Magnetic recording medium comprising protrusion on its surface and a plasma treated substrate and recording/reproducing method therefor

A magnetic recording medium comprising a ferromagnetic metal thin film layer formed on one surface of a flexible non-magnetic substrate and primarily comprising cobalt and oxygen is improved in durability by treating the substrate with a plasma at a frequency of 10 kHz to 200 kHz, and providing the magnetic layer on the surface with protrusions having a height of 30 to 300 .ANG. at an average density of at least 10.sup.5 /a.sup.2 per square millimeter of the surface, provided that the magnetic recording medium is passed for recording/reproducing operation across a magnetic head with a gap having a distance a as expressed n .mu.m.

BACKGROUND OF THE INVENTION 
This invention relates to magnetic recording media, and more particularly, 
to magnetic recording media of metal thin film type, and a method for 
conducting recording/reproducing operation in such media. 
Among magnetic recording media for use in video, audio and other 
applications, active research and development works have been made on 
magnetic recording media, usually magnetic tapes having a magnetic layer 
in the form of a continuous thin film because of the compactness of a roll 
of tape. 
The preferred magnetic layers for such continuous metal film type media are 
deposited films of Co, Co-Ni, Co-O, Co-Ni-O and similar systems formed by 
the so-called oblique incidence evaporation process in which cobalt and 
optional elements are evaporated in vacuum and directed at a given angle 
with respect to the normal to the substrate because such evaporated films 
exhibit superior characteristics. 
These magnetic recording media, particularly for use as magnetic tape and 
magnetic discs, must fulfil a number of requirements including low dynamic 
coefficient of friction, smooth and stable travel performance for a 
prolonged period, improved wear resistance, stability under storage 
environment to ensure consistent reproduction, and durability (durability 
of tape both during normal operation and in the still mode). 
A variety of pre-treatments have heretofore been made on various base films 
or substrates for the purpose of improving durability. Such pre-treatments 
include treatments with chemical solution, coating, corona discharge 
treatment, and the like. 
Chemical treatments may be treatments with acid and alkali. Among such 
chemical treatments most effective is by oxidizing the surface of a base 
film with a chemical solution of a strong acid and/or a strong oxidizing 
agent, for example, chromate solution, and introducing carbonyl or 
carboxyl radicals to etch the surface. The chemical treatments, however, 
require subsequent rinsing and drying of film surface and a great 
investment is needed for the treatment of spent liquid. Particularly, 
chromate treatment yields a spent liquid which must be severely treated 
for environmental pollution control, and its commercial utilization is now 
diminishing. 
The film coating technique is by coating a base film with an undercoat on 
which a magnetic film is formed. The interaction between a binder in the 
undercoat and the magnetic film is necessary. The composition of the 
undercoat must be selected to meet a particular binder and pigment used in 
the magnetic layer. The coating techniques reguire not only such a careful 
choice, but also coating and drying steps. Of course, the consumption of 
coating material leads to the increased cost of products. 
The corona discharge treatment is advantageous because of dry nature 
eliminating the need for additional steps of rinsing, drying, and disposal 
of spent liquid. Corona treatment has been carried out for many years and 
is effective in improving adhesion, wettability, and printability. The 
corona treatment, however, is not successful in improving the properties 
of magnetic recording media to such an extent as to fulfill the high 
performance which is imposed on the present day and future magnetic 
recording media. 
Another technique known in the art is a flame treatment which is difficult 
to apply to magnetic recording media which reguire a high degree of 
dimensional stability. 
Under these circumstances, a proposal is made to treat base films or 
substrates with a plasma. The plasma treatment is a one-step dry process 
and thus has the advantage that drying and disposal of spent solution are 
unnecessary and no extra material like binders is consumed. In addition, 
the plasma treatment enables high speed, continuous production so that it 
can be readily incorporated in the process of manufacturing magnetic 
recording media without sacrifying production speed and yield. 
One technique for plasma treatment of substrates is disclosed in Japanese 
Patent Publication No. 57-42889 (published on Sept. 11, 1982) wherein a 
treatment is effected with a plasma having a frequency in the range of 
radio frequency to microwave using a treating gas of air, oxygen, 
nitrogen, hydrogen, helium, argon, etc. The radio frequency of 13.56 MHz 
is only described in this publication. 
Also, Japanese Patent Application Kokai No. 58-77030 (laid open on May 10, 
1983) descloses a process of plasma treatment by applying an AC current at 
the commercial frequency between electrodes using a treating gas of 
oxygen, argon, helium, neon or nitrogen. These plasma treatments are 
somewhat successful in improving the adhesion of a treated base film to a 
magnetic layer and hence, the durability of magnetic recording media, but 
not fully satisfactory in bond strength and durability. 
These media should have a flat surface because of remarkable deterioration 
of their properties due to a spacing loss. However, as the surface becomes 
flatter, the friction becomes greater adversely affecting head contact and 
transport movement. 
Usually, the metal thin film type media have a magnetic layer as thin as 
0.05 to 0.5 .mu.m so that the surface property of the media depends on the 
surface property of the substrate. For example, Japanese Patent 
Application Kokai No. 53-116115 discloses the provision of gently sloping 
protrusions in the form of creases or wrinkles on the substrate surface. 
Also, Japanese Patent Application Kokai Nos. 58-68227 and 58-100221 
disclose the location of fine particles on the substrate surface, 
resulting in surface irregularities observable under an optical microscope 
with a magnifying power of 50 to 400 and actually measureable for height 
by means of a probe surface roughness meter (height 100 to 2000 .ANG.). 
These media are improved to a more or less extent in physical properties 
such as dynamic friction, runnability (the durability of tape which 
travels in firctional contact with rigid members in a recording machine), 
and moving stability as well as in electromagnetic properties. 
Various physical and electromagnetic properties of the ferromagnetic metal 
thin film layer can be further improved when the magnetic layer contains 
oxygen, and particularly when an oxide coating of ferromagnetic metal (Co 
and/or Ni) is formed at the magnetic layer surface, for example, by 
carrying out the formation of the ferromagnetic thin film layer in the 
presence of oxygen under a predetermined partial pressure. The 
above-mentioned gently sloping protrusions in the form of creases or 
wrinkles are less effective particularly when the oxide coating is formed 
at the magnetic layer surface. 
In Japanese Patent Application Kokai No. 58-68227, fine protrusions are 
distributed at a density of at most about 10.sup.6 per square millimeter. 
Video tape recorders utilize the minimum recording wavelength of less than 
1 .mu.m, for example, about 0.7 .mu.m. The magnetic layer having an oxide 
coating on its surface provides insufficient physical and electromagnetic 
properties at such a recording wavelength. 
Japanese Patent Application Kokai No. 58-100221 describes Examples 1 and 2 
where fine protrusions having a height of 300 to 500 .ANG. are distributed 
at a density of about 10.sup.4 to 10.sup.6 per square millimeter. These 
magnetic layers are regarded to be free of oxide at the surface and thus 
exhibit different travel durability behavior than magnetic layers having 
an oxide coating at the surface. The presence of an oxide layer on the 
magnetic layer surface requires that fine protrusions be distributed in 
optimum correlated size and density. Differently stated, prior art metal 
thin film type magnetic media are still not satisfactorily improved in 
durability, bond strength, and physical and electromagnetic properties. 
SUMMARY OF THE INVENTION 
It is, therefore, an object of the present invention to provide a novel and 
improved metal thin film type magnetic recording medium having a flexible 
substrate plasma treated so as to substantially improve durability and 
bond strength of the magnetic layer. 
It is another object of the present invention to provide a magnetic 
recording medium of such type having fine protrusions distributed in such 
size and population as to provide optimum physical and electromagnetic 
properties. 
It is a further object of the present invention to provide a method for 
conducting recording/reproducing operation on such a magnetic recording 
medium. 
According to a first aspect of the present invention, there is provided a 
magnetic recording medium comprising a flexible substrate having opposed 
major surfaces, and a ferromagnetic metal thin film layer on one surface 
of the substrate primarily comprising cobalt. The magnetic recording 
medium is used in combination with a magnetic head having a gap. According 
to the feature of the present invention, the substrate is plasma treated 
at a frequency in the range of 10 to 200 kilohertz. The metal thin film 
layer contains oxygen. Protrusions having a height of 30 to 300 .ANG. are 
distributed on the surface of the magnetic recording medium at an average 
density or population of at least 10.sup.5 /a.sup.2 per square millimeter 
of the surface where a is the distance of the magnetic head gap as 
expressed in .mu.m. 
According to a second aspect of the present invention, there is provided a 
method for conducting recording/reproducing operation on a magnetic 
recording medium comprising a flexible substrate having opposed major 
surfaces, and a ferromagnetic metal thin film layer on one surface of the 
substrate principally comprising cobalt, by passing the medium across a 
magnetic head having a gap. The feature of the present invention is that 
the ferromagnetic metal thin film layer contains oxygen and is formed on 
the flexible substrate which has been plasma treated at a frequency in the 
range of 10 to 200 kilohertz, and the medium has in average at least 
10.sup.5 /a.sup.2 protrusions per square millimeter of the surface, the 
protrusions having a height of 30 to 300 .ANG., where a is the distance of 
the magnetic head gap as expressed in .mu.m.

DETAILED DESCRIPTION OF THE INVENTION 
Referring to FIG. 1, there is illustrated a magnetic recording medium 
generally designated at 10 according to the present invention. The 
magnetic recording medium 10 includes a flexible substrate 11 having 
opposed major surfaces and a ferromagnetic metal thin film layer 12 formed 
on one major surface of substrate 11. The medium 10 has randomly 
distributed protrusions or bosses 16 on the surface, preferably on the 
magnetic layer surface. It is also contemplated in the present invention 
that a topcoat layer of any well-known composition is formed on the 
surface of metal thin film layer 12 and a backcoat layer of any well-known 
composition is formed on the other major surface of substrate 11, although 
the topcoat and backcoat layers are not shown in the figure and not 
critical to the present invention. These elements will be described in 
more detail hereinafter. 
Substrate 
The flexible substrates on which the ferromagnetic metal thin film layer is 
formed are not particularly limited as long as they are non-magnetic. 
Particularly preferred are flexible substrates, especially, of resins, for 
example, polyesters such as polyethylene terephthalate and polyimides. The 
substrates are not particularly limited in shape, size and thickness as 
long as they meet the intended application. Preferably, the substrates to 
be plasma treated according to the present invention have a thickness of 
about 5 to 20 .mu.m. 
Protrusion 
Fine protrusions or bosses 16 as shown in FIG. 1 have a height h of 30 to 
300 .ANG., and more particularly, 50 to 250 .ANG.. The protrusions 
provided in the present invention have such dimensions that they are not 
observable under an optical microscope or measureable by a probe type 
surface roughness meter, but can only be observable under a scanning 
electron microscope. Larger protrusions in excess of 300 .ANG. which are 
observable under an optical microscope are not desirable because of 
deterioration in electromagnetic properties and movement stability. 
Smaller protrusions of lower than 30 .ANG. are not effective in improving 
physical properties. 
The protrusions should be distributed on the surface of the magnetic 
recording medium at an average density of at least 10.sup.5 /a.sup.2, and 
more preferably 2.times.10.sup.6 /a.sup.2 to 1.times.10.sup.9 /a.sup.2 per 
square millimeter of the surface. A magnetic head 20 with which the 
magnetic recording medium of the present invention is used is provided 
with a gap 24 having a distance a (as expressed in .mu.m) as shown in FIG. 
2. The gap distance a usually ranges from 0.1 .mu.m to 0.5 .mu.m, and more 
preferably, from 0.1 .mu.m to 0.4 .mu.m. At protrusion densities of less 
than 10.sup.5 /a.sup.2 /mm.sup.2, and more particularly less than 
2.times.10.sup.6 /a.sup.2 /mm.sup.2, there result increased noise, 
deteriorated still performance, and other disadvantages, which are 
undesirable in practical applications. Higher protrusion densities of more 
than 10.sup.9 /a.sup.2 /mm.sup.2 are rather less effective in improving 
physical properties. 
The protrusions 16 may generally be provided by placing submicron particles 
15 on the surface of the substrate as clearly shown in FIG. 1. The 
submicron particles used herein have a particle size of 30 to 300 .ANG., 
and more preferably 50 to 250 .ANG.. Submicron protrusions are then formed 
on the surface of the magnetic recording medium which conform to the 
submicron particles on the substrate surface in shape and size. 
The submicron particles used in the practice of the present invention are 
those generally known as colloidal particles. Examples of the particles 
which can be used herein include SiO.sub.2 (colloidal silica), Al.sub.2 
O.sub.3 (alumina sol), MgO, TiO.sub.2, ZnO, Fe.sub.2 O.sub.3, zirconia, 
CdO, NiO, CaWO.sub.4, CaCO.sub.3, BaCO.sub.3, CoCO.sub.3, BaTiO.sub.3, Ti 
(titanium black), Au, Ag, Cu, Ni, Fe, various hydrosols, and resinous 
particles. Inorganic particles are preferred among others. 
The submicron particles may be placed on the substrate surface, for 
example, by dispersing them in a suitable solvent to form a dispersion, 
and applying the dispersion to the substrate followed by drying. Any 
aqueous emulsion containing a resinous component may also be added to the 
particle dispersion before it is applied to the substrate. The addition of 
a resinous component allows gently-sloping protrusions to form in 
conformity to the particles although it is not critical in the present 
invention. 
Plasma Treatment 
According to the present invention, substrates or base films are plasma 
treated on at least one surface which is to bear a magnetic layer. 
Although the substrate may be directly treated with plasma, it is 
preferable to apply the above-mentioned particle dispersion on the 
substrate surface prior to the plasma treatment. 
The plasma treatment is effected by feeding an inorganic gas as the 
treating gas, ionizing it, and contacting the gas-discharge plasma with 
the substrate, thereby plasma treating the substrate surface. 
The principle of plasma treatment will be briefly described. When an 
electric field is applied to a gas kept at a reduced pressure, free 
electrons which are present in a minor proportion in the gas and have a 
remarkably greater inter-molecular distance than under atmospheric 
pressure are accelerated under the electric field to gain a kinetic energy 
(electron temperature) of 5 to 10 eV. These accelerated electrons collide 
against atoms and molecules to fracture their atomic and molecular 
orbitals to thereby dissociate them into normally unstable chemical 
species such as electrons, ions, neutral radicals, etc. The dissociated 
electrons are again accelerated under the electric field to dissodiate 
further atoms and molecules. This chain reaction causes the gas to be 
instantaneously converted into highly ionized state. This is generally 
called a plasma. Since gaseous molecules have a less chance of collision 
with electrons and little absorb energy, they are kept at a temperature 
approximate to room temperature. Such a system in which the kinetic energy 
(electron temperature) of electrons and the thermal motion (gas 
temperature) of molecules are not correlated is designated a low 
temperature plasma. In this system, chemical species set up the state 
capable of chemical reaction such as polymerization while being kept 
relatively unchanged from the original. Substrates are plasma treated 
under these conditions according to the present invention. The use of a 
low temperature plasma avoids any thermal influence on substrates. 
FIG. 3 illustrates a typical apparatus in which substrates on the surface 
thereof are treated with a plasma. This plasma apparatus uses a variable 
frequency power source. The apparatus comprises a reactor vessel R into 
which a treating gas(es) is introduced from a source 41 and/or 42 through 
a mass flow controller 43 and/or 44. When desired, different gases from 
the sources 41 and 42 may be mixed in a mixer 45 to introduce a gas 
mixture into the reactor vessel. The treating gases may be fed at a flow 
rate of 1 to 250 ml per minute. 
Disposed in the reactor vessel R is means for supporting a base film to be 
treated, that is, a flexible substrate having particles distributed 
thereon. The support means used in this embodiment is a set of supply and 
take-up rolls 47 and 48 on which a substrate 10 for ordinary magnetic tape 
is wound. Depending on the particular shape of the magnetic recording 
medium substrate to be treated, any desired supporting means may be used, 
for example, a rotary support apparatus on which the substrate rests. 
On the opposed sides of the substrate to be treated are located a pair of 
electrodes 51 and 52, one electrode 51 being connected to a variable 
frequency power source 53 and the other electrode 52 being grounded. 
The reactor vessel R is further connected to a vacuum system for evacuating 
the vessel, including a liquefied nitrogen trap 55, a vacuum pump 57, and 
a vacuum controller 59. The vacuum system has the capacity of evacuating 
and keeping the reactor vessel R at a vacuum of 0.01 to 10 Torr. 
In operation, the reactor vessel R is first evacuated by means of the 
vacuum pump 57 to a vacuum of 10.sup.-3 Torr or lower before a treating 
gas or gases are fed into the vessel at a given flow rate. Then the 
interior of the reactor vessel is maintained at a vacuum of 0.01 to 10 
Torr. A take-up roll motor (not shown) is turned on to transfer the 
substrate to be treated. When the rate of transfer of the substrate and 
the flow rate of the treating gas mixture become constant, the variable 
frequency power 53 is turned on to generate a plasma with which a 
travelling substrate is treated. 
In this plasma treatment, the power source must have a frequency in the 
range of 10 to 200 kilohertz. Frequencies lower than 10 KHz and higher 
than 200 KHz result in a reduction in bond strength, and hence, durability 
imparted to magnetic recording media. It is to be noted that other 
parameters including supply current and treating time may be as usual or 
properly selected through experimentation. 
In the preferred embodiment of the invention, an inorganic gas containing 
oxygen is used as the treating gas. The inorganic gas may contain an 
effective proportion of, preferably 5 to 100% by volume of oxygen. The 
inorganic gas may be oxygen alone. As the inorganic gas mention may be 
made of argon, neon, helium, nitrogen, hydrogen and mixtures of two or 
more of them. It is also contemplated to use air as the oxygen-containing 
inorganic gas. 
Magnetic Layer 
The magnetic recording medium of the present invention has a magnetic layer 
on a substrate. The magnetic layer is of continuous ferromagnetic metal 
thin film type coextending over the substrate and is generally based on 
cobalt. In preferred embodiments of the present invention, the magnetic 
layer may preferably consist essentially of cobalt; cobalt and oxygen; 
cobalt, oxygen and nickel and/or chromium. That is, the magnetic layer may 
consist essentially of cobalt alone or a mixture of cobalt with nickel 
and/or oxygen. 
Where the layer consists essentially of cobalt and nickel, the weight ratio 
of Co/Ni may preferably be at least about 1.5. 
The magnetic layer may further contain oxygen in addition to cobalt or 
cobalt and nickel. The presence of oxygen contributes to further 
improvements in electromagnetic characteristics and runnability. In this 
case, the atomic ratio of O/Co (when nickel free) or O/(Co+Ni) is 
preferably not more than about 0.5, and more preferably from about 0.05 to 
0.5. 
Better results are obtained when the ferromagnetic metal thin film layer 
contains chromium in addition to cobalt; cobalt and nickel; cobalt and 
oxygen; or cobalt, nickel, and oxygen. The presence of chromium 
contributes to further improvements in electromagnetic characteristics, 
output level, signal-to-noise (S/N) ratio, and film strength. In this 
case, the weight ratio of Cr/Co (when , nickel free) or Cr/(Co+Ni) is 
preferably in the range of about 0.001 to 0.1, and more preferably about 
0.005 to 0.05. 
On the surface of the ferromagnetic metal thin film layer, oxygen forms 
oxides with ferromagnetic metals Co and Ni. In Auger spectroscopy, peaks 
indicative of oxides appear in a surface layer, particularly in a surface 
layer from the surface to a depth of 50 to 500 .ANG., more preferably 50 
to 200 .ANG.. This oxide layer has an oxygen content of the order of 0.5 
to 1.0 in atomic ratio. No particular limit is imposed on the 
concentration gradient of oxygen in the ferromagnetic metal thin film 
layer. 
The ferromagnetic metal thin film layer may further contain trace elements, 
particularly transition metal elements, for example, Fe, Mn, V, Zr, Nb, 
Ta, Ti, Zn, Mo, W, Cu, etc. 
The ferromagnetic metal thin film layer preferably consists of a 
coalescence of Co base particles of columnar structure oriented oblique to 
the normal to the substrate. More specifically, the axis of particles of 
columnar structure is preferably oriented at an angle of about 10 to 70 
degrees with respect to the normal to the major surface of the substrate. 
Each columnar particle generally extends throughout the thickness of the 
thin film layer and has a minor diameter of the order of 50 to 500 
angstroms. Cobalt and optional metals such as nickel and chromium form the 
columnar structure particles themselves while oxygen, when added, is 
generally present on the surface of each columnar structure particle in 
the surface layer essentially in the form of oxides. The ferromagnetic 
metal thin film layer generally has a thickness of about 0.05 to 0.5 
.mu.m, and preferably about 0.07 to 0.3 .mu.m. 
The magnetic layer is generally formed by the so-called oblique incidence 
evaporation process. The oblique incidence evaporation process may be any 
of well-known techniques preferably using an electron beam gun while the 
minimum incident angle with respect to the normal to the substrate is 
preferably 30 degrees. Evaporation conditions and post-treatments are well 
known in the art and any suitable ones may be selected therefrom. One 
effective post-treatment is a treatment for incorporating oxygen into the 
magnetic layer, which is also well known in the art. For further 
information about this evaporation process reference should be made to D. 
E. Speliotis et al., "Hard magnetic films of iron, cobalt and nickel", J. 
Appl. Phvs., 36, 3, 972 (1965) and Y. Maezawa et al., "Metal thin film 
video tape by vacuum deposition", IERE Conference Proceedings 54 (The 
Fourth International Conference on Video and Data Recording, The 
University of Southanmpton, Hampshire, England, 20-23 Apr., 1982), pp. 
1-9. 
The ferromagnetic metal thin film layer may be formed on the substrate 
either directly or via an undercoat layer of the well-known type. Further, 
the ferromagnetic metal thin film layer is genrally formed as a single 
layer, but in some cases, it may be made up from a plurality of laminated 
sub-layers with or without an intermediate non-ferromagnetic metal thin 
film layer interposed therebetween. 
The ferromagnetic metal thin film layer may be formed by any well-known 
techniques including evaporation, ion plating, and metallizing, and more 
preferably by the so-called oblique incidence evaporation process. The 
oblique incidence evaporation process may be any of well-known techniques 
preferably using an electron beam gun while the minimum incident angle 
with respect to the normal to the substrate is preferably at least 20 
degrees. Incident angles of less than 20 degrees result in deteriorated 
electromagnetic properties. The evaporation atmosphere may generally be an 
inner atmosphere of argon, helium or vacuum containing oxygen gas at a 
pressure of about 10.sup.-5 to 10.sup.0 Pa. Those skilled in the art will 
readily select other evaporation parameters including source-substrate 
spacing, substrate feed direction, can and mask configurations and 
arrangement, and the like, through a simple experiment if necessary. 
Evaporation in an oxygen-containing atmosphere causes a metal oxide film to 
form on the surface of the resulting magnetic layer. The partial pressure 
of oxygen gas necessary to allow for oxide formation may be readily 
determined through a simple experiment. 
A metal oxide coating may be formed on the surface of the magnetic layer by 
an oxidizing treatment. Any of the following oxidizing treatments may be 
employed for this purpose. 
(1) Dry treatment 
(a) Energy particle treatment 
Oxygen may be directed as energy particles to the magnetic layer at the 
final stage of evaporation process by means of an ion gun or neutron gun 
as described in Noguchi et al, U.S. Ser. No. 603,894 assigned to the same 
assignee as the present invention. 
(b) Glow treatment 
The magnetic layer is exposed to a plasma which is created by generating a 
glow discharge in an atmosphere containing O.sub.2, H.sub.2 O or O.sub.2 
+H.sub.2 O in combination with an inert gas such as Ar and N.sub.2. 
(c) Oxidizing gas 
An oxidizing gas such as ozone and heated steam is blown to the magnetic 
layer. 
(d) Heat treatment 
Oxidation is effected by heating at temperatures of about 60 to 150.degree. 
C. 
(2) Wet Treatment 
(a) Anodization 
(b) Alkali treatment 
(c) Acid treatment 
Chromate treatment, permanganate treatment, phosphate treatment 
(d) Oxidant treatment 
H.sub.2 O.sub.2 
Magnetic Head 
The magnetic recording medium of the present invention may be operated in 
combination with a variety of magnetic heads. It is preferred that at 
least a gap-defining edge portion of the magnetic head be of a 
ferromagnetic metal material. It is possible to form a core entirely of a 
ferromagnetic metal material although a part of the core including a 
gap-defining edge portion may be formed of a ferromagnetic metal material. 
FIG. 2 schematically shows a magnetic head generally designated at 20 as 
comprising core halves 21 and 22 formed of a ferromagnetic material such 
as ferrite. The core halves 21 and 22 are metallized at their gap-defining 
edge portions with ferromagnetic metal material layers 31 and 32 of about 
1 to 5 .mu.m thick by sputtering or any suitable metallizing techniques. 
The core halves 21 and 22 are mated so as to define a gap 24 therebetween 
which is filled with glass or dielectric material and has a distance a. 
This configuration, although the figure is not drawn to exact proportion 
and shape, provides improved electromagnetic properties and ensures smooth 
tape passage thereacross without head adhesion or clogging. Of course, the 
shape and structure of the head is well known. 
In the practice of the present invention, it is desirable that the head gap 
24 has a distance a of 0.1 to 0.5 .mu.m, and preferably 0.1 to 0.4 .mu.m, 
and a track width of 10 to 50 .mu.m, and preferably 10 to 20 .mu.m. 
The ferromagnetic metal materials used in the fabrication of the magnetic 
head may be selected from a variety of such materials including thin films 
and thin plates of amorphous magnetic metals, Sendust, hard Permalloy, 
Permalloy, etc. Among them, particularly preferred are amorphous magnetic 
Co-based alloys because they experience little head adhesion or clogging 
and have excellent electromagnetic properties. Preferred are amorphous 
magnetic alloys comprising 70 to 95 atom % of Co and 5 to 20 atom % of a 
vitrifying element(s) such as Zr, Nb, Ta, Hf, rare earth elements, Si, B, 
P, C, Al, etc., with the Zr and/or Nb being most preferred. Also preferred 
are alloys comprising 65 to 85 atom % of Co and 15 to 35 atom % of Si 
and/or B as a vitrifying element. The latter alloys may further contain 
less than 10 atom % of Fe, less than 25 atom % of Ni, less than 20 atom % 
(in total) of at least one member of Cr, Ti, Ru, W, Mo, Ti, Mn, etc. 
The amorphous magnetic alloys may be formed into core halves or 
gap-defining segments by sputtering or high speed quenching. 
Recording/reproducing operation may be performed on the magnetic recording 
medium of the present invention by means of the above-mentioned magnetic 
head in accordance with any well-known video recording/reproducing systems 
including the so-called VHS, Beta, 8-mm video and U-standard systems. 
The magnetic recording medium and recording/reproducing method according to 
the present invention has a number of benefits. 
The magnetic recording medium exhibits sufficiently reduced dynamic 
friction to provide stable movement because protrusion as high as 30 to 
300 .ANG. are distributed on the magnetic layer surface at an average 
density of at least 10.sup.5 /a.sup.2 /mm.sup.2. 
Since a controlled plasma treatment is carried out on the substrate at a 
frequency in the specified range in a treating gas having a controlled 
oxygen partial pressure, the bond strength between the substrate and the 
magnetic layer formed thereon with or without an intervening undercoat 
layer is significantly increased, resulting in improved durability. 
Runnability is outstandingly improved so that the dynamic friction 
increases little after repeated travel cycles in a recording/reproducing 
equipment. The medium tolerates an increased number of 
recording/reproducing operations and offers improved still characteristics 
(characteristics in the still mode reproduction). 
Improved stability ensures that the medium can be stored and operated in 
severely varying environments from high-temperature high-humidity to 
low-temperature low-humidity environments. 
Reproduction output is little affected by a spacing loss and contains less 
noise. 
The magnetic recording medium operated in contact with a head releases 
little materials which will adhere to and clog the head. 
These benefits are further enhanced when the medium is used in combination 
with ferromagnetic metal heads, and particularly in the case of high 
density recording at a minimum recording wavelength of less than 1 .mu.m. 
EXAMPLES 
Examples of the present invention are given below by way of illustration 
and not by way of limitation. 
EXAMPLE 1 
Colloidal silica was applied onto a substantially particulate-free smooth 
polyethylene terephthalate (PET) film of 10 .mu.m thick. There was 
obtained a substrate having submicron particles or protrusions distributed 
thereon. 
The particle-bearing substrate was plasma treated using argon and oxygen 
alone and mixtures of oxygen and argon as the treating gas. The plasma 
treating conditions were as follows. 
Gas flow rate: 100 ml/min. fixed for all cases of Ar alone, O.sub.2 alone 
and mixtures of Ar and O.sub.2 
Vacuum : 0.5 Torr 
Frequency: 100 kHz 
Power: 200 Watts 
Substrate transfer speed: 30 m/minute 
It is to be noted that the RF and microwave plasma treatments used herein 
correspond to 13.56 MHz and 2.45 GHz, respectively. 
The thus treated polyester substrate was moved along the circumferential 
surface of a cooled cylindrical can in a chamber which was evacuated to 
vacuum. A Co-Ni alloy was evaporated onto the substrate surface by heating 
with an electron beam gun while introducing oxygen into the evaporating 
atmosphere. The background pressure was set to 5.times.10.sup.-5 Torr and 
changed to 2.times.10.sup.-4 Torr after the introduction of O.sub.2 . The 
incident angle during evaporation was continuously reduced from 90.degree. 
to 30.degree.. The thus deposited layer has a composition of 80 Co/20 Ni 
(weight ratio) and a thickness of about 1500 .ANG.. 
For the samples prepared according to the present invention, no particular 
effect due to the application of colloidal silica was detected by 
observation under an optical microscope or by measurement with a 
probe-type surface roughness meter whereas protrusions were observed under 
a scanning electron microscope as being formed on the magnetic layer and 
having dimensions in conformity to the colloidal silica particles applied. 
The height and density of protrusions on the magnetic layer are shown in 
Table 1 along with the properties. 
Measurements are made as follows. The tests for evaluating electromagnetic 
properties used a signal having the minimum recording wavelength of 0.7 
.mu.m. 
The magnetic head used in examining the media was of the type shown in FIG. 
2 and having a gap distance a of 0.25 .mu.m and a track width of 20 .mu.m. 
The core halves 21, 22 were formed of ferrite, gap-defining edge portions 
31, 32 were amorphous layers of Co 0.8/Ni 0.1/Zr 0.1 (atomic ratio 
percent) formed by sputtering to a thickness of 3 .mu.m, and the gap 
filler was glass. For the head of this size, the minimum protrusion 
distribution density 10.sup.5 /a.sup.2 is calculated to be 
1.6.times.10.sup.6. 
Protrusion Observation 
Tape surface was observed under a scanning electron microscope (SEM) and a 
transmissive electron microscope (TEM). 
Still Life 
Signals were recorded in tape at 5 MHz and then reproduced in still mode. 
The still life is the lapse of time from the start of still mode 
reproduction until the reproduced output drops to 80% of the initial 
level. 
Surface State at the End of a Durability Test 
A tape was repeatedly moved on a commercially available video tape 
recorder. After 50 passes, the tape surface, that is, magnetic layer 
surface was observed under an optical microscope. Symbols used in Table 1 
have the following meaning. 
.circleincircle. : no flaw 
.circle. : flaw marks on less than 20% of the head traversing area. 
.DELTA.: flaw marks on more than 20% of the head traversing area. 
X: magnetic layer peeled off. 
Auger spectroscopy revealed that the magnetic layers formed in this example 
were covered with an oxide coating as thick as 100 to 200 .ANG.. 
Although colloidal silica was used as inorganic submicron particles in this 
example, it was found that similar results were obtained when the 
colloidal silica was replaced by other particulate materials, for example, 
alumina sol, titanium black, zirconia, and various hydrosols. 
For the measurement of sample No. 15, a ferrite head having the same size 
as mentioned above was used. 
TABLE 1 
__________________________________________________________________________ 
Sample 
Substrate 
Protrusion Still 
Surface 
No. treatment 
Height, .ANG. 
Density,/mm.sup.2 
Head life, min. 
state 
__________________________________________________________________________ 
1 100 kHz plasma 
50 5 .times. 10.sup.6 
amorphous 
&gt;120 .circleincircle. 
(O.sub.2 25%) 
2 100 kHz plasma 
50 1 .times. 10.sup.8 
amorphous 
&gt;120 .circleincircle. 
(O.sub.2 25%) 
3 100 kHz plasma 
100 5 .times. 10.sup.6 
amorphous 
&gt;120 .circleincircle. 
(O.sub.2 25%) 
4 100 kHz plasma 
100 5 .times. 10.sup.8 
amorphous 
&gt;120 .circleincircle. 
(O.sub.2 25%) 
5 100 kHz plasma 
200 5 .times. 10.sup.6 
amorphous 
&gt;120 .circleincircle. 
(O.sub.2 25%) 
6 100 kHz plasma 
200 5 .times. 10.sup.8 
amorphous 
&gt;120 .circleincircle. 
(O.sub.2 25%) 
7 100 kHz plasma 
300 5 .times. 10.sup.6 
amorphous 
&gt;120 .circleincircle. 
(O.sub.2 25%) 
8* 100 kHz plasma 
100 5 .times. 10.sup.5 
amorphous 
70 X 
(O.sub.2 25%) 
9* 100 kHz plasma 
1000 4 .times. 10.sup.6 
amorphous 
75 .DELTA. 
(O.sub.2 25%) 
10* 100 kHz plasma 
-- -- amorphous 
80 X 
(O.sub.2 25%) 
11 100 kHz plasma 
100 5 .times. 10.sup.6 
amorphous 
70 .DELTA. 
(Ar 100%) 
12 100 kHz plasma 
100 5 .times. 10.sup.6 
amorphous 
75 .DELTA. 
(N.sub.2 100%) 
13* -- 100 5 .times. 10.sup.5 
amorphous 
10 X 
14* -- -- -- amorphous 
5 X 
15 100 kHz plasma 
100 5 .times. 10.sup.8 
ferrite 
80 .DELTA. 
(O.sub.2 100%) 
__________________________________________________________________________ 
*Comparative Examples 
The data in Table 1 proves the effectiveness of the present invention. 
To examine how the surfacae of substrates was modified by plasma treatment, 
the contact angle of plasma-treated polyester films was measured. It was 
found that when the oxygen content of the atmosphere exceeds 5% by volume 
in the plasma treatment at frequencies of 10 to 200 kHz, functional groups 
are formed to a great extent such as to reduce the contact angle, 
providing a more wettable surface. In addition, the plasma treatment 
functions to clean the film surface to remove a weak boundary layer (WBL). 
These effects are reponsible to significant improvements in bond strength 
and still life. 
Along with the cleaninig effect by the plasma treatment, improved 
wettability resulting from a substantially reduced contact angle of the 
substrate surface contributes to an improvement in bond strength of the 
magnetic layer to the substrate, eventually resulting in an extended still 
life. 
EXAMPLE 2 
The procedure of Example 1 was repeated. Protrusions having a height of 100 
.ANG.. A were distributed at a density of 2.times.10.sup.8 /mm.sup.2. The 
plasma frequency and oxygen content were varied over a wide range. The 
resulting samples were measured for still life. The magnetic layer and the 
head were the same as in Example 1. 
The results are plotted in FIGS. 4 and 5. FIG. 4 illustrates the still life 
of samples as a function of the oxygen content of the treating atmosphere 
in various plasma treatments. Samples having substrates corona and 
microwave treated are also plotted. It is evident that the plasma 
treatment of substrates at a frequency of 20 kHz is outstandingly 
effective in extending the still life of magnetic tape. Plasma treatment 
in an atmosphere containing at least 5% by volume of oxygen is most 
effective. 
FIG. 5 illustrates the still life of samples as a function of the frequency 
of various treatments. It is evident that the plasma treatment in the 
frequency range of 10 to 200 kHz is outstandingly effective.