Magnetic bubble memory device

A magnetic bubble memory device comprises a magnetic bubble memory chip, a unit for generating a bias field, a magnetic shield, and a unit for compensating the bias field, wherein the thermodependency of the bias field is compensated to approximate that of the operation characteristics of the memory chip over a wide temperature range, thereby providing a wide region in which operation is ensured.

BACKGROUND OF THE INVENTION 
The present invention relates to a magnetic bubble memory device 
(hereinafter referred to as "bubble memory") and, more particularly, to 
the structure of a package provided with a bias field compensator for 
compensating the bias field in relation to temperature to ensure stable 
operation under a wide temperature range. 
A bubble memory functions as a solid file memory having no moving parts. 
Recently, as the scope of use of a bubble memory has increased, its high 
reliability in operation and resistance against vibration and shock has 
attracted attention to the possible use of this device as a memory device 
in aircraft. In a memory device used for this purpose, when an aircraft is 
flying at stratospheric heights, the memory device is subjected to 
temperatures as low as -55.degree. C., for example, and it must carry out 
stable operations at such low temperatures. However, stable operation of a 
conventional bubble memory cannot be ensured at such low temperatures, as 
described hereinafter. Accordingly, it is necessary to take measures such 
that the device is entirely accommodated in an isothermal box so that the 
temperature in the space surrounding the bubble memory cannot fall to 
0.degree. C. or below. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to eliminate the above-mentioned 
problem in the prior art, that is, to provide a superior magnetic bubble 
memory device which can ensure stable operation over a temperature range 
wider than the conventional range, without taking particular measures to 
keep the device warm. 
To achieve the above-mentioned object, it must be taken into consideration 
that the narrow temperature range for ensuring stable operation in a 
conventional device stems from the difference in thermodependency between 
the operation characteristic of a magnetic bubble memory chip and the bias 
field generated by permanent magnets. That is, as described hereinafter, 
the cause resides in the fact that the thermodependency of the upper limit 
and the lower limit of the operational region of the memory chip is 
nonlinear, while that of the bias field generated by the permanent magnets 
is linear, and as long as the thermodependency of the bias field is 
linear, it is impossible to ensure stable operation over a temperature 
range wider than that of a conventional device. 
According to the present invention, a bubble memory is provided with a bias 
field compensator for compensating the bias field so that its 
thermodependency approximates that of the operation characteristics of the 
memory chip over a wide temperature range, thereby ensuring a wide region 
of stable operation. 
In accordance with the above object, a magnetic bubble memory device 
according to the present invention comprises a magnetic bubble memory 
chip, means for generating a bias field for the maintenance of magnetic 
bubbles in the memory chip, a magnetic shield disposed externally to the 
bias field-generating means for isolating an external magnetic field, and 
means for compensating the bias field in accordance with the temperature. 
In an embodiment of the present invention, the bias field compensation 
means comprises means disposed externally to the magnetic shield and 
generating a compensation field having an opposite direction to that of 
the bias field. 
In a preferred embodiment of the present invention, the compensation field 
becomes strong as the temperature falls and, at a predetermined 
temperature, for example, 20.degree. C. or below, exceeds the shield limit 
of the magnetic shield, whereby the bias field is compensated. 
Another preferred embodiment comprises means for adjusting the compensation 
field, which means may be constituted of, for example, permanent magnets, 
or screws of magnetic material. 
In a further preferred embodiment, a spacer of a nonmagnetic material, in 
particular, a thermal conductive material such as aluminum, copper, or 
ceramic, is disposed between the compensation field-generating means and 
the magnetic shield, whereby a reduction in undesirable interference 
between the bias field and the compensating field and an improvement in 
heat radiation can be obtained. Moreover, the provision of heat-radiating 
elements for the compensation field-generating means results in a further 
improvement in heat radiation. 
In a still further preferred embodiment, the magnetic bubble memory chip, 
the bias field-generating means, and the magnetic shield are assembled 
together into a memory package, and one or more memory package(s) are 
mounted, preferably together with other electrical circuit elements which 
are not influenced by the magnetic field, on a mounting board, such as a 
printed-circuit board, so as to construct a unit. In this case, one set of 
the compensation field-generating means may be provided for each memory 
package, or one set for the entire unit, in such a manner that it can be 
used in common by all of the memory packages. 
The present invention is now described in detail based on the preferred 
embodiments and in comparison with the prior art, with reference to the 
accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
First, the prior art will be described with reference to FIGS. 1 through 6. 
FIGS. 1 and 2 are a partially broken, disassembled, perspective view and a 
diagramatic side view, respectively, illustrating a structure of a 
conventional bubble memory package. In these figures, the reference 
characters PA designate the package in general, which comprises a magnetic 
bubble memory chip 1 (hereinafter referred to as "memory chip"), a printed 
circuit board 2 for mounting the memory chip 1 thereon, permanent magnets 
3 and 4 generating a bias field H.sub.B for maintenance of the magnetic 
bubbles (hereinafter referred to as "bubbles") in the memory chip 1, coils 
5 for generating an in-plane rotating field H.sub.D for driving the 
bubbles, and a magnetic shield case 6 for isolating the above elements 
from the external magnetic field. 
The memory chip 1 basically comprises a substrate of a single crystal, such 
as gadolinium-gallium-garnet, on which a magnetic thin film (i.e., bubble 
crystal) of magnetic garnet is formed by a liquid phase epitaxial growth 
technique. Information is stored in the memory chip, in such a manner that 
"1" and "0 (zero)" of a binary coded information correspond to "existence" 
and "non-existence" of the bubble in the bubble crystal, respectively. The 
bubble is a bubble-shaped magnetic domain existing in the bubble crystal 
and can exist only under the application of a bias field having a suitable 
strength. The bubble collapses when the bias field is stronger than a 
certain strength, and strips out into a stripe domain when the bias field 
is weaker than a certain strength. The bias fields having such strengths 
are called a "collapse field" and a "strip-out (or stripe-out) field", 
respectively. Moreover, the memory chip 1 is provided, on the bubble 
crystal, with various functional sections performing functions such as 
generation, transfer, replication, and detection of the bubble, and with 
bubble propagation tracks defined by patterns of magnetically soft 
material, such as permalloy, or ion-implanted patterns. The application of 
the in-plane rotating drive field H.sub.D causes the bubbles to be 
propagated along the propagation tracks, and the application of control 
currents to the functional sections controls the bubbles, so that the 
writing and the reading of information are performed. 
The operation characteristics of such bubble memory chips are usually 
judged by an operational margin of the bias field H.sub.B with respect to 
the drive field H.sub.D. For example, the operation characteristic of a 
memory chip employing 2-.mu.m bubbles is shown in FIG. 3, in which the 
abscissa shows the drive field H.sub.D and the ordinate shows the bias 
field H.sub.B. In this figure, the reference characters H.sub.0 and 
H.sub.2 designate the collapse field and the strip-out field described 
hereinbefore, respectively, and the range FB between H.sub.0 and H.sub.2 
shows the free bubble existence range, i.e., the range of bias field where 
the bubbles can freely exist in the bubble crystal without the bubble 
propagation tracks or the like. On the other hand, a line L is a 
operational margin line, and the region (hatched for elucidation) 
surrounded by the line L is the operation region of the memory chip. The 
upper limit H.sub.0 ' of the operation region shows the strength of bias 
field, at which the bubble being propagated collapses, and the lower limit 
H.sub.2 ' shows the strength of bias field at which the bubble strips out 
along or from the propagation track. It is known that the operation region 
is usually slightly higher than the free bubble existence range FB, but 
the upper limit H.sub.0 ' and the lower limit H.sub.2 ' of the operation 
region are proportionate to the free bubble collapse field H.sub.0. 
The operation of the bubble memory chip depends on the temperature. FIG. 4 
shows the thermodependency of the free bubble collapse field H.sub.0. As 
illustrated, H.sub.0 varies nonlinearly with respect to the variation of 
the temperature T, but the average rate of variation over the range from 
0.degree. C. to 70.degree. C. is -0.2%/.degree.C. FIG. 5 shows the 
thermodependency of the operation characteristics of the memory chip under 
the drive field of H.sub.D =50 Oe. Both the upper limit H.sub.0 ' and the 
lower limit H.sub.2 ' of the operation region have the average rate of 
variation of about -0.2%/.degree.C. over the range from 0.degree. C. to 
70.degree. C. where the permanent magnet is of Sr-ferrite or Ba-ferrite, 
for example, the magnetic field generated thereby varies linearly with 
respect to the temperature at the rate of -0.2%/.degree.C. Conventionally, 
permanent magnets having such thermal characteristics have been used to 
generate the bias field H.sub.B as shown in FIG. 5, for example. FIG. 6 
shows the thermodependency of the difference .DELTA.H.sub.B between the 
bias field H.sub.B and the upper limit H.sub.0 ' and the lower limit 
H.sub.2 ' of the operation region shown in FIG. 5. The hatched region in 
FIG. 6 shows the region in which operation is ensured, and as can be 
clearly understood, the operation is ensured over the range from 0.degree. 
C. to 70.degree. C. However, according to the conventional structure, the 
operation cannot be ensured at a temperature as low as -55.degree. C., as 
mentioned above. 
Next, the embodiments of the present invention will be described. FIG. 7 is 
a side view of an embodiment of a magnetic bubble memory package according 
to the present invention. In the figure, the reference character PA 
designates a package in general which is the same as the conventional 
package shown in FIG. 2. A compensation field generator 10 is disposed 
externally of the package PA. The compensating field generator 10 
comprises plates 11 and 12 made of magnetically soft material, such as 
permalloy, disposed on upper and lower sides of the magnetic shield 6 of 
the package PA, and permanent magnets 13 and 14 disposed between the 
plates 11 and 12 at the opposite end portions of these plates 11 and 12. 
The permanent magnets 13 and 14 are arranged so that their north (N) poles 
and south (S) poles are oriented upwards and downwards in the figure, 
respectively, whereby a compensation field H.sub.CO opposite in direction 
to the bias field H.sub.B is generated between the plates 11 and 12. Where 
the permanent magnets 13 and 14 are of Ba-ferrite or Sr-ferrite, the 
compensation field H.sub.CO also varies at the rate of -0.2%/.degree.C. 
On the other hand, FIG. 8 shows the shield characteristic of the shield 
case 6. As can be understood from the figure, the internal field of the 
shield case abruptly varies when the external field exceeds 70 Oe. This 
means that the shield case is saturated by the external field of 70 Oe and 
thus permits the field component exceeding 70 Oe to pass therethrough. 
Accordingly, where the compensation field H.sub.CO generated by the magnets 
13 and 14 is predetermined to be 70 Oe at 20.degree. C., the strength of 
H.sub.CO and the reduction -.DELTA.H.sub.B of the bias field H.sub.B 
caused by the component of the field H.sub.CO passing through the shield 
case, in the range of temperature lower than 20.degree. C., are as shown 
in Table 1. 
TABLE 1 
______________________________________ 
T.degree. C. -55 -40 -20 0 
______________________________________ 
H.sub.CO Oe 80.1 78.6 75.7 72.8 
-.DELTA.H.sub.B Oe 
10.1 8.6 5.7 2.8 
______________________________________ 
As a result, as shown in FIG. 9 and similar to FIG. 5, the bias field 
H.sub.B is formed, as shown by the dashed line, beginning at the point of 
about 20.degree. C. In the figure, the hatched region shows the reduction 
-.DELTA.H.sub.B of H.sub.B, and the line H.sub.B ' shows the compensated 
bias field. Therefore, it can be understood from the figure that, over the 
range of temperature from -55.degree. C. to 20.degree. C., the rate of 
variation of the bias field H.sub.B ' approximates those of the upper 
limit H.sub.0 ' and the lower limit H.sub.2 of the operation region. It 
should be noted that, in the range of temperature of 20.degree. C. or 
more, the compensation field H.sub.CO is absorbed by the shield case 6 and 
the invasion is as small as 1 Oe and, accordingly, the bias field H.sub.B 
varies in accordance with the original thermodependency to present a 
thermal characteristic similar to the conventional one. FIG. 10 shows the 
region (hatched region) in which operation is ensured in relation to FIG. 
9, in the same manner as in FIG. 6. As can be seen from this figure, a 
stable operation can be ensured over a wide temperature range of from 
-55.degree. C. to 70.degree. C. 
It should be noted that the shield case 6 loses the shield function when 
saturated with the compensating field H.sub.CO, but because the 
magnetically soft plates 11 and 12 disposed externally of the shield case 
also have the shield functions, the external field still can be 
effectively isolated. 
FIGS. 11 through 14 show second through fifth embodiments of the bubble 
memory package according to the present invention. The basic structure and 
operation in these embodiments are the same as in the first embodiment 
described above, and accordingly, the same or similar parts are designated 
by the same references. 
In the second embodiment illustrated in FIG. 11, small permanent magnets 15 
and 16 are rotatably disposed at the opposite end portions of the 
magnetically soft plates 11 and 12. The rotation of these magnets causes 
the angle of their magnetic axes with respect to the direction of the 
compensation field to vary, thereby adjusting the strength of the 
compensation field H.sub.CO (FIG. 7) between the magnetically soft plates 
11 and 12. 
In the third embodiment illustrated in FIG. 12, the magnetically soft plate 
11 is provided with screws 17 and 18 of magnetic material at the opposite 
end portions. By screwing-in or -out these screws, the strength of the 
compensation field H.sub.CO can be adjusted similarly to the 
above-mentioned embodiment. 
In the fourth embodiment illustrated in FIG. 13, spacers 19 and 20 (hatched 
for elucidation) of non-magnetic material are disposed between the shield 
case 6 and the magnetically soft plates 11 and 12. This structure provides 
the reduction in undesirable interference between the bias field and the 
compensation field The spacers 19 and 20 are preferably of heat conductive 
material, such as Al, Cu, or ceramics, for providing good heat radiation. 
In the fifth embodiment illustrated in FIG. 14, the magnetically soft plate 
11 is provided with heat-radiating fins 21 on the surface thereof. 
Accordingly, the plate 11 also serves as a heat radiator, whereby the 
thermal characteristic in the high temperature range is also improved. The 
magnetically soft plate 11 may be provided with undulations on its 
surface, instead of the fins 21. In this case, the surface of the plate 11 
itself may be undulated, or another plate with the undulation preformed 
may be attached to the plate 11. 
Further, the spacers 19 and 20 and the heat-radiating fins 21 described 
above may be used in combination, and the spacers 19 and 20 and/or the 
heat-radiating fins 21 may be provided in the embodiments illustrated in 
FIGS. 7, 11, and 12. 
Usually, one or more of the magnetic bubble memory packages mentioned above 
are mounted on a mounting board, so as to construct a unit. FIG. 15 shows 
a first embodiment of such a magnetic bubble memory unit, in which three 
packages PA are mounted on a printed circuit board PB, and each of the 
packages PA is provided with the above-described compensation field 
generator 10, so that the bias fields in the packages are individually 
compensated. 
In this unit, the compensation field generator 10 may be any one of those 
illustrated in FIGS. 7 and 11 through 14 or may be constructed by a 
combination of those units. 
Further, it is taken as a matter of course that, in this unit, electric 
circuit elements other than the memory package, such as integrated 
circuits (IC), resistances, or capacitors, which are not influenced by the 
magnetic field, can be mounted together on the printed circuit board PB. 
FIGS. 16 and 17 show a second embodiment of the magnetic bubble memory 
unit, in which the compensation field generator is not provided to each of 
the packages, but a compensation field generator 100 which is common to 
all of the packages PA is disposed externally of the unit to accommodate 
the latter therein. The compensation field generator 100 has the same 
structure, except for the size, as that of the above-described 
compensation field generator 10, and its component parts are designated by 
the same references. The structure of this embodiment has an advantage in 
that modification in particular of the assembled unit or reconstruction of 
the unit in use is easy, because it is necessary only to add the 
compensation field generator to the unit externally of the latter. 
Further, this second embodiment of the unit also may be further provided 
with the compensation field-adjusting means, i.e., the permanent magnets 
15 and 16 or the screws 17 and 18, the spacers 19, and the heat-radiating 
fins 21, similar to the embodiments illustrated in FIGS. 11 through 14. 
FIGS. 18 through 21 show such embodiments. A third embodiment illustrated 
in FIG. 18 comprises permanent magnets 15 and 16, as in the embodiment 
illustrated in FIG. 11, and a fourth embodiment illustrated in FIG. 19 
comprises screws 17 and 18, as in the embodiment illustrated in FIG. 12. A 
fifth embodiment illustrated in FIG. 20 comprises spacers 19, as in the 
embodiment illustrated in FIG. 13, but the spacers 19 are disposed only 
between the upper magnetically soft plate 11 and the magnetic shields 6 of 
the packages. A sixth embodiment illustrated in FIG. 21 corresponds to the 
fifth embodiment illustrated in FIG. 20, but further comprises 
heat-radiation fins 21, as in the embodiment illustrated in FIG. 14. 
Furthermore, it is also taken as a matter of course that, in the 
embodiments illustrated in FIGS. 16 through 21, electric circuit elements 
which are not influenced by the magnetic field can be mounted together 
with the packages PA on the printed circuit board PB. 
As described above, the present invention can provide a magnetic bubble 
memory device in which operation is ensured over a very wide temperature 
range, together with a high reliability against vibration and shock, and 
accordingly, can be applied for use for many various purposes.