Multi-layered ferromagnetic film and method of manufacturing the same

The film is composed of an alternate lamination of unit iron layers and unit layers of ferromagnetic iron compound such as Fe.sub.3 Al, Fe.sub.3 Si, Fe.sub.3 Ge and Fe.sub.3 Ga. The thickness of the both unit layers is less than 70 .ANG.. The film has a high saturation magnetization more than 230 emu/g and a high thermal stability so that the film is particularly applicable to a magnetic head core.

The present invention relates to a multi-layered ferromagnetic film having 
a high saturation magnetization and a method of manufacturing the same, 
which is applicable, for example, to a magnetic head core and a recording 
medium such as a magnetic disc and a magnetic tape. 
BACKGROUND OF THE INVENTION 
The saturation magnetization of a ferromagnetic film is determined by the 
total vector sum of magnetic moments per iron atoms included in the 
ferromagnetic film. The magnetic moment per iron atom of a pure stable 
iron is about 2.2 .mu..sub.B (Bohr magneton) which is increasable when the 
interatomic distance of the pure stable iron, in other words lattice 
constant thereof, is increased. One known method of increasing the 
interatomic distance is effected to force to dissolve nitrogen atoms in 
solid state into the pure stable iron. The other known method is to grow 
iron on a simple metal which has a larger interatomic distance than iron 
and to increase the interatomic distance at the interface thereof as 
disclosed, for example, in C. L. Fu, J. Freeman, and T. Oguchi "Prediction 
of Strongly Enhanced Two-Dimensional Ferromagnetic Moments on Metallic 
Overlayers, Interfaces, and Superlattices" (PHYSICAL REVIEW LETTERS, 
volume 54, Number 25, pp 2700-2703), wherein an increase of magnetic 
moment per iron atom is predicted by growing iron on Copper (Cu), Silver 
(Ag) an Gold (Au) which have a larger interatomic distance than that of 
iron. However since Copper, Silver and Gold are non-ferromagnetic metals, 
a saturation magnetization of the combined film in total comprising a 
lamination of Copper, Silver or Gold layers and iron layers decreases 
below that of an iron film alone. 
SUMMARY OF THE INVENTION 
One of the object of the present invention is to provide a multi-layered 
ferromagnetic film having a comparable or higher saturation magnetization 
than that of an iron film alone. 
Another object of the present invention is to provide a method of 
manufacturing a multi-layered ferromagnetic film having a comparable or 
higher saturation magnetization than that of an iron film alone. 
A multi-layered ferromagnetic film of the present invention comprises an 
alternate lamination of unit iron layers and unit ferromagnetic iron 
compound layers. 
Although .gamma..sub.2 phase iron is in a semi-stable phase, the 
.gamma..sub.2 phase iron has a lattice constant larger than that of a 
stable body centered cubic (bcc) structure iron, thus has a larger 
magnetic moment per iron atom (2.8 .mu..sub.B) than that of the stable bcc 
structure iron (2.2 .mu..sub.B) used for the unit iron layers. 
For the unit ferromagnetic iron compound layers, ferromagnetic iron 
compounds such as Fe.sub.3 Al, Fe.sub.3 Si, Fe.sub.3 Ge, Fe.sub.3 Ga, 
Fe.sub.3 Pt and Fe.sub.3 Sn are used, which have a high thermal stability 
and a large lattice constant close to the semi-stable .gamma..sub.2 phase 
iron. 
Since the lattice constant of the ferromagnetic iron compound is larger 
than that of the pure stable iron, the interatomic distance of the unit 
iron layer deposited on the unit ferromagnetic iron compound layer is 
enlarged thus to increase the magnetic moment per iron atom included in 
the unit iron layer. 
Magnetic moments per iron atom of Fe.sub.3 Al, Fe.sub.3 Si, Fe.sub.3 Ge and 
Fe.sub.3 Ga are illustrated on Table 1, which is extracted from Table 2 of 
Nobuo KAWAMIYA, Kengo ADACHI and Yoji NAKAMURA "Magnetic Properties and 
Mossbauer Investigation of Fe-Ga Alloys (Journal of the Physical Society 
of Japan, Vol. 33, No.5 November, 1972, pp 1318-1327). 
TABLE 1 
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Magnetic Moment 
Crystal Per Fe Atom (.mu.B) 
Iron Compound 
Structure at 0.degree. K. 
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Fe.sub.3 A 
A bcc 1.65 (2.03) 
D 2.35 
Fe.sub.3 Si 
A bcc 1.28 (1.89) 
D 2.37 
Fe.sub.3 Ge hexagonal 2.2 (2.20) 
Fe.sub.3 Ga 
A bcc 1.5 (2.17) 
D 2.7 
Fe.sub.3 Ga fcc 2.4 (2.4) 
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D represents D site sublattice. 
bcc represents body centered cubic. 
fcc represents face centered cubic. 
( ) represents average magnetic moment per Fe atom. 
As seen from Table 1 the magnetic moments per iron atom in the 
ferromagnetic iron compounds depend on an atomic configuration of iron 
atom, the maximum magnetic moment per iron atom in the ferromagnetic iron 
compounds are larger than that of the pure stable iron (2.2 .mu..sub.B). 
Because of the large magnetic moments per iron atom of the ferromagnetic 
iron compounds, the saturation magnetization of the multi-layered 
ferromagnetic film as a whole of the present invention, which is formed 
through an alternate lamination of the unit ferromagnetic iron compound 
layers and the unit iron layers, is also enhanced. 
A variation of the thickness of the unit iron layer, in other words, the 
distance from the lower unit ferromagnetic iron compound layer, greatly 
affects the magnetic moment per iron atom in the unit iron layer, since 
the lattice constant or the interatomic distance of iron atom of the lower 
unit ferromagnetic iron compound layer controls that in the unit iron 
layer. 
Inventors found out that the unit iron layer having a thickness more than 
70 .ANG. does not substantially increase the saturation magnetization of 
the multi-layered ferromagnetic film as a whole and layer having a 
thickness less than 50 .ANG. greatly enhances the saturation magnetization 
thereof. The minimum thickness of the unit iron layer is that of the 
single atom layer, in that, 2-3 .ANG.. 
The alternate lamination of the unit ferromagnetic iron compound layers and 
the unit iron layers of the present invention is preferably subjected to a 
first heat treatment at a temperature of 200.degree.-300.degree. C. in a 
vacuum for a predetermined time so as to stabilize and match the 
interfaces between the unit iron layers and the unit ferromagnetic iron 
compound layers and to further enhance the saturation magnetization of the 
multi-layered ferromagnetic film as a whole of the present invention. 
Further the alternate lamination of the unit ferromagnetic iron compound 
layers formed of Fe.sub.3 Ga containing 20-35 atomic % of Ga and unit iron 
layers is preferably subjected to a second heat treatment at a temperature 
of 500.degree.-800.degree. C. so as to stabilize the face centered cubic 
(fcc) ordered lattice structure in the unit ferromagnetic Fe.sub.3 Ga 
layers and to further enhance the saturation magnetization of the 
multi-layered ferromagnetic film as a whole of the present invention. 
Further the total thickness of the multi-layered ferromagnetic film for use 
as the magnetic head core is preferably more than 1000 .ANG..

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The sputtering method was used to form the alternate lamination of the unit 
iron layers and the unit ferromagnetic iron compound layers. First, the 
ferromagnetic iron compound such as Fe.sub.3 Al, Fe.sub.3 Si, Fe.sub.3 Ge 
and Fe.sub.3 Ga was deposited by sputtering on a mono-crystalline silicon 
substrate, then, the iron was deposited by sputtering on the previously 
deposited unit ferromagnetic iron compund layer. The deposition of the 
ferromagnetic iron compound and the iron was alternately repeated to form 
the multi-layered ferromagnetic film. The sputtering conditions of the 
ferromagnetic iron compound and iron are illustrated in Table 2. The 
sputtering conditions are modified depending on the materials to be 
sputtered. 
TABLE 2 
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Substrate Mon-crystalline Si 
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Temperature of substrate 
150.degree. C. 
Initial degree of vacuum 
&lt;1.0 .times. 10.sup.-4 Pa 
Input power 0.5 to 1.5 kW 
Pressure of Ar gas 67- 0.67 Pa 
Ratio of two unit layer thickness 
1:1 
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The saturation magnetizations of multi-layered ferromagnetic Fe-Fe.sub.3 
Al, Fe-Fe.sub.3 Si, Fe-Fe.sub.3 Ge and Fe-Fe.sub.3 Ga films thus produced 
with varying unit Fe layer thickness and while keeping the ratio of the 
two units layer thickness as 1:1 were measured by a vibrating sample 
magnetometer (VSM) and are illustrated in FIG. 1. 
As seen from FIG. 1, when the unit Fe layer thickness is reduced, the 
saturation magnetizations of the respective multi-layered ferromagnetic 
films increase. When the unit Fe layer thickness is reduced less than 70 
.ANG., the saturation magnetizations of the respective multi-layered 
ferromagnetic films suddenly increase. The multi-layered ferromagnetic 
Fe-Fe.sub.3 Ga film with unit Fe layer thickness of 20 .ANG. showed a 
saturation magnetization of 258 emu/g. When the unit Fe layer thickness of 
the respective multi-layered ferromagnetic films is reduced less than 30 
.ANG., the saturation magnetizations of multi-layered ferromagnetic 
Fe-Fe.sub.3 Al, Fe-Fe.sub.3 Si, Fe-Fe.sub.3 Ge and Fe-Fe.sub.3 Ga films 
exceed that of the pure Fe film as indicated by the dotted line in FIG. 1. 
Further with respect to the respective multi-layered ferromagnetic films 
with the same unit Fe layer thickness, the multi-layered ferromagnetic 
Fe-Fe.sub.3 Ga film showed the highest saturation magnetization and 
followed by multi-layered ferromagnetic Fe-Fe.sub.3 Ge, Fe-Fe.sub.3 Si and 
Fe-Fe.sub.3 A films which trend partly corresponds to the average magnetic 
moments per iron atoms of respective ferromagnetic iron compounds 
themselves as shown in Table 1. 
The respective multi-layered ferromagnetic films with the unit Fe layer 
thickness of 50 .ANG. were heat treated at a temperature of 200.degree. C. 
in vacuum so as to stabilize and match the interfaces between the unit Fe 
layers and the unit ferromagnetic iron compound layers. FIG. 2 shows the 
effects of the low temperature heat treatment on the saturation 
magnetization of the multi-layered ferromagnetic Fe-Fe.sub.3 Al, 
Fe-Fe.sub.3 Si, Fe-Fe.sub.3 Ge and Fe-Fe.sub.3 Ga films. As seen from FIG. 
2 the saturation magnetizations of the films increase with the increase of 
the heat treatment time. 
Upon production of the multi-layered ferromagnetic Fe-Fe.sub.3 Ga film with 
unit Fe layer thickness of 50 .ANG., Ga content in Fe.sub.3 Ga sputtering 
target was varied in the range of 15 to 50 atomic % to see the effects of 
the Ga content on the saturation magnetization of the multi-layered 
ferromagnetic Fe-Fe.sub.3 Ga film as a whole. FIG. 3 is the results of the 
investigation. As seen from FIG. 3, when the Ga content is less than 27 
atomic %, the saturation magnetization of the multi-layered ferromagnetic 
Fe-Fe.sub.3 Ga film increases with the increase in Ga content, but when 
the Ga content exceeds 27 atomic %, the saturation magnetization decreases 
with the increase in Ga content. The Ga content in Fe.sub.3 Ga sputtering 
target which exhibits a higher saturation magnetization than that of the 
pure iron film is in the range of 20 to 35 atomic %. 
A multi-layered ferromagnetic Fe-Fe.sub.3 Ga film with unit Fe layer 
thickness of 50 .ANG. and formed by using Fe.sub.3 Ga sputtering target 
containing 27 atomic % Ga was heat-treated at a temperature of 600.degree. 
C. in vacuum to stabilize the fcc ordered lattices in the unit 
ferromagnetic Fe.sub.3 Ga layers and to further increase the saturation 
magnetization of the film. FIG. 4 shows the effects of the heat treatment 
on the saturation magnetization of the film as a whole. As seen from FIG. 
4, the saturation magnetization of the film increases with the increase of 
the heat treatment time. Substantially the same effects were obtained with 
the heat treatment of the multi-layered ferromagnetic films at a 
temperature of 500.degree.-800.degree. C. 
Further, without subjecing the alternate lamination of the unit 
ferromagnetic iron compound layers and the unit iron layers together with 
the substrate to at least one heat treatment explained above, 
substantially the same effects was obtained by heating up the substrate at 
a temperature 150.degree.-400.degree. C. during the sputtering operation. 
Multi-layered ferromagnetic Fe-Fe.sub.3 Ga films with varying unit Fe layer 
thickness were formed on the substrate which was kept at a temperature of 
400.degree. C. during the sputtering operation. The saturation 
magnetizations of the multi-layered ferromagnetic Fe-Fe.sub.3 Ga film thus 
produced are illustrated in FIG. 5, which shows an improvement of the 
saturation magnetization over that of the non-heat-treated multi-layered 
ferromagnetic Fe-Fe.sub.3 Ga film as illustrated in FIG. 1.