CONVERTISSEUR MAGNETIQUE COMPRENANT UN ELEMENT MAGNETORESISTANT

MAGNETIC CONVERTER HAVING A MAGNETORESISTIVE ELEMENT

Abrégé anglais

PHN 8050
2-2-1976
ABSTRACT:
A magnetic converter having a magnetoresistive
element. In order to be able to use a magnetoresistive
element which couples a large height (and hence good resistan-
ce to detrition) to a good reproduction of short wave signals,
the resistance of the element is measured by means of an
alternating current hving a frequency which is larger than
<IMG>, where ? is the resistivity, ? the magnetic per-
meability and D the cross-section of the element. The current
distrubution in the element is then controlled by the lateral
skin effect.

Revendications

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. A magnetic converter for detecting information
representing magnetic fields on a magnetic recording medium
which is movable relative to the converter, comprising a
substantially rectangular strip of magnetoresistive material,
the width of said rectangular strip being substantially greater
than the thickness thereof, and having a free edge which is
intended to face the recording medium, and two electric connec-
tions on opposite ends of the strip for connection to an electric
circuit for detecting resistance variations, said circuit com-
prising a current source, characterized in that the current
source is designed to supply a high frequency alternating current
having a frequency wt which is larger than <IMG>, where ? is the
magnetic permeability in Henry/m of the magnetoresistive material,
is the reisitivity in ohm.m and D is the cross-section in m2.
2. A device as claimed in Claim 1, characterized in that
the current source is a Gunn oscillator.
3. A device as claimed in Claim 1, characterized in that
the current source is an L.S.A. oscillator.

Description

"Magnetic converter having a magnetoresistive element".
.,
The invention relates to a magnetic converter
for detecting infor~ation-representing magnetic fields on
a magnetic recording medlum which is movable relative to the
converter, comprising a substantially rectangular strip of
magnetoresisti~e material ha~ing a free edge which is intended
to face the recording medium, and two electric connections on
opposite ends of the strip for connection to an electr$cal
circuit for detecting resistance variations, said circuit com-
prising a current source.
~agnetic converters in which a magnetoresistive
element is used as a sensing element are known~ for example,
from United States Patent Specifications 3,493,694 and 3,691,5400
The operatio~ of such magnetoresistive heads is based on the
use of a strip-shaped element of a ferromagnetic, metallic ~`
material~ for example ~i-Fe, which is provided on a non magnetic
substrate snd which is moved in the i~mediate proximity of, or
in contact with, a magnetic recording medium for reproducing
the information contents thereof. The field of the recording
medium produces variations in the magnetic contition of tbe
element and modulates the resistance thereof via the so~called
magnetoresistance effect. This means that, when the recording
medium passes the head9 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 an external electrical circuit
which is connected to the element assumes the form of current
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or voltage fluctuations representing the information stored in the recording
medium. For this purpose, the ends of the element are connected to a current
source which supplies a measuring current which may be both a direct current
and an alternating current. One end of the element is intended to face the
recording medium during operation. For good opPration it is important that
said edge be as near as possible to, and preferably in oontact with, the
recording mldium. AS a result o~ this, however, the dimension of the
element normal to said edge (the height) decreases during use as a result of
detrition. In order to maintain good opera~ion of the element as long as
possible, one would want to choose the height as large as possible, for
example, 100 microns. For a good reproduction of short wavelengths, however,
the height may not bP chosen to be too l æ ge and typically is from 5 to 10
micxons.
It is the object of the invention to provide a solution to this
dilemma. In ac~ordance with this invention there is provided a magnetic
con~er~er for detecting information-representing magnetic fields on a
magnetic recording medium which is movable relative to the converter,
comprising a 5ubstantially rectangular strip of magnetoresistive material,
the width of said rectangular strip being substantially greater than the
thickness thereof, and having a free edge which is intended to face the
reoording medium, and two elect~ic connections on opposite a~ds of the
strip for connection to an electric cixcuit for detecting resistance varia-
~ions, said circult oomprising a current source, characterized in that the
current source is ~esigned to supply a high frequency alternating current
having a frequency ~3t which is larger than ~ , where /u is the magne~ic
permeability in Henry/m of the magnetoresistive material,~ is ~he resistiv-
ity in ohm.m and D is the cross-section in m .
By opexating the strip of magnebDresistive material with such a
high frequency current, a current distribution occurs in the strip which is
given by the laberal skin effect. Said current distribution is characterized
by a concentration o~ the current at the sdges of the strip,
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6;253
namely at the edge facing the recording medium and the opp~sibe-
ly located ~dge. As a result of this it is possible with a
comparatively large height of the strip to nevertheless
achieve a large sensitivity.
A Gunn oscillator or L.S.A. oscillator is pre-
ferably used as a current source in the device according to
the invention. Oscillators of this type produce a sufficient
power at the frequencies required m the scope of the in-
vention.
The invention will be described in greater de-
tail, by way of example, with reference to the drawing.
Figure 1 shows diagrammatically a magnetic
converter having a magnetoresistive strip o the type to which
the present invention relates.
Figure 2 is a sectional view of the magneto-
resistive strip shown in Figure 1 ln which the current dis-
tribution is shown which occurs in the construction acoording
to the invention.
Figure 1 shcws a magnetic converter havin~ a
strip 1 of magneboresistive material provided on a non-magnetic
substrate 3 and connected to said strip is a current source 2.
The so~LAce 2 is a high-~requency ascillating current source
which controls the measuring curr~nt Is through the strip 1.
The voltage be~w~en the terminals 7 and 8 of the strip 1 is
denoted by Vs and is measured by means of the measuring m-
strument 4. This voltage is a measure of the information-
representing magnetic fields 6 originating fm m the magneto-
recording medium 5 bo which the strip 1 is coupled magnetically.
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The strip 1 is at right angles to the plane of the recording
medium 5. The dimensions of the strip l are denoted by the
latters a, b and c~ where a is the height of the strip, b is
the thickness and c is the track width. For the str-lp 1, thin
layers of material are used Qf which Ni-Fe having a ~hickness
between 200 and ~000 ~ is the most usual.
When it is ensured that the curren~ source 2
produces a measuring current Is having a frequency ~t which
is larger than 8 P b' and if a > > b~ which is just desired
for the application as a magnetic head, the measuring current
will be concentrated, as a result of the occurrence of the
so-called lateral skin effect, on the two opposite edges of
the s~rip l, as is shown in Figure 2, which is a sectional
view of the strip 1, by means of the shaded area. It is to be
ncted that the di~ension a in ma~netoresistive elemen~s is
indeed many times larger than the di~ension b. For example,
a = 100 /um and b = 0~1 /um. With a resistivity ~ = 15.10 8
ohm.m and a relative magne~ic pen~eability h r 10 the value
of the limit frequency for such a s~rip (the frequency above
which the lateraL skln effec~ occur~) then is 1519.8 MHz. This
frequency is just in the range for which a sufficient power
. . ,
is reached with Gunn or L. S.AA oscillators for the present
appli~ation (lO mW in c.w~ operation, with efficiency of 2 to
5 ~a ) ~
As a result of the concentration of the current
at the strip edges, the di~ensio~ a may be chosen to be so ~ --
large that a larger resis~ance to detrition is achiev~d than ;
iS posslble in the known co~verters of the kind mentioned in
', ~
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.. .. . . .. . , ,, . . . ~ .

,253
the preamble. As a is made larger, the demagnetizing field in
the interior of the s~rip becomes smaller, so that the sen-
sitivity to exte~nal fields increases and the loss of sen-
sitivity by concentration of the measuring current at the
edge of the strip 1 which is furthest away from ~he recording
medium 5, is compensated for.
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