Magnetic recording medium with an underlayer and a cobalt-based magnetic layer

An improved cobalt-platinum (CoPt) thin film metal alloy media for horizontal magnetic recording has a coercivity substantially greater than prior CoPt thin film metal alloy media. A tungsten underlayer between the substrate and the CoPt magnetic layer improves the coercivity above that of media wiht conventional underlayers, such as chromium. The coercivity of the CoPt layer can be increased even further if the CoPt film is deposited in such a manner as to form an intermetallic compound of Co.sub.3 W in the interface region between the tungsten underlayer and the CoPt magnetic layer. The tungsten underlayer also improves the magnetic properties of the media when the magnetic layer is an alloy of cobalt-platinum-chromium (CoPtCr).

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to thin film metal alloy magnetic recording media, 
and in particular to a thin film metal alloy disk for horizontal magnetic 
recording in which an alloy comprising cobalt and platinum forms the 
magnetic layer. 
2. Description of the Prior Art 
Alloys of cobalt and platinum with various percentages of platinum 
concentration have been used as the magnetic material in thin film 
magnetic recording disks for horizontal recording. In such disks, the 
hexagonal close packed (HCP) crystalline structure of the cobalt-platinum 
(CoPt) alloy is formed on the substrate, or on an intermediate underlayer, 
so that the C-axis, i.e. the [002] axis, of the CoPt film is either in the 
plane of the film or has a component in the plane of the film. 
The coercivity (H.sub.c) of CoPt films is dependent upon the composition of 
the platinum, with the maximum H.sub.c occurring at approximately 20 
atomic percent (at. %) platinum. See J. A. Aboaf, et al., "Magnetic 
Properties and Structure of Co-Pt Thin Films", IEEE Trans on Magnetics, 
MAG-19, 1514 (1983), and M. Kitada, et al., "Magnetic Properties of 
Sputtered Co-Pt Thin Films", J. Appl. Phys. 54 (12), December 1983, pp. 
7089-7094. The coercivity and other properties of cobalt-platinum films 
have been reported by Opfer, et al. in an article entitled "Thin-Film 
Memory Disc Development," Hewlett-Packard Journal, November 1985, pp. 
4-10. 
A thin film disk with a cobalt-platinum-chromium (CoPtCr) magnetic layer, 
wherein Cr is added to improve the corrosion resistance of magnetic layer, 
is described in Japanese patent application No. 198568, published May 22, 
1984. The CoPtCr magnetic layer is deposited onto a nickel-phosphorus 
(NiP) film formed on a suitable substrate. 
In order to improve the coercivity of the CoPt magnetic film in certain 
types of disks, a chromium (Cr) underlayer is often formed between the 
substrate and the CoPt magnetic layer. The use of a chromium underlayer in 
a CoPt thin film disk is described in the above referenced article by 
Opfer, et al. and in European patent application No. 145157, published 
June 19, 1985 and assigned to the Hewlett-Packard Company. 
The use of tungsten (W) as an enhancement layer in certain types of thin 
film disks for horizontal recording has been suggested in an article by 
Dubin, et al. in "Degradation of Co-based Thin-film Recording Materials in 
Selected Corrosive Environments," J. Appl. Phys. 53(3), March 1982, pp. 
2579-2581. The Dubin, et al. article states that a 1000 .ANG. thick 
underlayer of tungsten is used to promote epitaxial crystal growth in 
magnetic layers of cobalt-nickel-tungsten (CoNiW) and cobalt-nickel 
(CoNi). 
In Japanese unexamined patent application No. 59-227107, published Dec. 19, 
1984, a magnetic recording medium is described in which the magnetic film 
contains cobalt, platinum and tungsten, with platinum comprising between 4 
and 15 at. % and tungsten comprising between 0.5 and 8 at. % of the CoPtW 
alloy. This Japanese reference indicates that greatly improved coercivity 
is obtained by adding tungsten to the cobalt-platinum alloy. 
Co-pending application Ser. No. 791,963, assigned to the same assignee as 
this application, describes a specifically deposited layer of an 
intermetallic compound, such as cobalt-tungsten, (Co.sub.3 W), to form a 
nucleating layer for a subsequently deposited magnetic film for vertical 
recording. 
European patent application No. 140513, published May 8, 1985 and assigned 
to the same assignee as this application, suggests various combinations of 
underlayers and magnetic layers as structures for horizontal magnetic 
recording, including a CoPt magnetic layer formed on a nonmagnetic layer 
of a WCo alloy. 
SUMMARY OF THE INVENTION 
The present invention is an improved CoPt or CoPtCr thin film magnetic 
recording disk for horizontal recording and incorporates a nonmagnetic 
underlayer of tungsten between the substrate and the CoPt or CoPtCr 
magnetic layer to improve the coercivity. The tungsten underlayer and the 
magnetic layer are deposited so as to form an intermetallic compound, 
Co.sub.3 W, at the interface between the tungsten and magnetic layers. 
With the use of a tungsten underlayer and the deposition of the underlayer 
and magnetic layers in a manner so as to form the intermetallic compound 
in the interface region, the coercivity of CoPt or CoPtCr disks can be 
improved, or in the alternative, the same coercivity can be achieved with 
less platinum. 
For a further understanding of the nature and advantages of the present 
invention, reference should be made to the following detailed description 
taken in conjunction with the accompanying drawings

DESCRIPTION OF THE PREFERRED EMBODIMENT 
In order to note the improved coercivity of the CoPt disk made according to 
the present invention, a CoPt disk was first made with a chromium 
underlayer between the substrate and the CoPt magnetic layer. A chromium 
underlayer of 1000 .ANG. thickness was deposited by DC magnetron 
sputtering onto a silicon substrate at an Argon pressure of 
3.2.times.10.sup.-3 Torr and a substrate temperature of 128.degree. C. 
Thereafter, a 400 .ANG. thick cobalt-platinum alloy film with 20 at. % 
platinum (Co.sub.80 Pt.sub.20) was sputter deposited onto the chromium 
underlayer without breaking vacuum in the sputtering chamber. Curve A in 
FIG. 1 is an M-H hysteresis loop for this film and illustrates a 
coercivity H.sub.c of 1320 Oersteds (Oe) and a squareness S of 0.887. 
Curve B is an M-H loop for a cobalt-platinum film of similar thickness 
deposited onto a tungsten underlayer. In accordance with the present 
invention, a 1000 .ANG. thick tungsten underlayer was deposited by DC 
magnetron sputtering onto a silicon substrate with the substrate 
maintained at a temperature of 38.degree. C. Thereafter, without breaking 
vacuum in the sputtering chamber, a 425 .ANG. thick Co.sub.80 Pt.sub.20 
film was sputter deposited onto the tungsten underlayer. The resulting M-H 
hysteresis loop (Curve B of FIG. 1) illustrates for this film a coercivity 
of 2145 Oe and a squareness S of 0.864. For the two disks whose data is 
depicted in FIG. 1, the same Co.sub.80 Pt.sub.20 sputtering target was 
used. 
Cobalt-platinum films of various thicknesses were formed on tungsten 
underlayers at various substrate deposition temperatures, and the magnetic 
properties of these films were compared with cobalt-platinum films formed 
on chromium underlayers. As shown by Curve A in FIG. 2, the various 
experimental samples had Co.sub.80 Pt.sub.20 thicknesses ranging between 
approximately 240 and 640 .ANG. and were sputter deposited onto 1000 .ANG. 
thick tungsten underlayers while the silicon substrate temperature was 
maintained between approximately 30.degree. and 40.degree. C. The 
coercivity values for such films were substantially greater than the 
coercivity of Co.sub.80 Pt.sub.20 films deposited onto 1000 .ANG. thick 
chromium underlayers while the silicon substrate temperature was 
maintained at approximately 128.degree. C., as shown by Curve C in FIG. 2. 
(The coercivity of the Si/1000 .ANG. Cr/400 .ANG. Co.sub.80 Pt.sub.20 disk 
of FIG. 1 is significantly less than the corresponding disk in Curve C of 
FIG. 2 because the latter was made with a different Co.sub.80 Pt.sub.20 
sputtering target, which may have had slight variations in composition). 
When the temperature of the silicon substrate was increased from 
approximately 30.degree. to 40.degree. C. to 128.degree. C. and Co.sub.80 
Pt.sub.20 films of the same thickness range were deposited onto the 1000 
.ANG. thick tungsten underlayers, the resulting coercivities were 
substantially reduced, but still substantially higher than for the CoPt 
samples formed on chromium underlayers. (See Curve B in FIG. 2.) 
In order to understand the difference in coercivity of the Co.sub.80 
Pt.sub.20 films depicted by Curves A and B in FIG. 2, an X-ray diffraction 
analysis was performed on the silicon disks with the Co.sub.80 Pt.sub.20 
film deposited onto the tungsten underlayers. The X-ray diffraction curve 
of FIG. 3 is for a silicon disk with a 1000 .ANG. thick W underlayer and a 
625 .ANG. thick Co.sub.80 Pt.sub.20 magnetic layer deposited at a silicon 
substrate temperature of 38.degree. C. FIG. 3 depicts a peak intensity at 
2.theta. equal to 42.62 degrees, which corresponds to the (002) plane of 
the HCP Co.sub.80 Pt.sub.20 magnetic film and a peak intensity at 2.theta. 
equal to 39.7 degrees, which corresponds to the (110) plane of the 
body-centered-cubic (BCC) tungsten underlayer. In addition, a peak 
intensity at 2.theta. equal to 35.25 degrees corresponds to the (110) 
plane of the HCP Co.sub.3 W intermetallic compound. This peak in FIG. 3 
confirms that an intermetallic compound phase of cobalt and tungsten is 
present at the interfacial region between the Co.sub.80 Pt.sub.20 and W 
layers. The (110) reflection for the HCP Co.sub.3 W implies that a 
component of the C-axis [002] of the Co.sub.3 W is in the plane of the 
film. The (110) reflection for Co.sub.3 W was not detected in an X-ray 
diffraction analysis of Co.sub.80 Pt.sub.20 films on tungsten underlayers 
deposited at 128.degree. C. (i.e. those disks whose data is shown by Curve 
B in FIG. 2). Thus, the results shown in FIG. 3 indicate that the 
relatively low temperature deposition of Co.sub.80 Pt.sub.20 on a tungsten 
underlayer causes an interfacial reaction between Co and W, which results 
in a highly oriented hexagonal Co.sub.3 W phase at the interface. This 
interface enhances the C-axis orientation of the Co.sub.80 Pt.sub.20 film 
in the plane of the film. 
A thin film disk with a 625 .ANG. thick (Co.sub.90 Pt.sub.10).sub.80 
Cr.sub.20 magnetic layer deposited onto a 1000 .ANG. thick W underlayer on 
a silicon substrate also showed excellent magnetic properties. This disk 
was formed by DC magnetron sputtering using separate Co.sub.90 Pt.sub.10 
and Cr targets at a substrate temperature of 28.degree. C. The disk had 
H.sub.c =970 Oe, coercivity squareness S*=0.89 and a remanance-thickness 
product Mr.t=1.68.times.10.sup.-3 emu/cm.sup.2. The X-ray diffraction 
analysis of this disk is shown in FIG. 4 and depicts a strong peak 
intensity at 2.theta.=35.16.degree., which corresponds to the (110) plane 
of the Co.sub.3 W interface region, thereby confirming the formation of 
the Co.sub.3 W interface between the W underlayer and (Co.sub.90 
Pt.sub.10).sub.80 Cr.sub.20 magnetic layer. 
As used herein the term "intermetallic compound" refers to those chemical 
compositions which are more than a simple mixture in the form of an alloy, 
but in which the constituents are present in a fixed stoichometric ratio 
so that the composition can be essentially represented by a chemical 
formula. An intermetallic binary compound of two elements, such as cobalt 
and tungsten, is an intermediate phase which exists only at the discrete 
stoichometric ratio of three atoms of cobalt to one atom of tungsten. The 
Co.sub.3 W intermetallic compound is indicated on the published phase 
diagrams for cobalt and tungsten, such as in Constitution of Binary 
Alloys, McGraw Hill, 1958, p. 519. 
In all the experimental examples described herein the substrate was 
semiconductor grade single-crystal silicon. When a silicon substrate is 
used, an underlayer is required to cause the C-axis orientation of the 
magnetic layer to be in the plane of the film. The tungsten underlayer of 
the present invention serves this purpose. When the substrate is other 
material, such as a nickel-phosphorus (NiP) film formed on an aluminum 
alloy disk, however, an underlayer may not be absolutely necessary but 
only beneficial to improve the in-plane C-axis orientation of the magnetic 
layer. The tungsten underlayer of the present invention enhances the 
in-plane C-axis orientation of the magnetic layer and thereby improves the 
magnetic properties of the thin film disk when the substrate is a NiP film 
on an aluminum alloy disk. 
While the experimental examples described herein were limited to the use of 
tungsten as the underlayer, other refractory metals such as niobium (Nb), 
molybdenum (Mo), and vanadium (V) would also likely enhance the 
orientation of the CoPt and CoPtCr film because such metals are known to 
form HCP intermetallic compounds with cobalt. These compounds are, 
respectively, Co.sub.2 Nb, Co.sub.3 Mo and Co.sub.3 V. 
The above description relates only to the formation of the magnetic layer 
and underlayer on the substrate in horizontal recording media and not to 
the well known aspects of the media and the media fabrication processes. 
For example, in the fabrication of thin film metal alloy disks it is known 
to provide a protective overcoat, such as a sputtered, essentially 
amorphous carbon film, over the magnetic layer and in certain instances to 
provide an adhesion layer, such as a sputtered film of titanium, between 
the overcoat and the magnetic layer. 
While the preferred embodiments of the present invention have been 
illustrated in detail, it should be apparent that modifications and 
adaptations to those embodiments may occur to one skilled in the art 
without departing from the scope of the present invention as set forth in 
the following claims.