A MAGNETORESISTIVE READ TRANSDUCER ASSEMBLY

Abstract: A magnetoresistive (MR) element (10) is provided with elongate extensions (14h, 14v), which are made of the same material as the MR element and unidirectionally magnetised along their length in order to provide bias for the MR element. The bias provided by the elongate extensions (14h, 14v) may be supplemented, e.g by means of an exchange bias film (16) having the same dimensions as the MR element (10).

Description: This invention relates to a magnetoresistive read transducer assembly for reading data from a magnetic record medium. A known magnetoresistive (MR) sensor or head is 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 magnetisation 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 magnetisation, 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 method of bias 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. US-A-3,813,692 shows one example of this method of bias. Alternatively, the field from the sense current magnetises a proximate soft magnetic 'keeper film' whose stray field, in turn, exerts a transverse bias upon the MR sensor as is described in US-A-3,864,751. Another type of bias method is current deflection bias, as shown in US-A-4,280,158. Here, the quiescent magnetisation remains along the longitudinal direction, while, instead, the sense current is deflected to flow askew to the magnetisation. This effect is achieved with a superpositioned 'barberpole' conductor configura­ tion. This method produces a transverse bias condition along with a small (typically insufficient) longitudinal bias field underneath the conductor. A further type of prior method of bias 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 described in US-A-­ 3,840,898. Alternatively, the film may be antiferromagnetic and coupled to the sensor via an exchange mechanism as described in US-A-4,103,315. Either method can, at least in principle, provide a transverse and/or a longitudinal bias component. US-A-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 circumferen­ tially 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 methods of bias met previous requirements, but the additional structure required adds complexity to the MR read trans­ ducer which it is desirable to avoid. This invention seeks to provide an improved biased magneto­ resistive read transducer assembly. A magnetoresistive read transducer assembly including a mag­ netoresistive element having one edge which, in use, faces a record medium, is according to the invention characterised by having a plurality of elongate extensions of the same material as the ele­ ment, extending from at least one of the other edges of the element, the elongate extensions being unidirectionally magnetised along their length in order to provide bias for the element. In a specific embodiment, a plurality of elongate extensions extend parallel to the edge of the MR element which is adapted to face the magnetic medium and a plurality of elongate extensions extend in the direction normal to the edge of the MR element which is away from the magnetic medium. In another 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. How the invention can be carried out will now be described by way of example, with reference to the accompanying drawings, in which:- Figure 1 comprises seven diagrams which indicate how transverse bias works for a magnetoresistive (MR) sensor; Figure 2 comprises two sets of diagrams (a) and (b) which indicate how longitudinal bias works for an MR sensor; Figure 3 comprises three plan views (a)b, (b) and (c) of different sensor geometries; Figure 4 comprises three graphs (a), (b) and (c) which indicate the effect of different sensor configurations on the sensor opera­ ting characteristics; Figure 5 is a graph which provides a comparison of the sensor operating characteristics of a specific embodiment alone and with exchange bias; Figure 6 represents a cross-section of one MR sensor embodying the invention; and Figure 7 represents a cross-section of another MR sensor embodying the invention. Some of the operating principles of an MR sensor will first be reviewed with the aid of Figures 1 and 2. The desired shape of the bias profile of an MR sensor, will be described with the aid of Figure 1 relative to the transverse bias profile and with the aid of Figure 2 with respect to the longitu­ dinal bias profile. Figure 1(a) is a sketch showing the quiescent magnetisation, M, at some skew angle relative to the sense current, I, through the MR sensor 10. The plot of Figure 1(b) shows that the transverse magnetisation, My, varies linearly with bias strength, Hy, when Hy < Hu, where Hu is the total anisotropy (induced + shape) of the MR sensor. When Hy ≧ Hu, then My = Ms and the MR sensor is saturated. The relationship between the resistance R of the MR sensor and the transverse magnetisation My is given by where c mr is the Magnetoresistance coefficient which depends on the material used, and this constant is typically 2 to 3 × 10⁻² for permalloy, for example. The plot shown in Figure 1(c) shows that the sensor's resistive signal, ΔR/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 Figure 1(d) shows that the excitation, origina­ ting 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 Figure 1(e) typically, the excitation is not a 'small signal input' but, at least at the lower stripe edge, causes excur­ sions 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 permis­ sible without the combined bias and excitation producing saturation. The plot in Figure 1(f) shows the bias profile for maximum output signal. Bias skew increases in accord with the decreasing excita­ tion. With such a bias profile, the excursions of transverse magnetisation across the stripe, remain just short of saturation as is shown in Figure 1(g). Figure 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 Figure 2(a), so that the effective bias field, He, being the sum of the applied and the demagnetising field, Hd, peaks about the centre of the sensor. The resulting concave sensitivity profile, S, is undesirable since the magnetic head would have a sensitivity that is lowest over the centre of the track and highest about the edges of the track. A more advantageous convex sensitivity profile, Figure 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 centre segment at maximum sensitivity. The present invention involves the addition of elongate or 'comb' extensions 14 to the MR sensor 10 as shown in Figure 3. An elongate vertical comb extension 14v, as shown in Figure 3(a), provides transverse bias while the elongate horizontal extensions or comb 14H of Figure 3(b) provides longitudinal bias. Both bias components are obtained from a configuration like the one shown in Figure 3(c) in which both horizontal combs 14h and vertical combs 14v are provided on the same MR sensor 10. The combs 14 are contig­ uous with and made of the same material as the MR sensor element 10. It was found that these comb extensions 14h, 14v, when magne­ tised along their long axis, exert a magnetostatic bias on the MR sensor much as though each extension was a permanent magnet. The extensions do, in fact, behave much like permanent magnets in that they exhibit a large hysteresis to magnetisation reversal. This hysteresis can easily be ten times larger than the intrinsic coer­ cive force of the sensor material. As the extensions 14 are made of a soft magnetic material, typical 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 magnetisation reversal. Figure 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 magnetisation. The three curves show plots of ΔR/R vs Hx for different sensor geometries. Curve (a) shows the response of a 25 µm wide stripe without extensions (as a reference). Curve (b) shows the response of an MR stripe with transverse extensions on both sides, and curve (c) shows the response of an MR stripe with transverse extensions on only one side. 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 thres­ hold, 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 that can be achieved. Bias permeance requires the reversal threshold Ht to be larger than any disturbing fields seen by the comb structure. The distur­ bing 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 Figure 6. The amplitude of this residual disturbance has to be compared with the reversal threshold for Ht. Typically, Ht is in the tens of 0e (1 0e = 4π/10³ amp/metre). For example, one specific embodiment similar to Figure 5 has 2.4 µm wide extensions and a reversal threshold of about 30 0e. Narrower extensions, 1.4 µm wide, have a reversal threshold of about 50 0e. Although these values appear safe relative to the expected amplitudes of disturbing fields, there remains the concern that repetitive disturbance 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 disturbance cycles, even with disturbance ampli­ tudes just below Ht. Bias sufficiency too, depends on comb geometry. As is sugges­ ted in Figure 3, the magnetisation, 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 direc­ tion. The measured response characteristics reflect, of course, this bias averaged over the height of the MR stripe. Such measure­ ments 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 Figure 4, having a nominal width/spacing ratio of 1.7 and a 4 µm high MR stripe, exhibits an average internal bias of 15 0e for the one-sided configuration of Figure 4(c) and 30 0e for the two-sided configuration of Figure 4(b). Corresponding quiescent skew angles are 14 degrees and 28 degrees, respectively, based on a measured shape anisotropy of 64 0e. 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 sufficiency 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 Figure 3(c), and a section view of this sensor is shown in Figure 6. In this embodiment a plurality of horizontal combs 14h and a plurality of vertical combs (not shown in Figure 6) are provided on the same MR sensor 10, and electrical conductor leads 16 (shown in dashed line in Figure 3(c)) are provided to conduct the read signal to the external read circuits (not shown). The MR sensor 10 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 Figures 3(c) and Figure 6 and having the following dimensions, and this sensor had suitable performance characteristics. The sensor has an MR stripe height of 10 µm, and comb extensions that are 50 µm long, and 2.5 µm, wide with a spacing between comb extensions of 1 µm. By evaluating embodiments having varied values in the relevant geometric parameters, it was concluded that the length to width 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 can 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 Figure 7), the sensor consists of a sandwiched MR sensor 10 and exchange bias film 16, both patterned together to the same geometry such as that shown in Figure 3(a), for example. Figure 5 compares response characteristics of two identi­ cal 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 0e. 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 ex­ change 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 degrees from the x-axis. The curve shows a resulting transverse bias of 32 0e and one would hence estimate a 3 0e longi­ tudinal component.

Claims: 1. A magnetoresistive read transducer assembly including a mag­ netoresistive element (10) having one edge which, in use, faces a record medium, the magnetoresistive element being characterised by having a plurality of elongate extensions (14h, 14v) of the same material as the element, extending from at least one of the other edges of the element, the elongate extensions being unidirectionally magnetised along their length in order to provide bias for the element. 2. An assembly as claimed in claim 1, in which some of the elon­ gate extensions extend from an edge of the element which is normal to said one edge and some of the elongate extensions extend from an inner edge of the element which is parallel to said one edge. 3. An assembly as claimed in claim 1 or claim 2, in which each of the elongate extensions is at least ten times as long as it is wide. 4. An assembly as claimed in any preceding claim, in which each of the elongate extensions has a width which is equal to at least twice the distance between adjacent ones of the extensions. 5. An assembly as claimed in any preceding claim, including means to supplement the bias provided to the element by the elongate extensions. 6. An assembly as claimed in claim 5, in which the means to supplement the bias comprises an exchange bias film (16) disposed contiguously with the magnetoresistive element, the exchange bias film having the same dimensions as the magnetoresistive element.

Inventors: HUANG, SHIEZEN, VOEGELI, OTTO
Applicants: INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1986
Main classification: G11B5
IPC codes: G11B5