Magnetic Bubble Information Writing Device

1. Field of the Invention

This invention relates to a magnetic bubble device for utilizing magnetic bubbles (cylindrical magnetic domains) formed in a thin magnetic film having uniaxial anisotropy, and more particularly to a magnetic bubble information writing device which generates magnetic bubbles.

2. Description of the Prior Art

Customarily is used a well-known method called the field access method in which a pattern of thin film of soft ferromagnetic material such as permalloy is formed as a circuit to propagate magnetic bubbles therethrough on the thin magnetic film having uniaxial anisotropy and in which the pattern is magnetized by a rotating magnetic field rotating in the plane of the pattern. As such a pattern have been reported a T-Bar pattern, a chevron pattern and various other types of patterns. The magnetic bubble device requires, in addition to the above mentioned rotating magnetic field, another magnetic field perpendicular to the thin, uniaxially anisotropic magnetic film. This perpendicular field is called the bias magnetic field, necessary for stabilizing the existence of the generated magnetic bubbles.

The most favorable type of the conventional magnetic bubble information writing device is the field nucleation type in which magnetic bubbles are generated through nucleation caused by sending pulse current through a hairpin-shaped conductor loop disposed on a magnetic bubble propagation circuit. When a pulse current is sent through the conductor loop in such a direction as to decrease the intensity of the bias magnetic field, a magnetic bubble is created inside the loop of the conductor. The thus created magnetic bubble is propagated along the propagation circuit as the rotating magnetic field rotates. By sending and interrupting such a pulse current in accordance with the information "1" or "0", the information pattern "1" or "0" can be written in the magnetic bubble propagation circuit.

The conventional magnetic bubble information writing device described above has the following problem. Namely, if magnetic bubbles are generated repeatedly for a long time in the region of the magnetic film where the intensity of the bias magnetic field is low, an erroneous operation will be caused, that is, a great number of undesirable, unwanted magnetic bubbles (hereinafter referred to as stray bubbles) are generated in the region of the thin magnetic film inside the conductor loop. As the repulsive force among the stray bubbles increases with the increase in the concentration of the stray bubbles, some of the stray bubbles intrude into the propagation circuit to destroy the useful information stored in the circuit. In order to eliminate such an erroneous operation, a bias magnetic field must be used which has an intensity higher than a certain threshold value and in that case the margin of the bias magnetic field is very narrow.

The object of this invention, which has been made to eliminate the drawback of the conventional magnetic bubble information writing device, is to provide a novel magnetic bubble information writing device in which the information is not destroyed by stray bubbles and in which the bias field margin is not narrowed.

According to the basic concept of this invention, the stray bubbles generated in the region of the thin magnetic film inside the conductor loop are annihilated, before going out of the loop, by sending through the conductor loop a second pulse current (hereafter referred to as stray bubble annihilating pulse current) having a polarity opposite to that of the pulse current for generating magnetic bubbles (hereafter referred to as bubble generating pulse current).

As regards the magnetic bubble information writing device using such two kinds of pulse current, the following conditions must be satisfied in addition to the requirement that the timing relative to the rotating magnetic field of generating the bubble generating pulse current is matched to the period during which magnetic bubbles can be created. Namely, it is necessary that

(1) the stray bubbles generated by the bubble generating pulse current should be completely annihilated, before going out of the conductor loop, by the stray bubble annihilating pulse current and that

(2) the bubbles as information generated by the bubble generating pulse current should continue to stably exist, even if the stray bubble annihilating pulse current is sent through the conductor loop.

These two conditions seem incompatible with each other. On the one hand, if a stray bubble goes out of the conductor loop, it can by no means be annihilated and in order to annihilate it before it goes out of the conductor loop, it is necessary to supply the stray bubble annihilating pulse current as soon as the bubble generating pulse current has been delivered. Moreover, in order to surely eliminate the stray bubble, the amplitude and the width of the stray bubble annihilating pulse current should be as large as possible. On the other hand, in order to prevent a magnetic bubble written in as information from being annihilated by the stray bubble annihilating pulse current, the stray bubble annihilating pulse current should be supplied after the bubble as information has been propagated from inside the conductor loop to outside the loop through the propagation circuit. Accordingly, it is preferable to separate the epoch at which the stray bubble annihilating pulse current is to be delivered, far enough from the epoch at which the bubble as information had best be generated. Moreover, in order to reduce the influence of the stray bubble annihilating current pulse on the bubble as information, the amplitude and the width of the pulse current should be set as small as possible.

According to this invention, the two conditions as above appearing incompatible with each other can be satisfied simultaneously. Namely, according to the magnetic bubble information writing device embodying this invention, having a means for sending through the conductor loop disposed on the magnetic bubble propagation circuit a first pulse for generating a magnetic bubble and a second pulse having polarity opposite to that of the first pulse and succeeding the first pulse, the time interval from the cease of the first pulse current to the start of the second pulse current is shorter than the time required for a stray bubble generated inside the conductor loop to get out of the loop and the epochs relative to the phase of the rotating magnetic field at which the first and the second pulse currents are sent through the conductor loop, are respectively set within a first predetermined period in which any desired bubble can be generated and after the expiration of a second predetermined period so that the desired bubble thus generated cannot be eliminated, the first and the second predetermined periods being determined mainly depending upon the pattern of the propagation circuit.

This invention will now be described in further detail by way of embodiment with the aid of the attached drawings. It should be noted that this invention is not limited to the embodiments but permits of numerous variations or improvements.

FIG. 1 schematically shows a magnetic bubble information writing device.

FIG. 2 shows an example of the waveforms of pulse currents used in the magnetic bubble information writing device according to this invention.

FIG. 3 is a plan view showing the conductor loop and the chevron pattern of a magnetic bubble information writing device used in the experiment according to this invention.

FIGS. 4 and 5 show the cross sectional structures of the main parts of the device shown in FIG. 3.

FIG. 6 shows the parameters of the pulse currents sent through the conductor loop.

FIGS. 7 and 8 show an example of the results of experiments made to explain the operation of the magnetic bubble information writing device according to this invention.

FIG. 9 shows another example of the waveforms of pulse currents used in the magnetic bubble information writing device according to this invention.

FIG. 1 shows a magnetic bubble information writing device of field nucleation type. In FIG. 1 are shown a propagation circuit 1 and a conductor loop 2, the propagation circuit 1 consisting of a plurality of triple-chevron patterns 3 and the conductor loop 2 being so disposed as to enclose the gap A between the adjacent triple-chevron patterns 3. It is here assumed for convenience's sake that magnetic field 4 rotates counterclockwise as shown in FIG. 1. In this case, a magnetic bubble 30 is propagated from right to left on the sheet of the figure (as indicated by an arrow 5), following the propagation circuit 1. It is also assumed for convenience's sake that the orientation of the rotating magnetic field, which rotates in the plain of the sheet of the figure, is at the reference angle of 0.degree. when it points from top to bottom of the sheet of the figure. Accordingly, the orientations at 90.degree., 180.degree. and 270.degree. are respectively measured counterclockwise from the reference angle, as indicated in FIG. 1. Further, it is assumed for convenience's sake that the orientation of the bias magnetic field 6 is perpendicular to and points from front to rear surface of, the sheet of the figure as indicated conventionally. In this case, when the orientation (phase) of the rotating magnetic field is 0.degree., the region A inside the conductor loop 2 is one in which a magnetic bubble can exist stably. A magnetic bubble is generated by sending through the conductor loop 2 a pulse current corresponding to the information to be written in the medium from a pulse generator 7 when the orientation of the rotating magnetic field is approximately at 0.degree.. The pulse current is in such a direction as to weaken the intensity of the bias magnetic field. This pulse current generates a magnetic bubble 31 in the position A inside the conductor loop 2. As described before, however, stray bubbles 32 may be generated in the region B which is inside the conductor loop 2 and not on the propagation circuit, and be a cause of erroneous operation. According to this invention, the erroneous operation can be prevented by sending stray bubble annihilating pulse currents 50 - 52 through the conductor loop 2 after the bubble generating pulse currents 40 - 42, as shown in FIG. 2. In FIG. 2 is shown a case where the information pattern to be written in is . . . 1 0 1 1 . . . A pair of a bubble generating pulse and a stray bubble annihilating pulse, 40 and 50, 41 and 51 or 42 and 52, are sent through the conductor loop 2 in accordance with the information pattern "1". The stray bubble annihilating pulse currents 50 - 52 are respectively sent after the bubble generating pulse pulse currents 40 - 42. Since the bubble generating pulse current and the stray bubble annihilating pulse current appear in a pair, stray bubbles 32, even if generated by the bubble generating pulse current, can immediately be annihilated by the stray bubble annihilating pulse current just subsequent to the bubble generating pulse current. Namely, the stray bubbles can be annihilated before they intrude into the propagation circuit and destroy the useful information.

FIG. 3 shows the pattern of the magnetic bubble propagation circuit of a magnetic bubble information writing device fabricated for test performance. The propagation circuit 1 is in the quadruple-chevron configuration and the center position A inside the conductor loop 2 is located in the gap between the component chevrons. The dimensions of several portions of the pattern are as indicated in the figure. FIG. 4 gives a partial cross section of the pattern shown in FIG. 3. In FIG. 4 are shown a magnetic bubble propagation circuit 1 of the quadruple-chevron configuration, having a thickness of about 0.4 .mu.m; a conductor loop 2 having a thickness of about 0.5 .mu.m; a film 10 of SiO.sub.2 having a thickness of about 0.7 .mu.m; and a thin magnetic film 20 in which magnetic bubbles are to be generated, made of magnetic material (YSm).sub.3 (FeGa).sub.5 O.sub.12, the diameter of the generated bubbles being about 6 .mu.m. In the case where the magnetic material is conductive or where the strain in the conductor loop causes an adverse result, an insulating film 11 of SiO.sub. 2 having a thickness of about 0.4 .mu.m should preferably be interposed between the conductor loop 2 and the thin magnetic film 20, as shown in FIG. 5.

FIG. 6 shows the various parameters of the bubble generating pulse current 24 and the stray bubble annihilating pulse current 25. As the typical parameters for a pulse current are defined the timing (i.e. instant) at which the pulse current is initiated (relative to the rotating phase of the rotating magnetic field), the amplitude and the width of the pulse current. As shown in FIG. 6, the timing, the amplitude and the width of the bubble generating pulse current 24 are designated respectively by .theta..sub.g, I.sub.g and .tau..sub.g, while those of the stray bubble annihilating pulse current 25 are denoted respectively by .theta..sub.a, I.sub.a and .tau..sub.a. It is here assumed for convenience's sake that the reference point, i.e. 0.degree., of the timing is set equal to that orientation in terms of angle of the rotating magnetic field at which the position of a magnetic bubble stably existing on the propagation circuit 1 in FIG. 3 coincides with the center position A inside the conductor loop 2. Accordingly, since the bias magnetic field is pointing from front to rear surface of the sheet of the drawing, the timing is at 0.degree. when the rotating magnetic field points exactly toward the bottom of the sheet of the figure. Moreover, as shown in FIG. 6, the time interval from the cease of the bubble generating pulse current to the initiation of the stray bubble annihilating pulse current is indicated by .theta.'.sub.a (represented by the angle through which the rotating magnetic field rotates during the time interval).

The operation of a magnetic bubble information writing device using such a magnetic bubble propagation circuit (FIGS. 3 and 4) and pulse currents (FIG. 6) as described above, under the influence of a rotating magnetic field of 100 kHz, will be described below.

FIG. 7 shows an example of an experiment made to obtain the condition on pulse currents which meet the requirement (1) given above for the magnetic bubble information writing device according to this invention. It is conventionally believed that in order to completely annihilate a magnetic stray bubble before it gets out of the conductor loop, the interval .theta.'.sub.a from the cease of the bubble generating pulse current to the initiation of the stray bubble annihilating pulse current should be get as small as possible. The experiment has revealed that there exists the maximum value .theta.'.sub.a max in .theta.'.sub.a and that if the value of .theta.'.sub.a is set smaller than .theta.'.sub.a max, the requirement (1) can be fully satisfied. FIG. 7 shows the variation of .theta.'.sub.a max with the changing values of the timing .theta.'.sub.a of the stray bubble annihilating pulse current. The hatched area in the graphical representation in FIG. 7 corresponds to the region where the above requirement (1) is satisfied. This result has been obtained by maintaining the pulse amplitudes I.sub.g and I.sub.a and the pulse widths .tau..sub.g and .tau..sub.a at 330 mA, 300 mA, 250 nsec and 300 nsec, respectively. Even in the case, however, where I.sub.a and .tau..sub.a as well as I.sub.g and .tau..sub.g are varied, the same result as shown in FIG. 7 has been obtained, the values of .theta.'.sub.a max being independent. Thus, it has become clear that the maximum value .theta.'.sub.a max are characteristic of the magnetic bubble information writing device and especially determined by the geometrical configuration of the conductor loop. And in case of the embodiment shown in FIG. 7, the requirement (1) will be fully satisfied if .theta.'.sub.a does not exceed 11.degree..

FIG. 8 shows an example of another experiment made to obtain the condition on pulse currents which meet the requirement (2) given above. In the case where the stray bubble annihilating pulse current is sent as soon as possible (within 11.degree.) after the cease of the bubble generating pulse current, there is a possibility that the magnetic bubble 31 generated in the propagation circuit 1 by the bubble generating pulse current is also annihilated by the stray bubble annihilating pulse current. Accordingly, the upper limit of the margin of the bias field when the stray bubble annihilating pulse current is sent through the conductor loop, is considered to be lower than when the stray bubble annihilating pulse current is not supplied. The experiment has revealed that the upper limit of the margin of the bias magnetic field is largely affected by the timing .theta..sub.a at which the stray bubble annihilating pulse current is started to flow, but that the upper limit remains constant when .theta..sub.a exceeds a certain threshold value .theta..sub.a min, the requirement (2) being fully satisfied. FIG. 8 shows how the upper limit of the bias field margin varies with the changing values of the timing .theta..sub. a at which the stray bubble annihilating pulse current is started to flow.

The dashed line corresponds to the case where the stray bubble annihilating pulse current is not sent. This result has been obtained by maintaining the other parameters of the pulse currents such that .theta..sub.g = 250 nsec, I.sub.g = 330 mA, .theta..sub.a = 300 nsec and I.sub.a = mA. Especially, the interval .theta.'.sub.a is, in this case, set at 5.degree. for which the requirement (1) described with FIG. 7 is fully satisfied. However, even when the values of the parameters were variously changed, it turned out that the same result as shown in FIG. 8 was obtained independently of the change in the values of the parameters. Therefore, the value .theta..sub.a min is considered to be characteristic of the magnetic bubble information writing device and especially to be determined by the geometric configuration of the pattern of the propagation circuit. In case of the embodiment as shown in FIG. 8, the requirement (2) will be fully satisfied by setting .theta..sub.a larger than .theta..sub.a min (= 36.degree.).

The conclusion obtained above is interpreted as follows. When the phase of the rotating magnetic field exceeds .theta..sub.a min, the magnetic bubble generated by the bubble generating pulse current is propagated completely out of the conductor loop so that the bubble is not annihilated by the stray bubble annihilating pulse current as soon as the bubble generating pulse current ceases.

In sum, the timing of the bubble generating pulse current flowing in terms of the phase angle of the rotating magnetic field must be greater than .theta..sub.a min - .theta.'.sub.a max, with the pulse width neglected for simplicity's sake. The reason for this is that the generated bubble is propagated immediately to the position corresponding to the then assumed instantaneous phase of the rotating magnetic field as soon as the bubble generating pulse current has vanished. The timing in terms of the phase angle of the rotating field at which the bubble generating current flows (more strictly, it ceases to flow) must be smaller than a certain maximum value (.theta..sub.g + .tau..sub.g).sub.max within which a generated bubble can exist stably, the maximum value being 100.degree. for the device described above. Accordingly, the timing in terms of the phase angle of the rotating field at which the stray bubble annihilating pulse current is started to flow, has also an upper limit and if the stray bubble annihilating pulse is started to flow in a phase angle region not exceeding this upper limit, the useful bubble as information is never annihilated, even when the stray bubble annihilating pulse current is sent out immediately after the bubble generating pulse current.

The summary of the foregoing is as follows. The condition for the phase current to fully satisfy the requirement (1) is that

and the condition for the pulse current to fully satisfy the requirement (2) is that

The values .theta.'.sub.a max, .theta..sub.a min and (.theta..sub.g + .tau..sub.g).sub.max are all characteristic of the device so that they are determined if only the specification of the device is determined. Namely, the inequalities (1) and (2), which are for restricting the timing of the stray bubble annihilating pulse current, have no correlation to each other, that is, they are independent on each other. If therefore is easy to manage the conditions for the pulse currents in such a manner that the two inequalities above are simultaneously satisfied.

The feature of this invention is that pulse currents capable of satisfying the above inequalities (1) and (2) are sent through the conductor loop while the above described characteristic of the magnetic bubble information writing device is utilized. An electric circuit for generating such pulse currents can easily be realized by combining a positive and a negative pulse generators triggered by clock pulses in such a manner that their outputs are synthesized.

As described above, according to the magnetic bubble information writing device embodying this invention, even if stray bubbles are generated, they can be completely annihilated by the stray bubble annihilating pulse current while the stray bubble annihilating pulse current exerts no influence on the bubble in the propagation circuit. Consequently, the lower limit of the bias field margin when the stray bubble annihilating pulse current is sent through the conductor loop, is never higher than that attained by the bias field margin when the stray bubble annihilating pulse current is not sent. Moreover, there is no lowering of the upper limit as described with FIG. 8 and the margin becomes broad.

FIG. 9 shows another example of the waveforms of pulse currents sent through the conductor loop, used in the magnetic bubble information writing device according to this invention. The information pattern in FIG. 9 is the same as that in FIG. 2, i.e. . . . 1 0 1 1 . . . . In FIG. 9, reference numerals 60 - 62 indicate bubble generating pulse currents to be sent out in accordance with the information pattern "1", these pulse currents being the same as those in the previous embodiment. In this embodiment, however, the stray bubble annihilating pulse currents 70 - 73 are sent out even for the information pattern "0" as well as for "1". In this case, too, the stray bubbles are annihilated in the same manner as in the previous embodiment. The stray bubble annihilating pulse current 71 flows through the conductor loop 2 when there is no stray bubble so that this pulse 71 is superfluous for the purpose of annihilating stray bubbles. However, since the stray bubble annihilating pulse currents can be generated periodically independently of the difference between the information patterns "1" and "0", the electric circuit for generating the stray bubble annihilating pulse current can be simplified.

It should be noted that this invention has hitherto been described as applied to the magnetic bubble information writing device using the propagation circuit and conductor loop having such configurations as shown in FIGS. 3 or 4 but that this invention can similarly be applied also to other examples where the propagation circuit and the conductor loop take different configurations.