High density magnetic recording medium with high Hr and low Mrt

A magnetic recording medium exhibiting high in-plane anisotropy at low Mrt is formed employing a NiAl seedlayer and a CrMn underlayer thereon. Embodiments include magnetic recording media with a CoCrPtTa magnetic alloy layer exhibiting a Hr greater than 2800 Oe with a Mrt no greater than 0.5 memu/cm.sup.2. The resulting media also exhibit high S* and low media noise.

TECHNICAL FIELD 
The present invention relates to magnetic recording media, such as thin 
film magnetic recording disks, and to a method of manufacturing the media. 
The present invention has particular applicability to high areal density 
magnetic recording media exhibiting low noise and high remanent coercivity 
at low magnetic film thickness. 
BACKGROUND ART 
The requirements for increasingly high areal recording density impose 
increasingly greater demands on thin film magnetic recording media in 
terms of remanent coercivity (Hr), magnetic remanance (Mr), coercivity 
squareness (S*), medium noise, i.e., signal-to-noise ratio (SNR), and 
narrow track recording performance. It is extremely difficult to produce a 
magnetic recording medium satisfying such demanding requirements. 
The linear recording density can be increased by increasing the coercivity 
of the magnetic recording medium. However, this objective can only be 
accomplished by decreasing the medium noise, as by maintaining very fine 
magnetically non-coupled grains. Medium noise is a dominant factor 
restricting increased recording density of high density magnetic hard disk 
drives. Medium noise in thin films is attributed primarily to 
inhomogeneous grain size and intergranular exchange coupling. Accordingly, 
in order to increase linear density, medium noise must be minimized by 
suitable microstructure control. 
A conventional longitudinal recording disk medium is depicted in FIG. 1 and 
comprises a substrate 10, typically Aluminum (Al) or an (Al)-alloy, such 
as an Al-magnesium (AlMg.sub.-- alloy, plated with a layer of amorphous 
nickel-phosphorus (NiP). Alternative substrates include glass, ceramic and 
glass-ceramic materials, silicon, plastic, as well as graphite. There are 
typically sequentially sputter deposited on each side of substrate 10 and 
adhesion enhancement layer 11, 11', e.g., chromium (Cr) or a Cr alloy, a 
seedlayer 12, 12', such as NiP, an underlayer 13, 13' such as Cr or a Cr 
alloy, a magnetic layer 14, 14', such as cobalt (Co)-based alloy, and a 
protective overcoat 15, 15', such as a carbon-containing overcoat. 
Typically, although not shown for illustrative convenience, a lubricant 
topcoat is applied on the protective overcoat 15, 15'. 
It is recognized that the magnetic properties, such as Hr, Mr, S* and SNR, 
which are critical to the performance of a magnetic alloy film, depend 
primarily upon the microstructure of the magnetic layer which, in turn, is 
influenced by the underlying layers, such as the underlayer. It is 
recognized that underlayers having a fine grain structure are highly 
desirable, particular for growing fine grains of hexagonal close packed 
(HCP) Co alloys deposited thereon. 
It has been reported that nickel-aluminum (NiAl) films exhibit a grain size 
which is smaller than similarly deposited Cr films which are the 
underlayer of choice in conventional magnetic recording media. Li-Lien Lee 
et al., "NiAl Underlayers For CoCrTa Magnetic Thin Films", IEEE 
Transactions on Magnetics, Vol. 30, No. 6, pp. 3951-3953, 1994. 
Accordingly, NiAl thin films are potential candidates as underlayers for 
magnetic recording media for high density longitudinal magnetic recording. 
However, it was found that the coercivity of a magnetic recording medium 
comprising an NiAl underlayer is too low for high density recording, e.g. 
about 2,000 Oersteds (Oe). 
Lee et al. subsequently reported that the coercivity of a magnetic 
recording medium comprising a NiAl underlayer can be significantly 
enhanced by depositing a plurality of underlayers containing alternative 
NiAl and Cr layers rather than a single NiAl underlayer. Li-Lien Lee et 
al., "Effects of Cr Intermediate Layers on CoCrPt Thin Film Media on NiAl 
Underlayers," Vol. 31, No. 6, November 1995, pp. 2728-2730. It was found, 
however, that such a magnetic recording medium is characterized by an 
underlayer structure exhibiting a (110)-dominant crystallographic 
orientation which does not induce the preferred (1120)-dominant 
crystallographic orientation in the subsequently deposited Co alloy 
magnetic layer and is believed to contribute to increased media noise. 
Li-Lien Lee et al. were able to obtain an underlayer exhibiting a 
(200)-dominant crystallographic orientation by initially depositing a Cr 
sub-underlayer directly on the non-magnetic substrate at an undesirably 
high temperature of about 260.degree. C. using radio frequency (RF) 
sputtering. However, deposition of a Cr sub-underlayer at such an elevated 
temperature undesirably results in large grains, which is inconsistent 
with the reason for employing NiAl as an underlayer. On the other hand, it 
is very difficult to obtain a Cr (200)-dominant crystallographic 
orientation, even at elevated temperature such as 260.degree. C., on 
glass, ceramic and glass ceramic substrates using direct current (DC) 
magnetron sputtering, which is widely employed in the magnetic recording 
media industry. 
Li-Lien Lee et al. recognized the undesirability of resorting to high 
deposition temperatures to obtain a (200)-dominant crystallographic 
orientation in the underlayer structure. It was subsequently reported that 
an underlayer structure exhibiting a (200)-dominant crystallographic 
orientation was obtained by depositing a magnesium oxide (MgO) seedlayer 
using radio frequency (RF) sputtering. Li-Lien Lee et al., "Seed layer 
induced (002) crystallographic texture in NiAl underlayers," J. Appl. 
Phys. 79 (8), 15 April 1996, pp. 4902-4904; and David E. Laughlin et al., 
"The Control and Characterization of the Crystallographic Texture of the 
Longitudinal Thin Film Recording Media," IEEE Transactions on Magnetics, 
Vol. 32, No. 5, September 1996, pp. 3632-3637. Such a magnetic recording 
medium, however is not commercially viable from an economic standpoint, 
because sputtering systems in place throughout the industry making 
magnetic recording media are based upon direct current (DC) sputtering. 
Accordingly, RF sputtering an MgO seedlayer is not economically viable. 
The use of an NiAl underlayer is also disclosed by C.A. Ross et al., "The 
Role Of An NiAl Underlayer In Longitudinal Thin Film Media" and J. Appl. 
Phys. 81(a), P.4369, 1996. 
Various efforts have been made to optimize the magnetic properties of a 
magnetic recording medium by achieving a desirable crystallographic 
structure in a magnetic film employed to store information. These efforts 
involve the use of different materials for the seedlayer, underlayer or 
buffer layer, as well as varying sputtering parameters, including the 
substrate temperature, sputtering power density, substrate bias, film 
thickness, sputtering gas environment and sputtering pressure. In order to 
achieve a strong in-plane magnetic anisotropy with high Hr and high 
recording signal, it is necessary to form the easy magnetic axis of the 
magnetic layer so that it is substantially aligned in the film plane. 
As the demand for higher areal recording density increases, the thickness 
of the magnetic film employed in the magnetic recording medium decreases. 
However, there is a superparamagnetic limit where the grain size of the 
magnetic layer becomes less thermally stable with a reduction in grain 
size. Consequently, a small thermal agitation will deteriorate the stored 
magnetic information. Moreover, even before the superparamagnetic limit is 
reached, as the film thickness is reduced, additional factors negatively 
impact magnetic coupling, such as the smaller grain size and the non 
uniformity of the films. These factors all reduce in-plane magnetic 
anisotropy and, consequently, reduce Hr within a certain Mrt range. 
Currently, for most of the materials, the Mrt range at which a dramatic 
decrease in Hr is observed is about 0.5 memu/cm.sup.2. In fact, it is 
typically found that at for most, using current magnetic recording medium 
manufacturing process, the Hr is reduced by up to about 50% of its maximum 
value at an Mrt of about 0.4 memu/cm.sup.2. 
There exists a need for magnetic recording media having high in-plane 
anisotropy at a low film thickness. There exists a particular need for 
magnetic recording media suitable for high areal recording density 
exhibiting a high Hr at a Mrt less than about 0.5 memu/cm.sup.2. 
DISCLOSURE OF THE INVENTION 
An object of the present invention is a magnetic recording medium for high 
areal recording density exhibiting high in-plane magnetic anisotropy, high 
Hr and low noise at a low Mrt. 
Additional objects, advantages and other features of the present invention 
will be set forth in part in the description which follows and in part 
will become apparent to those having ordinary skill in the art upon 
examination of the following only to be learned from the practice of the 
present invention. The objects and advantages of the present invention may 
be realized and obtained as particularly pointed out in the appended 
claims. 
According to the present invention, the foregoing and other objects are 
achieved by a magnetic recording medium comprising a non-magnetic 
substrate; a nickel aluminum (NiAl) seedlayer on the substrate; a chromium 
manganese (CrMn) underlayer on the seedlayer; and a magnetic layer on the 
underlayer. 
Additional advantages of the present invention will become readily apparent 
to those skilled in this art from the following detailed description, 
wherein only the preferred embodiment of the present invention is shown 
and described, simply by way of illustration of the best mode contemplated 
for carrying out the present invention. As will be realized, the present 
invention is capable of other and different embodiments, and its details 
are capable of modifications in various obvious respects, all without 
departing from the present invention. Accordingly, the drawings and 
description are to be regarded as illustrative in nature, not as 
restrictive.

DESCRIPTION OF THE INVENTION 
The present invention provides magnetic recording media exhibiting high 
in-plane anisotropy at significantly reduced magnetic film thicknesses 
vis-a-vis conventional magnetic recording media. The present invention 
enables the manufacture of magnetic recording media for high areal 
recording density exhibiting high in-plane magnetic anisotropy at Mrt 
values significantly less than 0.5 memu/cm.sup.2, e.g. at Mrt values of 
about 0.26 memu/cm.sup.2. Thus, magnetic recording media in accordance 
with the present invention exhibit a higher Hr than conventional magnetic 
recording media for low Mrt values. Magnetic recording media in accordance 
with the present invention also exhibit a high S*, high SNR, a narrow 
pulse width and high overwrite. These objectives are achieved in 
accordance with embodiments of the present invention by providing a 
seedlayer-underlayer structure comprising a NiAl seedlayer and a chromium 
manganese (CrMn) underlayer. 
In embodiments of the present invention, the NiAl underlayer typically 
comprises about 40 to about 60 at. % Al while the CrMn underlayer 
typically comprises about 10 to about 50 at. % Mn. The NiAl underlayer 
typically has a thickness of about 10 .ANG. to about 1000 .ANG., e.g. 
about 100 .ANG. to about 500 .ANG.. The CrMn underlayer typically has a 
thickness of about 10 .ANG. to about 500 .ANG., e.g. about 50 .ANG. to 
about 200 .ANG.. 
The magnetic layer employed in accordance with the present invention can 
comprise any magnetic material employed in the manufacture of conventional 
magnetic recording media, such as Co alloys. Suitable Co alloys for use in 
the present invention include cobalt-chromium-platinum-tantalum 
(CoCrPtTa), CoCrTa and CoCrPt. The magnetic layer can advantageously be 
deposited at a low film thickness of about 10 .ANG. to about 300 .ANG., 
e.g. about 50 .ANG. to about 100 .ANG.. 
Magnetic recording media in accordance with the present invention can be 
provided on a non-magnetic substrate comprising any of the non-magnetic 
substrate materials employed in manufacturing conventional magnetic 
recording media, such as NiP-plated Al or Al alloys, glass, ceramic, or 
glass-ceramic materials. Conventional carbon-containing protective 
overcoats and lubricant top coats are also employed in producing magnetic 
recording media of the present invention. The present comprising a NiAl 
seedlayer and CrMn underlayer can, therefore, take the form of the 
magnetic recording medium depicted in FIG. 1 wherein seedlayer 12, 12' 
comprises NiAl and underlayer 13, 13' comprises CrMn. Another embodiment 
of the present invention is schematically illustrated in FIG. 2 and 
comprises substrate 20, seedlayer 21, 21' comprising NiAl, underlayer 22, 
22' comprising CrMn, magnetic layer 23, 23', carbon-containing protective 
overcoat 24, 24' and a lubricant topcoat (not shown). In this embodiment, 
an optional adhesion promoting layer has been omitted. 
EXAMPLES 
Example 1 
Six magnetic recording media were formed, each comprising a NiP-plated Al 
substrate having an average surface roughness (Ra) of 6 .ANG.. A NiAl 
seedlayer comprising 50 at. % Al was sputter deposited on the substrate 
and a CrMn underlayer containing 20 at. % Mn was sputter deposited on the 
NiAl seedlayer. Sputtering was conducted in a DC magnatron sputtering 
apparatus. The base pressure was maintained at about 10.sup.-7 Torr. The 
substrate was heated to in excess of 100.degree. C. and the sputtering 
pressure was maintained in the range of about 5 to about 15 mTorr. A 
CoCrPtTa alloy layer of varying thicknesses between about 50 .ANG. to 
about 300 .ANG. containing 16 at. % Cr, 5 at. % Pt and 4 at. % Ta, was 
deposited on the CrMn underlayer. Each magnetic recording medium contained 
the same film structure and was produced under substantially the same 
conditions except that the Mrt of the magnetic layer was varied and, 
therefore, the magnetic layer thickness was varied. The magnetic 
properties of the samples were tested on a calibrated non-destructive 
rotating disk magnetometer. The recording signal and medium noise was 
measured at 240 kfci (kiloflux reversal per inch) linear density using a 
Guzik tester with a MR (magnetoresistive) head having a gap length of 
about 0.5 .mu.m and flying at a height of 1.1 micro inch. The results are 
reported in FIGS. 3A-3C, showing the Hr, S* and SNR, respectively, as a 
function of Mrt. 
Example 2 
Six additional media were prepared having the same structure and the same 
manner as those in Example 1, except that the magnetic layer was a 
CoCrPtTa alloy containing 15 at. % Cr, 8 at. % Pt and 4 at. % Ta. The 
results showing the Hr, S* and SNR, respectively, are reported in FIGS. 
4A-4C. 
It is apparent from FIGS. 3A-3C and 4A-4C that the use of a NiAl/CrMn 
seedlayer underlayer structure in accordance with the present invention 
achieved a very high Hr at a very low magnetic film thickness range, with 
an Mrt value at 0.26 memu/cm.sup.2. In addition, the S* was maintained 
above 0.8, thereby indicating strong magnetic coupling in the extremely 
thin films. Additionally, the SNR was desirably high. 
In Table I below, Sample 1 is the magnetic recording medium of Example I 
which had the lowest Mrt value, while Sample 2 is the medium of Example 2 
which had the lowest Mrt value. As seen in Table 1, the SNR values were 
26.9 dB and 26.4 db, respectively, while the PW50 (pulse width) values 
were quite narrow. Moreover, the OW (overwrite) values for both samples 
exceeded 36 dB, thereby indicating superior recording performance. 
TABLE I 
______________________________________ 
SNR PW50 OW 
Sample 
Hr(Oe) Mrt(memu/cm.sup.2) 
S* (dB) (uin) (dB) 
______________________________________ 
1 2790 0.26 0.84 26.9 9.4 39.9 
2 2976 0.26 0.83 26.4 9.2 36.9 
______________________________________ 
The demagnetization transition width is proportional to the ratio of 
Mrt/Hr. Accordingly, in order to achieve a higher recording density, 
magnetic recording media having a low Mrt/Hr ratio are desirable while 
maintaining other appropriate magnetic properties. The results reported in 
FIGS. 3A-3C and 4A-4C illustrate that magnetic recording media in 
accordance with the present invention exhibit desirably low Mrt/Hr ratios, 
while maintaining strong magnetic coupling and achieving superior 
reading/writing performance. Accordingly, magnetic recording media in 
accordance with the present invention are suitable for ultra high density 
recording. The present invention enables the manufacture of magnetic 
recording media having a Hr in excess of 2900 Oe while maintaining a Mrt 
value less than 0.30 memu/cm.sup.2. 
The present invention can be employed to produce any of various types of 
magnetic recording media, particularly thin film disks. The present 
invention is particularly applicable to producing high areal recording 
density magnetic recording media requiring a low flying height and 
exhibiting a low Mrt/Hr ratio, low media noise and high S*. 
Only the preferred embodiment of the present invention and but of a few 
examples of its versatility are shown and described in the present 
disclosure. It is to be understood that the present invention is capable 
of various other combinations and environments and is capable of changes 
and modifications within the scope of the inventive concept as expressed 
herein.