Patent Application
20100328823
Magnetoresistive (MR) sensors and related devices have a wide range of applications in non-volatile data storage systems. MR sensors are typically formed as part of a magnetic transducer or read/write head, and can be configured for data storage operations on a variety of different data storage media. The writer uses a write current to generate magnetic flux in a main pole (or write pole), and data are stored by switching the write current as the transducer tracks along a magnetic medium. In particular, the write current determines the flux density and field direction at the tip of the write pole, and the writer generates a sequential bit pattern by defining magnetic domains in the medium according to the flux at the write pole tip.
The reader reads back the bit pattern as a function of a sense current, which depends upon the resistance across the MR sensor. In anisotropic MR (AMR) devices, for example, the volume resistance varies with the external magnetic field, allowing the bit pattern to be decoded from the sense current as the AMR sensor tracks along the magnetic medium. In spin valve sensors, the resistance depends on transition effects such as giant magnetoresistance (GMR) or tunneling magnetoresistance (TMR), and the sense current varies with the relative magnetic orientation of a free layer, where the free layer field rotates in response to the external field (i.e., the bit pattern), with respect to a reference layer, in which the field direction is fixed.
Simple spin valve devices typically utilize a single reference layer, while dual spin valve (DSV) devices have multiple reference layers. Other sensors utilize the colossal MR (CMR) effect, which is characterized by greater signal amplitude but also requires a stronger external field, and can be challenging to achieve with standard magnetic media.
In read head construction, the MR sensor is generally located between first and second read shields, which are oriented transversely to an external media-facing surface and separated from one another by a read gap. In some reader designs the MR sensor extends across the read gap from one shield to the other, and in other designs the MR sensor is spaced from one or both shields by a nonmagnetic material.
In typical stacked transducer designs, the reader and writer structures are formed one on top of the other, usually with the reader first and the writer on top. In merged/stacked configurations, which are generally less common, one of the read shields also functions as a first or bottom return pole for the writer. Alternatively, the reader and writer are formed side-by-side in a substantially coplanar arrangement, with the writer spaced from the reader along the media-facing surface of the transducer.
Bit density is a major consideration in transducer design. In particular, as the bit density (or areal density) increases, the corresponding read and write head structures must be reduced in size. In the case of the reader this affects both the read gap, which is measured between the shields along the media-facing surface, and the stripe height, which is defined along the spin valve, transverse to the media-facing surface. Reduced stripe height and read gap are associated with increased demagnetization fields, which result when flux lines overlap and cross layers of the MR stack, affecting free layer response. As a result, there is a continuing need for MR sensor designs that accommodate smaller reader dimensions and higher areal density, while maintaining response and sensitivity by addressing the effects of demagnetization fields.
This invention is directed to a magnetic reader. The reader comprises a magnetoresistive (MR) stack located between first and second shields. The MR stack extends from a proximal end oriented toward a media-facing surface to a distal end oriented away from the media-facing surface. A demagnetization flux guide extends from the first shield toward the second shield, and is spaced from the distal end of the MR stack. The flux guide magnetically couples the distal end of the MR stack to the first shield.
Reader 30 and writer 40 are formed as a number of closely spaced layers, for example by thin film deposition on a slider or other substrate material. In the particular embodiment of
MR stack 34 comprises a multilayer magnetoresistive stack or spin valve, which extends along tracking direction T between first (bottom) read shield 31 and second (top) read shield 32. Nonmagnetic insulator (read gap material) 35 comprises a dielectric such as alumina (aluminum oxide, or Al2O3), and extends between first and second read shields 31 and 32.
As shown in
Bit density scales with the physical dimensions of reader 30 and writer 40, particularly MR stack 34. Unfortunately, reduced stack dimensions are associated with increased demagnetization effects, as described above, which tend to limit reader sensitivity and the physically attainable density. To address this concern, reader 30 is provided with a demagnetization flux guide to direct stray (demagnetization) flux away from MR stack 34, increasing reader sensitivity and providing for higher areal densities and reduced reader dimensions.
Reader 30 comprises first read shield 31, second read shield 32 and MR stack 34, as described above, with flux guide 50 to reduce demagnetization effects. In the embodiment of
In some embodiments, reader 30 and writer 40 are provided with protective coating 58 at external surface 51. Typical protective coatings include encapsulants, diamond-like coatings (DLCs) and combination thereof, which protect magnetic transducer 15 and prevent hard particle contamination of magnetic medium 20.
In operation of transducer 15, magnetic medium 20 translates in tracking direction T with respect to media-facing surface 51. Writer 40 generates magnetic flux loops that cross media-facing surface 51 into data storage medium 20, in order to lay down a sequential bit pattern.
Reader 30 decodes the bit pattern as a function of a sense current across MR stack 34, where the sense current varies with the external field strength along media-facing surface 51 (that is, with the magnetic domains in the bit pattern). First and second read shields 31 and 32 shield MR stack 34 from stray flux, improving sensitivity to smaller bit patterns and increasing the attainable bit density.
As bit size decreases, however, reader 30 and writer 40 are subject to additional dimensional constraints. With particular respect to MR stack 34, reduced layer thickness increases field overlap and demagnetization effects, ultimately limiting the attainable areal density. Flux guide 50 addresses this issue by directing demagnetization flux away from MR stack 34, providing for smaller stack dimensions without sacrificing reader sensitivity.
First and second read shields 31 and 32 extend transversely to media-facing surface 51, separated by MR stack 34 and read gap material 35. MR stack 34 is located between first and second read shields 31 and 32, with proximal (media-facing) end 71 oriented toward media-facing surface 51 and distal (opposite) end 72 oriented away from media-facing surface 51.
In the embodiment of
Stripe height H is measured from proximal end 71 to distal end 72 of MR stack 34. Read gap G is measured along the tracking direction, between first shield 31 and second shield 32.
In compact bit spacing configurations, read gap G is limited to a particular width, for example about 100 nm or less. In some of these embodiments, read gap G is limited to about 50 nm or less, and in additional embodiments read gap G is limited to about 30 nm or less. In these dimensional ranges, MR stack 34 is subject to substantial demagnetization field effects, which are addressed via demagnetization flux guide 50.
Flux guide 50 and shields 31 and 32 are typically formed of a soft magnetic material such as NiFe, CoFe or a similar soft magnetic alloy. In some embodiments, flux guide 50 and one or both of read shields 31 and 32 are formed of the same magnetic material, in order to improve flux coupling and field uniformity, or to reduce manufacturing requirements. Alternatively, different magnetic materials are used, for example to provide field shaping.
Flux guide 50 is spaced from MR stack 34 by distal flux gap D at stripe height H, and extends along distal end 72 in order to magnetically couple MR stack 34 to one of shields 31 and 32. Flux guide 50 also extends transversely with respect to media-facing surface 51 along one of read shields 31 or 32, and across a portion of read gap G such that flux guide 50 is spaced from the opposite shield by shield gap S.
In the particular embodiments of
In ALD embodiments, the microscopic structure of the flux gap material is defined by a gas phase chemical process in which individual precursors (that is, the molecular components of read gap material 35) are sequentially deposited as a series of monolayer structures formed by self-limiting surface reactions. The ALD process results in conformal flux gap structures that are substantially free of pin holes and other defects, with the gap material chemically bonded to other reader components such as shields 31 and 32, flux guide 50 and MR stack 34 (at distal end 72). The ALD process also improves gap uniformity, with thickness tolerances on the order of 1 nm or less for both distal flux gap D and shield flux gap S.
Seed layer 61 and cap layer 66 are generally formed of materials selected to reduce magnetic couplings between MR stack 34 and first and second read shields 31 and 32, for example Cu, Ni, Fe, Cr, P, Ta and combinations thereof, including NiFe, NiFeCr, Ta/NiFe and NiCrFe/NiFe alloys. The material of seed layer 61 is also selected to enhance the crystallographic structure of the additional layers in MR stack 34, particularly the grain structure, grain size and domain orientation in pinning layer 62.
Pinning layer 62 is deposited adjacent seed layer 61, and is typically formed of a material having a permanent magnetic moment sufficient to pin or fix magnetic orientations within SAF 63. In particular, pinning layer 62 is magnetically coupled to pinned layer 63A, which in turn determines the magnetic orientation of reference layer 63C. Suitable materials for pinning layer 62 include magnetic alloys such as CoPt and CoPtCr, or antiferromagnetic (AFM) materials such as PtMn, MiMn or FeMn.
Pinned layer 63A and reference layer 63C of SAF 63 are typically comprised of ferromagnetic materials such as Fe, NiFe or CoFe, such that the magnetization of pinned layer 63A is determined by ferromagnetic coupling to pinning layer 62. Coupling layer 63B is formed of a non-ferromagnetic material such as Ru, which promotes exchange coupling such that reference layer 63C is antiferromagnetically coupled to pinned layer 63A, and the magnetization of reference layer 63C is oriented oppositely to that of pinned layer 63A.
Free layer 65 is typically formed of a ferromagnetic material in which the magnetic orientation is responsive to an external field across media-facing surface 51, and the resistivity of MR stack varies with the magnetic orientation of free layer 65 as compared to reference layer 63C. This allows reader 30 to read back bit patterns as a function of a sense current that depends on the resistivity of MR stack 34. In GMR embodiments, for example, spacer/barrier layer 64 is formed of a nonmagnetic conductor or spacer, and the sense current across MR stack 34 depends on the giant magnetoresistive effect.
In tunneling (TMR) embodiments, spacer/barrier layer 64 is formed of an insulating tunnel barrier material, and the sense current is a tunneling current. In these TMR embodiments, suitable materials for spacer/barrier layer 64 include dielectric oxides of Al, Mg, Hf, Ta, Nd, Ti or Zr, and combinations thereof.
In the CPP embodiments of MR stack 34 illustrated by
In current-in-plane (CIP) embodiments, the sense current contacts are formed on either side of MR stack 34 (that is, on the left and right in
The magnetic field orientations within individual layers of MR stack 34 depend on the detailed arrangement of seed layer 61, pinning layer 62, SAF layers 63A, 63B and 63C, spacer/barrier layer 64, free layer 65 and cap layer 66. Typically, individual magnetic orientations are either into or out of (that is, perpendicular to) the planar structure of MR stack 34, but longitudinal magnetic fields are also produced for biasing and other purposes, and surface and edge magnetization effects must also be taken into account.
Magnetic flux lines, moreover, are not constrained to remain within particular layers of MR stack 34, but tend to diverge and stray across layer boundaries, generating both longitudinal and perpendicular demagnetization field components, for example in free layer 65 and along the sides and distal end 72 of MR stack 34. These demagnetization fields tend to increase hysteresis, increasing response time and reducing the angular difference in magnetization direction in free layer 65 with respect to reference layer 63C.
As the dimensions of reader 30 and MR stack 34 are reduced to accommodate higher bit densities, moreover, demagnetization effects increase, further decreasing sensitivity and response. Flux guide 50 limits these demagnetization effects by directing stray flux away from MR stack 34. In particular, flux guide 50 directs demagnetization flux away from distal end 72 of free layer 65, reducing hysteresis and increasing the response of free layer 65.
The configuration of flux guide 50 varies with the corresponding configuration of MR stack 34 and the other elements of reader 30. In some embodiments, for example, flux guide 50 extends from top read shield 32 toward bottom shield 31, as shown in
Curves 81 and 82 (dashed) are forward and backward hysteresis curves, respectively, for the baseline design, and curves 83 and 84 (solid) are obtained after adding a demagnetization flux guide. The hysteresis area between baseline curves 81 and 82 is substantial, demonstrating demagnetization effects. In particular, demagnetization fields reduce sensitivity by requiring a greater change in effective field Hy before a commensurate change in effective resistance R is observed (that is, before the sense current changes). Baseline response curves 81 and 82 are also history-dependent, in that the transition point (in effective field Hy) depends not only upon the field strength but also prior magnetic history and field polarity. As a result. In addition the sense current is less responsive to changes in external magnetic fields, and the reader is less sensitive to higher-density bit patterns and smaller field gradients.
Curves 83 and 84, on the other hand, show that the demagnetization flux guide substantially limits hysteresis effects by reducing the demagnetization fields. In particular, hysteresis curves 83 and 84 exhibit a smoother, more continuous transition from low to high resistivity, with improved linearity and reduced free layer rotation. As a result, effective resistance R for curves 83 and 84 is substantially independent of field polarity and prior field excursions, the sense current is more responsive to changes in the external magnetic field, and the MR stack exhibits improved sensitivity to higher-density bit patterns and smaller field gradients.
MR stack 34 comprises seed layer 61, pinning layer 62, reference layer 63C, spacer/barrier layer 64, free layer 65 and cap layer 66, as described above. In these embodiments, however, the multilayer SAF structure is replaced by a single ferromagnetic (FM) reference layer 63C, which is directly ferromagnetically coupled to pinning layer 62, rather than antiferromagnetically coupled to pinned layer 63A via coupling layer 63B as shown in
The single-layer FM reference layer structure of
In the particular embodiments of
While this invention has been described with reference to particular embodiments, the terminology used is for the purposes of description, not limitation. 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, including the substitution of various equivalents for particular invention elements and adaptation of the invention's teachings to different materials, situations and circumstances. Thus the invention is not limited to the particular embodiments disclosed herein, but encompasses all embodiments falling within the scope of the appended claims.
