Magnetoresistive head

A magnetic reading device having two electrically series-arranged magnetoresistive elements, in particular for interference compensation and positioning control. A pattern of equipotential strips is provided on each element to adjust the quiescent angle .alpha. between the direction of magnetisation and the direction of current passage of one element between 30.degree. and 60.degree. and between 210.degree. and 240.degree., respectively, and that of the other element at 360.degree. - .alpha. and 180.degree. - .alpha., respectively, the device has for various applications an automatic compensation of the signals across the elements so that the use of difference amplifiers is superfluous. In addition, due to this adjustment, the reproduction characteristics has a linear variation.

The invention relates to a magnetic reading device for detecting 
information-representing magnetic fields on a magnetic recording medium, 
comprising a number of flat magnetoresistive elements each having an 
in-plane easy axis of magnetisation and each comprising at their ends 
electric contacts via which the elements are connected to an electric 
circuit for detecting their resistance variations. 
In IBM Technical Disclosure Bulletin, vol. 14, No. 8, January 1972, pp. 
2488-9 a device is described the operation of which is based on the use of 
strip-shaped elements of a ferromagnetic, metallic material, for example 
Ni-Fe, provided on a non-magnetic substrate, which elements are connected 
in a bridge circuit and one of which is brought in the immediate proximity 
of, or in contact with, a magnetic recording medium with one of its edges. 
The field of the recording medium produces variations in the magnetisation 
of the latter element and modulates the resistance thereof via the 
magneto-resistance effect. This means that, when the recording medium 
passes the device, the information-representing magnetic fields present on 
the medium rotate the spin system of the magnetoresistive element so that 
the resistance varies. In this manner the output signal of the bridge 
circuit in which the element is incorporated assumes the form of voltage 
fluctuations which represent the information stored in the recording 
medium. 
In order to compensate for the effect on the magnetoresistive element of 
external interference fields, at least one further magnetoresistive 
element is incorporated in the bridge circuit. This element does not 
"sense" the magnetic fields of the recording medium but does "sense" 
interfering external fields. 
Scanning devices are also known in which two magnetoresistive elements, 
each having a length equal to half the track width, are arranged beside 
each other. By supplying their signals to a difference amplifier, an 
indication is obtained about the place of such a magnetoresistance 
combination above the track. 
The two known devices require comparatively complicated electric circuits 
and/or magnetoresistive configurations to obtain the desired output 
signal. 
In addition there is the difficulty that the variation of the resistance of 
a magnetoresistive element under the influence of a magnetic field is 
quadratic, so that it is desirable upon scanning analog recordings to 
optimize the operation of the element by linearizing the resistance 
characteristic. It is known to do this by applying a transverse magnetic 
bias field by means of external means to an elongate element the easy axis 
of magnetisation of which coincides with the longitudinal direction of the 
element. Under the influence of this field, the direction of magnetisation 
of the element which in the absence of an external field coincides with 
the easy axis of magnetisation is rotated through a certain angle. The 
strength of the bias field is preferably such that the direction of 
magnetisation encloses an angle of 45.degree. with the direction of 
current passage through the element. The drawbacks of the use of the 
transversal magnetic bias field are that their is a danger that the 
information on the recording medium will be varied by it, and that it is 
difficult to adjust the strength of the field at the correct value to give 
all the magnetoresistive elements used the same bias. 
It is an object of the invention to provide a magnetic scanning device 
which does not have the said drawbacks. For that purpose, the device 
according to the invention is characterized in that it comprises a first 
and a second magnetoresistive element having equally oriented directions 
of magnetisation, on each of which is provided a pattern of mutually 
parallel equipotential strips to adjust the quiescent angle .alpha. 
between the direction of magnetisation and the direction of current 
passage of the first element between 30.degree. and 60.degree. and the 
quiescent angle of the second element, at 360.degree. - .alpha., the 
elements being connected in series. With an adjustment of .alpha. between 
210.degree. and 240.degree. the quiescent angle of the second element 
should be 180.degree. - .alpha.. 
The effect of the above-described configuration resides in the fact that, 
by providing a pattern of equipotential strips on an element, the current 
is forced to travel at a desired angle with the longitudinal direction so 
that the quiescent angle between the direction of magnetisation and the 
direction of current passage can easily be adjusted. In this manner it is 
particularly simple to cause the quiescent angles between the easy axis 
and the direction of current passage of two magnetoresistive elements 
connected in an electric circuit to differ such that the resistance 
variations of the elements under the influence of a magnetic field are of 
opposite sign. In other words, the compensation is in the configuration 
itself, so that the use of a difference amplifier as in the known 
magnetoresistance devices having interference compensation or positioning 
control, is not necessary. Moreover the desired adjustment is realised 
without it being necessary to apply a bias field which might vary the 
information on the recording medium. 
A preferred embodiment of the device according to the invention is 
characterized in that a magnet is present for generating at the area of 
each element a magnetic auxiliary field the direction of which coincides 
with the direction of magnetisation of the element. As will be explained 
in detail hereinafter, such a longitudinal auxiliary field which 
preferably has a strength which is of the order of magnitude of the 
coercive field strength of the material of the magnetoresistive elements, 
enables the selection of one of the two possible stable states of the 
magnetisation. This makes it possible to series-produce scanning devices 
of the present type in a reproducible manner, it being practical when of 
each element the direction of magnetisation is parallel to its edge to be 
facing the recording medium. 
In particular, the principle of the invention may be used advantageously in 
scanning devices having positioning control or interference compensation. 
For that purpose, a first modified embodiment of the device according to 
the invention is characterized in that the magnetoresistive elements are 
of equal length and are positioned in line and have a combined length 
which corresponds to the width of information tracks present on a 
recording medium to be scanned, the elements being connected to a current 
source, a voltage being formed between the final contacts of the pair 
formed by the first and second element when the pair is not centred on a 
track to be followed, positioning means controlled by said voltage being 
present to continuously keep the pair of elements centred on a track to be 
followed. 
For that purpose, a second modified embodiment of the device according to 
the invention is characterized in that the device comprises a substrate an 
edge of which is intended to cooperate with a recording medium to be 
scanned, the first and the second magnetoresistive element being provided 
at different distances from the said edge. 
In the latter case it is also possible to compensate for crosstalk of the 
signals of tracks present beside a selected track when it is ensured that 
the distance to the edge of the substrate of the element farthest remote 
from the said edge is of the order of magnitude of the distance between 
two adjacent information tracks on a recording medium to be scanned.

FIG. 1 shows a magnetoresistive element 1 consisting of an NiFe film 2 
having gold connections 3, 4 and covered with a pattern of stripes 5, 5', 
5'', 5''', 5'''' of gold which extend at an angle of 135.degree. with the 
direction of magnetisation M in the quiescent state. Under the influence 
of the -- equipotential -- stripes 5, 5', 5'', 5''', 5'''', the current I 
will travel at an angle of approximately 45.degree. with (M.). It has been 
suggested to designate a magnetoresistive element having such a stripe 
pattern as a "Barber Pole". This name will be used hereinafter. The static 
resistance characteristic as is shown in FIG. 3 belongs to said 
configuration. Because the magnetisation in the NiFe may have two stable 
states (solid line (M) and broken line (M') in FIG. 1), there are in fact 
two static resistance characteristics (shown in FIG. 3 as a solid line and 
as a broken line), dependent on which of the two stable states is present. 
As is usual, it has been assumed that the NiFe film comprises only one 
magnetic domain. 
One of the two stable states, for example M, can be selected by means of a 
longitudinal field H.sub.y having a strength of the magnitude of the 
coercive force of the NiFe (see FIG. 1). 
In the stable state (of FIG. 1), in which the solid line arrow hence 
indicates the actual direction of magnetisation, a field H.sub.x which 
represents the information on the recording medium 6 will rotate the 
magnetisation in such manner that the angle between the magnetisation and 
the current I (maintained by a current source 7) increases; in that case 
the resistance decreases (see solid - line characteristic of FIG. 3). 
In FIG. 2, in which the same reference numerals are used for the same 
components as in FIG. 1, the situation is shown that the stripes 5, 5', 
5'', 5''', 5'''', do not extend at an angle of 135.degree., but an angle 
of 45.degree. with M. Upon rotation of the magnetisation under the 
influence of the field H.sub.x, the angle between the current I and the 
direction of the magnetisation which in the quiescent state is -45.degree. 
will decrease; the resistance increases. This is denoted by the broken 
line static characteristic of FIG. 3. FIG. 3 shows the relative resistance 
variation .DELTA. R/R, as a function of the strength of the normalized 
external field H.sub.x /H.sub.o. H.sub.o is a field which indicates at 
what field saturation occurs while assuming that the quadratic character 
of the resistance characteristic is maintained up to an angle of 
90.degree. between the direction of current passage and the direction of 
magnetisation. H.sub.o depends on the height and the thickness of the 
magnetoresistive element in question. 
Hence two static resistance characteristics are possible which are replicas 
of each other relative to the H.sub.x /H.sub.o axis as a result of the two 
stable states of the magnetisation. 
If the stable state of the magnetisation is reversed while simultaneously 
reversing the direction of the gold stripes, then notching at all changes 
and the static characteristic remains the same. Hence, also two static 
characteristics which are replicas of each other with respect to the 
H.sub.x /H.sub.o axis are found in case the direction of the stripes is 
reversed. 
1. Interference Compensation 
Interference compensation can be obtained in known manner with an extra 
magnetoresistive element 12 parallel to a scanning element 8 but arranged 
at a larger distance from the magnetic medium 11 (FIG. 4). The two 
magnetoresistive elements 8, 12 should be incorporated in a bridge 
circuit. Both are controlled with a current source 10 and 14, 
respectively; one side, 9 and 13, respectively, of the elements 8, 12 is 
connected to earth. The difference of the voltage across the elements is 
amplified in a difference amplifier 15. 
For linearising the reproduction characteristics and for automatic 
interference compensation, two Barber Poles are used within the scope of 
the invention the stripes of which are replicas of each other relative to 
an axis perpendicular to the recording medium 22 (see FIG. 5). 
Furthermore, a longitudinal field H.sub.y is applied which has the same 
direction for both. The two Barber poles 16, 17 are arranged in series and 
operated with one current source 18. If now, as a result of a magnetic 
interference field, the resistance of one of them increases, that of the 
other one decreases; so the overall resistance does not vary as a result 
of a magnetic interference field. The voltage between the final contacts 
19, 20 of both elements is now supplied to a normal amplifier 21 instead 
of to a difference amplifier. So the compensation is already in the Barber 
poles due to the replicated characteristics. The field H.sub.x which 
represents the information on the recording medium 22 only influences the 
resistance of the element 17. 
2. Centering on the Track 
A track searching head as described in the published Dutch Patent 
Application No. 7,309,590 consists of two magnetoresistive elements which 
are positioned in line. This may also be considered as one 
magnetoresistive head 23 having a centre tapping 24 (FIG. 6). Said centre 
tapping 24 is connected to earth and both halves 25, 26 of the element 23 
are controlled by means of current sources 27 and 28, respectively. The 
difference of the voltages between the contacts 24, 29 and 24, 28, 
respectively, is applied to a difference amplifier 31. When the head is 
centrally above a track on the recording medium 31, both halves 25, 26 
supply the same amount of signal, so the difference signal then is equal 
to zero. When the head is partially above a track, one half will produce 
more signal than the other one. 
In this case also, within the scope of the invention, the elements may be 
constructed as replicated "Barber Poles", as is shown in FIG. 7. This 
figure shows a magnetoresistance configuration 33 which consists of two 
halves 34, 35 having stripe patterns which are replicas of each other 
relative to an axis normal to the medium. There is one longitudinal field 
H.sub.y. The configuration according to the invention needs only two 
connections 36, 37. When the head is present centrally above a track on 
the recording medium 38, the resistance fluctuations in the two halves 34, 
35 are opposite, so no output signal is generated. When the head is 
partially on a track, the resistance of one half varies more strongly than 
that of the other half (where said variation is opposite), so that a net 
resistance variation remains and the head generates an output signal. This 
signal may be used to control positioning means which keep the head 
centrally above the track. The assembly is operated by means of one 
current source 39 and the voltage across the final contacts 36, 37 is 
amplified with a normal amplifier 40. 
The following is remarked with reference to the embodiments shown in the 
figures. The magnetoresistive elements are always shown in a position in 
which they extend normal to the plane of the recording media. However, 
they may also be arranged parallel to the plane of the recording media. 
See, for example, the article by R. P. Hunt entitled: "A magnetoresistive 
transducer" published in "IEEE Transactions on Magnetics", vol. mag. -7 
No. 1, March 1971, pp. 150-154. 
For the rest, the substrates on which the films are provided and possible 
further envelopes are not shown in the Figures for reasons of clarity.