Magnetoresistive element

A shunt biased magnetoresistive element includes a sensor part sensitive to an external magnetic field and a pair of leads (electrodes) in contact with the sensor part. The width (W) of the contact between the sensor part and each of the leads is selected to be equal to or larger than the width (L) of the sensor part. The MR element further includes a center tap having a width (W') selected to be two or more times as large as the width (L) of the sensor part, because the quantity of current flowing through the center tap is two times as large as that flowing through each of the end leads.

BACKGROUND OF THE INVENTION 
This invention relates to the structure of a magnetoresistive element 
(which will be abbreviated hereinafter as an MR element), and more 
particularly to a structure which ensures uniform flow of current through 
a shunt biased MR element comprising a magnetoresistive film (which will 
be abbreviated hereinafter as an MR film) and a conductor film making 
electrical contact with the MR film. 
A magnetic head employing an MR film (which head will be abbreviated 
hereinafter as an MR head) is now being widely used. In the case of such 
an MR head, it is necessary to externally apply a predetermined magnetic 
field in order to improve the sensitivity and linearity of the MR film. 
This magnetic field is called a bias magnetic field, and various methods 
including (1) a method of disposing a permanent magnet in the neighborhood 
of the MR film, (2) a method of disposing a conductor film in contact with 
the MR film, and (3) a method of disposing a soft magnetic film in the 
neighborhood of the MR film, have been proposed hitherto. Especially, the 
method described in (2) is called a shunt biasing method, and such a shunt 
biasing method is disclosed in, for example, U.S. Pat. No. 3,967,368. This 
U.S. patent shows an arrangement as shown in FIG. 1. Referring to FIG. 1, 
both a sensor part 10 exhibiting a magnetoresistive effect in response to 
an externally applied magnetic field and leads 20 and 25 supplying a 
predetermined current to the sensor part 10 to produce a bias magnetic 
field and deriving a change in the resistance of the sensor part 10 as a 
voltage change are made of an MR film and a conductor film. However, in 
the structure of the shunt biased MR element shown in FIG. 1, no 
consideration was given to the widths W and W' of the leads 20 and 25 at 
the surface of the shunt biased MR element remote from the surface 15 
facing a recording medium. Further, the relation between the sensor part 
10 and the leads 20, 25 was not also considered. As a result, a uniform 
current did not always flow through the sensor part 10, and the desired 
bias magnetic field could not be applied to the MR film. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to clarify the dimensional 
relation between the sensor part and the leads and to provide an MR 
element structure which ensures uniform flow of current through the sensor 
part so as to produce the desired appropriate bias magnetic field. 
In accordance with an aspect of the present invention which attains the 
above object, the width W of the portions (the electrodes), where the 
sensor part sensitive to an externally applied magnetic field is in 
contact with the leads, is selected to be equal to or larger than the 
width L of the sensor part. In the case of the center tap, the quantity of 
current flowing therethrough is two times as large as that flowing through 
each of the both end leads. Therefore, the object of the present invention 
may be also attained when the width W' of the portion (the electrode) 
where the sensor part is in contact with the center tap is selected to be 
two or more times as large as the width L of the sensor part.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The basic principle of the present invention will be first described. 
When current flows through an L-shaped conductor, the current distribution 
is generally dependent upon the ratio between the width of one of the 
conductors and that of the other. FIG. 3a shows the result of computer 
simulation of current flow in such a case. In FIG. 3a, the density of 
curves 30 represents the current density. It will be seen in FIG. 3a that 
the width W of the conductor 40 is considerably smaller than that L of the 
other conductor 50, and the flow of current is concentrated in the inside 
portion 60 of the wider conductor 50. Thus, the current flow in the wider 
conductor 50 is not uniform. This is because equipotential lines 70 (which 
are orthogonal to the direction of current flow) in the conductors are 
distributed as shown in FIG. 3b, and the distribution of those in the 
wider conductor 50 changes more than that of those in the narrower 
conductor 40. 
The ratio of the quantity of current flowing through the inside portion 60 
(having a width Lu=1/2L) of the conductor 50 to the total quantity of 
current flowing through the plane A-A' of the conductor 50 shown in FIG. 
3a was calculated. FIG. 4 shows the relation between the conductor width 
ratio W/L and the ratio of the quantity of current flowing through the 
portion 60 having the width Lu. The value of the current quantity ratio of 
50% means that the current flows uniformly through the conductor 50. It 
will be seen from FIG. 4 that the current flow is substantially uniform 
when the conductor width ratio W/L is 1 or more. Thus, when the conductors 
50 and 40 are supposed to provide the sensor part 10 and the lead 20 
respectively of the MR element shown in FIG. 1, the conductor width ratio 
W/L is required to be equal to or larger than 1 in order that the current 
flows uniformly through the sensor part 10. 
The value of W/L should not be excessively large when a multi-track 
structure is provided by arranging a plurality of MR elements in 
side-by-side relation. This is because the value of M/L determines the 
lower limit of the track pitch, and it is difficult to increase the track 
density. While current does not flow in the direction of the track width T 
in the hatched regions 35 (FIG. 1) defined between the two end leads 20 
and the sensor part 10, which makes the bias for the MR element inproper 
and thus makes the sensitivity of the MR element low, when the MR element 
is used to form a magnetic head for overwriting information on a magnetic 
recording medium such as a magnetic tape, and when the magnetic head 
deviates from a recorded track, the MR film in the hatched regions 35 will 
be magnetized due to magnetization in the previously recorded track 
(because the hatched regions also have MR films being magnetic films) and 
this magnetization transferred to the sensor part 10 is read or reproduced 
as noise which degrades the S/N (signal-to-noise) ratio of the magnetic 
head. Therefore, it is undesirable to unnecessarily increase the value of 
W/L, and this value of W/L is generally preferably selected to be about 2 
to 2.5 at the maximum. 
The above description has referred to the leads 20 in contact with the both 
ends respectively of the sensor part 10. When the MR element is to be 
operated as a differentially sensed, shunt biased MR element by providing 
a center tap 25 as shown in FIG. 2, the quantity of current flowing 
through this center tap 25 is two times as large as that flowing through 
each of the two leads 20. Therefore, the ratio between the width W' of the 
center tap 25 and the width L of the sensor part 10 should be selected to 
be two times as large as the ratio W/L, in order that current can flow 
uniformly through the sensor part 10. That is, it is apparent that the 
current distribution becomes uniform when the ratio W'/L is selected to be 
2 or more. 
When the width W' of the center tap 25 is excessively large, it also leads 
to a source of trouble. That is, in the hatched sensor region 36 having 
the width W' in FIG. 2, current does not flow in the widthwise direction 
of the track. Accordingly, the region 36 having the width W' in the track 
width T becomes a non-sensitive region which does not contribute to signal 
reproduction, and a low output results. Therefore, the ratio W'/L is 
preferably about 4 to 5. 
FIGS. 2 to 4 illustrate the case where the leads 20 and 25 have a 
rectangular shape. However, the leads 20 and 25 may be narrowest at the 
portions contacting the sensor part 10 and may become progressively wider 
at portions remoter from the sensor part 10 as, for example, shown in FIG. 
5. It is apparent that, when the widths of the leads 20 and 25 at the 
portions contacting the sensor part 10 and W and W' respectively, the 
relations set forth above are the conditions required for ensuring uniform 
flow of current through the sensor part 10. 
An embodiment of the present invention will now be described. FIG. 6 is a 
schematic perspective view of a shunt biased MR head embodying the present 
invention. The method of making the shunt biased MR head shown in FIG. 6 
is basically the same as that disclosed in U.S. Pat. No. 3,967,368. First, 
a film 110 of an electrical insulator such as alumina is deposited by 
sputtering on a base 100 of ferrite. A film 120 of an electrical conductor 
such as titanium is evaporated on the insulator film 110, and an MR film 
130 is then formed on the conductor film 120 in such a relation that its 
magnetic easy axis extends in a direction as shown by the black arrow, in 
FIG. 6. A film of an electrical insulator such as alumina (not shown) is 
then deposited on the MR film 130 as a protective layer, and lead-out 
conductors are provided to form an MR element. A block 140 formed of the 
same material as that of the base 100 or of a film of a magnetic material 
such as permalloy is then mounted on the MR element to complete the MR 
head. This block 100 constitutes a magnetic circuit together with the base 
100 and acts as a shield, so that the MR head can operate with a high 
resolution. 
The shape of the MR element shown in FIG. 6 is the same as that having the 
center lead 25 shown in FIG. 2. In the illustrated embodiment, the values 
of Tw and L are Tw=20 .mu.m and L=10 .mu.m respectively. Two MR heads were 
manufactured for the purpose of comparison. In one MR head, the width W of 
the leads was 20 .mu.m according to the present invention, while the width 
W was 5 .mu.m in the other MR head. In each of the two MR heads, the width 
W' of the center lead was W'=2W. The MR films of the two MR heads had the 
same thickness of 600 .ANG., and the conductor films of the two MR heads 
had also the same thickness of 1800 .ANG.. 
The two shunt biased MR heads manufactured in the manner described above 
were used to reproduce information recorded on a commercially available 
1/2-inch magnetic tape 150 by the same recorder. In this case, current 
flowed into the center lead from the both end leads in each of the MR 
elements. The magnetic tape 150 was disposed in a relation very slightly 
spaced (generally, 1 .mu.m or less) from the MR head and was driven in a 
direction as shown by the arrow in FIG. 6. 
When the same quantity of current was supplied to each of the two MR heads, 
the output of the MR head having W=20 .mu.m according to the present 
invention showed an improvement of 4 dB when compared to that of the 
comparative MR head having W=5 .mu.m. The above improvement in the output 
of the MR head manufactured according to the present invention is attained 
by the fact that the current flow through the MR element, especially, the 
conductor film 120 is more uniform. 
The above advantage will be described with reference to FIG. 7. In the 
graph of FIG. 7, the horizontal axis represents the position in the MR 
film as measured in the direction of the width L of the MR film, and the 
vertical axis represents the intensity of a bias and a signal magnetic 
field. The curve .circle.1 indicates the distribution of the signal 
magnetic field. The curve .circle.2 indicates the distribution of the 
bias magnetic field when the current flow in the conductor layer is 
uniform. The curve .circle.3 indicates the distribution of the bias 
magnetic field when the current flow in the conductor layer is locally 
concentrated in the inside portion of the conductor layer. It will be seen 
from comparison between the bias magnetic field distribution curves 
.circle.2 and .circle.3 shown in FIG. 7 that the bias magnetic field is 
more intense at the MR film portion which faces the magnetic tape and 
where the signal magnetic field is intense. FIG. 8 shows changes in the 
resistance of the MR film relative to the intensity of the bias magnetic 
field so as to explain the relation between the input and the output of 
the MR head. It will thus be seen in FIG. 8 that a signal magnetic field 
having a higher intensity induces a greater change in the resistance of 
the MR film in the presence of an appropriate bias magnetic field, and, as 
a result, the output of the MR head increases. 
It will be understood from the foregoing description of the present 
invention that, because of the uniform flow of current through a sensor 
part of a shunt biased MR element, a bias magnetic field appropriate for 
the MR element can be applied so that an MR head can generate a higher 
reproduced signal output, and the linearity of the output can be improved. 
Further, because the current does not locally concentrate in the sensor 
part and leads of the MR element, the life time of the MR element can be 
extended.