Domain wall free shields of MR sensors

A magnetic reader of the present invention comprises an MR sensor shielded by a magnetic shield including single domain soft magnetic materials. The domain wall free magnetic shield includes an unbiased soft magnetic layer and a biased soft magnetic layer separated by a non-magnetic layer. The easy axis of the biased layer is oriented to create a path for magnetic flux through the biased and unbiased layers thereby reducing the demagnetization field of the shield. A biasing layer maintains the first and second magnetic layers as single domain magnets. The biasing layer is further shaped to define a quiet zone where the biasing layer does not overlay the MR sensor.

BACKGROUND OF THE INVENTION

The present invention relates generally to shields for magnetoresistive sensors used in electronic data storage and retrieval systems. In particular, this invention relates to biased domain wall free shields. The present invention is a further improvement on U.S. Pat. No. 6,437,949, herein incorporated by reference.

In an electronic data storage and retrieval system, a transducing head typically includes a reader portion having a magnetoresistive (MR) sensor for retrieving magnetically encoded information stored on a magnetic disc. MR sensors are comprised of at least one magnetic layer whose magnetic alignment responds to external applied magnetic fields. Typical MR sensors are of the anisotropic magnetoresistive (AMR) sensor or giant magnetoresistive (GMR) sensor type. When an MR sensor is passed over the surface of a magnetic storage disc, magnetic flux from the surface of the disc alters the magnetic alignment of the MR sensor, which in turn alters the magnetic resistivity of the MR sensor. The change in resistivity of the MR sensor can be detected by passing a current through the MR sensor and measuring a voltage across the MR sensor. External circuitry then converts the voltage information into an appropriate format and manipulates that information as necessary.

A response curve of the MR sensor compares the voltage across the MR sensor to the magnetic flux received from the disc by the sensor. This response curve has both linear and non-linear portions, of which it is preferred that the MR sensor operate along the linear portion. To force the MR sensor to operate along the linear portions, the sensor is magnetically biased at a biasing point that is located along the linear portion of the response curve.

MR sensors have a large read pulse width that must be reduced for high linear densities. The pulse width of the MR sensor is reduced by sandwiching the MR sensor between two magnetic shields. The magnetic shields generally incorporate soft magnetic materials for their permeability to magnetic flux. The purpose of the shields is to prevent the magnetic flux from sources other than the desired transition from interacting with the MR sensor. During a read operation, the upper and lower shields ensure that the MR sensor reads only the information (transition) stored directly beneath it on a specific track of the magnetic disc medium by absorbing any stray magnetic fields emanating from adjacent tracks and transitions.

Within a conventional shield exists a plurality of magnetic domains separated from each other by a plurality of magnetic domain walls. Each domain has a magnetization that is oriented in a direction different than the magnetization of all adjacent domains. As the domain walls move, the bias point, as well as the response of the MR sensor to signals emanating from the magnetic disc medium changes. The overall result is noise during the read operation.

To avoid the problems associated with domain wall movement, the ideal shield structure would have no domain walls. Attempts to achieve a single domain structure in MR sensor shields have been largely unsuccessful due to inability of weak exchange coupling interactions to overcome large demagnetizing fields in conventional structures. The result is multiple domains, noise and suppression of sensor signal. Consequently, there remains a need in the art for a domain wall free shield without suppressing MR sensor amplitude while reducing or eliminating sources of noise.

BRIEF SUMMARY OF THE INVENTION

The present invention is a thin film structure with soft magnetic portions maintained in a single domain state for use as a shield for an MR sensor. The thin film structure of the present invention has an unbiased soft magnetic layer, a biased soft magnetic layer, a non-magnetic layer and a biasing layer. The non-magnetic layer is positioned between the unbiased soft magnetic layer and the biased soft magnetic layer. The biasing layer is positioned adjacent the biased soft magnetic layer. The biasing layer is further shaped to reduce the effects of transmitted dispersion on the MR sensor while maintaining exchange coupling in areas where the demagnetization field is greatest.

The layered thin film structure of the present invention effectively reduces the demagnetization field to levels where exchange coupling maintains the biased soft magnetic layer as a single domain. The unbiased soft magnetic layer is maintained in a single domain by forming a flux path with the biased soft magnetic layer.

DETAILED DESCRIPTION

FIG. 1illustrates the major components of a disc drive system20in which the domain wall free shield of the present invention is used. Disc drive system20includes magnetic disc22mounted for rotational movement about an axis defined by spindle24within housing26. Disc drive20also includes actuator28mounted to base plate30of housing26and pivotally movable relative to disc22about axis32. Cover34covers a portion of actuator28. Drive controller36is coupled to actuator28. Drive controller36is either mountable within disc drive system20or is located outside of disc drive system20with suitable connection to actuator28. Actuator28includes actuator arm assembly38, a rigid support member40, and head gimbal assembly42. Head gimbal assembly42includes flexure arm44coupled to rigid member40and air bearing slider46coupled to flexure arm44by a gimbal. Slider46supports a magnetoresistive (MR) transducing head for reading information from disc22and encoding information to disc22. The domain wall free shields of this invention are part of the MR transducer located within slider46.

During operation, drive controller36receives position information indicating a portion of disc22to be accessed. Drive controller36receives the position information from either an operator, a host computer, or from another suitable controller. Based on the position information, drive controller36provides a position signal to actuator28. The position signal causes actuator28to pivot about axis32. This causes slider46to move radially over the surface of disc22in a generally arc-like path indicated by arrow48. Drive controller36and actuator28operate in a known closed loop, negative feedback manner so that the transducer carried by slider46is positioned over the desired portion of disc22. Once the transducer is appropriately positioned, drive controller36then executes a desired read or write operation.

FIGS. 2 and 3illustrate transducing head50.FIG. 2is a cross-sectional view of transducing head50taken along a plane normal to air bearing surface (ABS).FIG. 3is a layered diagram that illustrates the location of a plurality of magnetically significant elements of transducing head50as they appear along the ABS. The ABS of transducing head50faces disc surface52of magnetic disc54. Magnetic disc54travels or rotates in a direction relative to transducing head50as indicated by arrow A. Spacing between the ABS of the transducing head50and disc surface52is preferably minimized while avoiding contact between transducing head50and magnetic disc54.

Transducing head50includes writer portion56and reader portion58. The writer portion56and reader portion58and are presented in a merged configuration with an intermediate spacer layer60.

Writer portion56of transducing head50includes bottom pole62, write gap layer64, conductive coils66, insulator layer68, top pole seed layer70, and top pole72. A write gap is defined at the ABS by write gap layer64between terminating ends of bottom pole62and top pole72. Conductive coils66are positioned in insulator layer68between bottom pole62and top pole72, such that the flow of electrical current through conductive coils66generates a magnetic field across the write gap.

Reader portion58of transducing head50includes: lower shield74, first gap layer76, contact layer78, magnetoresistive (MR) sensor80, second gap layer82, and upper shield84. A read gap is defined on the ABS between terminating ends of lower shield74and upper shield84. MR read element80is positioned between terminating ends of first gap layer76and second gap layer82. First and second gap layers76and82are positioned between lower shield74and upper shield84. Lower shield74and upper shield84may be layered upon separate seed layers (not shown). The seed layers are selected to promote the desired magnetic properties in lower shield74and upper shield84.

MR sensor80is a multilayer device operable to sense magnetic flux from a magnetic media. MR sensor80may be any one of a plurality of MR-type sensors, including, but not limited to, AMR, GMR, TMR, spin-valve and spin-dependent tunnel junction (STJ) sensors. At least one layer of MR sensor80is a sensing layer that requires longitudinal biasing, such as a free layer of a GMR spin-valve sensor. The sensing layer is typically within a reader stack and may also be referred to as the active region of MR sensor80. Moreover, for several types of MR sensors, at least one layer of MR sensor80is an antiferromagnetic layer that requires annealing to set a magnetization direction therein.

Magnetic flux from the surface52of disc54causes rotation of the magnetization vector of a sensing layer of MR sensor80, which in turn causes a change in electrical resistivity of MR sensor80. The change in resistivity of MR sensor80can be detected, for example, by passing a current through MR sensor80and measuring a voltage across MR sensor80. External circuitry then converts the voltage information into an appropriate format and manipulates that information as necessary.

During a read operation, lower shield74and upper shield84ensure that MR sensor80reads only the information stored directly beneath it on a specific track of magnetic disc54. Stray magnetic flux emanating from adjacent tracks and transitions are absorbed by soft magnetic materials within lower shield74and upper shield84.

Lower shield74and upper shield84are preferably formed of a lamination of thin films including soft magnetic materials which are maintained in single magnetic domain states. By maintaining the soft magnetic materials in single magnetic domain states, the problems associated with domain wall movement are avoided. A shield of the present invention is a further improvement of the thin film structure with soft magnetic portions maintained in a single domain state described in U.S. Pat. No. 6,437,949, herein incorporated by reference. The layered thin film structure of the present invention effectively reduces the demagnetization field to levels where exchange coupling maintains adjacent soft magnetic material as a single domain.

Upper shield84of the present invention is shown relative to MR sensor80inFIGS. 4,5and6.FIG. 4is a layered diagram of shield84as viewed from the air bearing surface.FIG. 5andFIG. 6are cross-sectional views of two embodiments of shield84taken along line5-5ofFIG. 4. Upper shield84is formed of a lamination of materials in accord with the present invention. Upper shield84is comprised of unbiased layer92, non-magnetic layer90, biasing layer88, and biased layer86. MR sensor80is preferably placed adjacent biased layer86of shield84.

Biased layer86and unbiased layer92are preferably formed of a soft magnetic material having anisotropic properties, such as NiFe, cobalt amorphous alloys, FeN, permalloy or Sendust. In a preferred embodiment, a product of a thickness of biased layer86and a magnetic moment of biased layer86preferably equals a product of a thickness of unbiased layer92and a magnetic moment of unbiased layer92. Additionally, an easy axis of unbiased layer92is preferably parallel to an easy axis of biased layer86. A net magnetic moment of unbiased layer92is indicated by arrow94and a net magnetic moment for biased layer86is indicated by arrow98.

Non-magnetic layer90is preferably a pinhole free thin (approximately 300 ANG.) layer of a nonmagnetic material, such as: tantalum, chromium, alumina, or silica. Nonmagnetic layer90, which is positioned between unbiased layer92and biasing layer88prevents exchange coupling between unbiased layer92and biasing layer88. Thus, a magnetization of unbiased layer92, depicted by arrow94, is oriented antiparallel to the magnetization of biased layer86, depicted by arrow98, due to demagnetization fields from biased layer86.

Non-magnetic layer90has a dramatic effect on reducing demagnetization fields within shield84, thereby allowing shield84to operate in a single magnetic domain state. Furthermore, this multi-layer lamination of shield84allows for the magnetizations of biased layer86and unbiased layer92to each be saturated along respective easy axes while leaving virtually unaffected the coherent rotation of magnetizations of hard axes of respective biased layer86and unbiased layer92. This feature allows the shield to operate by absorbing stray magnetic flux from adjacent bits or tracks on the magnetic disc by a process of coherent rotation.

Biasing layer88is located adjacent to unbiased layer92, such that biasing layer88is positioned between non-magnetic layer90and biased layer86. Biasing layer88is preferably formed of either a permanent magnet material (e.g., a hard magnetic material), such as CoPt, CoCrPt, CoCrTa and CoPdCr, or an antiferromagnetic material, such as NiMn, NiMnCr, PtMn, PdPtMn, CrMnPt, CrMnCu, CrMnPd and PtRuMn. Biasing layer88is oriented with a net magnetic moment represented by arrows96. Biasing layer88is exchange coupled with biased layer86to bias or set magnetization of biased layer86.

FIGS. 5 and 6are cross-sectional views of two embodiments of biasing layer88. Each view is taken along line5-5ofFIG. 4with the relative location of MR sensor80shown by a dashed outline. Biasing layer88is shaped such that non-magnetic layer90lies over the position of MR sensor80.

Biasing layer88is shaped to create a void or cut-out adjacent to the MR sensor80thereby creating a quiet zone Q proximal to MR sensor80. Quiet zone Q corresponds to a gap in the hard magnetic biasing layer in the area near MR sensor80. The size and shape of quiet zone Q is generally related to the size and shape of MR sensor80. The minimum size of quiet zone Q is the size and shape of the active region of MR sensor80. The upper size limite of quiet zone Q is set by the size of the shield and the need for sufficient area of biasing layer888to maintain the desired domain structure in adjacent biased layer86. Preferably the width of the quiet zone Q is sufficiently wide that biasing layer88does not overlay the area above the MR sensor80.

One embodiment of quiet zone Q is a void in biasing layer88preferably slightly larger than the size and shape of MR sensor80as shown inFIG. 5. Typically, the distance between the area overlaying MR Sensor80and edges of biasing layer88defining the boundary of quiet zone Q is between approximately 1 to 6 microns. However, the distance may be greater in one dimension as demonstrated in an alternative embodiment shown inFIG. 6, InFIG. 6, quiet zone Q is a void roughly the width of MR sensor80and extends through biasing layer88.

Quiet zone Q reduces transmitted dispersion affecting MR sensor80and reduces detrimental effects of the biasing layer88on MR sensor amplitude and sensitivity. Permanent magnetic materials have transmitted dispersion attributed to variation in the magnetic moments within the permanent magnetic materials. Transmitted dispersion may extend from the permanent magnetic materials into adjacent soft magnetic materials; thereby creating a source of noise to the MR sensor. By shaping the biasing layer88, the biasing field is maintained at the edges of shield84parallel to the ABS where the demagnetization field is greatest; while creating a quiet zone adjacent to MR sensor80without noise caused by transmitted dispersion or Barkhausen effects. The size and shape of quiet zone Q may be adjusted to balance the interests of maintaining a sufficient biasing field versus MR sensor sensitivity and transmitted dispersion. Enlarging quiet zone Q reduces the effects of transmitted dispersion on the MR sensor; while decreasing the size of quiet zone Q improves biasing.

FIGS. 7-13illustrate a method of forming a transducing head50with a domain wall free upper shield.FIG. 7shows a partially formed transducing head50on a substrate100. MR sensor80and associated structures including: first gap layer76, contact layer78, and second gap layer82, have been deposited. Some structures are omitted from subsequent figures. The formation of upper shield84onto the structure ofFIG. 7is described below.

FIG. 8shows the soft magnetic material deposited to create biased layer86.FIG. 9shows a following step in which a portion of biased layer86is masked off by photoresist102. Photoresist102generally lies over an area relative to MR sensor80.FIG. 10shows the next step, in which hard magnetic material is deposited to form biasing layer88.

For upper shield84, a separate seed layer for biasing layer88is not present because direct contact between biased layer86and biasing layer88is desired. Photoresist102is subsequently removed resulting in the structure ofFIG. 11. Non-magnetic layer90is deposited, followed by additional soft magnetic material to form unbiased layer92as shown inFIG. 12. Additional layers, including insulating layers, may be subsequently deposited on upper shield84to complete transducing head50as shown inFIGS. 2 and 3.

In a preferred embodiment, the hard bias material forming biasing layer88is obliquely deposited. Oblique deposition induces a preferred axis of anisotropy in the hard magnetic film thereby reducing the transmitted dispersion in the biasing layer88. The preferred axis of anisotropy in the hard magnetic film is formed without application of the setting field, although a setting field may be additionally applied.

Oblique deposition to biasing layer88is shown inFIG. 13. The angle of oblique deposition θ is measured from a surface normal. A surface normal is shown inFIG. 13as the Z axis of the reference Cartesian coordinate axes. The oblique angle of incidence θ measured from a surface normal is preferably approximately 60° up to approximately 90°, more preferably 65° to 75° with approximately 70° being most preferred. During oblique deposition, substrate100is preferably non-rotating for deposition from a point source104. Point source104is preferably a physical vapor deposition (PVD) source, such as an ion beam sputter deposition source. Alternatively, a collimating device may be used to limit the incidence angles of deposition flux to the desired range of oblique angles.

Selection of the proper orientation of incidence of the deposition flux which form the magnetic film is required to direct anisotropy of the grains/magnetic domains in a preferred direction, for example along the ABS direction. Therefore, in addition to the oblique angle, the deposition direction as shown by arrow A is preferably normal to the preferred anisotropy direction. InFIG. 13, the preferred anisotropy direction is parallel to the ABS direction (X axis), therefore deposition is directed at angle α, approximately 90° from the X axis. Angle α may be varied or wobbled by approximately ±10° from normal to compensate for any shadowing that may be caused by photoresist102, the structure of which may vary according to the shield and transducing head design as well as the fabrication methods used.

Due to the nature of physical deposition techniques, the deposited material tends to be thicker nearer the source104. To compensate, a first layer of hard bias material is deposited according to position A. A second layer is subsequently deposited at the same angle θ, but from the opposite side, approximately 180° relative to angle α in the X-Y plane as shown by position B inFIG. 13. Deposition is alternated between position A and position B until the hard bias material has reached the desired thickness. A setting field is applied to the hard bias materials.

FIG. 14is a bottom view of lower shield74of the present invention as viewed from the air bearing surface.FIG. 14is a layered diagram of shield74shown relative to MR sensor80. Lower shield74formed of a lamination of materials consistent with that described for upper shield84. Lower shield74is comprised of unbiased layer92, a non-magnetic layer90, a biasing layer88, and a biased layer86. Lower shield74includes two embodiments for biasing layer88. Biasing layer88is cut-away or shaped for lower shield74as described above for upper shield84.

Lower shield74is formed by materials and methods similar to that described above for upper shield84. However, the shield structure is reversed such that MR sensor80is preferably located adjacent biased layer86of lower shield74. Therefore the order of layer deposition for lower shield74is reversed from the description given above for upper shield84. Due to the reversed deposition order, biasing layer88is not deposited onto biased layer86as in upper shield84, therefore a seedlayer for biasing layer88may be included in lower shield74.

A seedlayer suitable for hard magnetic material may be deposited prior to biasing layer88in lower shield74. The seedlayer (not shown) is deposited on non-magnetic layer90and may be deposited by normal deposition techniques or oblique deposition at an angle θ similar to biasing layer88.

Alternatively, non-magnetic layer90may act as a seedlayer. Suitable, materials for non-magnetic layer90when acting as a seedlayer for subsequently deposited biasing layer88include Cr, TiW or other non-magnetic materials known in the art for hard bias seedlayers.

Preferably, lower shield74and upper shield84are both used in transducing head50. When lower shield74and upper shield84are used together, biasing layer90of each shield preferably shares a common or aligned magnetization. Alternatively, either lower shield74or upper shield84may be replaced in transducing head50by a conventional shield structure.

In summary, the shield structure of the present invention has an unbiased soft magnetic layer, a biased soft magnetic layer, a non-magnetic layer and a biasing layer. The non-magnetic layer is positioned between the unbiased soft magnetic layer and the biased soft magnetic layer creating a flux path to reduce the demagnetizing field. The biasing layer is positioned to exchange couple with the biased soft magnetic layer. The unbiased soft magnetic layer is maintained in a single domain by forming a flux path with the biased soft magnetic layer. The biasing layer is further shaped to reduce the effects of transmitted dispersion on the MR sensor while maintaining exchange coupling in areas where the demagnetization field is greatest.

Transducing head50including either first example shield74or second example shield74may alternatively be fabricated by oblique deposition described herein or by using known methods of material deposition and patterning known to those of skill in the art. Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.