Magnetic thin film memory

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This invention relates to data storage apparatus employing magnetlo films.

British Patent Specification No. 942,674 describes data storage elements using anisotropic magnetic films.

These films,have an easy” direotion of magnetization with whioh the magnetization veotor is aligned in the absenoe of an applied magnetlo field. The films also have a hard” direotion of magnetization whioh is perpendicular to the easy direotion.

A storage element using a film of thia kind has two stable magnetlo states and may be used for the storage of binary information. The element may be said to be in the binary zero state when the magnetization veotor is aligned with the easy direotion in one sense. The application of a suitable magnetlo field will cause the magnetization veotor to rotate through 18CP to be aligned with the easy direotion lh the opposite sense and the element may then be said to be in the binary one state.

The patent specification referred to above desoribes in detail the arrangements for· controlling switching of the element from one state to the other to store binary information and also desoribes a manner In which suoh stored information may be read out. In the foregoing application the storage elements are switohed under control of driving currents applied to conductors linked with the film, a first one of these oonduotora being aligned with the plane of the film and arranged at a small angle with respeot to ths easy direotion of magnetization while a seoond conductor is also aligned with the plane of the film but is approximately at right angles to the first oonduotor. The driving currents are always applied to the oonduotors in the same

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sense, that is, the current in eaoh conductor is unidirectional» While this arrangement is satisfactory if the driving currents are derived from unidireotionally conducting driving sources, it ia frequently desirable to control the operation of the storage elements from bidirectional sources, for example, pulse-operated transformers or selection and driving networks including ferrite cores» Under these circumstances in order to produce the required driving currents it becomes necessary to incorporate unidirectional devices, such as diodes,in the driving circuits with a consequent increase in the complexity and cost of the apparatus»

Hence, it is an object of the invention to provide an improved data storage device employing a thin magnetic film in which the storage of a data item is controlled by magnetic fields each applied in two opposite senses during a cycle of operation»

It is another object of the invention to provide an improved data storage matrix utilising a plurality of magnetic film storage elements eaoh controlled by magnetic fields each applied in two opposite senses during a cycle of operation»

According to the invention a data storage device includes a thin magnetio film element having mutually perpendicular easy and hard axes of magnetization switohable predominantly by domain rotation between two opposite stable states of remanence respectively representing binary digits of opposite significance in whioh the magnetization vector is aligned with the easy axis in a direotion dependent upon the significance of the binary digit, means for producing during a reading phase of an operating cycle a magnetio field substantially aligned with the hard axis in one sense and for producing during a writing phase of the cycle a magnetio field substantially aligned with the hard axis in the opposite sense

I concurrently with a magnetio field substantially aligned with the easy axis in one or the opposite sense in dependence upon the

- 3 720985 significance of the binary digit to be written.

The magnetic fields may be derived from bidirectional driving currents applied to a pair of mutually perpendicular conductors lying in a plane parallel and adjacent to a planar substrate supporting the film. Output signals may be induced in a further conductor in response to the rotation of the magnetization veotor of the film. A number of storage devices may be supported in matrix formation on a oommon substrate, the film elements being in the form of independent spots of film or of discrete areas occurring in a continuous film. The devices may be arranged in rows and columns respectively substantially aligned with the easy and hard axes of the film and having common row and column driving conductors. Suoh a matrix may be used for storing words of binary digits, each word being stored in a location formed by a row of elements and a location may be seleoted during an operating oycle by applying a

Current to the requirad^oonductor, the applied current flowing in one direction during the reading phase and in the opposite direction during the writing phase.

Apparatus embodying the present invention will now be described, by way of example, with referenoe to the accompanying drawing, in which}

Figure 1 shows a single film storage element, and

Figure 2 is a diagram illustrating a storage matrix utilising a plurality of individual storage elements.

The physical Construction of a storage element is similar to β-β-xÇ· k cobsA V o ts that desoribed in the above-mentioned patent applteatiem· A spot of anisotropic magnetic film 1 is deposited on a substrate 2. Over the spot 1 is a strip conductor 3.

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Two other strip conductors 6 and 9 lie over the spot 1 and are at right angles to the conductor 3. The conductors 3 and 6

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E are used to carry drive ourrents, and the oonduotor 9 is used for pick-up purposes, signals being induced therein when the magnetic state of the film is changed» The conductors 3, 6 and 9 may be copper strips, or they may be deposited or plated, for example,,

As is wall/known in storage devices of this kind, the conductors are insulated from each other and from the film by suitable insulating layers» For the sake of clarity, however, the insulation layers are omitted from the drawing» The substrate 2 may be glass, for &lt;3— example, or it may be'non-magnetic oonduotor» This latter form of construction is shown and described in an article entitle^

Making Beproduoible Magnetic Film Memories by Ë»M» Bradley, published in Electronics dated September 9th, I960» The sequence in whioh the conductors are laid over the film 1 may be different» For example, the piok-up oonduotor 9 may b® next to the film to provide maximum coupling» The return path for the oonduotor 9 nay 4® provided by a further similar conductor, or by the substrate itself if it is conductive»

In the partioular form of storage device described in the article referred to above, the film deposited on the substrate is continuous, for example as indicated by the area referenced 2 in Figure 1, and the storage element consists of a discrete area of film as indicated by reference 1, the area 1 being that part of the film which is effective to be switched by the applied magnetic fields» The area 1 will not be circular in this oase, the actual shape depending upon the fields produced by the drive conductors»

It has previously been proposed, in thin film storage devices constructed in the manner described, to arrange that the driving conductors are aligned at a small angle to the easy and hard directions of the film» This mode of construction is shown in

Figure 1, where the easy direotion of magnetization lies in the

- 5 720985 direction indicated by arrows 10 and ll₀ It will be apparent, therefore, that if the arrow 10, for example, indicates the direction of the magnetization vector corresponding to the binary zero state then the arrow 11 indicates the vector direction corresponding to the binary one state₀ The drive conductor 3 then lies at a small angle O to the easy axis» The angle &amp; may be 5° ia a practical case but for the sake of clarity this angular displacement is exaggerated in the Figure»

The application of a drive current to the conductor 3 produces a magnetic field at right angles thereto» The sense of the magnetic field produced is suoh that the magnetization vector is turned either clockwise or anticlockwise in dependence upon the direction of flow of the drive ourrent» For the sake of simplicity of explanation, the magnetic field will be described as having a direction corresponding to the direction into whioh it tends to turn the vector» Thus a current flowing in the direction indicated by arrow 4 produces a substantially uniform field Hd in the film in the direction indicated by arrow 5 and conversely a current flowing in the direction of arrow 20 produoes a field Hr in the direction indicated by arrow 21»

Similarly a drive current applied to conductor 6 in the direction indicated by arrow 7 produoes a field He in the direction indicated by arrow 8 and if the direction of current flow is reversed into the direotion indicated by arrow 23 a field Hf is produced in the direction indicated by arrow 22»

In order to illustrate one mode of operation of the element in whioh a bidirectional driving current is applied to the conductor 3 it will be assumed that th® magnetization vector lies initially in the direction of arrow 10 and that this direotion corresponds to the binary zero state_o In the mode of operation to be considered initially the effeot of reversing the drive current applied to the Conductor 6

- 6 ~ will not ba considered» A drive ouvrent pulse of suffiolent amplitude to produce a field greater than that required to saturate the film in the hard direction is now passed through the conductor 3, in the direction of arrow 4.

Domain rotation occurs in the film and the magnetization , vector is aligned with the direction of the applied field, that is, in the direction of the arrow 5« The magnetization veotor thus rotates through an obtuse angle from its initial position in order to align in the direction of the arrow 5 and in oonsequenoe

1G passes the hard axis· Henoe, after Cessation of the drive pulse, the veotor will not return to ths original position but will instead align in the direction 11. Thus, the application of this drive ourrent only to the Conductor 3 ln the direction 4 when the film is in the zero state produces a permanent change in the state of the film by Causing tha magnetization veotor to rotate through 180°·

Henoe, the application of this driving Current alone to the element will Cause the element to store a binary one· However, if it is required to store a binary zero when operating in this mode the element is required to remain in an unswitched state. Consequently it is neoessary to provide an inhibiting field to prevent switching taking place whenever a binary zero is to bs stored.

For this purpose a seoond, inhibiting, driving ourrent is applied in the direction indicated by arrow 7 to the Conductor 6 to produce the field H in the direction indicated by the arrow 8. The magnitude of the field H issuoh that when it ocours Concurrently

Θ with the field H^ the magnetization 'vector does not rotate sufficiently to pass the hard axis and in oonsequenCe returns to its initial position, aligned in the direction indicated by the arrow 10, after the driving ourrents have Ceased. It is convenient to make the drive pulse on the conduotor 6 start before, and finish after, the drive pulse on the

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conductor 3. This ensures that the field H_q is applied to the film during the turn off time of the field Thus, concurrent application of these drive pulses to the two conductors prevents switching of the film and ensures that it remains in the binary zero state»

It will be appreciated that, using this arrangement to control switching of the film, the values of the fields and H₀ and consequently the values of the drive currents producing them axe not oritiàal» For example, the minimum magnitude of the field Hj is that whioh will cause the magnetization vector to rotate through the 90° position corresponding to the hard axis and the minimum value of the field H is that which, when applied to the film concurrently with the field is sufficient to prevent the rotation of the magnetization vector through the direotion of the hard axis.

A signal indicative of the state of the film may be obtained by applying an interrogating drive current pulse to the conductor 3 in,the direotion of the arrow 20, to produce the field H* in the direotion of the arrow 21. If this reversed drive ourrent is of the same value as the original drive current applied to this conductor, the field produced causes the magnetization vector to rotate into alignment with the direotion of the arrow 21. If the element is storing a binary zero, the rotation of the vector is in a olookwise sense and lnduoes a voltage pulse, for example of positive polarity, in the pick-up conductor 9· The vector will then restore to its original position in alignment with the direotion of the arrow 10 when the interrogating drive pulse eeases and since this rotation is in the opposite sense a negative pulse is induoed in the pick-up conductor 9»

If, however, the element had been storing a binary one when the interrogating pulse was applied, the magnetization vector

- 8 720985 would have rotated in an anticlockwise direotion from alignment with the direction of the arrow 11 to that of arrow 21, passing through the hard axis. This movement of the vector induces a voltage pulse of negative polarity in the piok-up oonduotor 9.

As in the previous oase this pulse is followed hy a further _&lt;ο-τκ. Ve-v-. ao cinegative pulse as the veotor restores—to—tts-init-ial position in alignment with the direotion of the arrow 10 after the interrogating pulse ceases.

Thus, it will he seen that the application of an interro10 gating pulse to the oonduotor 3 only resets the film to the state corresponding to binary zero if it was previously in the binary one state and thereby generates two pulses of the same polarity in the piok-up oonduotor. If the film is already in the zero state, the interrogating pulse leaves the state unaltered and generates two pulses of unlike polarity in the piok-up oonduotor. Henoe the state of the film prior to the application of the interrogating pulse may be determined by testing the polarity of the pulse produced in the piok-up oonduotor on the ocourrenoe of the leading edge of the drive pulse and after the interrogating pulse has ôeased the element will always be left in the zero or normal state. It will be appreciated that in an alternative mode of operation the element may be nondestructively read out by employing an interrogating drive current of a value less than the saturation value suoh that the magnetization veotor does not pass through the hard axis from the binary one direotion.

This mode of operation, however, requires that the value of the interrogating current should be accurately controlled.

Since the drive currents required for writing binary digits into the element and for reading out from the element are in opposite directions it is convenient to employ a mode of operation in which a cycle of operation of the element contains two phases, one for reading and the other for writing. The minimum magnitude of the field produced hy the interrogating pulse is set hy the same considerations as for the field Hd and it is convenient to derive the driving ourrents for produoing these fields from a single source produoing a symmetrical bidirectional output. During the first or reading phase a driving current is applied to the oonduotor 3 in the direotion of arrow 20 to produce the field Hr as desoribed above for the interrogating pulse. During this phase stored information oan he readout by gating the leading edge of the output from the piok-up oonduotor 9» Alternatively the output signals may he inhibited hy closing these gates and in this case the driving current merely constitutes a resetting pulse. This reading phase is followed immediately by a writing phase in whioh the driving current applied to the oonduotor 3 is reversed and now flows in the direotion of the arrow 4» This current, if neoessary in conjunction with current applied to theoonduotor 6, allows new information to be stored in the element in the manner previously described or may alternatively be used for re-writing Information read-out during the first phase_o If the operation is solely required to he one of reading out,the application of the inhibiting current to the oonduotor 6 ensures that the element remains in the reset state at the end of the operation.

Thus the foregoing operations are conveniently controlled by a current source suoh as a transformer, for example, having an output consisting of a current pulse of one polarity followed hy a seoond pulse of opposite polarity. It will also be apparent that the aeleotion ahd control of a storage element in this way may equally well be accomplished by the use of conventional ferrite core driving circuits.

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The nodes of operation described above have required the reversal of driving ourrent applied to the conductor 3 for the two phases during a single operating cycle. During the writing phase, however, only a unidirectional driving ourrent applied to the conductor 6 has been described and this driving ourrent is applied only for the writing of one of the binary digits.

The digit drive current is also conveniently derived from a bidirectional current source and undçr these circumstances the Current applied to the conductor 6 flows in one direction, for example in the direction of the arrow 7, to write a binary zero and in the opposite direotion, for example in the direction of arrow 23,to write a binary one. Current flowing in the direotion of the arrow 7 produoes a field He in the direction of the arrow 8, while current flowing in the direction of arrow 23 produoes a field Hf of similar magnitude in the direotion of arrow 22»

The use of bidirectional digit driving Currents in this way has the advantage that the tolerance of alignment of the driving conductors with the easy and hard axes is increased» To illustrate this point oonsider the modes of operation, described earlier, in which a unidirectional digit current is employed. In these Cases since the writing of only one digit is associated with a drive ourrent applied to conductor 6,the angle &amp; through^zthe conductors are tilted with respect to the axes of the film must be sufficient to ensure that in the absence of the inhibiting digit drive field He the application of the field Hd takes the magnetization veotor past the hard axÿs. The maximum limit of the angle O is determined by the magnitude of the inhibiting field He and in a practical case the tolerance limits for the angle O have been found to be from 2° to 8° approximately» Since the field He must be Capable of restraining rotation of the vector past the hard axis even when the maximum angle of tilt exists it follows that an equal and opposite field Hf will cause a like modification of the vector rotation with a tilt of opposite sense. Thus, the mode of operation using mutuallyreversed digit drive currents for writing binary digits zero and one respectively ensures correct operation irrespective of the sense of tilt of the conductors with respeot to the easy and hard axes in a practical ease under similar operating conditions and using driving currents of similar magnitudes, the tolerance limits for the angle θ noted above have been increased from — 8° to + 8° in this way.

It will be appreciated that the angle &amp; is in any case quite small and the alignment of the conductors will be referred to hereinafter as substantially aligned with the easy or hard axes of the film as the case may be, it being understood that this alignment is within the tolerance limits of the angle θ for correct operation under the particular operating conditions chosen. A number of storage elements may be arranged in matrix formation and the foregoing modes of operation may be employed for writing and reading digits into and from the elements. Figure 2 shows, by way of example, an arrangement of elements in a four-by-four matrix, although it will be obvious that the number of elements in the rows and/or Oolumns may be varied as may be required for any particular purpose.

Eaoh of the rows of elements may be considered as a storage looation for a word of binary digits. A particular location may then be seleôted by a word seleotion arrangement and the digits of the selected word may be read and written by the appropriate operation of a digit drive selector.

Each of the oonduotors 3a - 3d, corresponding to the conductor 3 of Figure 1, is common to four individual film elements 1, whioh are schematically indicated in Figure 2, and is connected to a conventional word selection network 12 having ferrite cores as driving elements for supplying the necessary driving currents. The conductors 6a - 6d corresponding to the conductor 6 of Figure 1, are common to a column of four film elements and are connected to a digit selector 13.

The conductors 9a. - 9â# corresponding t© the conductor 9 of figure 1, are each connected to an amplifier 14. The conductors 6a, “ 6d. have been shown separated from the conductors 9®. ~ 94$ and the positions of the film elements have been schematically indicated for the sake of clarity of illustration.

The required word storage looation is selected by the apparatus to whioh the storage matrix is linked, for example a computer 24.

The computer 24 oontains a conventional address register whioh specifies the required location address and passes ooded address signals over linos 15 to the word selection network 12. The network inoludes a number of ferrite cores, one of whioh is linked with each of the word drive conductor 3a, to 3d,. The ferrite core associated with the required looation, for example, that associated with the word drive conductor 3c^ is set and then reset under oontrol of timing or clook pulses derived over a line 25 from the computer.

The setting and resetting of the core induces a driving current in the linked word drive conductor 30.. The driving current flows first in one direction during the reading phase and then in the opposite direction during the writing phase. Hence_s during the reading phase, the magnetic field Hr is produced in each element linked with the conductor 3o and the field Hd is produced in each element during the writing phase.

The conductors 9«, to 94, carry signals during the reading phase the signals having leading edges of a polarity determined by the digits stored in the elements of the selected looation with which the conductors 9», to 94. are linked. The amplifiers 14 are conditioned to

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A respond to the leading edge polarity corresponding to the storage of a binary one and a connection from the olook pulse line 25 to eaoh of the amplifiers 14 provides a strobing signal so that the final output fro» an amplifier 14 ocours only if the associated element had been storing a binary one.

The computer 24 also includes a register whioh contains the binary digits to be written into the selected location, and during the writing phase signals representing these digits are applied over lines 16 to the digit drive selector 13» The digit drive selector 13 includes a group of transformers, one associated with eaoh of the conductors 6a, to 6d. The primary windings of the transformers are in a driving oirouit whioh is gated by a suitably delayed strobing pulse to produce,during the writing phase, anA energizing pulse corresponding in sign to the particular significance of the digit to be written. The secondary windings of the transformers are each connected to the appropriate one of the conductors 6a. to 6d.

Thus the digit drive currents applied to the oonduotors 6a, to 6d. to produce the required fields He or Hf depending on the significance of the digits to be written.

It will be seen that in the preceding descriptions the mode of operation of the storage elements either individually or in a storage matrix requires the provision of bidirectional driving ourrents to conductors substantially aligned with both easy and hard axes of magnetization of the film. The use of a bidirectional word driving current provides a simple read~write cycle of operation without requiring complex aeleotion oirouita and the use of a bidirectional digit driving current, while still retaining the advantages of simple driving circuits also allows a greater manufacturing tolerance on the permissible angle 0 by whioh the conductors are tilted with respect to the axes of the film.

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While it will he evident from the preceding description that the bidirectional ourrents may be provided for example by driving oirouits including bistable devices, such as ferrite cores, or transformers, for example, it will also he appreciated that the practical advantage of greater manufacturing tolerance stems from the use of magnetic fields eaoh of opposite senses linking with the film» Thus, the required mode of operation could be achieved hy other forms of drive Oonduotor, provided that the fields produced are respectively substantially aligned with the easy and hard axes of the film» For example, the required fields oould be produced by providing a driving winding encircling eaoh elementary area ox in the oase of a word oriented matrix by a word driving winding distributed over a complete word storage location» Further, the single driving oonduotors shown may he replaced by two oonduotors each oarrying current only in one direction» For example, two word driving conductors may be provided for eaoh storage location, the selection of the location conditioning them to carry a driving Current in succession under control of timing signals» In this oase the oonduotors would be Coupled in opposite senses with the elements in the location» .

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The embodiments of the invention in whioh an exclusive property or privilege