Magnetoresistant transducer for reading very high-density data

A magnetoresistant transducer is described for reading very high-density a. The magnetoresistant transducer (TMRI) includes at least one magnetoresistor (MRI) of an anisotropic magnetic material placed perpendicular to the direction in which the data of a medium (SM) pass. The axis (AFAI) of easy magnetization of the magnetoresistor is normal to the medium and includes a thin, mono-range or single domain magnetic anisotropic layer (CI) normal to the medium and to the direction in which the data pass. The mono-range or single domain layer is strongly coupled magnetically with the magnetoresistor, and its axis of easy magnetization (AFACI) is normal to the medium and opposite in direction to the axis of easy magnetization of the magnetoresistor. The invention is particularly applicable to the reading of magnetic tapes and disks containing high density data.

FIELD OF THE INVENTION 
The present invention relates to magnetoresistant transducers. It is in 
particular applicable to the reading of the data contained on magnetic 
recording media such as rigid or flexible magnetic disks and magnetic 
tapes, and is more particularly intended for the reading of data having a 
very high density, i.e. greater than 5000 pieces of information per 
centimeter. 
DESCRIPTION OF THE PRIOR ART 
Magnetic disks carry data on circular, concentric recording tracks, which 
have a radial width on the order of several tens of microns and generally 
cover the greater part of their two sides. Magnetic tapes, on the other 
hand, generally carry data on tracks parallel to the length of the tape. 
Generally, a stream of magnetic data on tracks parallel to the length of 
the tape. 
Generally, a stream of magnetic data recorded on the tracks of a disk or a 
tape appear in the form of succession of small magnetic ranges called 
"elementary ranges" distributed over the entire length of the track and 
having magnetic inductions with the same modulus and the opposite 
direction. 
The number of pieces of information per unit of length measured along the 
circumference of a track in the case of a magnetic disk or according to 
the length of the tape in the case of a magnetic tape is called 
longitudinal density (or linear density). When the density is greater than 
5000 pieces per centimeter the medium is considered to have a very high 
density and may be termed a VHD disc or VHD tape, as the case may be. The 
number of recording tracks per unit of length measured along the diameter 
of the disk is called the radial density of the data (in the case of a 
magnetic disk). The present trend in the development of magnetic disks is 
to increase both the linear density and the radial density of the data. 
The means which make it possible, either to record data on disks or tapes 
or to read data recorded thereon, or again to perform both of these 
functions are called "magnetic transduction systems". Generally, there is 
associated with a recording medium one or more magnetic transduction 
systems with the medium being driven so as to move past the system or 
systems. In current practice, when it is desired to read very high-density 
linear and/or radial data magnetic disks, more and more frequently resort 
is made to transduction systems which include one or more magnetoresistors 
designated under the name of "magnetoresistant transducers". It may be 
recalled that a magnetoresistor is an element consisting of a magnetic 
material the electrical resistance R of which varies according to the 
magnetic field to which it is subjected. In current practice, these 
magnetoresistors are electrical resistors having the form of very thin 
layers (several hundred angstroms to several microns), the length of which 
is much greater than the width. Frequently, these magnetoresistors are 
arranged over a substrate of an electrically insulating material. 
To provide a better understanding of the invention, a brief review of 
applicable magnetic principles will be undertaken. Consider, for example, 
a measurement magnetoresistor R connected to the terminals of a current 
generator which produces a current having an intensity I circulating in 
the direction of the length of the magnetoresistor and assume that the 
magnetoresistor is part of a magnetoresistant transducer associated with a 
magnetic recording medium and that it is located at a very small or even 
nil distance from the medium. When each of the elementary magnetic fields 
passes in front of the transducer, an overflow magnetic field H.sub.f 
created in the vicinity of the surface of the medium causes a variation 
.DELTA.R in its resistance, whence a variation .DELTA.V=I.times..DELTA.R 
at its terminals, which gives .DELTA.v/V=.DELTA.R/R, .DELTA.R/R being 
called the magnetoresistance coefficicent. Ordinarily this coefficient is 
on the order of 2%. It can, therefore, be seen that the variation in 
voltage picked up at the magnetoresistor's terminals increases with the 
resistance R. 
The electrical signal picked up at the terminals of a magnetoresistor is 
only a function of the magnetic value H.sub.f to which it is subjected. 
Because of this, its amplitude is independent of the speed of the 
recording medium face with which it is associated. 
It may be recalled that the ratio (B/H) between the induction of the 
magnetic field and the magnetic field itself when B and H are close to 
zero on the curve of first magnetization is designated as the "initial 
magnetic permeability of a magnetic material". (The curve of first 
magnetization is the curve which gives the variation of B as a function of 
H when the magnetoresistor is subjected to a magnetic field of 
magnetization beginning with an initial magnetic state of the material 
defined by B and H close to zero). In other words, the initial magnetic 
permeability of the material is equal to the slope of the curve of first 
magnetization in the vicinity of the point B=0 and H=0. 
It may further be recalled that a magnetic anisotropic material located in 
a plane and having a thickness far less than its length and also its 
width, presents in this plane two privileged magnetization directions, 
generally perpendicular to one another. One of them is called "direction 
of easy magnetization," while the other is called "direction of difficult 
magnetization". The initial permeability of the material in the direction 
of difficult magnetization is much greater than the initial permeability 
of the material in the direction of easy magnetization. 
Generally, magnetoresistors consist of a magnetically anisotropic material 
(for example, an iron-nickel alloy with 18% iron and 82% nickel). Their 
axis of easy magnetization is parallel to the direction of the current I 
and to their length, while their axis of difficult magnetization is 
perpendicular to the direction of the current. 
In current practice, the position of the magnetoresistor of a 
magnetoresistant transducer in relation to the recording medium associated 
with it is such that it is subjected to a component of the overflow field 
of the elementary ranges which is parallel to the axis of difficult 
magnetization, the magnetoresistor itself being perpendicular to the 
surface of the medium. When a magnetoresistor is not subjected to any 
magnetic field, we say that it is at rest. In this case, the magnetization 
(i.e., the magnetic induction inside the magnetoresistor) has the same 
direction as the axis of easy magnetization. 
Generally, the efficiency and likewise the sensitivity, of a 
magnetoresistor of a magnetically anisotropic material, i.e., the voltage 
of its output signal as a function of the magnetic field applied to it, 
may be determined by subjecting the magnetoresistor to a magnetic 
polarization field H.sub.pol parallel to its axis of difficult 
magnetization as described in French Pat. No. 2,165,206 entitled "Improved 
Magnetoresistors and Electromagnetic Transducer Incorporating Them". 
The value of the polarization field H.sub.pol is selected such that the 
magnetization of the magnetoresistor turns at an angle .theta., preferably 
close to 45.degree., in relation to its position at rest. 
In present practice, the linear and radial densities of the data obtained 
are such that the length of the elementary magnetic ranges is slightly 
greater than a micron and the radial width L.sub.p of the tracks is on the 
order of 10 to 20 microns. Under these conditions, the component of the 
overflow field of the elementary ranges to which the magnetoresistor is 
subjected is relatively low. In order to obtain a maximum signal/noise 
ratio, the dimensions of the magnetoresistor must be such that its height 
h measured according to a direction normal to the recording medium, and 
its length measured perpendicularly to the direction in which the data 
pass must be respectively on the order of 5 and 15 microns. 
Technologically, it is extremely difficult to achieve magnetoresistors 
having such dimensions. In current practice, the height h of 
magnetoresistors is on the order of 20 microns and may even reach 40 to 50 
microns. In the latter case, these magnetoresistors are intended for 
reading data on magnetic tapes and the magnetoresistors are maintained in 
contact with the magnetic tape causing the magnetoresistive material to be 
quickly worn away. 
It can be shown that the signal/noise ratio drops if h increases. Indeed, 
the signal at the terminals of a magnetoresistor subjected to the 
component of the magnetic overflow field normal to the medium drops as h 
increases, since the section of the magnetoresistor increases and 
consequently its resistance R diminishes. Furthermore, the noise signal 
increases and consequently its resistance R diminishes. Furthermore, the 
noise signal increases, since the component of the magnetic overflow field 
H.sub.f acts on the magnetoresistor only to a height h' lower than h. To 
increase the S/N signal, we are therefore let to increase the resistance 
of the magnetoresistor, i.e., to increase its length, which makes it 
possible to increase the useful signal S. In this case, the width of the 
track is less than the length of the magnetoresistor. The latter is then 
subjected to a part of the magnetic overflow fields produced by the tracks 
neighboring the track which the magnetoresistor faces. 
Under these conditions, to improve the S/N ratio, magnetoresistant 
transducers are used which have two parallel magetoresistant elements 
separated by a distance on the order of a tenth of a micron, less than the 
length of the elementary magnetic ranges, such that these magnetoresistors 
are subjected to the same component of the magnetic overflow, i.e., that 
produced by the range in front of which they are located. The two 
magnetoresistant elements are each polarized to a value on the order of 
45.degree. (in absolute value), their magnetization then being at a value 
of 90.degree. relative to one another. The output signal from the first 
magnetoresistant element is sent over a first lead-in of a differential 
amplifier, while the output signal from the second magnetoresistant 
element is sent over a second lead-in of the same amplifier. The absolute 
value of the output signal of the amplifier is proportional to twice the 
absolute value of the output signal of a single magnetoresistor and the 
signal/noise ratio is appreciably improved. 
In such an arrangement, the two magnetoresistors are subjected not only to 
the component H.sub.f of the magnetic overflow field produced by the 
magnetic range of the track P which they face, but also to the resultant 
of the magnetic overflow fields produced by the magnetic ranges located on 
this same track P, on either side of the range which they face. This 
resultant produces a noise signal which increases when the linear density 
of the data increases. It is then more difficult to discern any useful 
signal corresponding to a piece of data information of a track in a medium 
in relation to the noise signal. To cancel out the effects of this 
resultant on the output signal of the magnetoresistors, resort may be made 
to the use of magnetic screening means disposed on either side of the 
magneresistors. The magnetic screening means may consist of magnetic 
material, preferably anisotropic, the plane of which is perpendicular to 
the recording medium and to the direction in which the tracks move. 
However, such magnetoresistant transducers have a certain number of 
drawbacks, the most important of which can be categorized into three types 
as follows: 
1. When the linear density of the data reaches a value on the order of 5000 
inversions of magnetic flux per centimeter (which corresponds to 5000 
changes in the direction of the magnetic induction), the following 
phenomena are produced: 
(a) the resultant of the magnetic overflow fields produced by the ranges of 
the track P which surround the range which the two magnetoresistors of the 
magnetoresistant transducer face becomes significant (on the order of the 
component H.sub.f of the magnetic overflow field); 
(b) the distance between the magnetic screening means and the 
magnetoresistant elements becomes so low (on the order of a micron) that 
the magnetic coupling between the magnetoresistant elements and the 
screening means becomes significant. 
As a result of the two phenomena described above, a major mutual induction 
is produced between the magnetic screening means and the magnetoresistant 
elements which appreciably modifies the magnetization in them (intensity 
of the magnetization, angle of polarization), and this increases with the 
linear density of the data. This mutual induction results in an 
interference signal which may destroy the information which is to be read, 
i.e., which may totally cancel out the effects of the component H.sub.f of 
the magnetic overflow field on the two magnetoresistors. 
2. There is a phase difference between the two magnetoresistors such that 
the distance between them and the magnetic recording medium is different. 
This results in the value of the magnetic field to which each of the two 
magnetoresistors is subjected being different, and consequently the output 
signal at their terminals is different. This difference may be relatively 
major. As a result, it becomes difficult to exploit the signals delivered 
by the differential amplifier connected to this magnetoresistant 
transducer. 
3. The technological realization of magnetoresistant transducers of this 
type is delicate, and the cost of these tranducers increases when the 
linear density of the data increases. 
SUMMARY OF THE INVENTION 
The present invention makes it possible to remedy or at least minimize the 
above drawbacks in that there is provided a magnetoresistant transducer of 
extremely simple design, utilizing only one magnetoresistor in which the 
S/N ratio is relatively large, the magnetoresistor being virtually 
insensitive to the magnetic overflow fields other than the component 
H.sub.f of the magnetic overflow field produced by the elementary magnetic 
range which it faces. Furthermore, the useful signal is increased by using 
a magnetoresistor with a length greater than the radial width of the 
tracks (which increases resistance, as has been stated above). 
The principle of the invention consists of using a magnetoresistor in which 
the axis of each magnetization is perpendicular to the recording medium, 
this magnetoresistor being strongly coupled magnetically to a thin, 
magnetic mono-range or single domain of an anisotropic magnetic material, 
in which the axis of easy magnetization is likewise normal to the 
recording medium. The axes of easy magnetization of the magnetoresistor 
and the mono-range or single domain layer are antiparallel, the plane of 
the mono-range layer being perpendicular to the recording medium and to 
the direction in which the data move. 
When the component H.sub.f of the magnetic overflow field produced by the 
range which the magnetoresistor faces has a sign opposite that of the axis 
of easy magnetization of the mono-range or single domain layer, there is a 
reversal in the direction of the axis of easy magnetization in the 
mono-range or single domain layer, and because of the significant magnetic 
coupling between the layer and the magnetoresistor there is a reversal in 
the direction of the axis of easy magnetization in the magnetoresistor, 
which produces a variation .DELTA.R in it which is relatively significant. 
The reversal in the direction of the magnetization takes place only if the 
value of the component H.sub.f is greater than the coercive field H.sub.c 
of the magnetic material constituting the mono-range or single domain 
layer. 
It may be recalled that by definition the magnetic induction (magnetization 
of a thin, magnetic, mono-range or single domain layer has the same 
direction at any point whatsoever on that layer. When reference is made 
herein to mono-range layer, it should be understood to refer to a single 
domain layer and the terms "mono-range" and "single domain" are used 
interchangeably throughout the specification. 
According to the invention, there is provided a magnetoresistant transducer 
for reading data contained on a recording medium having at least one 
magnetoresistor of an isotropic material placed perpendicular to the 
direction in which the data pass, through which a current I runs in the 
direction of its length. The axis of easy magnetization of the 
magnetoresistor is normal to the medium and it has a thin, magnetic, 
anisotropic layers and mono-range normal to the medium and to the 
direction in which the data pass. The magnetic mono-range is strongly 
coupled magnetically with the magnetoresistor, and the axis of easy 
magnetization is normal to the medium and opposite in direction to that of 
the magnetoresistor. There is thus provided an extremely simple, 
inexpensive magnetoresistant transducer in which the signal/noise ratio is 
significantly improved over conventional devices.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In order to better understand how the magnetoresistant transducer according 
to the invention is constituted, it is advantageous to first summarize the 
operating principles of the magnetoresistors illustrated by FIGS. 1a, 1b, 
and 2; the problems of utilizing these magnetoresistors in cases where the 
data density is very high, as illustrated by FIGS. 3, 4 and 5; and finally 
the problems associated with magnetoresistant transducers having two 
magnetoresistors according to the prior art as illustrated by FIGS. 5 and 
6. 
Referring to FIGS. 1a and 1b, there is illustrated an elementary 
magnetoresistant transducer TMRE consisting of a single magnetoresistor 
MR. In FIG. 1a, magnetoresistor MR is shown arranged facing the track P of 
a magnetic recording medium SM, such as magnetic disk. The width L of the 
magnetoresistor is appreciably greater than the width L.sub.p of the track 
P. Its height h measured perpendicularly to the medium SM is, for example, 
on the order of 20 to 30 microns. The length L is greater than the width 1 
(also called thickness of the magnetoresistor). At both of its ends, the 
magnetoresistor MR has junction condcutors (not shown in order to simplify 
FIG. 1a) allowing it to be connected to circuits for reading data 
contained on the medium SM. The magnetoresistor MR consists of an 
anisotropic magnetic material. Its axis Ax.sub.f of easy magnetization is 
parallel to its length, and its axis Ax.sub.d of difficult magnetization 
is perpendicular to this large dimension and to the medium SM. The 
magnetoresistor MR is supplied by a current I which circulates, for 
example, in the direction indicated in FIGS. 1a and 1b, i.e., parallel to 
the axis Ax.sub.f. 
The magnetoresistor MR is subjected to the component H.sub.f of the 
magnetic overflow field created by the elementary magnetic ranges of a 
track P of the medium (some of the ranges, namely A.sub.1, A.sub.2 . . . 
A.sub.i, are shown in FIG. 1a). The component H.sub.f is normal to the 
recording medium and thus parallel to the axis of difficult magnetization 
Ax.sub.d. 
FIG. 2, which shows the curve of variation R of the resistance R of the 
magnetoresistor MR as a function of the magnetic field H applied to it 
along its axis of difficult magnetization Ax.sub.d, makes it possible to 
better understand the operation of the elementary magnetoresitant 
transducer TMRE. It can be seen that, for a value of H called "field of 
anisotrophy of the material" and also defined as H.sub.k, the magnetic 
material constituting the magnetoresistor is saturated in its direction of 
difficult magnetization so that the resistance R no longer varies. It is 
possible to give the magnetiation so that the resistance R no longer 
varies. It is possible to give the magnetoresistor MR a maximum 
sensitivity by displacing the axis of the ordinates in FIG. 2 from the 
origin O.sub.1 to the origin O.sub.2 by subjecting it to a field H.sub.pol 
of polarization as is indicated in the aforenoted French Pat. No. 
2,165,206. This field H.sub.pol is produced by an outside source not shown 
in order to simplify FIG. 1a and is parallel to the axis of difficult 
magnetization Ax.sub.d and consequently parallel to the component H.sub.f 
of the magnetic overflow field of data in the medium SM and normal to the 
plane of the recording medium. In the case where the magnetoresistor is 
subjected to this magnetic field of polarization, the variation R in its 
magnetoresistance is relatively significant and may even be maximum for a 
given value of the field H.sub.pol corresponding to a rotation in the 
direction of magnetization in relation to its position at rest by an angle 
of close to 45.degree.. Hence a relatively weak modification H in the 
magnetic field applied to the magnetoresistor leads to a relatively 
significant variation R in its resistance. We thus define an operating 
point PF by the abscissa of which O.sub.1 O.sub.2 is equal to H.sub.pol. 
If the magnetoresistor is subjected to the component H.sub.f of the 
magnetic overflow field of the data in the medium, there results a 
variation in resistance .DELTA.r.sub.f and a voltage picked up at the 
output terminals .DELTA.v= I .times..DELTA.R.sub.f. It is thus shown that 
around the operating point PF the variation in resistance is a function of 
the magnetic overflow field applied to the magnetoresistor in the 
direction of its axis in this field. 
Referring to FIG. 3, it can be seen that the reading of a piece of 
information by the magnetoresistor MR takes place when the latter is 
placed facing two elementary magnetic ranges A.sub.i-1 and A.sub.i of the 
track P of the medium SM. The magnetoresistor takes a relative position 
facing the boundary FR.sub.i of the two magnetic ranges A.sub.i-1 and 
A.sub.i. The magnetoresistor is not only subjected to the component 
H.sub.f of the magnetic overflow field produced by the two ranges A.sub.i 
and A.sub.i-1, but also to the resultant of the magnetic overflow fields 
produced respectively by the couples of neighboring magnetic ranges and 
opposite magnetic induction couples, for example, the magnetic couples 
EQU A.sub.i-1, -A.sub.i-2, A.sub.i-2, -A.sub.i-3, A.sub.i -A.sub.i+1, A.sub.i+1 
-A.sub.i+2, etc. 
This resultant, called H.sub.iv, becomes significant in relation to the 
component H.sub.f when the linear density of the data on the track P 
becomes extremely high (greater than 5000 pieces of information per 
centimeter). Referring to FIG. 4, let us consider the magnetoresistor MR 
which is placed facing the track P and is assumed to be perfectly centered 
above it, which means that the axis of symmetry of the magnetoresistor and 
the circular axis of symmetry of the track P which are normal to the 
medium SM are confused. 
When the radial density of the data becomes very significant, the 
magnetoresistor MR is subjected to the resultant H.sub.envi of the 
magnetic overflow fields produced by the two neighboring tracks P' and P" 
of the track P and by the two residues RES.sub.1 and RES.sub.2 of data 
which bear witness to the prior state of the medium SM, i.e., the state 
this medium had before recording of the tracks P, P', P", etc. It can be 
seen that the residue RES.sub.1 is located between the track P and the 
track P', while the residue RES.sub.2 is located betwen the track P and 
the track P", etc. This resultant H.sub.envi produces a significant noise 
signal at the terminals of the magnetoresistor in relation to the signal 
produced by the component H.sub.f. 
Referring to FIG. 5, when the density of the data is very high, the value 
of the component H.sub.f drops in such a way that only a part of the 
magnetoresistor MR is subjected to this field, which part is indicated by 
oblique dashes in FIG. 5. It can be seen that the magnetoresistor is 
subjected to this field only over a height h.sub.u that is appreciably 
lower than the total height h of the magnetoresistor. As hereinbefore 
indicated, under these conditions, i.e., those shown in FIGS. 3, 4 and 5, 
the signal/noise SN ratio of the magnetoresistor MR is considerably 
weakened when the linear and radial data density is increased. 
In current practice, in order to eliminate the drawbacks mentioned above 
with the aid of FIGS. 3, 4 and 5, transducers are used such as the 
transducer TMRA shown in FIG. 6. Such a transducer has two 
magnetoresistors MR.sub.1 and MR.sub.2 parallel to one another, and first 
and second magnetic screening means MB.sub.1 and MB.sub.2. This transducer 
TMRA is shown placed facing the track P of a recording medium SM, of which 
several elementary magnetic ranges have been shown, namely the ranges 
A.sub.1, A.sub.2, A.sub.i-1, . . . A.sub.i, A.sub.j, A.sub.j+1. The 
elements MR.sub.1 and MR.sub.2 are strictly identical to the element MR 
shown in FIGS. 1a and 1b, and a current I runs through them in the 
direction of their length. Let L.sub.1, AX.sub.f1, AX.sub.d1 be 
respectively, the length, the axis of easy magnetization and the axis of 
difficult magnetization of the magnetoresistor MR.sub.1. Likewise, let 
L.sub.2, AX.sub.f2, AX.sub.d2 be respectively the length, the axis of easy 
magnetization and the axis of difficult magnetization of the 
magnetoresistor MR.sub.2. The lengths L.sub.1 and L.sub.2 are essentially 
equal to one another, and their width is essentially greater than the 
width L.sub.p of the track P. 
The two magnetoresistors are polarized as follows: the magnetization 
AM.sub.1 of the magnetoresistor MR.sub.1 creates an angle of more than 
45.degree. with the axis of easy magnetization AX.sub.f1, i.e., with the 
position which the magnetization AM.sub.1 had when the magnetoresistor 
MR.sub.1 was at rest, while the magnetization AM.sub.2 of the 
magnetoresistor MR.sub.2 creates an angle of -45.degree. with the axis of 
easy magnetization AX.sub.f2, i.e., with the position which the 
magnetization AM.sub.2 had when the magnetoresistor MR.sub.2 was not 
subjected to any magnetic field. The two magnetizations AM.sub.1 and 
AM.sub.2 of the magnetoresistors between therefore make an angle of 
90.degree.. 
The magnetic screening means MB.sub.1 and MB.sub.2 are of an anisotropic 
magnetic material and have an axis of easy magnetization, respectively 
AF.sub.1 and AF.sub.2, and an axis of difficult magnetization, 
respectively AD.sub.1 and AD.sub.2. The axes AF.sub.1 and AF.sub.2 are 
parallel to the axes Ax.sub.f1 and Ax.sub.f2, while the axes of difficult 
magnetization AD.sub.1 and AD.sub.2 are parallel to the axes of difficult 
magnetization Ax.sub.d1 and Ax.sub.d2. 
The distance betwen the two magnetoresistors is sufficiently short for them 
to be subjected to virtually the same component H.sub.f of the magnetic 
overflow field produced by the data couple A.sub.i-1 and A.sub.i, reading 
of the data taking place when MR.sub.1 and MR.sub.2 are essentially 
equidistant from the boundary FR.sub.i separating these two ranges. 
The screening means MB.sub.1 and MB.sub.2 make it possible to channel and 
pick up the magnetic field lines from the resultant H.sub.iv of the 
magnetic overflow fields produced by the magnetic range couples of the 
track P found on either side of the range couple A.sub.i-i -A whose 
boundary the two magnetoresistors face. 
As indicated above, the voltage .DELTA.v.sub.1 delivered by the resistor 
MR.sub.1 and the voltage .DELTA.v.sub.2 delivered by the magnetoresistor 
MR.sub.2 are sent over a first and second lead-in repsectively of a 
differential amplifier, at the outlet of which a signal is picked up 
proportional to twice the absolute value of .DELTA.v, which is essentially 
equal to the absolute value of .DELTA.v.sub.1 and .DELTA.v.sub.2. The use 
of a differential amplifier also allows reducing the noise signal (due in 
particular to the magnetic data on the tracks neighboring the track P and 
the data residues such as RES.sub.1 and RES.sub.2 shown in FIG. 4). 
As indicated above, the magnetoresistant transducers such as the TMRA 
transducer have a certain number of drawbacks when the data density 
becomes very high (greater than 5000 pieces of information per 
centimeter). These transducers become technologically difficult to 
achieve, and therefore costly. Phenomena of mutual inductance develop 
between the magntic screening means MB.sub.1, MB.sub.2 and the 
magnetoresistors MR.sub.1 and MR.sub.2 which modify the magnetic state of 
the magnetoresistor (intensity of magnetization, value of the angle of 
polarization). This results in detection of the component H.sub.f of the 
magnetic overflow field produced by the range couple A.sub.i-1 and A.sub.i 
being appreciably perturbed, which may go as far as the complete 
destruction of the useful information, i.e., of the two voltages 
.DELTA.v.sub.1 and .DELTA.v.sub.2 resulting from the variation in 
resistance of the two magnetoresistors subjected to this component 
H.sub.f. Finally, a third drawback is illustrated in FIG. 7. Because of 
the technological difficulties of achieving the TMRA transducer, the two 
magnetoresistors MR.sub.1 and MR.sub.2 are very often staggered in 
relation to one another, so that the distance between the first 
magnetoresistor MR.sub.1 and the medium SM is different from the distance 
separating the magnetoresistor MR.sub.2 from this same medium. This 
results in the two signals .DELTA.v.sub.1 and .DELTA.v.sub.2 being quite 
appreciably different, which further considerably perturbs the outlet 
signal from the differential amplifier connected to the transducer TMRA. 
The transducer TMRI according to the invention makes it possible to remedy 
the drawbacks listed above. Its principle, illustrated by FIGS. 8a and 8b, 
consists of utilizing a magnetoresistor MRI in which the axis of easy 
magnetization AFAI is perpendicular to the plane of the recording medium 
SM. This magnetoresistor MRI is strongly coupled magnetically with a thin, 
magnetic, mono-range layer CI. The plane of the latter is perpendicular to 
the plane of the recording medium and to the direction F in which the data 
pass. This mono-range layer consists of an anisotropic magnetic material, 
and its axis of easy magnetization AFACI is perpendicular to the plane of 
the recording medium SM and opposite in direction to the axis of easy 
magnetization AFAI of the magnetoresistor MRI. A nonmagnetic layer CISI is 
placed between the magnetoresistor MRI and the magnetic mono-range layer 
CI. 
The operation of the magnetoresistant transducer TRMI according to the 
invention will be better understood by referring to the FIGS. 9 and 10. 
In FIG. 9 the transducer TMRI has been shown occupying three different 
positions POS.sub.1, POS.sub.2, POS.sub.3 before three magnetic range 
couples, namely A.sub.i-1 -A.sub.i, A.sub.i -A.sub.i+1, A.sub.i+1 
-A.sub.i+2, of respective boundaries FR.sub.i, FR.sub.i+1, FR.sub.i+2. 
When the transducer TMRI is placed facing the boundary FR.sub.i, it 
occupies the position POS.sub.1 ; when it is facing the boundary 
FR.sub.i+1 it occupies the position POS.sub.2, and finally when it is 
facing the boundary FR.sub.i+2 it occupies the position POS.sub.3. This 
manner of illustration makes it possible to consider the manner in which 
data is read by a single transducer as the medium is driven past the 
reading head. 
When the transducer TMRI occupies the position POS.sub.1 the component 
H.sub.f of the overflow field produced by the range couple A.sub.i-1 
-A.sub.i has the same direction as the axis of easy magnetization AFACI of 
the magnetic mono-range layer CI. 
Referring to FIG. 10, which represents the hysteresis cycle of the magnetic 
material constituting the layer CI, it can be seen that whatever the value 
of the magnetic field H, if it remains positive the induction B 
(magnetization) in the layer CI remains positive and equal to B.sub.r. If, 
consequently, there is no variation of direction in the magnetization (of 
the axis of easy magnetization AFAI) of the magnetoresistor MRI and, 
consequently, there is no variation in resistance in it (therefore, no 
signal at its terminals). 
When the transducer TMRI occupies the position POS.sub.2, the component of 
the magnetic overflow field H.sub.f(i+1) produced by the magnetic range 
couple A.sub.i -A.sub.i+1 is opposite in direction to the component 
H.sub.fi. This is defined as being negative in direction. By referring to 
the hysteresis cycle in FIG. 10 (this cycle, which is rectangular, is an 
ideal cycle corresponding to an ideal magnetic material, and it is obvious 
that in practice the real hysteresis cycles of the real magnetic materials 
are not strictly rectangular), we see then that we go from a positive 
magnetic field value H, as was the case for H.sub.fi, to a negative 
magnetic field value, as is the case for H.sub.f(i+1), H.sub.f(i+1) being 
less than -H.sub.c where H.sub.c is the coercive field of the magnetic 
material constituting the layer CI; the magnetization in the latter (and 
also the axis of easy magnetization AFACI) changes direction completely, 
going from the value+B.sub.r to -B.sub.r. 
This complete change in direction of the magnetization (also of the axis of 
easy magnetization AFACI), because of the strong magnetic coupling between 
the layer CI and the magnetoresistor MRI, brings on a complete change in 
direction of the magnetization (complete change of the axis of easy 
magnetization AFACI) in the latter. 
It is assumed that the coercive field HC of the magnetoresistor is less (in 
absolute value) than the coercive field H.sub.c of the layer CI. 
To determine the variation in resistance of the magnetoresistor, reference 
may be had to FIG. 11a. The magnetic field applied to this magnetoresistor 
varies from a positive value (H.sub.fi) to a negative value (H.sub.fi+1)) 
lower than -HC (i.e., greater in absolute value than 
.vertline.HC.vertline.). We have in fact 
.vertline.H.sub.fi+1).vertline.&gt;.vertline.H.sub.c .vertline., itself 
greater than .vertline.HC.vertline.. 
In the case of an ideal magnetic material with a perfectly rectangular 
hysteresis cycle, the curve of variation of resistance .DELTA.R/R as 
function of the magnetic field applied is given by the abscissa half-line 
-HC, which half-line is called PDT.sub.1. This curve is also called a 
DIRAC peak. To simplify, this curve will be called the theoretical DIRAC 
peak. 
When the magnetic material constituting the magnetoresistor is a real 
magnetic material having a nonrectangular hysteresis cycle, the curve of 
variation of magnetoresistance is given by the curve PDR.sub.1, which to 
simplify will be called the real DIRAC peak. 
When the transducer TMRI occupies the position POS.sub.3, it faces the 
boundary FR.sub.i+2 between the two magnetic ranges A.sub.i+1 and 
A.sub.i+2 and is subjected to the magnetic field component H.sub.f(i+2) 
which is positive and created by these two ranges, in the vicinity of 
their boundary FR.sub.i+2. It can be seen that the component H.sub.f(i+2) 
is opposite in direction to the direction of the magnetization in the 
layer CI, when the transducer occupies the position POS.sub.2 (referring 
still to FIG. 9). 
H.sub.f(i+2) being positive and greater than the coervice field H.sub.c of 
the magnetic material consituting the layer CI, by referring to the 
hysteresis cycle for this material shown in FIG. 10 we can see that the 
magnetization in the layer CI (and consequently the axis of easy 
magnetization AFACI) changes direction, going from the value -B.sub.r to 
the value+B.sub.r, since the magnetic overflow field to which the layer is 
subjected goes from a negative value to a positive value. This brings 
about a change in direction of the magnetization in the magnetoresistor 
MRI (and consequently a change in direction of the axis of easy 
magnetization AFAI). The curve of variation of the magnetoresistance 
coefficient of the magnetoresistor MRI, which makes it possible to 
determine the variation in resistance of it, is then indicated by the FIG. 
11b, since the magnetic field to which the magnetoresistor is subjected 
varies from a negative value (H.sub.f(i+1) in FIG. 2) to a positive value 
greater than HC. As in the case of FIG. 11a, for an ideal magnetic 
material having a perfectly rectangular hysteresis cycle, the curve of 
variation is given by the abscissa half-line+HC, namely the half-line 
PDT.sub.2, called theoretical DIRAC peak, while for a real magnetic 
material having a nonrectangular hysteresis cycle the curve of variation 
of the magnetoresistance coefficient is given by the curve PDR.sub.2, 
which we call real DIRAC peak for simplification. 
Whether the curve of variation of the magnetoresistance coefficient is that 
indicated in FIG. 11a or that indicated in FIG. 11b, with the 
magnetoresistor MRI being traversed in the direction of its length by a 
current I, there results a corresponding variation in voltage 
.DELTA.v=I.times..DELTA.R which is a voltage pulse having the form of a 
DIRAC peak. 
When the medium SM passes before the transducer TMRI according to the 
invention, the phenomena of variation in the direction of the 
magnetization in the layer CI and in the magnetoresistor MRI, as well as 
the variations in resistance of the magnetoresistor resulting from this 
are reproduced identically to those described above, when the transducer 
according to the invention occupied the position POS.sub.2 or the position 
POS.sub.3. 
FIG. 12 shows a preferred embodiment of a transducer according to the 
invention TMRI.sub.1. This transducer includes a thin, mono-range magnetic 
layer CI.sub.1 having a height h.sub.cm, in which the plane is 
perpendicular to the direction in which the data pass defined by the arrow 
F, and also perpendicular to the plane of the recording medium SM which 
carries these data, a nonmagnetic insulating layer CISI.sub.1, the height 
of which measured perpendicularly to the medium SM is essentially equal to 
h.sub.cm and the thickness of which is between 100 and 500 angstroms, 
approximately, a first and second magnetic layer CMI.sub.11 and CMI.sub.12 
of essentially equal dimensions, a magnetoresistor MRI.sub.1 placed 
between the two layers CMI.sub.11 and CMI.sub.12 and on the insulating 
layer CISI. The thin magnetic layers CMI.sub.11 and CMI.sub.12 and the 
magnetoresistor MRI.sub.1 are preferably made of the same anisotropic 
magnetic material. 
The axes of easy magnetization AFAM.sub.11, AFAM.sub.12, AFAI.sub.1 of the 
magnetic layers CMI.sub.11 -CMI.sub.12 of the magnetoresistor MRI.sub.1 
have the same direction (and consequently magnetizations in these layers 
and in the magnetoresistor). These axes of easy magnetization are parallel 
to the axis of easy magnetization AFACI.sub.1 and opposite in direction to 
it. The length of the magnetoresistor is slightly greater than the width 
L.sub.p of a track P of the medium SM, as can be seen in FIG. 12b. 
The purpose of the thin magnetic layers CMI.sub.11 and CMI.sub.12 which, as 
can be seen in FIGS. 12a and 12b, are in the same plane as the 
magnetoresistor MRI.sub.1, is to minimize the demagnetizing fields at the 
level of the magnetoresistor MRI.sub.1 tending to break them up into 
different ranges with antiparallel magnetization (i.e., the magnetizations 
of two adjacent ranges have the same direction and different senses) and 
consequently to render the magnetoresistor ineffective, that is, to cancel 
out the effect which consists of recording a variation in resistance when 
it is subjected to any magnetic field. 
Let us consider FIG. 13, showing a transducer TMRI.sub.2 according to the 
invention, which is a variant of preferred realization of the transducer 
shown in FIG. 12. This transducer TRI.sub.2 has a mono-range magnetic 
layer CI.sub.2, thin magnetic layers CMI.sub.2 1 and CMI.sub.2 2 which are 
identical and surround the magnetoresistor MRI.sub.2. The latter and the 
two magnetic layers CMI.sub.21 and CMI.sub.22 are disposed on a layer of 
nonmagnetic material CISI.sub.2, the arrangement being disposed on the 
thin mono-range magnetic layer CI.sub.2. The exposed face of layer 
CMI.sub.22 being placed against the upper flat face of mono-layer 
CI.sub.2. That part of the mono-range layer CI.sub.2 closest to the medium 
SM and which is also called the lower part of this layer may be broken 
down into three parts, namely a central part PCI.sub.2 and two lateral 
parts PLGI.sub.2 and PLDI.sub.2. The central part PCI.sub.2 is located 
at,a distance from the magnetic recording medium SM equal to d.sub.1, 
extremely short and less than the distance d.sub.2 separating the parts 
PLGI.sub.2 and PLDI.sub.2 from the recording medium SM. 
The length L of the magnetoresistor (which is essentially equal to the 
length of the thin mono-range layer CI.sub.2 and the thin layers 
CMI.sub.21 and CMI.sub.22) is noticeably larger than the radial width 
L.sub.p of the tracks of the medium (on the order of 2.4 times or more). 
FIG. 13 shows three of these tracks, namely the adjacent racks P', P and 
P". 
Let H.sub.f1 and H.sub.f2 be respectively the components of the magnetic 
overflow field of the data produced by the magnetic range couples of the 
track P and the tracks P' and P", which penetrate respectively into the 
central part PCI.sub.2 and into the lateral parts PLGI.sub.2 and PLDI. It 
is clear that H.sub.f1 is appreciably greater than H.sub.f2 in absolute 
value. 
The distances d.sub.1 and d.sub.2 are established such that Hhd f1 is 
greater in absolute value than the coercive field H.sub.c of the 
mono-range magnetic layer CI.sub.2, while H.sub.f2 is less than this value 
of the coercive field. This results in only the component H.sub.f1 of the 
magnetic overflow field created by the magnetic range couples of the track 
P being able to produce a reversal of the magnetization in the layer 
CI.sub.2, and consequently a reversal of the magnetization of the 
magnetoresistor MRI.sub.2, and hence a variation in voltage at its 
terminals. The transducer TMRI.sub.2 detects only the data on the track P, 
while having a relatively significant output signal, since the 
magnetoresistor MRI.sub.2 has a high resistance because of its great 
length. 
FIG. 14 makes it possible to determine how the nature of the data read by a 
transducer according to the invention can be detected (in FIG. 14, the 
transducer TMRI has been shown, but it is obvious that the reasoning is 
identical for the transducers TMRI.sub.1 or TMRI.sub.2), i.e., in fact, 
the direction of the magnetic overflow field created by the magnetic range 
couples of a track P of the medium SM at right angles to the boundary 
separating them. On each track P at its start (the start of the tracks is 
found by the particular piece of information placed on the magnetic disk), 
there is created two special pieces of information I.sub.00 and I.sub.0, 
defined by the boundary between the elementary ranges A.sub.00 and A.sub.0 
on the one hand and by A.sub.0 and A.sub.1 on the other. The component of 
the magnetic overflow field corresponding to the piece of information 
I.sub.00 is designated by H.sub.f00, while the component of the magnetic 
overflow filed corresponding to the piece of information I.sub.0 is 
designated by H.sub.f0. The direction of the two components H.sub.f00 and 
H.sub.f0 is assumed to be that indicated in FIG. 14. Two cases may then be 
produced: 
First case: We assume that the magnetization (also the axis of easy 
magnetization (AFACI) in the mono-range layer CI has the same direction as 
the component H.sub.f00. When the transducer TMRI is placed facing the 
piece of information I.sub.00. the component H.sub.f00 having the same 
direction as the axis of easy magnetization AFACI, there is no reversal of 
direction of magnetization in the layer CI and consequently no signal at 
the terminals of the magnetoresistor MRI. The piece of information 
I.sub.00 is then not found. When the transducer TMRI, after having passed 
before the piece of information I.sub.00, passes before the piece of 
information I.sub.0, the component H.sub.f0 having a direction opposite to 
the direction of the axis of easy magnetization AFACI, there is then a 
reversal in the direction of magnetization in the layer CI and 
consequently the appearance of a signal at the terminals of the 
magnetoresistor MRI; the piece of information I.sub.0 can thus be found. 
Second case: before passing before the piece of information I.sub.00, the 
monorange layer CI of the transducer TMRI has a direction opposite that 
indicated in FIG. 14, i.e., a direction opposite the component H.sub.f00. 
When the transducer TMRI passes at right angles to the piece of 
information I.sub.00, there is a consequent reversal in the magnetization 
in the layer CI and consequently the appearance of a signal at the 
terminals of the magnetoresistor MRI. Likewise, when the transducer TMRI, 
after having passed before the information I.sub.00 (the direction of its 
axis of easy magnetization is then that indicated by FIG. 14), passes 
before the piece of information I.sub.0, the magnetization in the layer CI 
then having a direction opposite the component H.sub.f0, there is again a 
reversal in the magnetization and consequently the appearance of a signal 
at the terminals of magnetoresistor MRI, and consequently the piece of 
information I.sub.0 is found. 
It can be seen that in both the first case and the second case, the piece 
of information I.sub.0 is always found. The nature of the latter is 
perfectly well known, i.e., the direction of the component H.sub.f0, and 
consequently the direction of the magnetization in the later CI is known 
because it is known that it passed before the piece of information 
I.sub.0. As of that moment it is possible to determine the direction of 
the other pieces of information by simple deduction. 
While the invention has been described in connection with particular 
embodiments, variations thereof will readily suggest themselves to those 
skilled in the art. Accordingly, it should be understood that the foregong 
description is not intended to be one of limitation and resort should be 
made to the appended claims intended to include all such variations which 
come within the full scope and true spirit of the invention.