Patent Application
20120050921
This disclosure is directed to a read/write head for a data storage system. The head comprises a main pole configured to emit flux, and a return pole configured to return flux to the main pole.
A magnetic damper is configured to dampen response of the return pole to the transitions in the flux positioned. The magnetic damper comprises a first conductor spaced from a first side of the return pole, a second conductor spaced from a second side of the return pole, and a third conductor connecting the first and second conductors. The magnetic damper is inductively coupled to the return pole.
Reader 14 comprises read sensor 20 with first (bottom) read shield 22 and second (top) read shield 24. Read sensor 20 comprises an MR element such as a spin valve or MR stack, located between read shields 22 and 24.
Read shields 22 and 24 may be formed of a soft magnetic material such as a nickel-iron (NiFe), cobalt-iron (CoFe) or nickel-cobalt-iron (NiCoFe) alloy, extending transversely from external (or media-facing) surface 26. Depending on embodiment, read shields 22 and 24 shield MR sensor (or MR stack) 20 from stray flux or noise.
Dielectric material 28 may be formed of silica (SiO2) or another nonmagnetic insulator. Depending on embodiment, dielectric material (or dielectric) 28 sometimes forms a read gap layer between first and second read shields 22 and 24, extending from the distal end of MR sensor 20 opposite surface 26 of read/write head 10.
MR sensor 20 extends between read shields 22 and 24, with the stack layers oriented perpendicularly to surface 26. In current-perpendicular-to-plane (CPP) embodiments, MR sensor 20 may span the read gap between read shields 22 and 24, as shown in
In current-in-plane (CIP) configurations, additional side contacts may be used to conduct a sense current through MR sensor 20 in a substantially parallel sense with respect to the stack layers. In these embodiments, MR sensor 20 may be spaced from read shields 22 and 24 by additional layers of dielectric 28.
Writer 16 comprises write pole (or main pole) 30, located between first (leading) return pole 18 and second (trailing) return pole 32. Write pole 30 directs flux from pole tip 34 across surface 26 to magnetic medium (or recording medium) 36. Flux loops from pole tip 34 close back across surface 26 via return poles 18 and 32.
Write pole 30 and return poles 18 and 32 may be formed of magnetically soft, high magnetic moment materials such as NiFe, CoFe and NiCoFe alloys. In some embodiments, write pole 30 has a composite or layered structure, in which the various layers include additional components such as niobium (Nb), zirconium (Zr), tantalum (Ta), aluminum (Al) and copper (Cu). In additional embodiments, one or both of return poles 18 and 32 may have a multilayer structure (e.g., layers 18A and 18B of first return pole 18), for example to enhance flux return. Alternatively, one or both of return poles 18 and 32 may accommodate a pole tip heater or other auxiliary component for writer 16.
Coil (or coils) 38 may be formed of a low resistivity material such as copper (Cu), or, alternatively, silver (Ag) or gold (Au). Coil 38 may be inductively coupled to write pole 30, for example to generate flux through pole tip 34 when energized by a write current. In some embodiments, coil 38 may be formed in helical or solenoidal fashion about write pole 30 and yoke 40, as shown in
Write pole 30 and yoke 40 extend from proximal ends oriented toward surface 26 to distal ends proximate magnetic vias 42. Yoke 40 and magnetic vias 42 may be formed of a magnetically soft material such as NiFe or NiCoFe. Depending on embodiment, yoke 40 and magnetic vias 42 may enhance flux delivery to pole tip 34 or close flux loops from return poles 18 and 32 back to yoke 40 and write pole 30, or yoke 40 and magnetic vias 42 may perform both functions.
Additional dielectric or nonmagnetic insulator 44 may surround write pole 30 and yoke 40, electrically insulating coil 38 and forming a nonmagnetic write gap between pole tip 34 and return poles 18 and 32, proximate surface 26. Depending upon embodiment, writer 16 sometimes includes a combination of leading and trailing write shields 46 and 48, or side shield structures, or both. In these various embodiments, one or both of write shields 46 and 48 may be configured with or without side shield elements to adjust the write gap spacing and to provide additional field shaping at pole tip 34.
Flux damper 12 comprises first (bottom) conducting layer 58 and second (top) conducting layer 60. Conductors 58 and 60 may be electrically connected by conducting vias 62 (shown in dashed or hidden lines), forming an inductive loop about return pole 18. The inductive loop may be spaced from return pole 18 by layers of nonmagnetic insulating material such as dielectric or insulating material 44 or 52.
In some embodiments, protective layer 50 covers pole tip 34 and other elements of reader 14 and writer 16 at surface 26. In these embodiments, protective layer 50 may include a diamond-like coating (DLC) or other hard coating, an encapsulant, or both. Depending on embodiment, protective layer 50 can be configured to protect sensitive structures of read/write head 10 or to reduce hard particle contamination of medium 36, or to provide a combination of protection and contaminant reduction.
In manufacture of magnetic head 10, reader 14 and writer 16 may be formed as a number of closely spaced layers, for example by thin film deposition onto a slider body or other substrate material. In the stacked head embodiment of
During write operations of magnetic head 10, writer 16 writes data in response to a current generated in coil 38, producing magnetic flux in yoke 40 and write pole 30. Flux loops exit write pole 30 at pole tip 34, crossing surface 26 to enter medium 36. The flux loops close back from medium 36 via return poles 18 and 32, through magnetic vias 42 to yoke 40 and write pole 30. Magnetic domain orientations in medium 36 can be determined by the polarity and magnitude of the write current in coil 38, allowing writer 16 to record a sequential bit pattern or other magnetic domain structure onto medium 36 as medium 36 translates along surface 26 in tracking direction S.
Reader 14 reads back data as a function of a sense current across MR sensor 20. The sense current varies with the magnetic domain orientations in medium 36, for example as reflecting the bit pattern laid down by writer 16, or another magnetic domain structure. First and second read shields 22 and 24 may be positioned to shield MR sensor 20 from stray flux or noise, improving speed, sensitivity and reliability at high areal densities.
As flux loops from pole tip 34 close back through return poles 18 and 32, spikes of magnetic flux (that is, regions of high flux density) may form along the media-facing boundary of return poles 18 and 32. These flux spikes may occur, for example, due to changes or polarity transitions in the write current in coil 38, or in the flux through pole tip 34 and return poles 18 and 32. In layered return pole embodiments, flux spikes sometimes form at an interface between pole layers, for example top layer 18A and bottom layer 18B of first return pole 18. Alternatively, flux spikes may occur at other discontinuities in the return pole structure, including edges, cracks, protrusions and sites of hard particle contamination.
Flux spiking can be associated with off-track erasure, side writing or data overwrite. In repeated scans of low-frequency transition patterns, for example, where the transition length may be greater than the trackwise length of magnetic head 10, side erasures in return pole 18 are sometimes associated in time with flux transitions at write pole tip 34. Flux spiking or side erasure (overwrite) regions 54 (projected lines) in medium 36 may also be spaced from low-frequency transitions 56 by approximately the same distance that pole tip 34 is spaced from the center of leading return pole 18.
In addition, return poles 18 and 32 may be wider than pole tip 34, and regions 54 may not occur along the data track. In some embodiments, for example, regions 54 are spaced up to ten, fifteen or twenty-five track widths (or more) on either side of pole tip 34, as described below with respect to
In the schematic view of
Magnetic damper 12 comprises first (bottom) conductor 58, which may be positioned opposite top conductor 60, as shown in
In the particular embodiment of
This contrasts with bucking coils and other planar designs, which may be oriented about one or both of magnetic vias 42 rather than return pole 18 or 32. In terms of tracking direction S, moreover, first conductor 58 leads the return pole and second conductor 60 trails the return pole, each occupying different planes that can be separated in or along the trackwise direction. Alternatively, the first and second conductor designations may be reversed, and, depending upon the particular orientation of the other writer components, second conductor 60 may lead return pole 18 or 32, and first conductor 58 may trail return pole 18 or 32 (see, e.g.,
During operation of writer 16, bits may be laid down along track T as a function of the magnetic flux emitted by write pole tip 34. Depending on embodiment, polarity transitions can be generated by switching the write current in coil 38, for example via inductive couplings to yoke 40 and write pole 30 with pole tip 34. Flux transitions may also propagate back to return poles 18 and 32 through a back magnetic via or other magnetic coupling structure, as shown in
Because return poles 18 and 32 may be wider than write pole tip 34, flux spiking or side erasure regions 54 may be separated from track T in a cross-track direction, as described above, for example with a lateral spacing of many times the track width. As a result, regions 54 may not pass under pole tip 34 in an ensuing write operation, raising the possibility of off-track data corruption when flux spikes reduce the signal-to-noise ratio (SNR) of off-track bits, increasing the bit error rate (BER). The cross-track spacing may not be known a priori, moreover, and the resultant bit corruption (and other related effects) may be difficult to predict, diagnose, trace and correct.
Magnetic flux damper 12 addresses this issue by generating an inductive electromotive force (EMF) to slow the response of return pole 18 to flux transitions generated in write pole 30. In particular, conducting vias 62 connect first and second conductors 58 and 60 to form a damping loop that substantially surrounds return pole 18 or return pole 32, or both.
Magnetic damper 12 is thus inductively coupled to one or both of return poles 18 and 32. Depending on embodiment, for example, the inductive loop may encompass magnetic coupling in regions 54 that are laterally spaced from track T by many track widths, and the induced EMF may have sufficient magnitude to reduce flux spiking, side writing and off-track erasure or over-writing during bit or polarity transitions, or due to other changes or transitions in the write current or emitted flux density, without substantially affecting flux loop closure during the write plateau.
As shown in
In the embodiment of
As shown in
Conductors 58 and 60 provide more or less complete conductive coupling, depending upon geometry. As shown in
The number, position and shape of conducting vias 62 vary with the geometry of conductors 58 and 60, and with the configuration of writer 16 and return pole 18. In particular, conducting vias 62 may be positioned to form an inductive loop about return pole 18, and the induced EMF may oppose flux transients propagating from write pole 30 through magnetic vias 42 to return pole 18. The specified degree of coupling varies with the geometry of conductors 58 and 60, and with the presence or absence of additional circuit elements used to help generate or modify the damping current.
In both passive and active embodiments, the effects of magnetic damper 12 are based on Faraday's Law of magnetic induction. That is,
where the induced EMF is related to flux Φ through the inductive loop formed by magnetic damper 12. According to Faraday's Law, the induced EMF is proportional to the time rate of change in magnetic flux Φ, which in turn depends on the geometry of magnetic damper 12, and on the magnitude and timing of the flux transitions as they propagate through write pole tip 34 and return pole 18.
The negative sign in Faraday's Law can be interpreted to indicate that the induced EMF acts to oppose changes in flux Φ, in accordance with Lenz' Law. Thus the induced EMF may generate a damping current in magnetic damper 12, and the damping current may generate a counter-flux that acts to oppose the change in flux Φ. Magnetic damper 12 may thus damp or slow the response of return pole 18 to flux and write current transitions, as described above, and the damping effects may apply to transitions or changes in flux Φ that propagate either through the magnetic medium or through a direct coupling to the write pole such as a yoke or magnetic via. Depending on embodiment, magnetic damper 12 may further support a damping current that generates sufficient opposing flux (or counter-flux) to limit flux spiking from return pole 18 into the magnetic medium, or to reduce the effects of off-track erasure and side-track writing, or both, as compared to read/write head operations performed without a damping current as provided by magnetic damper 12.
The damping current depends on the induced EMF and on the overall impedance of the damping circuit, as based on the generalized form of Ohm's Law. That is,
Where Z is the impedance of the damping loop and damping current ID is driven through impedance Z by the induced EMF from Faraday's Law. In general, impedance Z can be frequency dependent, so the damping current may vary with the sharpness of the transitions in flux Φ, as emitted by pole tip 34 and as propagating through return pole 18.
Based on Faraday's Law, the induced EMF (and thus the damping current) increases as a function of the number of loops formed by damper 12 about return pole 18. As opposed to the one-loop structure of
In some embodiments, resistor 68 (e.g., a resistive trace or thin-film resistor element) may be provided to complete the damping circuit, and resistance R may be substantial as compared to loop impedance Z. In these embodiments, resistor 68 may limit the damping current or damping effect, or both, and resistor 68 may enhance flux return from the magnetic medium. Alternatively, the loop circuit may be closed with a highly conducting material, with small resistance R as compared to total loop impedance Z, and the damping current may be determined primarily by the geometry and composition of conductors 58 and 60, along with conducting vias 62.
In the actively driven embodiment of
The value of capacitance C may further be selected primarily to slow the response of return pole 32 (or return pole 18) during polarity transitions. The current through capacitor 74 may thus be small through the write plateau, and there may be no substantial effect on flux loop closure except during the transition period, when spike suppression is achieved.
In RC coupling, the damping current may be generated primarily during write current transitions, and may allow for additional shaping of the damping current based on resistance R. In some embodiments, RC coupling provides for frequency filtering of the damping current, for example via high-pass filtering to address rapid switching of the write current while limiting effects on low-frequency components associated with the write plateau. In further embodiments, frequency filtering is performed in combination with inductive coupling of magnetic damper 12, as described above.
Damping current driver 76 may be located off the magnetic head, for example on flex circuit 72 with write driver 70. Off-head positioning allows damping current driver 76 to employ sophisticated pulse shaping, due to the greater area available for active circuit components. In particular, damping current driver 76 may be configurable to generate a damping current pulse that substantially reduces flux spiking and off-track erasure during write current transitions. In some of these embodiments, damping current driver 76 may be coupled to write driver 70 (e.g., via control link CTRL), for example to address timing jitter and pulse shape variations.
While this disclosure 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 technology, including the substitution of various equivalents for particular elements thereof, and adaptation of the disclosure's teachings to different materials, situations and circumstances. Thus the present disclosure is not limited to the particular embodiments disclosed herein, but encompasses all embodiments falling within the scope of the appended claims.
