JPS63205585A - Reluctance sensor small piece - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention This invention relates to magnetoresistive sensors, and more particularly to magnetoresistive sensor strips for magnetic disk drives.

BACKGROUND OF THE INVENTION Magnetoresistive sensors that respond to changes in resistivity caused by the presence of a magnetic field are increasingly being employed as in-head read transducers in magnetic disk drives;
This is because the change in resistivity is independent of disk speed and depends only on magnetic flux, and secondly because the sensor output can be calibrated by the sense current.

Typically, these sensors consist of a piece of NiFe alloy (permalloy) wJ magnetized along the low coercivity easy axis. Many other ferromagnetic alloys are also candidates. Typically, the strips are placed so that their axis of easy magnetization is transverse to the direction of rotation of the disk and parallel to the surface of the disk. The magnetic flux from the disk rotates the magnetization vector of the strip,
This changes the resistivity for the sense current flowing between the lateral contacts. The resistivity varies approximately according to the square of the cosine of the angle between the magnetization vector and the current vector.

(i.e. δ-p -ρ-1Ilax *cos2θ
, coconi, θ is the angle between the magnetization and current vectors,
ρ is the resistivity). Due to this cosine square relationship, if the magnetization and current vectors initially match, the initial change in resistivity due to the disk magnetic flux is low and in a fixed direction. Therefore,
Typically, the easy axis magnetization vector or current vector is biased by approximately 45 degrees to increase responsiveness to angular changes in the magnetization vector and to linearize the sensor output.

[Problem to be Solved by the Invention] A problem encountered with magnetoresistive sensor strips is Barkhausen noise caused by irreversible motion of the magnetic domains in the presence of an applied magnetic field, i.e., coherent rotation of the magnetization vector. is nonuniformly suppressed and depends on domain wall behavior. This noise mechanism is eliminated by creating a single magnetic domain within the sense current region of the strip.

Measures to Solve the Problem Many different measures have been taken to linearize the sensor output and provide a single magnetic domain within the sense region. It is known, for example, to increase the quality of the strip relative to its height in order to generate a single magnetic domain in the sense region. It is known that a large number of closed magnetic domains occur at both ends of a long strip. These move towards the center under the influence of an external magnetic field. However, long strips are subject to crosstalk in the lateral portions of the strip, which may pass magnetic flux from adjacent tracks into the sense region of the strip. In contrast, short strips almost always spontaneously "fracture" into multiple domains.

Efforts have been made to provide a single magnetic domain within the sensor region by forming strips with relatively short physical dimensions within the sensor region to reduce edge field fields. Soil, etc. (niaW)
U.S. Pat. No. 4,503.
Referring to Figure 4a of No. 394, an endless loop is constructed by connecting vertical parts to both ends of upper and lower horizontal parts having easy magnetization axes in opposite directions. U.S. Patent No. 4,555,740
No. 5, the strip has two intermediate upwardly extending legs. However, the strong transverse magnetic field (tra
nsVerse 1llallnetic) is present, even the formed pieces are “fractured” into multiple domains.
.

(The magnetic pole acts as a soft magnetic shield, insulating the sensor from magnetic fields that are not directly adjacent to the sensor).

Efforts have also been made to create single magnetic domains by applying a w1 field in a "long" or formed strip before reading. Such a magnetic field must be strong enough to form a relatively stable single magnetic domain within the central sensor region. This initialization field is typically provided by a barber pole, which is also used to tilt the direction of the sense current relative to the easy axis magnetic vector.

For short strips, efforts have been made to maintain a single domain with atomically bonded antiferromagnetic materials that create a longitudinal or exchange bias from adjacent permanent magnets. As previously mentioned, such biasing means are also provided in some applications to transversely bias the magnetic vector away from the easy axis to linearize the sensor output.

Both of these proposals (initialization and permanent) suggest that biasing the magnetic field will have an adverse effect on the information previously recorded on the magnetic disk, and that the permanent bias field (both transverse and longitudinal) will affect the sensor's effective variation. It has the disadvantage of increasing the orientation and decreasing the sensitivity to disk magnetic flux. The Barber pole (gradient current) design has another drawback in that the effective length of the sensor area is less than the longitudinal distance of the sensor contact. Barber poles also require precision lithographic steps to provide the angled contacts and shorting strips.

2 (of magnetoresistive material and antiferromagnetic material) at the exposed interface.

Because of the presence of dissimilar materials, the exchange bias is effectively
Not commonly used. This can cause corrosion and destroy the sensor strip. Furthermore, since the exchange bias is a mother-child mechanical interaction effect, highly reliable atomic interactions are required, but such a process is difficult and has a low yield. In general, this effect is strongly temperature dependent and is substantially low F under typical operating environments of conventional disk drives.

OPERATION The present invention comprises a series of improvements that address several problems of conventional magnetoresistive sensor strips, either singly or in combination, to form an improved head. This invention is common to co-filtered applications, each claiming individual improvements which may be combined to form an improved magnetoresistive sensor and head.

These improvements include forming the strips into pseudo-elliptical shapes. This shape has a very stable single domain in the central sense region of the strip. A subsequent exchange bias antiferromagnetic material can be atomically bonded to the ends of any strip for the purpose of maintaining the central region in a single domain state. Due to the nuclear effects of the replacement material, the material does not need to cover all ends of the strip, but
The exposed interface area can be recessed to reduce corrosion susceptibility. Once stability is established by pseudo-elliptical and/or boundary controlled exchange stabilization, only two cant contacts need to change current direction for the purpose of linearizing the MR sensor.

This completely eliminates the need for any barber poles used to stabilize the domain states, and the elimination of barber poles reduces the number of electrical contacts to only two sense contacts.

canted current design
ion) is further improved by patterning the easy axis of the strip to be inclined with respect to the horizontal plane of the magnetic disk and loosening the inclination of the contacts accordingly. This provides a large effective longitudinal sense area. Furthermore, by operating the sensor in its nonlinear mode, transverse bias can be completely eliminated in coded digital applications where data location is more important than magnetic strength or direction. While reducing the dynamic range, zero crossing determination from the derivative of the sensed read signal is improved by increasing the slope of the nasal linear response. Finally, the sensor is preferably placed outside the inductive write gear 9 to avoid the deleterious effects of multi-domain formation due to the strong magnetic fields present during the write operation.

Another gap structure is added with a wide central shield/pole that shields the elongated magnetoresistive sensor while providing good write/read characteristics.

Example 1 FIG. 1 is a pseudo-elliptical structure of a magnetoresistive sensor strip 10 magnetized along the easy axis M. The central portion, indicated by the reference numeral, is not curved like a true ellipse, but has relatively flat sides. Aspect ratio between total length and height, AR
is smaller than 3, but can be made larger without loss of effectiveness. From the central region, the sides naturally have small magnetic domains 12
and 14 converge to the apex where they are formed. Preferably, W≦L, and the minimum length E of the terminal end is about L, and the maximum value is not particularly limited. The structure TI forms an extremely stable single magnetic domain in the central region indicated by a large arrow.

Through experiments using this structure, 200 to 500 people with a total length of 25 microns, an L part of 9 microns, and a width W of 8 microns,
Ni: 82 F (3: 35 to switch the magnetization vector in the central region of the 18 alloy layer to the hard magnetization axis)
0e and 0.75 for non-patterned bulk
It turns out that 0et is not necessary. This is a coefficient of 4
This is interpreted as an improvement of 6.

If high transverse magnetic fields are expected, such as when an unshielded sensor is placed between or next to the poles of an inductive back-mounted sensor strip, Requires longitudinal bias. As mentioned above, there are many different ways to accomplish this. For example, the Barber pole bias generates a m1ll field. In general, permanent magnetic bias and exchange bias can also provide a longitudinal magnetic field. A new stabilizing means is disclosed in FIG.

Conventional exchange stabilization/biasing techniques are typically prepared by first depositing a ferromagnetic layer on the substrate and then depositing an antiferromagnetic layer on the ferromagnetic layer such that i1m matches after patterning. Ta.

Exchange bias can cause signal loss due to shunt effects. The Ill field has a negative m degree dependence.

Finally, there is the possibility of corrosion due to the bimetal membrane structure.

The domain stabilization process can be understood by considering that if the magnetization is somewhat suppressed at the boundary of the thin 1III piece, the planar 4Iiii magnetization direction can be changed to w4va in the central region of the boundary. By depositing FeMn within the crosshatch region of FIG. 2, the drawbacks of the standard exchange bias technique can be avoided. First, there is no exchange material in the central active region, so there is no signal loss due to current shunting.

Second, this stabilization technique is extremely temperature insensitive, provided that only the direction of magnetization is fixed, not the magnitude of the longitudinal magnetic field. Finally, proper buttering can eliminate bimetallic interfaces at any exposed edges.

In the example, the exchange bias material is FeMn because it is 110%.

An embodiment having a stable single domain central region employing an exchange bias end is shown in FIG. Here, the strip is C-shaped and has a relatively narrow central region and lateral ends with upwardly extending legs 26,28 which conduct the demagnetizing field further from the central region. This improves the stability of the single magnetic domain in the central region. Inclined i! (not shown) applied later! Exchange bias material 32, 34 and contact metallization (not shown) are applied to these terminations using the steps described below for the generally matching illustrated pattern. This pattern of exchange material eliminates edge and terminating domains and provides a stable central single domain sense region.

To avoid the aforementioned corrosion problems at exposed interfaces, the resist pattern is shaped to provide a recess S between the replacement material and the lower grade of the strip 10, i.e. the edge exposed to the magnetic disk in most designs. .

FIG. 3 shows the steps for forming the structure shown in FIG. 2. Step 1:! In a uniform magnetic field along the easy axis, strips of magnetoresistive material are deposited, sputtered, etc., and patterned onto a substrate (not shown for clarity). Step 2: Apply a photoresist layer and pattern using conventional processes to create an island resist 1 with inwardly sloping sides.
Form ii20. Step 3: Next, deposit, sputter, etc. a replacement material 22 onto the assembly. Step 4: Deposit contact metallization 23. Step 5: Using a lift-off process, remove the resist, replacement material 24 and metal 23 that is attached to it.

FIG. 4 shows a pseudo-elliptical strip with exchanging material at both ends extending into a flattened central region. A similar recess 836 must be provided.

FIG. 5 shows a cross-sectional view of the exchange bias strip 10 of FIG. 2 or 4 mounted between shields 42 and 44 of a typical head on a magnetic disk 50. In the figure, the replacement material 32 is placed over a short distance on the head surface! 136, the contact metal 38 has legs 40 extending into the strip 10 to shield the replacement material 32 from exposure. At least one of the shields 42, 44 is one of the inductive writing devices 11
It has a bridge. The shields are separated by spacers 52, typically made of a non-magnetic material such as Al2O3. By providing the recesses 36, the contacts 38 have legs 40 that are in direct contact with the magnetoresistive material 10. Thereby, the replacement material 32 is shielded from being exposed. Most of the sensor strips descend onto the disk surface 50 when the disk stops rotating, abrading the raw sensor strip material. The amount of denting vs. abrasion determines the life of the sensor strip until the replacement material is exposed and causes latent corrosion.

In the presence of a strong transverse magnetic field, the relatively stable single-domain region is "fractured" into multiple domains, which becomes the source of Barkhausen noise. The strong vi1s exists between the magnetic pole tips of the inductive writing device, that is, at the conventional most coercive head position. To reduce the effect of the inductive write pole tip on the magnetoresistive sensor strip,
It is known to place sensor strips side by side with an inductive write pole tip.

For example, Lee U.S. Patent Application No. 4.321゜6
Please refer to No. 41. This type of structure requires a soft magnetic shield, a shield/pole back end, and a pole tip. Mainly MR material 76.78 (U.S. Patent No. 4,321,64
The design of the present patent is not completely satisfactory because the magnetic field (see FIGS. 4 or 7 of No. 1) extends beyond the shield of the pole trailing end 90. The designs of FIGS. 6 and 7 provide a very magnetically quiet area for the magnetoresistive sensor.

The residual magnetic flux from the magnetic poles of the inductive writing transducer is very low, allowing highly stable and formed single domain sensors (such as the pseudo-ellipse 10 in Figure 1) to operate reliably without longitudinal bias. be able to.

FIG. 6 is a cross-sectional view of the basic element of the improved design. An oxide layer, preferably aluminum oxide 62, is deposited on the soft magnetic substrate 60, preferably NiZn. A magnetoresistive sensor@64 is then deposited and patterned in a magnetic field. [If desired, exchange bias material is then deposited and patterned. ] Next, a metal contact 66 is deposited on the magnetoresistive strip 64. Next, the second oxide layer 6
8 is deposited. These two oxide layers 62 and 68 constitute the read gap. Polyimide or photoresist 70 is then deposited and patterned as shown to remove the layer adjacent the gap edges of the sensor strip.

Next, a layer of ferromagnetic material 70, preferably N1Fe (permalloy), is applied. This 170 constitutes the trailing pole/shield. Next, a write gap oxide 75 (aluminum oxide or silicon dioxide) is deposited, followed by a second polyimide or photoresist 74. A metal coil 78 is deposited and patterned. Two layers of polyimide or photoresist 76 are deposited and patterned and the portions not adjacent coil 28 are removed. Finally, a final ferromagnetic material W479 is deposited to surround the coil and contact the other ferromagnetic material [72] to form a continuous flux path. After the package is formed, it is typically encapsulated in a suitable non-magnetic material and the gap edges are treated (usually wrapped) to expose the gap and provide a reliable gap height.

FIG. 7 is an end view of the basic elements of the double gap head of the embodiment. For clarity, spacer purchases are omitted. Shown are a ferrite substrate 60, a magnetoresistive strip 64, a lateral metal contact 66 defining an elongated central sensor area 65, and a ferromagnetic trailing pole/shield 7.
2 and leading magnetic pole 79. As shown, the length of the leading pole 79 through the magnetic mirroring by the trailing pole/shield defines the write track width. This length corresponds to the length of the central region 65 of the magnetoresistive strip 64 (plus processing protection frequency band). (To avoid crosstalk,
The length L is intentionally made smaller than the write track width. ) Typically, the magnetoresistive strip is longer than the track width to help provide a stable central region single domain. It is important that the trailing pole/shield 72 be the same length as the magnetoresistive sensor 64 to completely shield it from the side m1i13Xlii fields that occur during the write process. This causes the leading and trailing magnetic poles 79, 72 to have different lengths. However, this did not seem to affect the write track width, which was found to be defined by the length of the leading pole 79 and the mirror effect.

For many applications, such as audio, linear operation of magnetoresistive sensors is desirable. As mentioned above, linearization requires a slope of the easy axis magnetization vector or a slope of the current vector. Tilting the magnetization vector typically increases the anisotropy and reduces the range of resistivity variation and thus the sensitivity of the sensor. Similarly, current ramping results in comparable sensitivity losses as shown in FIG.

FIG. 8 shows a typical gradient current biasing technique in which a length of magnetoresistive strip 92 and a closely spaced conductor 80.
provides a ramped current from source 88 generally in the terr direction between the contacts. The current direction is generally perpendicular to the contact surface 84,86. These surfaces are generally tilted at angles θb of 40° and 45° for maximum linearity and sensitivity. (If 88 is a constant current source, it can be a voltage sensor; if it is a constant voltage source, it can be a transimpedance current sensor 1; if it is a 'soft' source, it can be a current sensor); The change in resistivity has been shown by investigation to be generally proportional to the length Left, which is shorter than the length between the longitudinal contacts.Then, Left is approximately equal to the track width of the narrow track and the length of the sense region. In this way, the sensitivity of the device is reduced by the ratio Leff/L, making terr comparable to the track width, since L is long enough to pick up significant glue O stalks from adjacent tracks. I don't like letting it happen.

Figure 9 shows the slope of the contact surface 84.86 at approximately 50°.
An improved gradient current sensor is shown that loosens to an angle θb'. This results in approximately 40° to 4° relative to the easy magnetization axis.
Left and therefore the sensitivity increases substantially while maintaining the 5° angle. The reason is that the magnetoresistive strip is patterned so that its easy axis itself is tilted by an angle θ,A of approximately 10°.

In the figure, the contact surfaces 84,86 are each inclined at an angle θb' of preferably 50°.

The lower grade of magnetoresistive strip 96 is parallel to the magnetic disk surface as in the prior art, but the upper edge 98 is patterned at an angle θP and at an angle θEA of approximately 10° with the lower edge.
yields an easy-axis magnetization vector of .

Strip 94 is formed from a bulk film deposited on a suitable substrate in a uniform magnetic field parallel to lower edge 96. The bulk film is then patterned using conventional lithographic techniques to form an angled pattern with the top edge extending upwardly relative to the bottom edge. Due to this shape, the easy axis magnetization vector is tilted upward, although it is smaller than the angle of the upper edge. To achieve a net easy-axis rotation of 10°, designers must adjust the strength of the undeflected easy-axis vector by adjusting its size, length,
The thickness must be balanced and the composition of the magnetoresistive material must be balanced with the upward edge angle.

In an exemplary embodiment, the strip 94 is comprised of an 80:2ONiFe alloy, approximately 500 mm thick, approximately 9 μm thick, and approximately 9 μm thick.
(sensor height at point 104) is approximately 8μ, θP is 10@, and θb' is 50° and θ is 10°. FIG. 10 shows the gradient magnetization easy axis pseudo-ellipse 100 and its relative orientation to the contact surfaces 84, 86 (contact equilibrium not shown).

For most digital applications, data is written onto the disk in codes (eg, variable length 2.7), where only the transition positions (pulse peaks) are important, rather than their direction or magnitude. The pulse amplitude triggers a correction sieve to distinguish between signal and noise. In this way, there is no reason to operate the sensor linearly, except to improve the initial sensitivity of magnetization vector rotation. Thus, the final improvement of the magnetoresistive sensor was to provide no transverse bias other than the patterned bias, operate the sensor in a nonlinear mode, and reduce the magnetization vector rotation in response to disk flux by 40 to 50 °
The objective is to design the magnetoresistive sensor and the disk magnetic flux so that the

Since the transition position It (pulse peak) is important, the signal from the disk is usually differentiated to detect zero crossings. Noise ultimately limits data density because noise obscures zero-crossing locations. However, without biasing the sensor, the sensor operates in a nonlinear mode (see equation in the prior art description) and the differential has a steeper zero-crossing slope than a linearly biased sensor. This increased zero-crossing slope reduces noise sensitivity and allows more accurate detection of zero-crossing locations, all else being equal.

In order to obtain a proper nonlinear signal from the sensor, the magnetization vector must rotate more than it would if it were biased; see FIG. 11 for an explanation of the principle. The upper part of the figure shows half of the normalized magnetoresistive response (cosine squared response) graph. At the bottom of the figure is a graph of two magnetic flux input signals, on the left 104 is a nonlinear magnetoresistive sensor “
The right side shows the input signal of the linear magnetoresistive sensor. Although the two signals are shown to have significantly different magnitudes, they can actually be the same magnitude if the relative responses of the magnetoresistive sensors are proportional to the relative differences shown. In fact, it is preferable to adjust the relative responses of the disk and sensor.

In the linear mode of operation, the input pulse 106 is in state 1.
2.3 and 4, the sensor has a resistivity shape @1', 2
', 3' and 4' in response (for counterproductive pulses, the state is opposite to 1'). For all states, the input and output have a linear response.

In nonlinear mode, input signal 104 is in state A-+F
, the sensor responds with states A'→F' (opposite polarity signal pulses produce the same output, but from the other half of the resistivity curve). The output is nonlinear from region D' to F', where it again becomes a linear response to the input.

It can be seen from the figure that the total response of the non-linear sensor (A' to F') is greater than the total response from the linear sensor (1' to 4'). In this way, the total sensitivity is increased and the transition center (pulse peak) can be located more accurately. Actual sensor output increases by 25-30%.

Although several materials can be selected to achieve the response shown in Figure 11, the preferred choice is to use a sensor consisting of permalloy and the head mounted on a conventional flyer to provide sufficient magnetic flux to produce the magnetization vector rotation shown. It is a magnetic disk material with

Figure 12 shows a pseudo-ellipse 10 non-inclined contact 84°86,
A preferred magnetoresistive sensor consisting of a constant current source 88 and a voltage sensor 90 is shown. Preferably, this sensor is mounted within the double gap sensor of FIGS. 6 and 7.

If no bias is applied, the sensor operates in a non-linear mode. Shielded second part of double gap sensor
The shape and position of the sensor within the gap maintains the sensor in a single domain state. If such an embodiment is not robust enough for a given application, regions 110 and 112 can be provided with replacement material to further increase stability, as described above.

[Brief explanation of the drawing]

Figure 1 is an elevational view of a pseudo-elliptical magnetoresistive sensor strip; Figure 2 is an illustration of the strip of Figure 1 with exchange bias material at both ends; Figure 3 is an illustration of the strip of Figure 1 with exchange bias material at both ends; Figure 3 is an elevation view of a pseudo-elliptical magnetoresistive sensor strip; Basic step diagram for performing bias deposition; FIG. 4 is a diagram of an elongated magnetoresistive strip with exchange bias material at both ends and an upwardly projecting termination; FIG. 5 is a diagram of a magnetoresistive sensor with exchange bias material recessed. 6 is a layer structure diagram of a double-gap magnetoresistive sensor; FIG. 7 is an elevational view of the basic elements of a double-gap magnetoresistive sensor; and FIG. 8 is a prior art gradient current contact. and an electrical circuit diagram connected thereto; FIG. 9 is a diagram illustrating the easy axis pattern bias strip and loosened gradient current contacts of the present invention;
Figure 10 shows an easy axis pattern biased pseudo magnetoresistive strip, Figure 11 shows the relative response characteristics of magnetoresistive sensors in linear and non-linear modes, and Figure 12 shows a non-tilted contact for a non-tilted response. FIG. [Explanation of reference symbols] 10.64.92.94...Magnetic resistance strip 12.14
... Magnetic domain 20 ... Island resist layer 22.24 ... Exchange material 23 - ... Contact metallization 26.28 ... Upward extending leg 32.34 - ... Exchange bias material 38.60 ...Metal contact 40...Leg 42.44...Shield 50...Magnetic disk 52...Spacer 60...Soft magnetic substrate 62.68...Oxide layer 70.74.76--・Photoresist 72.79--Successive leading magnetic pole 75--Gap oxide 78--Metal coil 80.82--Conductor 84.86--Non-tilted contact 88--Constant voltage! %El1 90-...Voltage sensor

Claims (14)

[Claims] (1) A magnetoresistive sensor strip having a lower edge, a central sense region having an upwardly curved upper edge and an easy axis of magnetization tilted upward from a line parallel to the lower edge. (2) a central sense region having a lower edge, an upwardly bent upper edge and an easy axis of magnetization tilted upward from a line parallel to the lower edge; An elongated magnetoresistive sensor strip having an extension of the strip towards the upper edge. (3) A magnetoresistive sensor strip according to claim (2), wherein the lower edge of the extension is bent upwardly. (4) The magnetoresistive sensor strip according to claim (2), wherein the strip has a pseudo-ellipsoid shape with one end bent upward. (5) A magnetoresistive sensor strip according to claim (4), wherein said shape has a transverse tip. (6) A magnetoresistive sensor strip according to claim 5, wherein an exchange biasing material is atomically bonded to the lateral tips but not to the central sense region. (7) A magnetoresistive sensor strip according to claim 6, wherein the exchange biasing material is spaced a short distance from the bottom edge. (8) A magnetoresistive sensor strip according to claim (5), each comprising two electrical contacts in contact with a lateral tip of the strip. (9) A magnetoresistive sensor strip according to claim (8), wherein said contact has parallel sloped surfaces in said central sense region. 10. The magnetoresistive sensor strip of claim 8, wherein the contact has non-slanted parallel surfaces in a central sense region. (11) In claim (1), the strip includes a pair of inductive write magnetic poles, a first gap between the magnetic poles, and a second gap between a trailing magnetic pole in the magnetic poles and a soft magnetic shield. a magnetic head having said soft magnetic shield forming two gaps between said strips, said strip being placed within said second gap, and said leading pole of said magnetic poles defining a track width; a magnetoresistive sensor strip having a length of about 100 psi, wherein the central sense region is about the same length as the leading pole and the trailing pole is at least as long as the strip. (12) A magnetoresistive sensor strip according to claim (11), wherein the soft magnetic shield is at least as long as the strip. (13) In claim (8), the strip includes a pair of inductive write magnetic poles, a first gap between the magnetic poles, and a second gap between a trailing magnetic pole in the magnetic poles and a soft magnetic shield. a magnetic head having said soft magnetic shield forming two gaps between said magnetic poles, said strip being disposed within said second gap, and said leading pole of said magnetic poles defining a track width; a magnetoresistive sensor strip having a length that is approximately the circumferential length of the leading magnetic pole, and the trailing magnetic pole is at least the circumferential length of the strip; (14) The magnetoresistive sensor strip according to claim (13), further comprising a constant current source connected between each of the contacts and a voltage sensing circuit connected between each of the contacts.