MAGNETORESISTIVE READ TRANSDUCER ASSEMBLY

Abstract: A magnetoresistive (MR) read transducer assembly comprises a thin film MR layer (10) which is longitudinally biased only in the end regions (12) by exchange bias developed by a thin film of antiferro­ magnetic material (16) that is deposited in direct contact with the MR layer in the end regions. The longitudinal bias is of a level suffi­ cient to maintain the end regions of the MR layer in a single domain state and thereby induce a single domain state in the central region (14) of the MR layer. Transverse bias is produced within the central region of the MR layer of a level sufficient to maintain that region of the MR layer in a linear response mode. Spaced conductive elements (18,20) are connected to the MR layer within the central region to define a detection region so that sensing means connected to the con­ ductive elements can determine the resistance changes in the detection region of the MR layer resulting from the magnetic fields intercepted by the MR layer.

Description: This invention relates to a magnetoresistive read transducer assembly for reading data from a magnetic record medium. Magnetoresistive (MR) sensors or heads are known which are capable of reading data from a magnetic surface at great linear densities. An MR sensor detects magnetic field signals through the resistance changes of a read element made from a magnetoresistive material as a function of the amount and direction of magnetic flux being sensed by the element. It is also known that in order for an MR element to operate opti­ mally, two bias fields should be provided. In order to bias the material so that its response to a flux field is linear a transverse bias field is generally provided. This bias field is normal to the plane of the magnetic media and parallel to the surface of the planar MR element. The other bias field which is usually employed with MR elements is referred to in the art as the longitudinal bias field which extends parallel to the surface of the magnetic media and parallel to the length of the MR element. The function of the longitudinal bias field is to suppress Barkhausen noise which originates from multi-domain activities in the MR element. Numerous prior art biasing methods and apparatus for MR sensors have been developed which use both longitudinal and transverse bias together, and these prior art biasing methods and apparatus have been effective to meet prior art requirements. However, the drive towards increased recording density has led to the requirement for narrower recording tracks and increased linear recording density along the tracks. The small MR sensors which are necessary to meet these requirements cannot be made with the use of the prior art techniques due to competing bias requirements. On the one hand, the longitudinal bias must be strong enough for domain suppression and this bias produces a field along the MR element. On the other hand, the transverse bias field is normal to the longitudinal bias field, so the longitudinal bias field competes against the transverse bias field and the transverse data signal. As a result of this bias competition, the MR sensor is chronically under-­ biased and signal sensitivity is significantly reduced. These high recording density applications require a very efficient output which prior art techniques are unable to produce. US-A-3,887,944 discloses an integrated array of side-by-side MR reading heads. To eliminate crosstalk between adjacent MR read heads, a region of high coercivity material is formed between adjacent MR sen­ sors. One of the ways discussed to produce the region of high coerci­ vity material is by exchange coupling between an antiferromagnetic material and the MR sensor. However, there is no consideration of the possibility of obtaining exchange bias fields from this coupling, the direction of such bias fields, if they do exist, and the subsequent domain states of the MR sensor. US-A-4,103,315 discloses the use of antiferromagnetic-ferromagnetic exchange coupling to produce a uniform longitudinal bias along the entire MR sensor for domain suppression. According to the invention, there is provided a magnetoresistive read transducer assembly comprising: a thin film magnetoresistive layer means for producing a longitudinal bias in only the end regions of the magnetoresistive layer, the longitudinal bias being of a level sufficient to maintain the end regions of the magnetoresistive layer in a single domain state, means for producing a transverse bias in at least a part of the central region between said two end regions of said magnetoresistive layer the transverse bias being of a level sufficient to maintain said part of the central region of the magnetoresistive layer in a linear response mode; and conductive means connected to said magnetoresistive layer within said central region to define a detection region so that resistance changes in the detection region produced by magnetic fields which are intercepted by the magnetoresistive layer can be detected. 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 is a diagram showing conceptually how the longitudinal bias is non-uniformly applied to the MR layer according to the present invention; Figure 2 is a diagram showing the aligned state of the longitudinal bias field; Figure 3 is a diagram showing the relationship of various regions of an MR layer according to the present invention; Figure 4 is an end view of a specific embodiment of a MR read transducer assembly embodying the invention; Figure 5 is a plan view of the transducer shown in Figure 4. Figure 6 is a section view on line 6-6 of Figure 4. The principles of operation of a magnetoresistive (MR) sensor, according to the invention, will be described with reference to Figures 1-3. A magnetoresistive (MR) sensor 10 (Figure 1) is provided with longitudinal as well as transverse bias, and the resulting bias is such as to render the sensor with excellent sensitivity and side reading characteristics. However, the bias is not uniformly applied to the MR sensor. The longitudinal bias (H BL ) for domain suppression is applied only to the end regions 12 of the MR sensor 10, and the transverse bias (H TB ) for a linear response mode is applied to at least a part of the central region 14 of the MR sensor 10. The longitudinal bias can be produced by any suitable method known in the art for producing a longitudinal bias. In the preferred embodi­ ment, the longitudinal bias is produced by an exchange biasing method. The exchange bias is produced by patterning an antiferromagnetic layer 16 so that it covers only the end regions 12 of the MR sensor 10, and, as a result, only the end regions 12 of the MR sensor 10 are exchange biased. The antiferromagnetic layer 16 creates an interfacial exchange interaction in the hatched areas 16a in Figure 1 that results in an effective bias field experienced by the MR sensor 10, and this bias field is oriented longitudinally for domain suppression. The effects of the longitudinal biasing are indicated in Figure 2. The end regions 12 of the MR sensor 10 are maintained in a single domain state by the longitudinal bias with the bias field oriented along the MR sensor in the same direction, to the right as shown in Figure 2. However, the central region 14 of the MR sensor 10 experiences no direct field from the longitudinal bias, H BL , and for this reason no signifi­ cant loss of transverse sensitivity of MR sensor 10 occurs. The aligned single domain magnetic state at the end regions 12 of MR sensor 10 renders reverse domain creation in the central region 14 very unfavour­ able, thereby inducing a single domain state in the central region 14 of the MR sensor 10 as shown in Figure 2. Once the longitudinal bias, H BL , has been produced, at least a part of the central region 14 of MR sensor 10 is provided with a transverse bias of a level sufficient to maintain that part of central region 14 of the MR sensor 10 in a linear response mode. The transverse bias can be provided by shunt biasing, soft-film biasing, or permanent magnet biasing as is known in the art to produce a desired rotation in magnetic moment M, as shown in Figure 3. An output signal i s can then be coupled out to a sensing means, with the aid of electrical conductor means 18,20 which are connected to the MR sensor 10 within the central region 14 of the MR sensor 10. The signal i s enables the sensing means to determine the resistance changes in a detection region within said central region 14 as a function of the fields which are intercepted by the MR sensor 10 from previously recorded data on a magnetic medium, for example. The distance between the inner edges of conductor means 18 and 20 comprises the detection region over which the output signal is sensed. The simplest configuration is an unshielded MR sensor. In this case, since the signal flux from recorded magnetic transitions can enter the sensor through its entire height, linear recording density is usually low. The recording density is limited primarily by practical lower limits to the height of the sensor. To improve the linear record­ ing density, the MR element is usually placed in the gap between two soft-magnetic shields. The signal performance of an MR sensor embodying the present invention is minimally affected by variations in the level of longitu­ dinal bias so long as the level of longitudinal bias is sufficient to maintain the end regions 14 of the MR sensor 10 in a single domain state. In direct contrast, the competing bias requirements of prior art MR sensors cause a significant effect on the signal performance of the MR sensor due to changes in the level of longitudinal bias. The present invention is based on the premise that with the end regions in a single domain mode, the central region is forced into a single domain mode and this is true so long as the longitudinally unbiased gap (L NLB ) is not too large in comparison to the height of the MR sensor. The relationship between the longitudinally unbiased length, L NLB , the transverse biased length, L TB , and the length of the detection region, L D , is shown in Figure 3. The relationship between these lengths is such that L NLB → L TB → L D is generally true also, L TB → L D and L NTB → L D holds true. While it is true that generally there is no overlap of L NLB and L TB , some overlap is permissible so long as the above conditions rela­ tive to L D are met. When the additional constraint of side reading suppression is considered, it is clear that L TB should not be greater than L D since L D is normally chosen substantially equal to the recording track width and the existence of an area of transverse biased sensor outside the detec­ tion region L D would result in increased side reading from adjacent tracks. Also, it is clear that L NLB should not be greater than L D since the end regions, due to the strong longitudinal bias, are made insensi­ tive to signals from neighbouring tracks. As a result, side-reading would be suppressed. These considerations lead to the conclusion that L NLB = L TB = L D is the optimum arrangement for maximum sensitivity and also greatest side reading suppression. A specific embodiment of a MR read transducer assembly embodying the invention is represented in Figures 4, 5 and 6. Referring to Figure 4, a MR thin film layer 10 is in intimate electrical contact with an antiferromagnetic thin film layer 16. The thin film layers 10 and 16 produce exchange bias in the covered area of the MR layer 10 so that it is longitudinally biased in a single domain mode in the end regions 12. On the opposite side of MR layer 10, there is a means for producing a transverse bias field comprising a soft magnetic film layer 22, the layer 22 being separated from the MR layer 10 by a thin spacer layer 24. The purpose of layer 22 is to provide a soft film transverse bias of the central region 14 of the MR layer 10. Magnetic shields 26 and 28 are provided to minimise the effects of stray magnetic flux which would decrease the resolution of the readback signal, and electrical insulating layers 30 and 32 are provided to isolate these shields from the active components of the transducer assembly. The choice of materials for the MR layer 10 and the antiferro­ magnetic layer 16 in Figure 4 is a significant consideration. For the MR layer 10, a soft magnetic material is desired which has low magneto­ restriction and a reasonably high MR coefficient. In general, a very thin film would be desirable since thinner films are more sensitive. However, as a practical matter, the thickness of the film must be chosen to avoid very thin films whose thickness and properties are hard to control. The MR layer 10 may be formed reliably if it has a thickness in the range of approximately 20 to 50 nm. In a specific embodiment, Permalloy NiFe alloy was used. Layer 16 must be of a material that is antiferromagnetic not only when deposited over the material chosen for the MR layer but also when deposited in a thin film form. In addition, the material should have a Neel temperature substantially higher than the operating temperature of the read transducer. The antiferromagnetic layer 16 is deposited to produce a unidirectional bias along the length of the assembly. Alter­ natively, the direction of the unidirectional bias field can be set by heating the MR-antiferromagnetic material layer above the ordering temperature, and then cooling the layer in an externally applied uni­ directional magnetic field. The material should be thick enough for reliable fabrication with a minimum thickness of about 10 nm, and ranging up to several tens nm. In a specific embodiment MnFe was used. The soft film material of layer 22 in Figure 4 must exhibit high permeability, high resistivity, and no appreciable MR effect of its own. The high permeability ensures adequate biasing from bias source 21 with modest bias currents, the high resistivity is desirable to minimise electrical shunting of signal, and the low MR coefficient compared with that of the MR layer 10 is needed to avoid cancellation of biasing and signal responses. The thickness of the soft-film biasing layer 22, usually in a range of 10 to 50 nm, is chosen so that it will saturate. The characteristics of the material being used for the MR layer 10 must also be considered so that the ratio of thicknesses of the MR layer 10 and the soft film bias layer 22 can be chosen to produce the transverse bias to bias the central region 14 of MR layer 10 to the chosen angle. In a specific embodiment, NiFeRh was used for layer 22. The purpose of spacer layer 24 is to provide physical spacing between the MR layer 10 and soft film bias layer 22, since separation is necessary for these layers to produce opposite magnetisation rotations. Spacer layer 24 should have high resistivity, should not diffuse into either of the layers 10 and 22, and should have the property of being easily deposited in thin layers. The layer 24 should be as thin as can be reliably fabricated such as 20 nm, for example. In a specific embodiment, Ta was used for this layer, but other materials such as Al₂O₃ or SiO₂ could also be used. A magnetoresistive read transducer embodying the invention can be made by any of the known thin film manufacturing processes in a manner similar to making integrated circuit chips. These manufacturing pro­ cesses include the deposition of the thin film layers on a substrate, and appropriate shaping of the layers by suitable techniques such as selective etching and photolithographic techniques.

Claims: 1. A magnetoresistive read transducer assembly comprising: a thin film magnetoresistive layer (10) means (16) for producing a longitudinal bias in only the end regions (12) of the magnetoresistive layer, the longitudinal bias being of a level sufficient to maintain the end regions of the magnetoresis­ tive layer in a single domain state, means (18,20,21) for producing a transverse bias in at least a part of the central region (14) between said two end regions of said magneto­ resistive layer, the transverse bias being of a level sufficient to maintain said part of the central region of the magnetoresistive layer in a linear response mode; and conductive means (18,20) connected to said magnetoresistive layer within said central region to define a detection region so that resis­ tance changes in the detection region produced by magnetic fields which are intercepted by the magnetoresistive layer can be detected. 2. A magnetoresistive read transducer assembly as claimed in claim 1, wherein said magnetoresistive layer is composed of NiFe. 3. A magnetoresistive read transducer assembly as claimed in claim 1 or claim 2, wherein said magnetoresistive layer has a thickness in the range of 20 to 50 nm. 4. A magnetoresistive read transducer as claimed in any preceding claim, in which the means for producing a longitudinal bias in only the end regions of the magnetoresistive layer comprises a thin film of antiferromagnetic material in direct contact with only said end regions of said magnetoresistive layer to produce said longitudinal bias by exchange biasing. 5. A magnetoresistive read transducer assembly as claimed in claim 4, wherein said thin film of antiferromagnetic material is composed of MnFe. 6. A magnetoresistive read transducer assembly as claimed in claim 4 or claim 5, wherein said thin film of antiferromagnetic material has a thickness in the range of 10 to 50 nm. 7. A magnetoresistive read transducer assembly as claimed in any preceding claim, wherein said conductive means comprises spaced con­ ductive elements, and said detection region is defined by the inner edges of said conductive elements. 8. A magneticoresistive read transducer as claimed in any preceding claim, wherein the central region of said magnetoresistive layer which is not longitudinally biased, said detection region, and said at least a part of said central region in which a transverse bias is produced, are of substantially the same size. 9. A magnetoresistive read transducer assembly as claimed in any preceding claim, further comprising: a thin film of soft magnetic material disposed parallel to but spaced from the magnetoresistive layer; and means for supplying a bias current to said conductive means to magnetically bias said magnetoresistive layer with transverse bias of a level sufficient to maintain said magnetoresistive layer in a high sensitivity condition whereby signal sensing means connected to said conductive means can determine the resistance changes in said magneto­ resistive layer as a function of the fields which are intercepted thereby. 10. A magnetoresistive read transducer assembly as claimed in claim 9 wherein said thin film of soft magnetic material is composed of NiFeRh. 11. A magnetoresistive read transducer assembly as claimed in Claim 9 or Claim 10, wherein said thin film of soft magnetic material is thinner than said magnetoresistive layer. 12. A magnetoresistive read transducer assembly as claimed in any preceding claim, disposed between magnetic shields.

Inventors: TSANG, CHING HWA
Applicants: INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1986
Main classification: G11B5
IPC codes: G11B5, H01L43, G01R33