Magnetic bubble domain structure

A magnetic bubble domain structure comprising a bubble domain layer supported by a nonmagnetic substrate. A control layer is superimposed on the major surface of the domain layer which remote from the substrate. The control layer has an easy axis of magnetization in the plane which defines at least one bubble domain propagation path. The unique control layer according to the invention may be a garnet layer which comprises two sublayers: a first, continuous, sublayer, and a second, discontinuous, sublayer. The discontinuous sublayer defines the required propagation patterns. An advantage of the invention is that the in-plane rotary field to be applied for propagating the bubble domains can be considerably weaker than in bubble domain structures which are equipped with nickel-iron propagation patterns.

BACKGROUND OF THE INVENTION 
The invention relates to a magnetic bubble domain structure comprising a 
nonmagnetic substrate bearing a bubble domain layer of a soft magnetic 
material having a positive magnetic anisotropy constant. Magnetic bubble 
domains can be propagated in the bubble domain layer. The structure 
further comprises a control layer of a soft magnetic material which is 
superimposed on the major surface of the bubble domain layer which is 
remote from the substrate. The control layer defines at least one bubble 
domain propagation path. 
Magnetic bubble domain memories generally comprise one or more memory 
storage loops, each of which accommodate a number of single-walled 
magnetic domains. Each magnetic domain represents one binary information 
bit. These bubble domains are rotated along the loop in a synchronized and 
controlled manner so as to enable one to access the stored information. 
See, for example, the article "Bubble memories come to the boil" by P. K. 
George et al, (Electronics, August 2, 1979, pp. 99-109). A number of 
different approaches are used in forming the bubble domain propagation 
paths or circuits. 
In so-called "field-access" systems a path or track is usually defined by 
providing, on the bubble domain layer, a thin film pattern which consists 
of a series of geometrical forms or elements of a soft magnetic 
nickel-iron alloy. When an in-plane magnetic driving field is rotated, the 
elements are polarized successively positively and negatively for 
propagating or moving, step-by-step the bubble domains along the paths. 
Bubble domain propagation elements of nickel-iron used so far have, for 
example, alternate T and I shapes, alternate Y and I shapes, asymmetric 
chevron shapes, and C-shapes. 
However, a disadvantage of the use of such nickel-iron elements is that the 
magnetostatic interaction ("adhesive force") between bubble domains and 
elements is so large that undisturbed propagation of bubble domains is 
possible only with strong rotating fields. This "adhesive force" is 
related to the fact that a bubble domain magnetizes its surroundings, 
notably the nickel-iron elements. This is the same effect as a permanent 
magnet which is laid on an iron table. 
SUMMARY OF THE INVENTION 
It is an object of the invention to provide a magnetic bubble domain 
structure in which the disturbing interaction between the bubble domains 
and the propagation elements does not occur so that a comparatively weak 
rotating field is sufficient for an undisturbed propagation of bubble 
domains. 
This object is achieved by a bubble domain structure in which the control 
layer (a) has a negative magnetic anisotropy constant, and (b) comprises 
two sublayers. The first sublayer is continuous. The second sublayer is 
discontinuous and it defines at least one bubble domain propagation path 
for guiding the movement of bubble domains in the bubble domain layer 
under the influence of a variation in the orientation of a rotating 
magnetic field having a direction of magnetization parallel to the plane 
of the bubble domain layer. The first and second sublayers are 
superimposed one upon the other. For defining the path or paths of 
propagation, the discontinuous sublayer may comprise propagation elements 
whose shapes correspond to shape of the known nickel-iron propagation 
elements, for example, T and I, Y and I, C, or chevron shapes. 
The operation of the bubble domain structure according to the invention is 
based on the fact that the rotary field magnetizes the first, continuous, 
sublayer of the control layer. The magnetization of the entire continuous 
sublayer rotates with the rotary field. By exchange coupling the 
magnetization of the second, discontinuous, control sublayer which defines 
the propagation paths is also rotated with the rotary field. The 
continuous sublayer functions as an amplifier, so that it is possible to 
use a comparatively weak rotating field. The disturbing interaction, which 
occurs when soft magnetic propagation elements of nickel-iron are used, is 
absent. 
The advantages of the bubble domain structure according to the invention 
become more prominent as the domain dimensions become smaller. In 
particular the invention is very suitable for the propagation of bubble 
domains having a diameter of 1 .mu.m or smaller. 
According to a first preferred embodiment of the bubble domain structure 
according to the invention, the planar bubble domain layer and the control 
layer are garnet layers. With the structure, the control layer may be 
formed either by ion implantation of the top layer of the bubble domain 
layer, or by growth of a second garnet layer, having a slightly different 
composition from the bubble domain layer, on the bubble domain layer. 
According to a second preferred embodiment, the bubble domain consists of a 
material having a magnetoplumbite structure, and the control layer 
consists of a material which combines a hexagonal crystal structure with a 
negative anisotropy constant.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Reference numeral 1 in FIG. 1 denotes a part of a magnetic bubble domain 
structure. It comprises a pattern of Y and I-shaped elements as denoted by 
2 and 2' and 3 and 3', respectively. These elements define a propagation 
path for a magnetic bubble domain. The structure of Y and I-shaped 
elements shown enables one to move domains transverse to the line II--II. 
However, the invention is not restricted to the use of Y and I-shaped 
elements. They are shown mainly for reasons of simplicity. More 
sophisticated elements are, for example, C-shaped elements. Whether 
movement takes place in the upward direction or in the downward direction 
depends on the sense of rotation of the rotary magnetic field M (FIG. 5) 
under the influence of which magnetic N and S poles are generated 
successively at the ends of the Y and I-shaped elements. 
As shown in FIG. 2, bubble domain structure 1 comprises a substrate 4 
formed by a disc which is cut from a single crystal with a given 
crystallographic orientation. A characteristic material for the substrate 
is a rare earth-gallium garnet or an equivalent material. The substrate 4 
supports a planar bubble domain layer 5, for example, an epitaxially 
deposited rare earth-iron garnet layer having a thickness of one or a few 
microns and having an easy direction of magnetization in a direction 
perpendicular to the plane of the layer. (This means that the layer 5 has 
a positive magnetic anisotropy constant: K.sub.1 &gt;0.) For illustration, a 
bubble domain 6 is shown in the layer 5 having a direction of 
magnetization opposite to that of the remainder of the layer. A control 
layer 7, which comprises a continuous sublayer 8 and a discontinuous 
sublayer 9 is present directly on top of the planar layer 5. To obtain the 
desired exchange coupling, the sublayers 8 and 9 preferably each have a 
thickness of at most 0.5 .mu.m (a thickness of 0.2 .mu.m is more 
preferrable) and have an easy direction of magnetization in the plane of 
the layer. (This means that the layers 8 and 9 have a negative magnetic 
anisotropy constant: K.sub.2 and K.sub.3 &lt;0). 
When bubble domain structure 1 is subjected to a rotating in-plane magnetic 
driving field M (assuming the usual bias field H to be present at right 
angles to the layer 5 so as to maintain the bubble domain 6), the 
following will happen. The rotating field M magnetizes the continuous 
sublayer 8, the whole magnetization of the sublayer 8 co-rotating with the 
field M. By exchange coupling, the parts of the discontinuous sublayer 9 
which form the propagation elements are also co-rotated. The sublayer 9 
generates magnetic poles analogous to those of known propagation 
structures of soft magnetic nickel-iron alloys. The interfering 
interaction between bubble domain and propagation element ("adhesion") 
which results from soft magnetic alloys, however, is absent in this case. 
The sublayer 8 functions as an amplifier so that a weaker rotary field 
will suffice than when conventional propagation structures are used. Since 
more trouble is experienced from the interfering interaction as the bubble 
domains are smaller, the advantage of the present invention becomes more 
and more prominent as the bubble domain dimensions become smaller. (For 
propagating 1 micron bubble domains, normally a rotary field of 50 to 100 
Oersted is necessary. When the invention is used, rotary fields of from 10 
to 20 Oersted are sufficient.) 
The structure shown in FIGS. 1 and 2 can be manufactured in various 
manners. 
According to a first method, a bubble domain layer 5 is grown on a 
monocrystalline substrate 4. The upper part of layer 5 is then implanted 
with ions to form a control layer 7 having in-plane magnetization. The 
upper part of the control layer 7 is etched away locally to form 
propagation elements 2 and 3 and 2' and 3', which remain as islands. 
For the realization of small bubble domains (cross-section 1 micron and 
smaller) the bubble domain material must have a very high magnetic 
anisotropy. In that case it is difficult to change the anisotropy constant 
from positive to negative by implantation. 
A second method involves the direct growth of a layer with K&lt;0 on a bubble 
domain layer with K&gt;0. This may be done, for example, by a liquid epitaxy 
process, in which a bubble domain layer of nominal composition 
(Y,La).sub.3 (Fe,Ga).sub.5 O.sub.12 is grown. In this process, the gallium 
content of the grown layer increases from a first value to a second value 
in order to obtain the reversal of the anisotropy. 
Where the above methods relate to layers having garnet crystal structures, 
a third method relates to bubble domain layers of materials having a 
magnetoplumbite structure, which in themselves have a positive anisotropy 
constant. A characteristic material of this type is barium ferrite 
(BaFe.sub.12 O.sub.19). Such domain layers are used, according to the 
invention, in combination with control layers of a material having a 
hexagonal crystal structure and a negative anisotropy constant. A 
characteristic material of this latter type is barium zinc ferrite 
(Ba.sub.2 Zn.sub.2 Fe.sub.12 O.sub.22). For this purpose, there is 
provided on a suitable substrate first a thin layer of BaFe.sub.12 
O.sub.19 having an easy axis of magnetization perpendicular to the plane 
of the layer. A layer of Ba.sub.2 Zn.sub.2 Fe.sub.12 O.sub.22 having an 
easy axis of magnetization in the plane of the layer is then provided on 
the BaFe.sub.12 O.sub.19 layer. 
In all these methods, the last step is to locally etch away material from 
the upper part of the control layer so as to form propagation elements. 
In the bubble domain chip shown in FIGS. 1 and 2, the control layer is 
constructed so that the sublayer 9 with the propagation pattern is 
separated from the layer 5 in which bubble domains are propagated by the 
continuous sublayer 8. In the bubble domain chip 11 shown in FIGS. 3 and 
4, the order of the sublayers is reversed. A bubble domain layer 15 
provided on a substrate 14 is provided with a control layer 17 which 
consists of a continuous sublayer 18 having in-plane magnetization which 
is obtained by implanting the layer 15 throughout its surface with ions 
down to a depth of, for example, 0.5 micron. By deeper implantation in 
places which correspond to a propagation pattern consisting of elements 
12, 13 12', and 13', for example down to an overall depth of 1 micron, a 
discontinuous sublayer 19 is obtained having propagation elements directly 
adjoining the bubble domain layer 15. The propagation of a bubble domain 
16 in layer 15 takes place in the same manner as described above in 
connection with the bubble domain chip 1 shown in FIGS. 1 and 2. 
In manufacturing the bubble domain chip shown in FIGS. 3 and 4, a rare 
earth-iron garnet layer was grown on a monocrystalline substrate 14 up to 
a thickness of 2 microns. The garnet layer was then ion implanted at two 
levels. The layer 18 (the first level) was obtained by implantation with a 
dose of 2.times.10.sup.14 Ne.sup.+ ions per cm.sup.2 at 100 KV. In order 
to obtain the recesses 19 (second level), implantation with a dose of 
10.sup.14 Ne.sup.+ ions per cm.sup.2 at 300-400 KV was carried out locally 
through apertures in a gold mask 20. 
Besides being suitable for the manufacture of a continuous sublayer with a 
discontinuous sublayer consisting of propagation elements with 
intermediate spaces ("gapped propagation elements"), the method of ion 
implantation at two levels is also very suitable for the manufacture of a 
combination of a continuous sublayer with a discontinuous sublayer 
consisting of propagation elements without intermediate spaces 
("contiguous propagation elements") as shown in FIGS. 6 and 7. 
FIG. 6 shows a part of a bubble domain chip 21 having a bubble domain 
propagation path 22 which is formed by the circumference of overlapping 
propagation elements 23, 23' and 23". As shown in FIG. 7, bubble domain 
chip 21 comprises a monocrystalline substrate 24 on which a bubble domain 
layer 25 is grown. The upper part of the layer 25 is implanted with ions 
at two levels so as to obtain a bipartite control layer 27 according to 
the invention. During a first implantation step the area of the layer 27 
is implanted with a dose of, for example, 10.sup.14 Fe.sup.+ ions per 
cm.sup.2 so as to form a continuous sublayer 28 with in-plane 
magnetization. During a second implantation step the layer 25 is implanted 
with the same dose down to a larger depth via apertures 32 and 33 in a 
thin gold mask 30 and 31 (200 nm thick) provided after the first step. The 
second implantation forms a discontinuous sublayer 29 with an in-plane 
magnetization consisting of the local recesses of the layer 28. In other 
words, the sublayer 29 has been formed with an in-plane magnetization and 
forms therein a propagation pattern of overlapping elements 23, and 23', 
23" with an easy axis of magnetization in the plane of the layer. In such 
a configuration when the exchange coupling between the sublayers 28 and 29 
does not dominate, so-called charged walls can be formed under the 
influence of an in-plane magnetic field. These walls emanate radially from 
the circumference of the propagation elements. When the bubble domain chip 
21 is subjected to an in-plane rotary field M (FIG. 5) the charged walls 
co-rotate so that bubble domain 26 is also propagated. 
In itself this does not differ from what happens in so-called contiguous 
disc bubble domains (see for example IEEE Transactions on Magnetics, Vol. 
MAG. 13, No. 6, November 1977, pp. 1744-64). However, the unique 
continuous sublayer 28 with in-plane magnetization of the inventive bubble 
domain structure ensures that the range of the charged walls is restricted 
so that the propagation of bubble domains along an adjacent bubble domain 
path 34 is not disturbed, not even when it is closed. In the known 
contiguous disc bubble domain devices, the charged walls have a large 
range so that the bubble domain path cannot be located very close 
together.