Combed MR sensor

A magnetic head assembly includes a magnetoresistive (MR) element for reading magnetically recorded information from a record medium. The MR element comprises an elongated member having a predetermined height and a plurality of elongated attachments which are contiguous with and made of the same material as the MR element. The elongated attachments have a predetermined width and a predetermined spacing and the attachments are unidirectionally magnetized along their length so that a bias is produced to maintain the MR sensor in a single domain state and to obtain a substantially linear component in the MR response in the MR element so that Barkhausen noise is suppressed and so that previously recorded information is read from the medium with optimum sensitivity.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates in general to magnetic transducers for transferring 
information signals to and from a magnetic medium and, in particular, to 
an improved magnetoresistive read transducer. 
2. Description of the Prior Art 
The prior art discloses a magnetic transducer referred to as a 
magnetoresistive (MR) sensor or head which has been shown to be capable of 
reading data from a magnetic surface at great linear densities. The MR 
sensor operates on the principle that the resistance of the read element 
is a function of the amount and direction of magnetic flux being sensed by 
the element. 
The prior art also teaches that in order for an MR element to operate 
optimally, two bias fields should be provided, a first bias field along 
its transverse direction as well as a second bias field along its 
longitudinal direction. The transverse bias orients the magnetization at 
some skew angle relative to the sense current, as is necessary to obtain a 
linear component of response to the flux field in the otherwise quadratic 
response. This bias field is normal to the plane of the magnetic media and 
parallel to the surface of the planar MR element. The longitudinal bias 
has the purpose of inducing a single domain state of magnetization, as is 
required for the suppression of Barkhausen noise. The longitudinal bias 
field extends parallel to the surface of the magnetic media and parallel 
to the lengthwise direction of the MR sensor. 
One type of bias method disclosed in the prior art is current induced bias 
in which a parallel auxiliary current produces a transverse bias field on 
the MR sensor strip. U.S. Pat. No. 3,813,692 shows one example of this 
bias method. Alternatively, the field from the sense current magnetizes a 
proximate soft magnetic "keeper film" whose stray field, in turn, exerts a 
transverse bias upon the MR sensor as is described in U.S. Pat. No. 
3,864,751. 
Another type of bias method is current deflection bias, as shown in U.S. 
Pat. No. 4,280,158. Here, the quiescent magnetization remains along the 
longitudinal direction, while, instead, the sense current is deflected to 
flow askew to the magnetization. This effect is achieved with a 
superpositioned "barberpole" conductor configuration. This method produces 
a transverse bias condition along with a small (typically insufficient) 
longitudinal bias field underneath the conductor. 
A further type of prior art bias method comprises permanent magnet bias in 
which the bias condition is produced with a proximate hard magnetic film. 
The hard magnetic film may be ferromagnetic and coupled magnetostatically 
to the sensor as shown in U.S. Pat. No. 3,840,898. Alternatively, the film 
may be antiferromagnetic and coupled to the sensor via an exchange 
mechanism as shown in U.S. Pat. No. 4,103,315. Either method can, at least 
in principle, provide a transverse and/or a longitudinal bias component. 
U.S. Pat. No. 4,418,372 discloses a magnetic rotary encoder comprising a 
substrate having a surface opposite to a rotary body having plural pieces 
of magnetic information recorded on at least one circumferentially running 
track. The substrate has a magnetoresistive element formed on its surface 
having a pattern including at least two portions extending in the radial 
direction of the rotary body and connected at the ends by a 
circumferentially extending portion. The disclosed arrangement is not 
designed to produce a bias in the MR element, but to produce an additive 
signal from each of the portions in the radial direction and phasing 
produced by the circumferential portions. 
The prior art bias methods have been effective to meet prior art 
requirements, but the additional structure required adds complexity to the 
MR read transducer which it is desirable to avoid. The prior art does not 
disclose a bias method to provide transverse as well as longitudinal bias 
solely through the use of a particular MR sensor geometry. 
SUMMARY OF THE INVENTION 
It is the principal object of this invention to provide transverse as well 
as longitudinal bias solely through the use of a particular sensor 
geometry. 
In accordance with the invention, a magnetic head assembly includes a 
magnetoresistive (MR) element for reading magnetically recorded 
information from a record medium. The MR element comprises an elongated 
member having a predetermined height measured in a direction normal to the 
magnetic medium from a first edge of the MR element which is adapted to 
face the recording medium. The MR element has a plurality of elongated 
attachments which are contiguous with and made of the same material as the 
MR element. The elongated attachments have a predetermined width and a 
predetermined spacing and the attachments are unidirectionally magnetized 
along their length so that a bias is produced to maintain the MR sensor in 
a single domain state and to obtain a substantially linear component in MR 
response in said MR element so that previously recorded information is 
read from said medium with optimum sensitivity, and so that Berkhausen 
noise is suppressed. 
In a specific embodiment, a plurality of elongated attachments extend in 
the direction of the edge of the MR element which is adapted to face the 
magnetic medium and a plurality of elongated attachments extend in the 
direction normal to the edge of the MR element which is away from the 
magnetic medium. 
In an alternate embodiment, an exchange bias film, patterned to the same 
geometry as the MR element, and the MR element are sandwiched together. 
The resultant combined bias improves the bias profile.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
To provide a better understanding of the magnetoresistive (MR) sensor 
according to the invention, some of the operating principles of an MR 
sensor will be reviewed with the aid of FIGS. 1 and 2. 
The MR sensor according to the invention is provided with transverse as 
well as longitudinal bias, and the resulting bias profile is such as to 
render the sensor with optimal sensitivity and side-reading 
characteristics. The desired shape of the bias profile, will be described 
with the aid of FIG. 1 relative to the transverse bias profile and with 
the aid of FIG. 2 with respect to the longitudinal bias profile. 
FIG. 1(a) is a sketch showing the quiescent magnetization, M, at some skew 
angle relative to the sense current, I, through the MR sensor 10. The plot 
of FIG. 1(b) shows that the transverse magnetization, My, varies linearly 
with bias strength, Hy, when Hy&lt;Hu, where Hu is the total anisotropy 
(induced+shape) of the MR sensor. When Hy.gtoreq.Hu, then My=Ms and the MR 
sensor is saturated. 
The relationship between the resistance R of the MR sensor and the 
transverse magnetization My is given by 
##EQU1## 
c.sub.mr is the Magnetoresistance coefficient which depends on the 
material used, and this constant is typically 2 to 3.times.10.sup.-2 for 
permalloy, for example. The plot shown in FIG. 1(c) shows that the 
sensor's resistive signal, .DELTA.R/R, depends in a quadratic fashion on 
My. Sensitivity, as measured by the slope of this response curve increases 
monotonically with My until My=Ms. At saturation, the sensitivity drops to 
zero. 
The sketch of FIG. 1(d) shows that the excitation, originating from 
magnetic charges in the medium 11, is largest at the lower edge of the MR 
sensor 10. On moving up on the sensor stripe, the flux leaks to the 
adjacent shields 12, and excitation decreases. As shown in FIG. 1(e) 
typically, the excitation is not a "small signal input" but, at least at 
the lower stripe edge, causes excursions over a substantial portion of the 
sensor's operating range. The excitation vanishes at the upper edge, h. 
Considering the bias dependence of sensitivity, and, the profile of 
excitation, it follows that maximum sensitivity is obtained by locally 
biasing the sensor to the largest skew permissible without the combined 
bias and excitation producing saturation. The plot in FIG. 1(f) shows the 
bias profile for maximum output signal. Bias skew increases in accord with 
the decreasing excitation. With such a bias profile, the excursions of 
transverse magnetization across the stripe, remain just short of 
saturation as is shown in FIG. 1(g). 
FIG. 2 illustrates relationships helpful in understanding the desired 
longitudinal bias profile. The longitudinal bias must be strong enough to 
retain the sensor in a single domain state, as is required for the 
suppression of Barkhausen noise. However, the stronger the longitudinal 
bias, Hx, the lower the sensitivity of the MR sensor. Typically, the 
applied longitudinal bias field, Ha, is uniform, as indicated in FIG. 
2(a), so that the effective bias field, He, being the sum of the applied 
and the demagnetizing field, Hd, peaks about the center of the sensor. The 
resulting concave sensitivity profile, S, is undesirable since the 
magnetic head would have a sensitivity that is lowest over the center of 
the track and highest about the edges of the track. A more advantageous 
convex sensitivity profile, FIG. 2(b), is attainable with a bias field 
that increases in strength toward the ends of the sensor. Such bias 
renders the sensor single domain, while leaving its center segment at 
maximum sensitivity. This is the sensitivity profile that is provided by 
the present invention. 
The present invention includes the addition of elongated or "comb" 
attachments 14 to the MR sensor 10 as is shown in FIG. 3. An elongated 
vertical comb attachment 14v, as shown in FIG. 3(a), provides transverse 
bias while the elongated horizontal attachment or comb 14h of FIG. 3(b) 
provides longitudinal bias. Both bias components are obtained from a 
configuration like the one shown in FIG. 3(c) in which both horizontal 
combs 14h and vertical combs 14v are provided on the same MR sensor 10. 
The combs 14 are contiguous with and made of the same material as the MR 
sensor element 10. 
It was found that these comb extensions 14h, 14v, when magnetized along 
their long axis, exert a magnetostatic bias on the MR sensor much like 
each extension was a permanent magnet. The extensions do, in fact, behave 
much like permanent magnets in that they exhibit a large hysteresis to 
magnetization reversal. This hysteresis can easily be ten times larger 
than the intrinsic coercive force of the sensor material. As the 
extensions 14 are made of a soft-magnetic material, typically Permalloy, 
such hysteresis may be surprising but is explainable considering their 
geometry. The long, narrow, extensions have a large shape anisotropy which 
imposes a substantial magnetostatic energy barrier to magnetization 
reversal. 
FIG. 4 shows the effects of transverse extensions. These MR characteristic 
curves were measured versus a varying transverse field, Hy, plus a 
constant longitudinal field, Hx, to retain a single domain magnetization. 
The three curves show plots of .DELTA.R/R vs Hx for different sensor 
geometries. Curve (a) shows the response of a 25 .mu.m wide stripe without 
extensions (as a reference). Curve (b) shows the response of transverse 
extensions on both sides of the MR stripe, and curve (c) shows the 
response of transverse extensions only on one side of the stripe. It can 
be observed from curves (b) and (c) that the sensor is put into either a 
positive or a negative state of internal transverse bias, depending on 
whether the applied field is reduced to zero from positive or from 
negative saturation. The sensor remains in that state until, at some 
threshold, Ht, of reversed field, the bias state reverses. The curves 
demonstrate that a transverse bias condition producing a linear response 
characteristic can be obtained simply by means of the transverse 
attachments to the sensor geometry. Test data on specific embodiments of 
the comb MR structure has shown that this structure is capable of 
providing a self-sufficient bias. The question of whether the bias is 
self-sufficient involves two issues, bias permeance and the level of bias 
than can be achieved. 
Bias permeance requires the reversal threshold Ht to be larger than any 
disturbing fields seen by the comb structure. The disturb fields may be 
environmental or originate from the storage medium. Whatever their 
external value, the strength of such fields will be much reduced at the 
comb location, at least within the usual MR configuration employing 
magnetic shields as shown in FIG. 6. The amplitude of this residual 
disturb field has to be compared to the reversal threshold for Ht. 
Typically, Ht is in the tens of Oe. For example, one specific embodiment 
similar to FIG. 4 has 2.4 .mu.m wide extensions and a reversal threshold 
of about 30 Oe. Narrower extensions, 1.4 .mu.m wide, have a reversal 
threshold of about 5 Oe. Although these values appear safe relative to the 
expected amplitudes of disturb fields, there remains the concern that 
repetitive disturb cycles may degrade the bias condition in some fashion. 
Testing was done regarding the possibility of such an occurrence, and no 
degradation of bias was found after 30,000 disturb cycles, even with 
disturb amplitudes just below Ht. 
Bias sufficiently too, depends on comb geometry. As is suggested in FIG. 3, 
the magnetization, M, is roughly perpendicular to the MR sensor where the 
comb extension meets the sensor. Between extensions, M tends to be 
parallel to the MR sensor. Along the sensor's upper edge, the average skew 
angle is determined by the width to spacing ratio of the extensions. On 
moving downward from the upper edge, the biasing flux from the comb 
extensions leaks to the adjacent shields. This results in the familiar 
hyperbolic decay of the transverse bias condition, the same as the decay 
of the excitation coming from the lower edge, just in the opposite 
direction. The measured response characteristics reflect, of course, this 
bias averaged over the height of the MR stripe. Such measurements show 
that the average bias increases with the width to spacing ratio of the 
comb extensions, and decreases with the height of the MR stripe. One 
specific embodiment having a geometry similar to FIG. 4, having a nominal 
width/spacing ratio of 1.7 and a 4 .mu.m high MR stripe, exhibits an 
average internal bias of 15 Oe for the one-sided configuration FIG. 4(c) 
and 30 Oe for the two-sided configuration FIG. 4(b). Corresponding 
quiescent skew angles are 14 deg. and 28 deg. respectively, based on a 
measured shape anisotropy of 64 Oe. 
The degree of bias permeance and sufficiency is hence dictated by geometry 
and limited by resolution of the fabrication process. Bias permeance 
improves with a decreasing width of the extensions, while bias 
sufficiently improves with an increasing width/spacing ratio. These 
competing requirements can both be satisfied only by having small spacings 
between extensions. 
In a specific embodiment, the MR sensor has a geometry such as that shown 
in FIG. 3(c), and a section view of this sensor is shown in FIG. 6. In 
this embodiment a plurality of horizontal combs 14h and a plurality of 
vertical combs (not shown in FIG. 6) are provided on the same MR sensor 
10, and electrical conductor leads 16 (shown in dashed line in FIG. 3(c)) 
are provided to conduct the read signal to the external read circuits (not 
shown). The MR sensor 10 is positioned to face the magnetic recording 
medium 20 and is preferably flanked by soft-magnetic shields 12, but in 
some cases the shields can be omitted. In the specific embodiment, the MR 
sensor was fabricated having the geometry of FIG. 3(c) and FIG. 6 and 
having the following dimensions, and this sensor had suitable performance 
characteristics. The sensor has an MR stripe height of 3 .mu.m, and comb 
extensions that are 50 .mu.m long, and 2.5 .mu.m, wide with a spacing 
between comb extensions of 1 .mu.m. By evaluating embodiments having 
varied values in the relevant geometric parameters, it was concluded that 
the length to width ratio of the comb extensions should be at least 10, 
and that the ratio of the width of the comb extensions to their spacing 
should be at least two. 
The comb configuration need not necessarily be used as a self-sufficient 
bias method. It can also, at no expense, be used to improve upon some 
other bias provisions. There is no reason, in principle, why the comb 
configuration cannot be used to enhance any of the bias techniques 
discussed in the Prior Art section of this application. For example, it 
may be used in combination with exchange bias. In this case (see FIG. 7), 
the sensor consists of a sandwiched MR sensor 10 and exchange bias film 
18, both patterned together to the same geometry such as that shown in 
FIG. 3(a), for example, FIG. 5 compares response characteristics of two 
identical comb configurations, one with and one without exchange bias. 
Without exchange bias, as shown in curve (a), we have the familiar 
transverse response, indicating an average, comb-induced bias field of 15 
Oe. With exchange bias, which, in effect, acts just like a superpositioned 
DC magnetic field, the loop is offset relative to Hy. The amount of offset 
equals the transverse component of exchange bias. Since the direction of 
exchange bias is adjustable, one can adjust relative bias components in 
the transverse and the longitudinal direction. In the present example, the 
bias direction was set at 85 deg. from the x-axis. The curve shows a 
resulting transverse bias of 32 Oe and one would hence estimate a 3 Oe 
longitudinal component. 
While the invention has been particularly shown and described with 
reference to a preferred embodiment thereof, it will be understood by 
those skilled in the art that various other changes in the form and 
details may be made therein without departing from the spirit and scope of 
the invention.