments realign themselves along the horizontal axis. Due to easy axis dispersion, some of the magnetization in an actual film remains locked close to the hard direction. (,5 The magnetic rotational switching properties of the film element F are illustrated in the plot of FIG. 2 by the curve generally designated C. Magnetic fields applied in the horizontal or easy direction (arrows 12 in FIG. 1) 70 are represented along the abscissa and fields applied in the hard axis direction (arrows 14 in FIG. 1) are represented along the ordinate. The points Hx along the vertical and horizontal axes, where the portions of the curve C are tangent to these axes, represent the uniaxial anisotropy 75 4 field for the film element. This is the minimum field effective of and by itself to produce an irreversible rotation of the magnetic moments. When the film is in a quiescent state with no field applied and the moments are oriented along the easy axis pointing either to the right or to the left, the state of the element is represented at 16, the intersection of the ordinate and abscissa in FIG. 2. If it is assumed that the film has its ma,-netization oriented to the ri,-ht along the easy axis, then it is possible to change this orientation by applyin- to the film a field sufficient to exceed the threshold represented by the curve C. Consider, for example, a hard axis field in excess of the critical value shown at HK, to be applied to the film. The magnetization moments are then oriented in the vertical direction, and upon termination of this field in the absence of any horizontal field, the moments may ali.-n themselves either to the right or to the left in the plane of the film. When a horizontal field is applied at the time the vertical field is removed, the nioments reali@-n themselves in the direction of the applied horizontal field. When the applied fields are of insufficient intensity to exceed the threshold indicated by the curve C of FIG. 2, some reversible rotational switching takes place and the moments, upon tern-lination of the applied fields, realign themselves in the original horizontal direction. More specifically, if the magnetic moments in the film are originally oriented along the easy axis in the negative or left direction in FIGS. I and 2, and a vertical field +Hy, and a horizontal field -Hxl are simultaneously applied, the magnetic moments are rotated in a clockwise direction. The amount of rotation can be shown on the plot by drawing a line 18A from a field Point 18, for applied fields +Hy, and -Hxl, tan.@ent to the curve in the first quadrant of FIG. 2. The arrow adjacent field point 18 indicates the direction in which the moments are oriented parallel to the line. A detailed explanation of the manner in which such lines are drawn is incltided in an article by Hsu Chang, appearing in the IBM Journal of ReseaTch and Development, vol. 6, No. 4, pp. 419-429, October 1962. For the present case, with a field applied of - Hxl and +Hy, to the film with the moments ori-inally oriented in the left or negative direction, the vector is drawn parallel to the characteristic curve in the first quadrant indicating that the moments ori,-inally pointin.- to the left are now rotated by an angle 18B from the original horizontal direction. Since the point 18 defined by the applied fields is within the characteristic curve, upon termination of the applied fields, the magnetic moments reassume their initial orientation to the left along the horizontal axis. The operation is similar if the horizontal field applied is in a direction opposite the initial direction of orientation alon.- the easy axis as long as the field point defined by the vertical and horizontal field is within the characteriStiC CUTve. Thus, if along with the vertical field +Hy,, a horizontal field +Hxl in the opposite direction to the initial magnetization is applied to define a field point 21, the magnetic moments are rotated through an angle 21B to the direction indicated on a line 21A. However, when the fields are removed, the element reassumes its initial state with the moments oriented in the negative direction pointin- to the left in the figures. If the applied fields are such as to define a field point outside of the characteristic curve, rotational switching from one state to the other can be effected. A combination of a field in the horizontal direction +Hx2 and a field iTi the vertical direction +I-IY2, defining a field point 22 which is outside the curve C, is sufficient to produce an irreversible rotation. In this case, the state of the magnetization when the two fields are applied is obtained by drawing a line 22A from the point 22 tangent to a portion of the characteristic curve C in the second quadrant. This directional line indicates that in the presence of the applied fields, +HX2 and +HY2 the moments are oriented to the right at an an,-le 22B to the horizontal axis. If now the applied fields are terminated, the moments align themselves alon- the easy axis in the plus direction, pointing to 0
[3]3,440,626 5 the right in the flgUTes. The moments may be realigned back to the original direction by applying a field in the vertical direction +HY2 in combination with a field in the horizontal direction -HX2. These combined fields represented at a point 25, rotate the moments to the direction indicated by the arrow on a line 25A at an angle 25B 5 with the horizontal axis. Upoii removal of these fields, the ori.-irial magnetization in the negative direction alon.@ the easy axis is reassumed. In the above description of the manner in which the 10 moments are rotated by fields applied to the film element of FIG. 1, only a single film element is employed. Further, when the film element is in a stable or quiescent state witli the moment oriented in one direction or the other no external field is applied to the films. In the case of storage 15 devices of the type in which this invention is directed, two films are arranged, one above the other so that the magnetization in one produces a field which is applied to the other. This is illustrated in FIG. 3 wherein two films F, and F2 are shown placed one above the other. These films 20 are anisotropic and have their easy axes parallel to each other in the horizontal direction indicated by the arrows 32. The two films may, for example, be made of Permalloy and have substantially the same thickness, approximately 1000 a-@igstroms. The primary difference between the two 25 film elements F, and F2 is that they are prepared such that one requires a larger field to produce irreVCTsible rolational switchin.- than the other. Stated another way, the film F, has a lower anisotropy fleld Hy,, than the anisotropy Hjc2 for the upper film F2. This is shown in FIG, 4 where the 30 characteristic cutve for the film F, is represented at C, and the characteristic curve for the upper film F2 is represented at C2. The anisotropy field for the films HR, and HK2 are located at the points at which the curves are tangent to the ordinate and abscissa. Because of the arrangement of the two films, it can 35 be seen that the st@ray field from each of the films is present as a field applied to the other film. These fields are rel@@- resented in FIGS. 3 and 4 at H12 by the field lines H12 and H21 for the case when the magnetic moments in the lower film F, a.-e oriented to the left in the negative direction 40 and the magnetic moments in the upper film F2 are oriented to the right in the positive direction. Referrin@ to FIG. 4, it can be seen therefore, that the film F2 hav' ing its moments oriented in the positive direction along the easy axis has applied to it the stray field H21 Of film Fl. 45 Similarly, the lower film F, with its ma.-netic moments oriented to the left, and in the negative direction, is stibjected to the stray field H12 of the upper film F2. With films F, and F2 i-Tl close proximity, these stray fields are essentially equal to and oppositely directed to the de- 50 magnetizing fields and the result is an essential reduction of the demagnetizing field to zero. The films F, and F2 are arranged to be tightly coupled so that essentially all of the stray flux from one passes through the other, when in the quiescent state shown in 55 FIG. 3. The same is true, of course, wben the moments are rotated in both films so that the orientation in the lower film is positive and to the right and in the upper film negative and to the left. However, becatisc of the difference in the uniaxial anisotropy fields for the films, 60 the moments are rotated different amounts by externally applied fields. When, for example, a drive conductor 48 is energized to apply a vertical field to both films, the rotation is not the same in both films, and therefore, the magnetization components both in a vertical and hori- 65 zontal direction are different. During this rotation, the tight coupling between the films is disrupted and stray fields are created which close in the space outside the films as indicated by thedotted flux lines Hs. This change can be sensed by a conductor such as 50 to produce an 70 output indicative of the state of the two films. The manner in which the storage device of FIG. 3 is operated in accordance with the principles of the present invention can be understood by consideration of this figure, alon.@ with FIGS. 4 and 5. The storage element is 7,@) 6 considered to be storing a binary "I" when the moments are ali,-ned as shown in FIG. 3, that is, with the moments in the upper fllm F2 ali@ned in the positive direction and the moments in the low@er film F, aligned in the negative direction. A binary "O" is stored when the magnetization in both films is reversed. In either case, when the device is in a quiescent stable state, the flux produced by the magnetization of each film closes throu,-h the other film with substantially no leakage flux in the space surrounding th-. device. Energization of conductor 48 to read out the stored information causes a vertical hard axis field HR to be applied to both films. When this field is applied, the film F2 is also bein,- - subjected to a horizontal field H21 from film F, and s-milarly, the film F, is subject to a horizontal fi-@ld H12'from film F2- The applied hard axis read field H,:@ causes the moments in both films to rotate towards the upward vertical direction. Both films are subjected to the same externally ara,,plied field but because of the lower uniaxial anisotropy characteristics of film Fl, the moments in this film are rotated through a larger angle than are the moments in film F2. Thus, during th-- rotation, the horizontal component of the field applied by each film to the other film decreases from the inilial equal values H12 and H2, and since the moments in film F, are rotated through a greater an.-le from the horizoiital, the horizontal component of the field applied by the film F, to the film F2 decreases to a smaller valut than the oppositely directed horizontal component of the field applied by film F2 tO film Fl. At the same time, the rotation of the moments in both films away -'rom the horizontal axis causes the field applied by each film to the other film to include a vertical component which is in a direction opposite to the direction of the applied field HR. The vertical component of the field applied by film F, to film F2 is greater than the vertical component of the field applied by film F2 tO film F, due to the greater an.-ular rotation of the moments in film Fl. As a result the orientation of the moments in the film F, is, as is depicted, on a line 60A drawn from a field poiiit 60 to curve C, in FIG. 4, and the orientation of the moments in film F2 is as shown on a line 62 drawn froiii a field point 62A tan,@ent to curve C2. The angle through which the moments in film F, are rotated is shown at 60B and is, as stated above, greater than the angle through which the moments in film F2 are rotated, which is shown at 62B. This rotation is shown in FIG. 5 wherein magnetization in th,- borizontal direction parallel to the easy axis is plotted along the abscissa and magnetization in the hard direction along the ordinate. The initial magnetization of the film eleme-tit F2 is indicated in FIG. 5 by arrow 70 extending along the horizontal axis to the right and the initial magnelization of the film F, is represented by an arrow 72 extending to the left. Vaen the vertical fi--Id HR is applied as described above by eper.-izing conductor 48, the moments in film F2 are rotated through the angle 62B to the direction indicated on line 62A. The moments in film F, are rotated throtigh the larger an@le 60B to the direction ir@dicated on line 60A. The magnetization in film F2 applies a field H,2 in the opposite directio@i to film Fl. Similarly, the magnetic field applied to film F2 as a result of the magietization of film F, is represented by the vector H21. The horizontal component of the stray field H21 of the film F, is shown at HX2, and th@- horizontal component of the stray field H12 iS shown at Hxl2. The difference bet@veen these two horizontal components is designated at Hs in the drawing, and this represents the field which does not close through the two films ' but in the space around the films as indicated in FIG. 3. About half of this flux surrounds conductor 50. It is evident from the above description that as the moments are rotated in the two films as a result of the field applied in the vertical direction, the ti,-ht coupling between th-- el,-ments is disrupted and a magnetic field is
[4]7 pro.duced which links the conductor 50 and produces an output. The polarity of this output is indicative of the direction in which the moments were aligned alon@ the horizontal axis prior to the application of the interrogation sigr@al to conductor 48. When this signal is terminated and, therefore, the field HR is removed, the horizontal field applied by each film to the other film causes both to reassume their initial condition in the binary "1" state. It should be noted that the field HR applied to achieve the nond.-structive readout described above is greater than the anisotropy field HKI for filrn 1. However, the readout is nondestructive, since the stray fields serve to reset each film in its original condition when the vertically applied field HR is removed. The readout field may have an intensity less than the value HR and may, in fact, be less than the field HKI, in which case, a lesser rotation is produced in each film. In such a case, however, the output is not as lar.-e as that produced when a field of the intensity HR is applied, since the difference in rotation is not as -reat. A field in excess of the field HR ni,,ty also be applie.d even to the point that the anisotropy field HX2 for fi]M C2 iS exceeded. Thou.-h in a normal case the application of such a field would destroy the information stored in a thin film element, here where each film is continually subjected to a horizontal component of the stray field of the other, the moments are not ali.oned completely vertically alon.- the hard axis. As a result, the films reassume their iriitial condition storing a binary "1" on termination of the applied field. When a field having an intensity -reater than H K2 is applied, though each film undergoes a larger rotation than when a field HR iS applied, the difference in rotation is not as great. Therefore, the applicatiori of lar,-er readout field does not necessarily produce a lar.-er output si---nal on conductor 50. When the stora,@e elenient of FIG, 3 is storing a binary "O" with the magnetiz,,ttion in each of the films reversed, the operation is essentially the same. The interrogation or readout signal applied to conductor 48 rotates the moments in each film a different amount, thereby producing ,t field which links output conductor 50. In this case, the polarity is opposite to that achieved before, when a binary "I" is stored. In both cases, as the vertical field is removed, a subsequent pulse of opposite polarity is produced as the flux linkage around conductor 50 is reduced. Thus, it is the polarity of the initial output produced as the interrogation si,lnal is applied which indic,,ites the binary state of the storage device. Writin.- is accomplished in the stora,@e device of FIG. 3 by ener.-izing conductor 48 with a pulse of sufficient magnitude to produce a word field Hw (FIG. 4) which is applied to both films F, and F2- If a binary "I" is to be written, conductor 50 is energized at the same time with a pulse in a direction and ma.-nitude to apply to both films a di@it field +HD in FIG. 4. Upori the application of these fields, the moments in both films are oriented in almost a vertical direction, the rotation beina sufficient to reduce the horizontal component in each film to a point where the net horizontal field applied is in the positive direction of digit field +HD. The pulse applied to conductor 48 is then terminated, thereby removing field Hw. Initially, both films in the presence of the horizontal field HD tend to orient themselves in the positive direction. However, as the rotation in the film F2, the one having the larger anisotropy field HK2 takes place, the horizontal component of this ma.-netization increases. This produces a growing stray field in the horizontal direction, which is applied to film F, and which beconies larger than the applied di-@-it field HD so that the morinents in the film F, are rotated counterclockwise in FIG. 4. Thus, even in the presence of the applied field +HD in the positive direction, upon removal of the write field, Hw, film F, assumes a stable state with the moments oriented along the easy axis in the negative direction, as 31440,626 8 a result of the overriding influence of the horizontal stray feld froni film F2. Film F2, of course, is oriented in the plus direction so that the binary "1" state sbown in FIG. 3 is achieved. When a binary "O" is to be written, the operation is essentially the same, with the exception that the di.-it field in this case, applied by energizin-.- condlictor 50 is in the ne-ative direction represented by -HI) in FIG. 4. It sholild be emphasized that during a writing operation when the hard axis word field Hw is 10 applied, though both films have their moments initially orierited in the same direction by the digit field (-HD or +HD), the rotational interaction is such when the field Hw is removed that the film F2 havin- the high anisotropy field HK2 rotates to a final stable state in the direction of 15 this applied di-it field and film F, rotates to a stable state in the opposite direction. The operation is not a two-step operation of the type in which both films first assume a condition with the moments oriented in the same direction alona the easy axis, and when the write field Hw is re20 moved a-@id the film F, is switched by domain wall switchinas a result of the stray field from film F2 when the di.-it field is removed. It is not necessary that the writing field Hw exceed the valtie HK2, as lon- as it, tooether with the digit field, 25 is sufficient to exceed the threshold for film F2 represented by curve C2- Further, the horizontal or digit field (+HD or -HD) need not be maintained until rotation back to the easy axis direction is completed in both films, but this field may be terminated along with the write field, A 30 write operation may also be achieved during which the or -H are applied as above and fields H@v and +HD the write field is @riot removed in one step. In such a case, the energization of the conductor 48 is controlled so that the write field is first reduced, for example, to a value 35 Hwl at which time the digit field +HD or -HD is terminated and the then vertical fi,Id is reduced to "O." The size of the output pulse realized during the now destructive readotit operation previously described is dependent not only upon the intensity of the read field Hy, 40 but on a number of the parameters. In the embodiment of FIG. 3, the film has a uniaxial anisotropy field three times as larae as that of film Fl. The ratio between these two values, termed the anisotropy ratio A, is three. Larger nondestructive readout signals can be obtained by increasin@ the ratio, for example, by substituting for the film 45 F2, a filn-1 havin- a larger anisotropy field. When a higher anisotropy field film is substituted for film F2 to obtain a larger output signal outptit, la"er write signals must, of course, be applied during write operations to store inforrnation in the device. 50 A further embodiment of the invention is shown in FIG. 6, wherein a magnetic thin film storage element generally designated 74 is shown with the associated read and write circuitry necessary for its operation. As is indicated in the cross-sectional view of FIG. 6, storage ele55 ment 74 is formed of two film elements FIA and F2A arranged one above the other and separated by a layer of insulating material 73. These films have different uniaxial anisotropy fields HK, the upper film F2A having an anisotropy field essentially three times that of the lower 60 film F,A. As shown in FIG. 6, the storage device is mounted on a ground plane 75 separated from the lower film F2A by a layer of insulating material 76. A vertically extending conductor 77 serves as both a sense conductor and a digit driver. A horizontally extending conductor 65 78 is the word driver and is set)arated from conductor 77 by a layer of insulating material 79. The film elements FIA and F2A in FIG. 6 are rectangular in form and in this speciflc, differ from those shown in FIG. 3. However, this change in geometry, thou,-h it does affect the overall 10 magnetic characteristics in some degree, does not alter the basic and underlyin.- principles of operation of the invention. In the operation of the storage device of FIG. 6, non75 destructive readout operations are achieved by controlling
[5]a read driver 80 to apply si,-nals to conductor 78 wbich cause outputs to be induced on conductor 77 indicative of the storage state of the device. During a read operation, a pair of switches 81 and 82 are transferred to the dotted position shown in the figure and the output is directed to a load 85. During a write operation, a write driver 83 applies a signal to word conductor 78 and a bit driver 84 applies a signal to conductor 78. During this operation, the switches 81 and 82 are in the full line portion shown in the drawing. FIG. 7 is a cross sectional view showing a further embodiment of the invention, which is similar to that of FIG. 6 in all respects but one, this being that the two film elements F2A and FIA are so fabricated that a completely closed magnetic flux path is provided. This type of structure is fabricated for example, by evaporation using masks such that the layer of insulating material separating the two magnetic film elements is narrower than these elements so that when the second film element F2A is evaporated, it contacts the lower film element F,A tO provide the desired closed flux path. -In the cross sectional view of FIG. 7, the magnetic connections between the films are exa.-gerated for illustrative purposes. A f urther embodiment is shown in FIGS. 8 and 8A which is similar to the embodiment of FIG. 6 and differs primarily in that a separate sense conductor 77A is provided between the two films FIA and F2A. The word driver 78 and bit driver 77 are arranged as before, external to the two films and these drivers are operated in the same manner as above, to apply the fields necessary to read and write. With the sense conductor 78A arranged between the two films, a larger output signal is produced upon switching during a nondestructive readout, since this conductor links the closed flux path formed by both films. A further embodiment is shown in FIG. 9 in the form of an array of magnetic stora.-e elements constructed in accordance with the principles of this invention. In this fl-ure, the stora,-e elements 74 are arran.-ed in columns and rows in a conventional manner. The word drive lines are energized by word selection circuitry generally designated 88 and the bit drive lines 78 are energized by digit selection circuitry generally designated 90. In the embodiment of FIG. 9, as is shown in detail in the cross sectional view of FIG. 9A, each stora.-e element is provided with a separate sense line 77B which is somewhat narrower than the bit driver 78 and is arranged between the bit driver and the two storage Mms FlA and F2A. The operation is the same as that described above. The sense conductor 78B produces an output dliring a nondestructive read operation when a readpulse is applied to a word conductor 77. A.-ain, afl of the switching for this nondestructive read operation is by rotation and, therefore, at very hi.-h speeds. Further, the nondestructive read operation is achieved with the films returning by rotation to their original states directly. Writing may be accomplished in the magnetic memory array of FIG. 9 in either a di.-it mode or a word mode. Thus, for example, if the upper horizontal drive line 78 is energized to apply a hard axis write field to the three storage devices 74 in the upper row of the memory, the interacting fields between the coupled films, return the films to their original condition upon termination of the applied field even thou.-h it exceeds the uniaxial anisotropy field for both films. Writing of new information takes place only in those storage devices in the upper row which has its associated di.- it drive conductor 77 ener.aized by the digit selection and drive circuit represented at 90. lt is apparent from the embodiments described above that improved cotipled film stora.-e devices are provided which can be operated in a thin film form at high speeds by rotational switching, - using drive and sense conductors which are external to the coupled films. With this type of device it is possible to use fairly thick drive and sense conductors, and, therefore, conductors having relatively low resistivity without in any way affecting the coup"ng 3,440,626 10 between the two films forming the storage element. Where one or more of the drive lines is placed between the film it is obvious that if the conductor is made thick to achieve a lower resistivity the separation between the films is also increased. Close coupling is therefore only possible at the expense of the resistivity of the conductor separating the film. In the embodiment of FIG. SA, both the di.ait and word drive conductors are external to the coupled films and it is only the sense conductor 77A which is located 10 between the films. Further, the device may be operated using only two conductors, a word conductor which is energized for both'reading and writing and a bit conductor which is ener.-ized during writing and serves as a sense conductor during reading operations. Extremely fast non15 destructive interrogating operations involving rotational switching only are achieved since the only drive field applied for interro.-ation is in a direction transverse to the easy axis of both filn-is. While the invention has been partictilarly shown and 20 described with reference to preferred embodiments thereof, it will -be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention. 25 What is
