Magnetic recording medium and method for making

In a magnetic recording medium comprising a magnetic layer containing a magnetic powder, 5 to 20% by weight based on the weight of the magnetic powder of a non-magnetic abrasive having a Mohs hardness of at least 6, and a binder on a non-magnetic substrate, the magnetic layer becomes more durable while maintaining electromagnetic properties by distributing the abrasive in the magnetic layer such that its concentration is highest in a surface-adjoining portion and continuously decreases therefrom toward the substrate side. The medium is prepared by applying a magnetic coating to a substrate and continuously passing the coated substrate through reversing magnetic fields prior to curing of the coating.

This invention relates to a magnetic recording medium and a method for 
preparing the same. 
BACKGROUND OF THE INVENTION 
Magnetic recording media are generally manufactured by forming magnetic 
layers on non-magnetic substrates. The magnetic layers contain magnetic 
powder in binders, to which various additives such as antistatic agents, 
lubricants, abrasives, dispersants, and stabilizers are added to provide a 
good profile of electromagnetic properties, durability, and reliability. 
In recent years, the requirement of high density recording is imposed on 
magnetic recording media in accordance with the size reduction of the 
equipment. 
One approach for increasing the recording density of magnetic recording 
media is to furnish magnetic powder having a higher coercive force, higher 
saturation magnetic flux density or smaller particle size. Also attempts 
have been made for achieving a higher packing density and a higher degree 
of orientation of magnetic powder as well as smoothing the surface and 
reducing the layer thickness. These attempts for higher recording density, 
however, have arisen many problems including losses of reliability and 
durability, causing magnetic head clogging, frequent occurrence of 
dropouts and deterioration of still performance. 
It is known that reliability and durability can be improved through a 
proper choice of the type and amount of abrasive. Japanese Patent 
Application Kokai No. 57036/1986, for example, proposes to control the 
population or density of abrasive to at least 0.25 particles/.mu.m.sup.2 
on the magnetic layer surface. In the case of magnetic layers which are 
reduced to a thickness of about 4 .mu.m or less for high density recording 
purposes, an increase of the density of abrasive by an ordinary 
distribution technique can lead to a loss of electromagnetic properties 
such as sensitivity and C/N and an increased abrasion of the associated 
magnetic head despite improved reliability and durability. 
In turn, Japanese Patent Publication No. 14482/1979 proposes a magnetic 
layer of double layer structure for preventing magnetic head clogging and 
improving electro-magnetic properties. However, the magnetic layer 
contemplated therein is relatively thick as understood from the example in 
which two magnetic layers of 6 .mu.m and 4 .mu.m are stacked to form a 
composite magnetic layer of 10 .mu.m thick. That is, thin magnetic layers, 
say about 4 .mu.m or less, required for high density and long term 
recording are not borne in mind. This is partially because such extremely 
thin layers can be stacked with difficulty or at the sacrifice of 
productivity. Even if the upper layer to be stacked can be as thin as 
about 0.6 .mu.m, the abrasive is distributed uniformly in a thickness 
direction of the upper layer and at a relatively high density. In 
addition, such a thin upper layer cannot be effectively worked as by 
calendering. Consequently, electromagnetic properties are adversely 
affected and productivity is lost due to the complex manufacturing 
process. 
As discussed above, high density recording media must meet ambivalent 
requirements of electromagnetic properties and reliability and durability. 
The prior art techniques for abrasive addition are difficult to find a 
compromise therebetween. 
SUMMARY OF THE INVENTION 
Therefore, an object of the present is to provide a magnetic recording 
medium adapted for high density recording having a single magnetic layer 
and exhibiting high reliability and durability as well as improved 
electro-magnetic properties. Another object is to provide a method for 
preparing the medium. 
The present invention which achieves these and other objects provides a 
magnetic recording medium comprising a non-magnetic substrate and a 
magnetic layer thereon containing a magnetic powder, 5 to 20% by weight 
based on the weight of the magnetic powder of a non-magnetic abrasive 
having a Mohs hardness of at least 6, and a binder. The magnetic layer has 
a surface remote from the substrate. The abrasive is distributed in the 
magnetic layer such that the concentration of the abrasive is highest in a 
surface-adjoining portion and continuously decreases therefrom toward the 
substrate side. 
Typically, the magnetic layer has a thickness of up to 4 .mu.m. The ratio 
of the concentration p1 of the abrasive in the surface-adjoining portion 
extending from the surface to a depth of 0.6 .mu.m to the concentration p2 
of the abrasive in the remaining portion of the magnetic layer, p1/p2, is 
at least 1.5. The non-magnetic abrasive has a mean particle diameter of up 
to 0.6 .mu.m. 
According to another aspect of the present invention, there is provided a 
method for preparing a magnetic recording medium comprising the steps of: 
applying a magnetic coating composition containing a magnetic powder, 5 to 
20% by weight based on the weight of the magnetic powder of a non-magnetic 
abrasive having a Mohs hardness of at least 6, and a binder to a 
non-magnetic substrate, and 
applying an external magnetic field in which the direction of magnetic line 
of force is successively reversed to the coated substrate, thereby forming 
a magnetic recording medium having a magnetic layer in which said 
non-magnetic abrasive is distributed at a higher concentration in a 
portion of the magnetic layer adjoining the outside surface thereof than 
in the remaining portion. 
Preferably, the external magnetic field is created by a plurality of unit 
magnets arranged on the side of the substrate remote from the coated 
surface such that the polarity of one unit magnet is opposite to the 
polarity of adjoining unit magnets. 
Typically, the coated substrate is continuously passed through the 
reversing external magnetic fields. Orientation is carried out after the 
step of applying an external magnetic field to the coated substrate. 
The magnetic recording medium according to the present invention includes a 
single thin magnetic layer in which the total content of abrasive is 
reduced by causing some part of the abrasive to shift upward or locally 
concentrate in the surface-adjoining portion of the magnetic layer while 
reducing the content of abrasive from the surface-adjoining portion toward 
the substrate side. The local concentration of abrasive near the surface 
ensures high reliability and durability while maintaining the excellent 
electromagnetic properties of the thin magnetic layer for high density 
recording. 
Prior to orientation of magnetic powder, the magnetic coating is moved 
through the reversing magnetic fields whereby the magnetic powder is drawn 
toward the substrate so that relatively much abrasive is distributed on 
the surface side. The passage through the reversing magnetic fields 
assists in debubbling from the coating and orientation, contributing to 
improvements in electromagnetic properties.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
The magnetic recording medium of the present invention includes a magnetic 
layer on a non-magnetic substrate having a magnetic powder and an abrasive 
dispersed in a binder. 
The magnetic powder used herein may be selected from magnetic powders 
commonly used in conventional magnetic recording media, for example, iron 
oxide particles such as .gamma.-Fe.sub.2 O.sub.3, Co-doped iron oxide 
particles such as Co-doped .gamma.-Fe.sub.2 O.sub.3, magnetic metal 
particles, barium ferrite particles and CrO.sub.2. A proper type of 
magnetic powder may be selected for a particular purpose while its 
coercive force, remanence, specific surface area and other factors may 
vary to meet the purpose. 
In the case of video tape and other magnetic recording media adapted for 
high density recording, magnetic metal particles having a coercive force 
of at least 1,300 oersted (Oe), especially 1,300 to 3,000 Oe are 
preferred. 
Where magnetic metal particles are used as ferromagnetic powder, those 
particles having an oxide coating on the surface may be used and one or 
more types of metal particles used. 
The ferromagnetic powder used herein is generally in needle and particulate 
forms, between which a choice may be made in accordance with the intended 
application of magnetic recording medium. For use as video tape, needles 
are preferred, especially having an average aspect ratio l/d of about 3 to 
about 20 wherein l is an average length in the range of 0.1 to 0.5 .mu.m 
and d is an average breadth. Also for such use, a specific surface area of 
20 to 70 m.sup.2 /g as measured by the BET method is preferred. 
The binder used herein may be selected from binders commonly used in 
conventional magnetic recording media, for example, thermoplastic binders, 
thermosetting binders, and electron beam curable binders. Preferably, 0.1 
to 0.3 parts by weight of the binder is used per part by weight of the 
magnetic powder. 
The abrasive used herein is a non-magnetic abrasive having a Mohs hardness 
of at least 6. Examples of the abrasive include alumina Al.sub.2 O.sub.3, 
non-magnetic chromium oxide Cr.sub.2 O.sub.3, silicon carbide SiC, 
titanium oxide TiO.sub.2, silica SiO.sub.2, zirconia ZrO.sub.2, and 
mixtures thereof. Among these, at least one abrasive selected from 
Al.sub.2 O.sub.3, Cr.sub.2 O.sub.3, and SiC is preferred from the 
standpoints of the dispersion of abrasive and head abrasion. 
The abrasive preferably has a mean particle diameter of up to 0.6 .mu.m, 
more preferably from 0.1 to 0.4 .mu.m. Electromagnetic properties are 
sometimes poor in excess of 0.6 .mu.m whereas extremely fine abrasive 
particles are ineffective for durability improvement. The abrasive may 
have spherical, angular or other particulate form. The particle diameter 
is usually determined by taking an image of particles under a transmission 
electron microscope (TEM) and calculating the diameter from the projected 
areas of particles on the assumption that the particles are circular. 
The abrasive is present in the magnetic layer in a total amount of 5 to 
20%, preferably 8 to 18% by weight based on the weight of the magnetic 
powder. On this basis, less than 5% by weight of abrasive is ineffective 
for durability and stability improvement whereas more than 20% by weight 
of abrasive adversely affects electromagnetic properties. 
The abrasive is distributed in the magnetic layer of a single layer 
structure. The abrasive is locally concentrated in a portion of the layer 
adjoining the surface remote from the substrate, that is, a 
surface-adjoining portion. The concentration of the abrasive is highest in 
the surface-adjoining portion and lowest on the substrate side. In the 
lower portion of the magnetic layer between the surface-adjoining portion 
and the interface with the substrate, the abrasive concentration 
continuously varies or decreases from the surface-adjoining portion toward 
the substrate side. In this sense, the lower portion is referred to as a 
graded region. 
The abrasive is preferably localized such that p1/p2 is at least 1.5, more 
preferably from 1.5 to 10, provided that p1 is the concentration of 
abrasive in the surface-adjoining portion of the magnetic layer extending 
from the surface to a depth of 0.6 .mu.m and p2 is the concentration of 
abrasive in the remaining or lower portion of the magnetic layer. The 
benefits of the invention would be somewhat lost with a p1/p2 ratio of 
less than b 1.5. Extreme localization as represented by a p1/p2 ratio in 
excess of 10 would rather detract from the reinforcement of the magnetic 
layer by the abrasive. 
The concentration of the abrasive in the magnetic layer is determined by 
observing an image on a section of the magnetic layer under a transmission 
electron microscope (TEM), and calculating the percentage of the projected 
areas of abrasive particles per unit area. Preferably, the concentration 
of abrasive in the surface-adjoining portion p1 is 10 to 20% by area 
although it will somewhat vary with the total content of abrasive. 
Durability is often low with a p1 of less than 10% by area whereas 
electromagnetic properties will become low with a p1 in excess of 20% by 
area. 
The concentration of the abrasive throughout the magnetic layer is 
generally 2 to 8% by area provided that the total content of the abrasive 
is 5 to 20% by weight based on the weight of the magnetic powder. This 
contrasts with the prior art uniform layer in which the abrasive is added 
in an amount of about 25% by weight based on the weight of the magnetic 
powder in order to form a magnetic layer having an abrasive concentration 
of 10% by area. 
In the magnetic recording medium of the invention, the abrasive localized 
region having an abrasive concentration of 10 to 20% by area is normally 
0.3 to 1.0 .mu.m thick and adjoined on the substrate side by a graded or 
transition region where the abrasive concentration continuously decreases 
toward the substrate interface. Then better results are obtained. 
The magnetic layer may further contain well-known additives such as 
antistatic agents, lubricants, dispersants, and film hardeners depending 
on the particular application of medium. 
The magnetic layer preferably has a thickness of up to 4 .mu.m, especially 
about 2.0 to about 4.0 .mu.m. For the 8-mm video tape, the magnetic layer 
is about 2.5 to about 3.5 .mu.m. 
Preferably, the magnetic layer has a coercive force Hc of about 800 to 
2,500 Oe, a residual magnetic flux density Br of about 1,500 to 3,000 G, 
and a squareness ratio of about 0.8 to 0.9. Further, the magnetic layer 
has a porosity of up to 7%, especially 2 to 7%. The porosity may be 
determined either by comparing the saturation magnetization .sigma..sub.s 
of magnetic powder with the residual magnetic flux density Br of medium or 
from a TEM image on a section of the magnetic layer. 
Although the magnetic layer used in the invention is a single layer having 
the above-defined distribution of abrasive, an underlying magnetic layer 
may be provided between the magnetic layer and the substrate. 
In general, the medium of the invention has such a magnetic layer on one 
surface of the non-magnetic substrate. A double side medium having a 
magnetic layer on either surface of the substrate is also contemplated 
herein and the application of an external magnetic field to be described 
later may be conducted upon formation of each magnetic layer. 
As to the coating type magnetic layer, reference is made to Japanese Patent 
Application Kokai No. 38522/1987 by the same assignee as the present 
invention. 
The non-magnetic substrate used in the magnetic recording medium of the 
invention is not particularly limited. A desired one for a particular 
purpose may be selected from a variety of known flexible materials and 
rigid materials and processed to desired shape and dimensions in 
accordance with the selected standard. Exemplary flexible materials are 
polyesters such as polyethylene terephthalate. 
If desired, an underlying layer, either magnetic or non-magnetic, may be 
provided on the substrate while a backcoat layer may be provided on the 
surface of the substrate remote from the magnetic layer. The backcoat 
layers which can be used herein include well-known coating type backcoat 
layers containing conductive fillers and pigments and plasma-polymerized 
films. 
Now, the manufacture of the magnetic recording medium according to the 
present invention is described. 
The magnetic recording medium is manufactured by first blending a magnetic 
powder with 5 to 20%, preferably 8 to 18% by weight based on the weight of 
the magnetic powder of a non-magnetic abrasive having a Mohs hardness of 
at least 6 and a binder to form a magnetic coating composition. The 
composition is then applied to a non-magnetic substrate by a coating 
technique. Prior to drying of the coating, an external magnetic field in 
which the direction of magnetic line of force is successively reversed is 
applied to the coated substrate. The external magnetic field applying 
means used herein includes a plurality of magnetic field creating means 
which may be permanent magnets or electromagnets. 
FIG. 1 illustrates one preferred example of the means for creating an 
external magnetic field. Most often, a magnetic strip 1 having a magnetic 
coating 2 on a substrate 3 is passed through an external magnetic field in 
which the direction of magnetic line of force is successively reversed. 
The means 5 for creating reversing magnetic fields includes a plurality of 
unit magnets 4 which are arranged on the side of the substrate 3 remote 
from the coated surface such that the polarity of one unit magnet is 
opposite to the polarity of adjoining unit magnets. The number of unit 
magnets 4 corresponds to the number of reversal of magnetic fields and is 
usually at least 4, preferably 8 to 25. Less than four unit magnets are 
often ineffective to provide the desired distribution profile of abrasive 
whereas too many reversing magnetic fields are unnecessary and sometimes 
result in a coating having a rough surface. 
The unit magnets 4 may be either juxtaposed in close contact relationship 
as shown in the figure or spaced apart. It is advantageous to use unit 
magnets which are equal in magnetic field intensity and size. Any desired 
unit magnets may be used insofar as they create magnetic fields which are 
uniform in the transverse direction of the strip. They may be sized in 
accordance with the desired magnetic field reversal cycle to be described 
later. Preferably the unit magnets 4 have a maximum energy product (BH)max 
in the range of 3.5 to 37 MGOe. 
The strip 1 is usually passed over the external magnetic field means 5 in 
the form of an alternate magnet arrangement at a spacing of about 3 to 20 
mm. Then a magnetic field having an intensity of at least 300 G, more 
preferably 800 to 2,000 G can act on the magnetic coating 2. A better 
distribution profile of abrasive is accomplished with such a spacing or 
magnetic field intensity. Too high magnetic field intensity is 
unnecessary. The strip can be placed closer to the magnet arrangement, but 
without contact of the substrate 3 with the unit magnets 4. 
Most often, the coated substrate 1 is continuously passed across the 
external magnetic field creating arrangement 5 as shown by an arrow in the 
figure whereby reversing external magnetic fields alternately act on the 
wet coating or magnetic layer 2. Alternatively, the unit magnet 
arrangement 5 may be moved relative to the coated substrate. As the coated 
substrate is passed through reversing magnetic fields, magnetic particles 
in the magnetic coating oscillate along the lines of magnetic flux 
whenever the direction of magnetic flux is reversed so that the magnetic 
particles are cyclically drawn toward the magnetic field creating 
arrangement 5. Consequently, the non-magnetic abrasive particles are moved 
outward and distributed at a relatively high concentration in the 
surface-adjoining portion of the magnetic coating 2. As a result of 
fluctuation, the magnetic particles undergo a kind of preliminary 
orientation leading to a higher degree of orientation and a higher 
squareness ratio. The fluctuation is also effective in removing bubbles, 
resulting in reduced porosity. 
The number of reversing magnetic fields is represented by the number of 
unit magnets 4 for simplicity's sake and is therefore preferably at least 
4, more preferably 8 to 25. The cycle of magnetic field reversal may be 50 
to 400 cycles/sec. To this end, the coated strip 1 is continuously 
transferred in a longitudinal direction across the external magnetic field 
creating arrangement 5 with the substrate 3 faced toward the arrangement 
5, whereby the magnetic field acting on the magnetic coating 2 is 
alternately reversed. 
The magnetic fields act on the magnetic coating 2 from the back side of the 
substrate because the magnetic particles are fluctuated or drawn toward 
the substrate. If the magnetic fields act on the magnetic coating 2 from 
above, fluctuation of magnetic particles occurs near the coating surface, 
not inducing the outward movement of abrasive particles. 
Therefore, it is sufficient and effective for the present purposes to place 
the magnetic field creating means 5 on only the back side of the substrate 
as shown in the figure although it is possible to place the magnetic field 
creating means 5 on both the coating and back sides of the substrate. In 
the latter case, the magnetic field acting from the back side should have 
higher intensity than the magnetic field acting from the coating side. 
The strip 1 is then passed between an orienting pair of opposed magnets 6, 
6 for achieving longitudinal orientation. The orienting magnets 6 are 
spaced from the alternating magnetic field creating means 5 such that they 
do not interfere with each other. The polarity of the orienting magnets 6 
at the opposed ends (N in the figure) should preferably be opposite to the 
polarity of the last stage unit magnet 4 of the magnetic field creating 
means 5 (S in the figure) because smooth orientation is achievable. The 
magnetic field for orientation preferably has an intensity of 1,500 to 
10,000 G in the magnetic layer or coating 2. 
Thereafter, the coated strip is subjected to surface smoothing as by 
calendering and to curing. 
The strip is finally cut to the predetermined size by means of a slitter or 
the like, obtaining standard magnetic tapes. 
EXAMPLE 
Examples of the present invention are given below by way of illustration 
and not by way of limitation. 
EXAMPLE 1 
A magnetic coating composition was prepared according to the following 
formulation. 
______________________________________ 
Ingredients Parts by weight 
______________________________________ 
Magnetic metal powder 100 
mean length 0.2 .mu.m, aspect ratio 8, 
Hc: 1500 Oe, .sigma..sub.s : 130 emu/g 
Vinyl chloride-vinyl acetate-vinyl 
10 
alcohol copolymer (VAGH by UCC) 
Polyurethane resin 10 
(N-2304 by Nihon Polyurethane K.K.) 
Low molecular weight polyisocyanate 
5 
(Colonate L by Nihon Polyurethane K.K.) 
Abrasive 3-30 
Stearic acid 0.2 
Lecithin 0.5 
Toluene 50 
Methyl ethyl ketone 50 
Methyl isobutyl ketone 50 
______________________________________ 
The abrasive used herein was a mixture of Cr.sub.2 O.sub.3 having a mean 
particle diameter of 0.25 .mu.m and Al.sub.2 O.sub.3 having a mean 
particle diameter of 0.20 .mu.m in a weight ratio of 1:1. It was added in 
varying amounts of from 3 to 30 parts by weight. 
The magnetic coating composition was coated on a polyester film of 10 .mu.m 
thick, and then subjected to localization by reversing external magnetic 
fields, orientation, calendering, and thermosetting. 
The localization was carried out by using an arrangement of unit magnets 
for creating reversing magnetic fields as shown in FIG. 1. The unit 
magnets each had a width of 20 mm in the transfer direction of the coated 
film and created an equal magnetic field at the surface. The magnetic 
fields were reversed in the number shown in Table 1 by varying the number 
of unit magnets. The number of reversal of magnetic fields is represented 
by the number of unit magnets. 
Further, the intensity of the magnetic field acting on the magnetic coating 
from the unit magnets was varied as shown in Table 1 by changing the 
spacing between the coated film and the magnet arrangement. 
For orientation, a pair of opposed magnets were used which created an 
orienting magnetic field of 4000 G. 
The magnetic layer had a final thickness of 3.0 .mu.m. 
The coated film was finally slit into magnetic tape samples 8 mm wide. 
For each sample, using a TEM image on a section of the magnetic layer, p1 
of a surface-adjoining portion of 0.6 .mu.m deep from the surface and p2 
of the remaining portion were determined. It is to be noted that p1 and p2 
are percentages of the projected areas of abrasive particles in the unit 
cross sectional area. 
In Sample Nos. 1 to 5, a region extending about 0.2 .mu.m from the level of 
0.2 .mu.m deep from the surface had the reported value of p1 and the 
underlying region was a graded region where the concentration of abrasive 
continuously decreased toward the substrate side. 
For comparison purposes, sample No. 13 of double layer structure was 
prepared by coating a lower layer on a film and then coating an upper 
layer thereon. The lower layer contained 12.5 parts by weight of the 
abrasive per 100 parts by weight of the magnetic powder and the upper 
layer contained 25 parts by weight of the abrasive per 100 parts by weight 
of the magnetic powder, with the remaining ingredients being the same as 
above. The lower and upper magnetic layers were 2.4 .mu.m and 0.6 .mu.m 
thick, respectively. 
The squareness ratio, residual magnetic flux density Br, and porosity (%) 
of the samples are also shown in Table 1. The porosity was calculated from 
Br. 
The samples were further measured for RF output at 5 MHz. The RF output was 
reported in dB relative to the output of sample No. 11. 
Furthermore, the samples were evaluated for still performance and head 
clogging. The still performance was examined by operating the tape at room 
temperature (23.degree. C.) and RH 65% and measuring an output drop after 
2 hours. Evaluation was based on the following criterion. 
______________________________________ 
E: RF output drop &lt; 1.0 dB 
G: 1.0 dB .ltoreq. 
RF output drop &lt; 2.0 dB 
F: 2.0 dB .ltoreq. 
RF output drop &lt; 3.0 dB 
P: 3.0 dB &lt; RF output drop 
______________________________________ 
The head clogging was examined by feeding the tape a number of passes at 
40.degree. C. and RH 80%. Evaluation was based on the following criterion. 
E: no clogging beyond 100 passes 
G: 50 passes.ltoreq.clogging&lt;100 passes 
F: 20 passes.ltoreq.clogging&lt;50 passes 
P: clogging within 20 passes 
The results are shown in Table 1. 
TABLE 1 
__________________________________________________________________________ 
Riversing magnetic Square- 
Sample 
field Abrasive 
.rho.1 ness Br Porosity 
RF output 
No. Intensity (G) 
Reversal 
(pbw) (%) .rho.1/.rho.2 
ratio 
(G) (%) (dB) Still 
clogging 
__________________________________________________________________________ 
1 1000 10 15 10 1.6 0.84 2570 
6.9 +0.6 .smallcircle. 
.smallcircle. 
E 
2 1000 15 15 13 2.5 0.84 2590 
6.2 +0.7 .smallcircle. 
.smallcircle. 
3 1500 20 10 11 3.4 0.85 2750 
3.3 +0.8 .smallcircle. 
.smallcircle. 
4 1500 20 15 14 2.7 0.84 2630 
4.3 +0.7 .smallcircle. 
.smallcircle. 
5 1500 20 20 20 3.3 0.84 2540 
6.4 +0.4 .smallcircle. 
.smallcircle. 
11* -- -- 15 7 1.0 0.81 2340 
14.3 0.0 xP xP 
12* -- -- 25 10 1.0 0.80 2120 
18.7 -0.2 .smallcircle. 
.smallcircle. 
13* -- -- upper 25 
10 2.0 0.83 2400 
8.3 -0.2 .smallcircle. 
.smallcircle. 
(double lower 12.5 
layer) 
__________________________________________________________________________ 
*outside the scope of the invention 
The effectiveness of the present invention is evident from Table 1. 
Comparative sample No. 13 having a thin upper layer was less smooth in 
calendering and therefore, had a rough surface, resulting in a lowering of 
RF output. In contrast, sample Nos. 1 to 5 within the scope of the 
invention showed improved electromagnetic properties and durability. 
Equivalent results were obtained with magnetic tape samples which were 
prepared by replacing the ferromagnetic powder by another, and replacing 
the abrasive by Al.sub.2 O.sub.3, Cr.sub.2 O.sub.3 and SiC alone or 
mixtures thereof. 
There has been described a magnetic recording medium comprising a magnetic 
layer in which the abrasive is locally distributed such that its 
concentration is highest near the surface and decreases toward the 
substrate side. The medium is satisfactorily reliable and durable while 
the electro-magnetic properties of the magnetic layer for high density 
recording are maintained. 
The method of the invention is not only effective in localizing the 
abrasive, but also helps debubbling and orientation of the magnetic 
coating, contributing to a further improvement in electromagnetic 
properties. 
While the invention has been described with reference to a preferred 
embodiment, it will be understood by those skilled in the art that various 
changes may be made and equivalents may be substituted for elements 
thereof without departing from the scope of the invention. In addition, 
many modifications may be made to adapt a particular situation or material 
to the teachings of the invention without departing from the essential 
scope thereof. Therefore, it is intended that the invention not be limited 
to the particular embodiment disclosed as the best mode contemplated for 
carrying out this invention, but that the invention will include all 
embodiments falling within the scope of the appended claims: