The present invention relates to a process for regulating to desired values the dimensions of bubbles of magnetic bubble elements during their production by liquid phase epitaxy.
It is pointed out that a magnetic bubble element is constituted by a magnetic layer with small magnetic domains having an opposite magnetic induction to that of the material surrounding them in the layer.
In a monocrystalline magnetic layer, such as a magnetic garnet film having a uniaxial magnetic anisotropy perpendicular to the plane of the layer, it is possible to create generally cylindrical magnetic domains in which the magnetic induction is of the opposite direction to that in the remainder of the layer.
These domains, which are normally "bubbles" are stabilized at their operating size under the action of a continuous magnetic field, called the polarization field. The latter must be perpendicular to the layer and the domains can be displaced in the plane of the layer under the action of propagation means magnetized by a rotary magnetic field applied in the plane of the layer. In this way, it is possible to produce circuits, comparators, memories, etc.
More specifically, the present invention relates to the preparation of magnetic bubble elements constituted by ferrimagnetic garnet films deposited by liquid phase epitaxy on a non-magnetic garnet substrate, said films preferably being magnetized perpendicular to the plane of the film. In such films, the magnetic domains appear in the form of cylinders with a circular cross-section, for example, positive on the upper face of the layer and negative on the lower face from the magnetic standpoint and in this way they form magnetic dipoles having an axis perpendicular to the displacement plane.
In connection with the construction of magnetic bubble memories, it is known that their capacity is directly linked with the diameter of the magnetic bubbles. Thus, to obtain a capacity of 256 kbits, elements are used, whose bubbles have a diameter of 2.7 .mu.m, whilst to obtain capacities up to 0.5 and 1 megabit it is necessary to use elements whose magnetic bubbles have a diameter of 3 to 1.5 .mu.m, preferably 2.5 and 1.8 .mu.m.
Thus, in connection with the construction of magnetic bubble memories, considerable importance is attached to processors making it possible to adjust the diameter of the magnetic bubbles of such elements to the desired values.
Hitherto, in the processes for the production of magnetic bubble elements by liquid phase epitaxial deposit, the diameter of the element bubbles has been controlled by acting on the composition of the epitaxy bath comprising oxides or carbonates of the elements used in the composition of the film.
Thus, in the case of garnet films of formula: EQU T.sub.a.sup.1 T.sub.b.sup.2 T.sub.c.sup.3 Ca.sub.d) (Fe.sub.e Ge.sub.f)O.sub.12
in which T.sup.1, T.sup.2 and T.sup.3, which differ from one another, represent an element in the series of rare earths including yttrium and a, b, c and d are numbers such that their sum is substantially equal to 3, whilst e and f are numbers such that their sum is substantially equal to 5, the diameter of the bubbles has been controlled by modifying the composition of the epitaxy bath with respect to the quantities of the different rare earths oxides for influencing the anisotropy and on the quantity of germanium oxide for modifying the magnetization. (Materials Research Bulletin, Vol. 10, No. 1, 1975 and Journal of Crystal Growth, Vol. 12, No. 1, December 1977).
Thus, in processes for the production of bubble memories as desired in the Journal of Crystal Growth, Vol. 12, No. 1, December 1977, certain conditions must be respected in order to obtain films with a satisfactory quality.
Thus, for reducing the diameter d of bubbles of the element, it is necessary to reduce the characteristic length l of the film on wishing to respect the condition according to which said diameter d is similar to the thickness h of the film in order to obtain a good stability of the bubbles. This can be achieved by increasing the magnetization of the film because the characteristic length l is defined by the formula: ##EQU1## in which A represents the exchange constant, Ku the uniaxial anisotropy constand and M.sub.s the saturation magnetization.
However, on increasing the saturation magnetization M.sub.s of the film, the anisotropy field H.sub.k is generally reduced making it difficult to respect the condition: EQU H.sub.k -4.pi.M.sub.s .gtoreq.700 oersteds
necessary for preventing spontaneous nucleation of the bubbles. In addition, in order to respect this condition, it is necessary to influence the respective quantities of the rare earths to increase the anisotropy field H.sub.k.
However, this control method involving on the one hand the respective quantities of the different rare earths and on the other the germanium quantity in the epitaxy bath has the disadvantage of requiring a relatively large change in the epitaxy bath composition to change from one bubble diameter to another.