Process for the production by catalysis of a layer having a high magnetic anisotropy in a ferrimagnetic garnet

Process for producing a ferrimagnetic garnet layer having a high magnetic anisotropy on an amagnetic substrate, wherein it comprises the stages of forming at least one ferrimagnetic garnet layer by epitaxy from the amagnetic substrate, implanting ions in the ferrimagnetic garnet layer in order to produce defects therein, depositing on the ferrimagnetic garnet layer a metal layer able to activate and diffuse hydrogen into said garnet layer and heating under a hydrogen atmosphere of the complete structure, in order to bring about the diffusion of hydrogen into the ferrimagnetic garnet layer. Application to the production of bubble stores with non-implanted propagation patterns.

BACKGROUND OF THE INVENTION 
The present invention relates to a process for the production by catalysis 
of a layer having a high planar magnetic anisotropy in a ferrimagnetic 
garnet. It more particularly applies to the field of producing magnetic 
bubble stores and particularly non-implanted disk bubble stores, as well 
as in the field of producing magneto optical or semiconductor material. 
In general terms, the production of a bubble store firstly consists of 
producing by epitaxy a ferrimagnetic garnet layer with growth anisotropy 
perpendicular to the layer on an amagnetic substrate, mainly a garnet. It 
is pointed out that magnetic bubbles are small magnetic domains, whose 
magnetization, directed perpendicular to the surface, is reversed compared 
with that of the material containing the bubbles. The ions are then 
implanted in the epitactic layer. 
This ion implantation makes it possible to produce on the surface of the 
ferrimagnetic garnet layer a planar magnetization layer, i.e. a layer 
whose magnetization is parallel to the surface of said layer. This planar 
magnetization layer has the object of increasing the stability of the 
magnetic bubbles. This ion implantation makes it possible to produce 
planar magnetization layers over a thickness of approximately 0.5 .mu.m. 
By using an appropriate implantation mask, it is possible to define in the 
case of bubble stores with non-implanted patterns, propagation patterns, 
which are contiguous patterns, having the shape of a disk, lozenge, etc. 
As ion implantation is only carried out around these patterns, the latter 
are called non-implanted patterns. 
In the case of bubble stores with patterns based on iron and nickel, ion 
implantation, apart from serving to form the surface layer with planar 
magnetization, is also used for eliminating the "hard" bubbles, i.e. the 
bubbles having structures with complex walls. 
The propagation of the magnetic bubbles along the propagation patterns is 
realised by applying a rotary d.c. field in a direction parallel to the 
surface of the ferrimagnetic layer. The bubbles positioned below the 
planar magnetization surface layer are bonded to non-implanted propagation 
patterns via a potential well due to the stress field between the 
implanted and non-implanted zones. The displacement of the magnetic 
bubbles along the propagation patterns results from the action of the 
rotary field, which produces a mobile charged wall entraining the bubbles. 
For a considerable time use has been made of the magnetostriction 
properties of the ferrimagnetic garnet layers to obtain said magnetic 
anisotropy of the surface layer. Thus, ion bombardment produces on the 
surface of the epitactic garnet layer, defects which consequently lead to 
a deformation of the mesh parameter in the direction perpendicular to said 
ferrimagnetic garnet layer. Within the garnet layer, said defects produce 
high mechanical stresses oriented parallel to the surface of said layer. 
It has been proved that an expansion of the mesh parameter could not be 
carried out parallel to the surface of the ferrimagnetic layer. 
The ferrimagnetic garnet layers are produced so as to have a negative 
magnetostriction coefficient. In this case, a compressive stress obtained 
by ion implantation induces magnetic anisotropy in the plane of the 
implanted surface layer which exceeds the growth anisotropy of the 
starting material, i.e. the non-implanted material. 
Unfortunately this magnetostriction mechanism has limits depending on the 
size of the growth anisotropy of the material (growth by epitaxy), as well 
as its negative magnetostriction coefficient. Thus, it is not possible to 
increase the implanted ion dose indefinitely, because beyond a certain 
threshold of the defects, the magnetism of the implanted surface layer is 
cancelled out and it is no longer possible to move the bubbles along the 
non-implanted propagation patterns. 
However, in view of the fact that new generations of magnetic bubble stores 
and in particular non-implanted pattern stores tend to store ever higher 
information densities, it is necessary for ever decreasing sizes of the 
magnetic bubbles, which cannot be achieved using a material with a high 
growth anisotropy. Unfortunately, with such materials, it is no longer 
possible to obtain a planar magnetization in the implanted layer by a 
simple magnetostriction mechanism. 
In order to increase the magnetic anisotropy of the implanted layer, no 
matter what the growth anisotropy of the starting material, consideration 
has recently been given to carrying out a reverse sputtering of argon ions 
in said implanted layer. This is carried out by heating a sample to above 
100.degree. C. This process is described in the article entitled "Magnetic 
and Crystalline Properties of Ion-implanted Garnet Fibres with Plasma 
Exposure" by K. Betsui et al, published at the Intermag Conference, 
Hamburg in 1984. 
SUMMARY OF THE INVENTION 
The present invention relates to another process for producing a layer 
having a high planar magnetic anisotropy in a ferrimagnetic garnet making 
it possible to obviate the disadvantages referred to hereinbefore. 
It is based on the introduction by catalysis of hydrogen into the upper 
part of the implanted ferrimagnetic garnet layer. 
More specifically the present invention relates to a process for producing 
a ferrimagnetic garnet layer having a high magnetic planar anisotropy on 
an amagnetic substrate, wherein it comprises the stages of forming at 
least one ferrimagnetic garnet layer by epitaxy from the amagnetic 
substrate, implanting ions in the ferrimagnetic garnet layer in order to 
produce defects in said layer, deposition on the ferrimagnetic garnet 
layer of a metal layer able to activate and diffuse hydrogen into said 
garnet layer and heating under a hydrogen atmosphere of the complete 
structure, in order to obtain hydrogen diffusion into the ferrimagnetic 
garnet layer. 
According to the invention, the diffusion of hydrogen into the 
ferrimagnetic garnet layer using a metal layer serving as the catalyst 
makes it possible to very considerably increase the magnetic anisotropy of 
said layer. This magnetic anisotropy increase can, it would seem, be 
explained by a chemical interaction of the hydrogen at the defects 
produced during ion implantation, said defects giving rise to pending or 
free bonds. 
According to a preferred embodiment of the process according to the 
invention, the metal layer is a palladium layer. Advantageously heat in 
the presence of hydrogen takes place at between 100.degree. and 
300.degree. C. 
According to another preferred embodiment of the process according to the 
invention, the metal layer has a thickness between 20 and 50 nm. 
According to another embodiment of the process according to the invention, 
the implanted ions are neon ions. 
The process for producing a ferrimagnetic garnet layer with a high planar 
magnetic anisotropy according to the invention can advantageously be used 
in the production of a bubble store with non-implanted propagation 
patterns. 
In such an application, the process according to the invention comprises 
the stages of forming a ferrimagnetic garnet layer by epitaxy from the 
amagnetic substrate, implanting ions in the upper part of the 
ferrimagnetic garnet layer in order to produce defects in said part and 
form the propagation patterns, deposition on the ferrimagnetic garnet 
layer of a metal layer able to activate and diffuse hydrogen into the 
implanted part of the garnet layer and heating under a hydrogen atmosphere 
of the complete structure, in order to bring about hydrogen diffusion into 
the implanted part of the garnet layer.

As shown in the drawing, the first stage of the process consists of forming 
in per se known manner by epitaxy on an amagnetic substrate 2, such as of 
gadolinium gallate (Gd.sub.3 Ga.sub.5 O.sub.12), a ferrimagnetic garnet 
layer 4, whereof the magnetization vector is oriented perpendicular to the 
surface of layer 4. In said ferrimagnetic layer 4 with a thickness of 
approximately 1000 nm there can be magnetic bubbles 5, in the presence of 
a polarizing field. The ferrimagnetic garnet can be a known material in 
accordance with the following formula (YSmLuCa).sub.3 (FeGe).sub.5 
O.sub.12. 
The orientation of the magnetization vectors in the ferrimagnetic garnet 
layer 4 is due to a growth anisotropy of the materials obtained by an 
appropriate choice of the epitaxy conditions, which are well known in the 
art. 
The following stage of the process consists of carrying out ion 
implantation in the ferrimagnetic layer 4 in order to obtain in the upper 
6 thereof and over a thickness of approximately 300 nm the formation of 
defects. This ion implantation can be carried out with different types of 
ions, such as hydrogen, neon, nitrogen, oxygen, argon, etc. at a dose and 
energy such that the implanted upper part 6 of the ferrimagnetic layer 4 
is not made amorphous and particularly so that the garnet layer loses its 
magnetic properties. In particular neon ion implantation can take place at 
a dose of 2.times.10.sup.14 atoms/cm.sup.2 and an energy of 200 keV. 
Apart from the formation of defects in the upper part 6 of the 
ferrimagnetic garnet layer 4, ion implantation makes it possible to form 
in said part using an appropriate mask non-implanted propagation patterns 
7 of magnetic bubbles 5. 
Following said ion implantation, on ferrimagnetic layer 4 is deposited a 
metal layer 8 with a thickness between e.g. 20 and 50 nm. This metal layer 
8 has the property of activating and diffusing hydrogen into the implanted 
part 6 of the ferrimagnetic layer 4, when the complete structure is 
brought into the presence of hydrogen. 
As material constituting the metal layer 8, it is possible to use palladium 
or an alloy thereof, such as e.g. an alloy of palladium and silver or 
nickel, pure platinum or in the form of an alloy thereof. These different 
materials make it possible by different mechanisms to decompose the 
hydrogen gas into hydrogen atoms (nascent hydrogen formation), thus 
permitting its diffusion into the upper part of the ferrimagnetic layer 4. 
They serve as a diffusion catalyst. Advantageously use is made of a 
palladium material layer. 
The following stage of the process consists of heating the complete 
structure in the presence of hydrogen, so as to permit the diffusion 
thereof into the upper part 6 of the ferrimagnetic layer 4. Heating is 
advantageously carried out at a temperature between 100.degree. and 
300.degree. C. A temperature below 100.degree. C. would lead to a 
relatively long diffusion time (several days), whereas a temperature above 
300.degree. C. would be prejudicial to obtaining a high planar magnetic 
anisotropy in the upper part 6 of the ferrimagnetic layer 4. 
Thus, an excessively high temperature would lead to the reinstatement of 
the defects and in particular the closure of the pending bonds formed in 
the upper part 6 of the ferrimagnetic layer 4 during ion implantation. 
However, it is the presence of these defects which permits the formation 
of chemical bonds with the diffused hydrogen. 
The heating time is a function of the heating temperature, as well as the 
hydrogen pressure used. The higher the heating temperature, the shorter 
the duration thereof for a given hydrogen pressure. In the same way, the 
higher the hydrogen pressure, the shorter the heating time for a given 
heating temperature. The stage of heating the structure in the presence of 
hydrogen can be carried out in one or more individual stages. 
The stages described hereinbefore make it possible by varying the magnetic 
anisotropy to form a surface layer 6 having a planar magnetization and 
which is used more particularly for stabilizing the underlying magnetic 
bubbles 5. 
The final stage of the process consists of either eliminating, particularly 
by chemical etching, the metal layer, or by forming therein by chemical 
etching the different electrical conductors necessary for producing within 
the bubble store the functions of writing, recording information, 
non-destructive reading, register-to-register transfer and erasure. 
The embodiment of the inventive process described hereinbefore illustrates 
the significant increase obtained in the planar magnetic anisotropy of 
that part of the implanted ferrimagnetic layer 6 more particularly 
containing the non-implanted propagation patterns of the magnetic bubbles. 
Following the implantation in the ferrimagnetic garnet layer 4 made from 
(YSmLuCa).sub.3 (FeGe).sub.5 O.sub.12 of neon ions with a dose of 
2.times.10.sup.14 atoms/cm.sup.2 and at an energy of 200 keV, 
determination took place of the magnetic anisotropy variation between the 
anisotropy of the new ferrimagnetic material and the implanted 
ferrimagnetic material by measuring the variation of the anisotropy 
magnetic field .DELTA.H.sub.K (in A/m) before and after treatment. 
This was followed by the deposition of a palladium layer on said 
ferrimagnetic layer with a thickness of approximately 50 nm by vacuum 
evaporation and the anisotropy magnetic field variation was again 
measured. This was followed by a first heating of the structure obtained 
in the presence of hydrogen for 24 hours at 135.degree. C. in a furnace 
and under a hydrogen pressure of 1 atmosphere (10.sup.5 Pa). This was 
followed by a measurement of the magnetic anisotropy variation between the 
anisotropy of the new ferrimagnetic layer and the anisotropy of the 
treated ferrimagnetic layer. 
Finally, a second heating of the structure obtained was carried out in the 
presence of hydrogen, the pressure thereof still being 1 atm., at a 
temperature of 144.degree. C. for 22 hours 15 minutes. The variation of 
the anisotropy field between the anisotropy field of the new ferrimagnetic 
layer and that of the ferrimagnetic layer treated in this way was again 
measured. 
In parallel, the same treatment and the same measurements as hereinbefore 
were carried out on a sample not covered with palladium and at each stage 
of the process the hydrogen concentration was determined by nuclear 
reaction with boron ions. The results obtained are given in the following 
table. It can be seen from this table that the magnetic anisotropy of the 
implanted ferrimagnetic layer had more than doubled through using the 
process according to the invention. 
It should be noted that that part of the non-implanted ferrimagnetic layer 
containing the magnetic bubbles was not modified by the heating stages of 
the structure in the presence of hydrogen. 
TABLE 
__________________________________________________________________________ 
BEFORE HEATING AFTER FIRST HEATING 
AFTER SECOND HEATING 
Without Pd With Pd 
Without Pd 
With Pd 
Without Pd 
With Pd 
__________________________________________________________________________ 
.DELTA.H.sub.K 
183 .multidot. 10.sup.3 
179 .multidot. 10.sup.3 
180 .times. 10.sup.3 
427 .times. 10.sup.3 
193 .times. 10.sup.3 
458 .times. 10.sup.3 
in A/m 
Hydrogen 
None None Little Much Little Much 
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