Longitudal magnetic recording medium having a multi-layered underlayer and magnetic storage apparatus using such magnetic recording medium

A longitudinal magnetic recording medium and a magnetic storage apparatus using the longitudinal magnetic recording medium. The recording medium has a multi-layered underlayer and a magnetic layer formed on it. The underlayer has a first layer of a body-centered cubic structure, a second layer of a hexagonal-close packed structure formed on the first layer, and a third layer of a bcc structure formed on the second layer. The underlayer may further have a fourth layer of an hcp structure layer formed on the third layer.

BACKGROUND OF THE INVENTION 
The present invention relates to magnetic storage apparatus and 
longitudinal magnetic recording media and more particularly, to a 
longitudinal magnetic recording medium which is low in media noise and is 
less susceptible to thermal fluctuation as well as to a magnetic storage 
apparatus which has a recording density as high as, e.g., 3 giga-bits or 
more per inch.sup.2. 
As computers having higher performances are developed, the quantity of 
information to be treated such as image data has been steadily increased. 
This also has required a larger capacity of a magnetic disk device as an 
external storage device. At present, a recording density of several 
hundreds of mega-bytes per inch.sup.2 is realized. As a magnetic head for 
such a high-density magnetic disk device, there has been recently employed 
an inductive/magneto-resistive composite head which is separated into 
recording and reproducing parts so that an inductive head is used as the 
recording part and a magneto-resistive (MR) head is used as the 
reproducing part. The magneto-resistive head is higher in reproducing 
sensitivity than the prior art inductive head, so that, even when the size 
of recorded bits is much reduced and leakage flux is decreased from the 
medium, a sufficient reproducing or read output can be obtained. Further, 
there has been developed a spin-valve type giant magneto-resistive head 
having a much increased reproducing sensitivity. 
A longitudinal magnetic recording medium now commercially available is made 
up of a magnetic layer of a Co alloy such as CoNiCr, CoCrTa or CoCrPt and 
a Cr underlayer for control of crystallographic orientation of the 
magnetic layer. Since the Co alloy magnetic layer has a hexagonal 
close-packed (hcp) structure having a c axis as an axis of easy 
magnetization, when the magnetic layer is used for a longitudinal magnetic 
recording medium, it is desirable to align the c axis to be in-plane. To 
this end, a technique has been employed such that a Cr underlayer having a 
body-centered cubic (bcc) structure is first formed on a substrate, and a 
Co alloy magnetic layer is epitaxially grown on the Cr underlayer so that 
the c axis is oriented to be in-plane. When CoCrPt alloy is employed for 
the magnetic layer, a technique has been proposed in which Ti or V is 
added to a Cr underlayer to increase a lattice spacing and to improve a 
lattice matching between the underlayer and magnetic layer, thus orienting 
the c axis more closely to be in-plane (refer to JP-A-63-197018, laid open 
on Aug. 15, 1988 and JP-A-62-257618 laid open on Nov. 10, 1987). 
When a MR head is used as a reproducing head, not only a signal but also 
noise from the longitudinal recording magnetic medium would be picked up 
with high sensitivity, for which reason the longitudinal magnetic 
recording medium is required to be lower in noise than the prior art. The 
media noise results mainly from a region (magnetization transition region) 
in which magnetization is disturbed between recording bits. Thus narrowing 
the region leads to reduction in the media noise. As is well known, this 
is effectively achieved by making fine magnetic crystal grains or magnetic 
grains of the magnetic layer and weakening interaction between the 
particles to thereby make magnetization reversal size small. As has been 
mentioned above, since an epitaxial relationship is established between 
the magnetic layer and underlayer, magnetic grains can be made fine by 
making fine particles of the underlayer. Methods for making the underlayer 
particles fine are considered to include a method for making the thickness 
of the underlayer small and a method for adding doping elements to the 
underlayer. 
These methods, however, have problems that it is difficult to keep desired 
crystallographic orientation and to form a film having an excellent 
crystalline structure, and further it is hard to establish a good 
epitaxial relationship between the underlayer and the magnetic layer. 
There occurs another problem of thermally unstable magnetization involved 
by excessive fine magnetic grains, which results in that a recorded signal 
decreases with time. An important factor of fabricating a microstructure 
desirable for high-density recording is to make fine particles and also to 
make dispersion of particle sizes small to thereby suppress formation of 
particles which are too fine to be immune to the influence of thermal 
fluctuation. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a magnetic storage 
apparatus which is high in recording density and reliability and also to 
provide a longitudinal magnetic recording medium for use in the magnetic 
storage apparatus, which medium is low in media noise and less susceptible 
to thermal fluctuation. 
According to one aspect of the present invention, a longitudinal magnetic 
recording medium has a multi-layered underlayer and a magnetic layer 
formed thereon, wherein the multi-layered underlayer includes a first 
layer substantially having a body-centered cubic (bcc) structure, a second 
layer substantially having a hexagonal-closed pack (hcp) structure formed 
on the first layer, and a third layer substantially having a bcc structure 
formed on the second layer. 
According to another aspect of the present invention, a longitudinal 
recording magnetic storage apparatus has a magnetic recording disk, a 
magnetic head associated with the disk, a signal processing circuit 
connected with the magnetic head, a first driver for rotating the disk and 
a second driver for actuating the magnetic head for recording/reproducing 
a signal on the disk, wherein the recording disk comprises a multi-layered 
underlayer and a magnetic layer formed thereon, the multi-layered 
underlayer including a first layer substantially having a body-centered 
cubic (bcc) structure, a second layer substantially having a 
hexagonal-closed pack (hcp) structure formed on the first layer, and a 
third layer substantially having a bcc structure formed on the second 
layer. 
Thus, the multi-layered underlayer includes at least one bcc-hcp layer pair 
having a bcc structure layer and a hcp structure layer formed on the bcc 
structure layer. The hcp structure layer in the bcc-hcp layer pair is 
preferably non-magnetic, but may be magnetic to such an extent that the 
hcp structure should not exert an ill effect on magnetization of the 
magnetic layer. 
Each of the bcc structure layers is preferably (100)-oriented, while each 
of the hcp structure layers is preferably (11.0)-oriented. 
Description will now be made of knowledge obtained by inventors of the 
present application through their experiments and researches, on which the 
present invention is based. 
The inventors prepared a longitudinal magnetic recording medium by forming 
a magnetic layer on a multi-layered underlayer of a 3-layer structure in 
which a layer substantially having an hcp structure is sandwiched by 
layers substantially having a bcc structure on a substrate. 
More specifically, a longitudinal magnetic recording medium, as shown in 
FIG. 1, is made up of a multi-layered underlayer 12, a magnetic layer 13 
and a protective layer 14 sequentially formed on a substrate 11, e.g., by 
a sputtering process. The multi-layered underlayer 12, as shown in FIG. 2, 
has a 3-layer structure in which a layer (which will be referred to merely 
as the hcp layer, hereinafter) 22 having a hexagonal close-packed (hcp) 
structure is sandwiched by a layer (which will be referred to merely as 
the bcc layer, hereinafter) 21 having a body-centered cubic (bcc) 
structure and another bcc layer 23. 
It has been experimentally found, when there is employed such a 
multi-layered underlayer that is made of the bcc layer 21 (100)-oriented, 
the hcp layer 22 (11.0)-oriented on the bcc layer 21, and the bcc layer 23 
(100)-oriented on the hcp layer 22 by an epitaxial growth process, the 
particle size of the magnetic layer can be made fine. This is considered 
to result from the fact that, as reported in J. Appl. Phys., Vol. 73, 
(10), May 15, 1993, the magnetic layer has a so-called bi-crystal 
structure in which a plurality of hcp particles (11.0)-oriented are grown 
on a single bcc particle so that their c axes are perpendicular to each 
other. That is, reduction of the crystal grain size takes place at an 
interface between the bcc and hcp layers 21 and 22 and also takes place at 
an interface between the bcc layer 23 and magnetic layer 13. 
Further, as shown in FIG. 3, when an additional hcp layer 31 is formed on 
the bcc layer 23 as the multi-layered underlayer 12 so that the magnetic 
layer 13 is formed on the hcp layer 31, reduction of the crystal grain 
size also takes place at an interface between the bcc layer 23 and hcp 
layer 31. It is generally considered that the larger the size of the bcc 
grains is, the more easily the grains tend to have a bi-crystal structure, 
though it also depends on the used film forming process. For this reason, 
a relatively large grain is divided into finer grains, but it is hard for 
an originally fine grain to be further divided into excessively fine 
grains, so that the dispersion of the grain size can be suppressed. 
Further, since the hcp layer 31 has the same hcp crystalline structure as 
the magnetic layer 13, the hcp layer 31 also can act to improve the 
crystalline structure in the initial growth part of the magnetic layer. 
This enables reduction of deterioration of magnetic properties (such as 
coercivity, coercivity squareness, etc.) involved by the formation of the 
thin magnetic layer. 
With respect to the material of the multi-layered underlayer, in accordance 
with the lattice constant of the magnetic layer, the bcc layer is 
preferably made of one selected from the group consisting of Cr and 
chromium alloys such as Cr--Ti alloy, Cr--V alloy, Cr--Mo alloy and Cr--Ta 
alloy. The hcp layer is preferably made of one selected from the group 
consisting of CoCr alloys such as Co--Cr alloy, Co--Cr--Ta alloy, 
Co--Cr--Pt alloy and Co--Cr--Pt--Ta alloy. 
It is preferable that the thicknesses of the respective layers of the 
multi-layered underlayer be 50 nm or less in order to prevent increase in 
the grain size. 
The hcp layer of the multi-layered underlayer is desirably substantially 
non-magnetic. However, even when the hcp layer has weak magnetization, its 
influence exerted on magnetic properties of the magnetic layer can be 
suppressed by making the hcp layer thin. For example, when the underlayer 
has such a 3-layer structure as shown in FIG. 2, reduction in the 
coercivity of the medium can be prevented by setting a product 
(Br1.times.t1) of a thickness t1 and a residual magnetic flux density Br1 
of the hcp layer 22 to be not larger than 20% of a product (Br2.times.t2) 
of a thickness t2 and a residual magnetic flux density Br2 of the magnetic 
layer 13, as shown in FIG. 4. 
It is preferable that the hcp layer of the multi-layered underlayer is set 
to have a Cr concentration not smaller than 30 at % and not larger than 40 
at % in order to weaken the magnetization of the hcp layer and obtain a 
good crystalline structure. 
Meanwhile, the magnetic layer is required to be made of a material having a 
large crystal magnetic anisotropy in order to suppress the influence of 
thermal fluctuation involved by the formation of fine grains. More in 
detail, it is preferable that the magnetic layer contains Co, Cr and Pt as 
main elements and its Pt concentration be 10 at % or more. 
The substrate should have an excellent surface smoothness. More 
specifically, an Al--Mg substrate having an NiP layer formed thereon, a 
glass substrate, an SiO2 substrate, an SiC substrate, a carbon substrate 
or the like can be used as the substrate.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The present invention will be detailed in connection with preferred 
embodiments which follow. 
Embodiment 1 
An Al--Mg substrate 11 having an outer diameter of 65 mm, an inner diameter 
of 20 mm and a thickness of 0.4 mm and further having an NiP layer formed 
on a surface of the substrate, was subjected to a DC magnetron sputtering 
process to sequentially form thereon a multi-layered underlayer 12, a 
magnetic layer 13 and protective layer 14 to thereby obtain a longitudinal 
magnetic recording medium, as shown in FIG. 1. More in detail, the 
multi-layered underlayer 12 was made up of a 10 nm-thick Cr layer (bcc 
layer) 21, a 10 nm-thick CoCr alloy layer (hcp layer) 22 and a 10 nm-thick 
CrTi alloy layer (bcc layer) 23, as shown in FIG. 2. The magnetic layer 13 
in FIG. 1 was a 15 nm-thick CoCrPt alloy layer having an hcp structure, 
and the protective layer 14 was a carbon layer. The above film formation 
was carried out under conditions that Argon gas partial pressure is 6 
mTorr, an input power is 1 kW and a substrate temperature is 300.degree. 
C. 
The respective layers of the recording medium in accordance with the 
present embodiment were evaluated with respect to their crystallographic 
orientation by an X-ray diffraction analysis, which results are shown in 
FIG. 5. It has been found that the bcc Cr layer 21 is (100)-oriented, the 
hcp CoCr alloy layer 22 grown on the layer 21 is (11.0)-oriented, the bcc 
CrTi alloy layer 23 grown on the layer 22 is (100)-oriented, and the hcp 
CoCrPt magnetic layer 14 grown on the layer 23 is (11.0)-oriented. 
When the Cr concentration in the CoCr alloy layer 22 in the underlayer is 
varied in a range from 25 at % to 45 at %, it has been found that the CoCr 
alloy layer 22 is magnetized as weakly as about 150 emu/cc at a Cr 
concentration of 25 at %. In general, when the CoCr alloy is a bulk, the 
alloy becomes non-magnetic at a Cr concentration of about 25 at % or more. 
However, when the film forming temperature is increased to about 
300.degree. C. to form a CoCr alloy thin film, the alloy film becomes 
magnetized. This is considered to be because the alloy is separated into a 
phase (ferromagnetic) with low Cr concentration and a phase (non-magnetic) 
with high Cr concentration. It has also been found that, when the Cr 
concentration is set at 45 at % or more, the crystal structure of the CoCr 
alloy layer cannot have an hcp single phase and thus cannot satisfy an 
epitaxial relationship with the upper and lower layers. 
As a result of evaluating read/write characteristics of the recording 
medium according to the present embodiment, good read/write 
characteristics were obtained, in particular, when the Cr concentration is 
not smaller than 30 at % and not larger than 40 at %. When reading and 
writing operations were carried out by use of a magneto-resistive head 
with a linear recording density of 260 kFCI and with a longitudinal 
magnetic recording medium having the CoCr alloy layer with 35 at % of Cr 
concentration, the medium had a signal-to-noise ratio of 1.8. In the case 
of a longitudinal magnetic recording medium using the CoCr alloy layer 
having a Cr concentration less than 30 at % or higher than 40 at %, a 
product (Br.times.t) with the residual magnetic flux density Br becomes 
large or the crystallographic orientation of the magnetic layer will be 
insufficient, so that the medium had a low signal-to-noise ratio of 1.3 or 
less. 
It has been obvious from the above examinations that, in order to suppress 
the magnetization of the underlayer which affects magnetic properties of 
the magnetic layer and also to realize good epitaxial growth, the Cr 
concentration of the CoCr alloy layer is required to be set at a level not 
smaller than 30 at % and not larger than 40 at %. 
Embodiment 2 
An Al--Mg substrate 11 having an outer diameter of 65 mm, an inner diameter 
of 20 mm and a thickness of 0.4 mm and further having an NiP layer formed 
on a surface of the substrate, was subjected to a DC magnetron sputtering 
process to sequentially form thereon a multi-layered underlayer 12, a 
magnetic layer 13 and protective layer 14 to thereby obtain a longitudinal 
recording magnetic medium, as shown in FIG. 1. More in detail, the 
multi-layered underlayer 12 was made up of a 10 nm-thick Cr layer (bcc 
layer) 21, a 10 nm-thick CoCr alloy layer (hcp layer) 22, a 10 nm-thick 
CrTi alloy layer (bcc layer) 23 and a 10 nm-thick CoCr alloy layer (hcp 
layer) 31, as shown in FIG. 2. The magnetic layer 13 in FIG. 1 was a 15 
nm-thick CoCrPt alloy layer having an hcp structure, and the protective 
layer 14 was a carbon layer. The above film formation conditions were the 
same as in the embodiment 1. 
FIG. 6 shows a relationship between the thickness of the magnetic layer in 
the recording medium of the present embodiment and coercivity Hc measured 
with a magnetic field applied in a relative running direction of a 
magnetic head to the medium in a write mode, together with measurement 
results of the longitudinal magnetic recording medium of the embodiment 1. 
In the case of the longitudinal magnetic recording medium of embodiment 2, 
it is seen that, when the thickness of the magnetic layer is made small, 
reduction in the coercive force is smaller than that of the longitudinal 
magnetic recording medium of embodiment 1. This is considered to be 
because the uppermost layer of the multi-layered underlayer has the same 
hcp structure as the magnetic layer, thus improving the crystalline 
structure of the initial growth part of the magnetic layer. In this way, 
it has become clear that, when it is desired to make the magnetic layer 
thin, it is effective for the uppermost layer of the multi-layered 
underlayer to have an hcp structure. It has also been found that, when a 
Pt concentration of the magnetic layer 13 is high, a similar result can be 
obtained by adding Pt, Ta or the like to the CoCr alloy layer 31 in the 
multi-layered underlayer to improve a lattice matching between the 
magnetic layer and underlayer. 
Embodiment 3 
A plan view of a magnetic storage apparatus in accordance with an 
embodiment of the present invention as well as a vertical cross-sectional 
view of the apparatus are shown in FIGS. 7a and 7b. More in detail, FIG. 
7b is a cross-sectional view taken along line VIIB--VIIB in FIG. 7a. The 
magnetic storage apparatus includes a longitudinal magnetic recording 
medium 71, a driver 72 for rotation-driving the medium, a magnetic head 
73, a driver 74 for driving the magnetic head, and a read/write signal 
processing circuit 75. 
Shown in FIG. 8 is a structure of the magnetic head used in the magnetic 
storage apparatus. This magnetic head is of a composite type which is a 
combination of a reading inductive head formed on a magnetic head slider 
base 87 and a reproducing magneto-resistive head. The recording inductive 
head section is of a thin film type which has a pair of recording magnetic 
poles 81 and 82 and a coil 83 intersected therewith. A layer gap between 
the recording magnetic poles was set to be 0.3 .mu.m. The magnetic pole 82 
also functions as a magnetically shielding layer. More in detail, the 
magnetic pole 82 forms a pair with a magnetic shield layer 86, both of 
which have a thickness of 1 .mu.m and function also to magnetically shield 
the reproducing head. A distance between the shield layers is 0.25 .mu.m 
(FIG. 8 being not to scale for clarification of the structure). The 
reproducing magneto-resistive head section has a magneto-resistive sensor 
84 and conductor layers 85 as electrodes. 
FIG. 9 shows a vertical cross-sectional view of a structure of the 
magneto-resistive sensor. A signal detection region 91 of the 
magneto-resistive sensor includes a transversal biasing layer 93, a 
separating layer 94 and a magneto-resistive ferromagnetic layer 95 
sequentially formed in this order on a gap layer 92 of Al oxide. The 
magneto-resistive ferromagnetic layer 95 was made of NiFe alloy and 20 nm 
thick. The transversal biasing layer 93 was made of NiFeNb and 25 nm 
thick. However, the transversal biasing layer 93 may be made of any alloy, 
so long as the alloy is of a ferromagnetic type which is relatively high 
in electric resistance and good in soft magnetic properties. The 
transversal biasing layer 93 is magnetized by a magnetic field induced by 
a sense current flowing through the magneto-resistive ferromagnetic layer 
95 in a film in-plane direction (transversal direction) perpendicular to 
the current to apply a transversal biasing magnetic field to the 
magneto-resistive ferromagnetic layer 95. Thus, this forms a magnetic 
sensor which can provide a linear reproduction output with respect to a 
leakage magnetic field from the medium. The separating layer 94 for 
preventing branching of the sense current from the magneto-resistive 
ferromagnetic layer 95 was made of Ta that is relatively high in electric 
resistance and 5 nm thick. The signal detection region 91 has tapered 
regions 96 provided at its both ends. The tapered regions 96 include 
permanent magnet layers 97 for converting the magneto-resistive 
ferromagnetic layer 95 to a single magnetic domain and a pair of 
electrodes 98 formed on the permanent magnet layer 97 for extracting a 
reproduced signal thereon. Since the permanent magnet layer 97 is required 
to be high in coercivity and not to be easily changed in magnetization 
direction, for which reason the permanent magnet layer 97 is made of alloy 
such as CoCr or CoCrPt. The longitudinal magnetic recording medium 71 (see 
FIG. 7) employed was such as described in embodiment 1 and had a 
coercivity Hc of 3.2 kOe. 
When read/write characteristics were evaluated by use of the magnetic 
recording system of the present embodiment under conditions of a head 
flying height of 30 nm, a linear recording density of 260 kFCI and a track 
density of 13 kTPI, a signal-to-noise ratio of 1.8 were obtained. Namely, 
a sufficiently high level read output was obtained with such a large 
signal-to-noise ratio. Further, when an input signal of the magnetic head 
was subjected to an 8/9 coding operation, the recording/reproducing 
operation was possible with a recording density of 3 Giga-bits per 
inch.sup.2. In addition, after 50,000 cycles of head seek test from inner 
circumference to outer one, the number of bit errors was 10 bits or less 
per surface and a mean time interval between failures was 300,000 hours. 
Embodiment 4 
In a magnetic storage apparatus having structures similar to those shown in 
FIGS. 7a, 7b and 8 in embodiment 3, it is preferable that the 
magneto-resistive sensor 84 be of such a spin-valve type as shown in FIG. 
10, because it provides a larger output. A signal detection region 101 of 
the magneto-resistive sensor has such a structure that a 3 nm-thick Ta 
buffer layer 103, a 7 nm-thick first magnetic layer 104, a 1.5 nm-thick Cu 
intermediate layer 105, a 3 nm-thick second magnetic layer 106 and 10 
nm-thick, Fe-20 at % Mn anti-ferromagnetic alloy layer 107 are 
sequentially formed in this order on a gap layer 102 made of Al oxide. The 
first magnetic layer 104 was made of a Ni-20 at % Fe alloy and the second 
magnetic layer 106 was made of Co. Exchange field from the 
anti-ferromagnetic alloy layer 107 causes the magnetization of the second 
magnetic layer 106 to be fixed in one direction. The magnetization 
direction of the first magnetic layer 104 adjacent to the second magnetic 
layer 106 via the non-magnetic intermediate layer 105, on the other hand, 
is varied by a leakage magnetic field from the longitudinal magnetic 
recording medium. Relative change in the magnetization direction between 
the two magnetic layers causes a change in the total resistance of the 3 
layers. This phenomenon is known as spin-valve effect. In the present 
embodiment, the magneto-resistive sensor utilizes the spin-valve effect. 
Tapered regions 108 made up of a permanent magnet layer 109 and a pair of 
electrodes 110 are similar to those in the magneto-resistive sensor 
explained in connection with the embodiment 2. The longitudinal magnetic 
recording medium 71 (see FIG. 7) employed was such as described in 
embodiment 2 and had a coercivity Hc of 3.2 kOe. 
When recording/reproducing characteristics were evaluated by use of the 
magnetic storage apparatus of the present embodiment under conditions of a 
head flying height of 30 nm, a linear recording density of 260 kFCI and a 
track density of 13 kTPI, a signal-to-noise ratio of 2.0 was obtained. 
Namely, a sufficiently high level read output was obtained with such a 
large signal-to-noise ratio. Further, when an input signal to the magnetic 
head was subjected to an 8/9 coding operation, the recording/reproducing 
operation was possible at a recording density of 3 Giga-bits per 
inch.sup.2 in a temperature range of 10 to 50.degree. C. In addition, 
after 50,000 cycles of head seek test from inner circumference to outer 
one, the number of bit errors was 10 bits or less per surface and a mean 
time interval between failures was 300,000 hours. 
In accordance with the aforementioned embodiments, since magnetic grains 
can be made fine and dispersion of the particles can be made small, a 
signal-to-noise ratio can be made high. As a result, when a 
high-sensitivity magneto-resistive head is used, there can be provided a 
small-size, large-capacity magnetic storage apparatus which has a 
recording density of 3 Gigabits or more per inch.sup.2, a high 
signal-to-noise ratio, a low error rate, and a mean time interval between 
failures of 300,000 hours or more.