Abstract:
The present invention provides a modified perpendicular magnetic recording head which generates a relatively small magnetic field in the soft underlayer of a magnetic recording disk in order to reduce unwanted noise from the underlayer. The noise-suppressing magnetic field is sufficiently strong to effectively drive magnetic domains out of the soft underlayer underneath the head.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of U.S. Provisional Patent Application Serial No. 60/175,265 filed Jan. 10, 2000; U.S. Provisional Patent Application Serial No. 60/175,271 filed Jan. 10, 2000; U.S. Provisional Patent Application Serial No. 60/180,293 filed Feb. 4, 2000; U.S. Provisional Patent Application Serial No. 60/189,365 filed Mar. 15, 2000; U.S. Provisional Patent Application Serial No. 60/191,974 filed Mar. 24, 2000; and U.S. Provisional Patent Application Serial No. 60/191,775 filed Mar. 24, 2000. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to perpendicular magnetic recording heads, and more particularly relates to recording heads which include means for suppressing unwanted noise from the soft magnetic underlayer of the recording disk. 
     BACKGROUND INFORMATION 
     Perpendicular magnetic recording systems have been developed for use in computer hard disk drives. An approach to perpendicular magnetic recording requires the use of recording media with a magnetically soft underlayer which provides a flux path from the trailing pole to the leading pole of the writer. The soft underlayer enables substantially stronger fields than can be generated with a ring head in conventional longitudinal recording systems. The soft underlayer also provides sharper field gradients which enable writing on high coercivity media. In addition, the soft underlayer also helps during the read operation. During the read back process, the soft underlayer produces the image of magnetic charges in the magnetically hard layer, effectively increasing the magnetic flux coming from the media. This provides a higher playback signal. 
     One of the challenges of implementing perpendicular recording is to resolve the problem of soft underlayer noise. The noise is caused by fringing fields generated by magnetic domains, or uncompensated magnetic charges, in the soft underlayer that can be sensed by the reader. For the write process to be efficient, high moment materials, e.g., B S &lt;20 kG, may be used for the soft underlayer. If the magnetic domain distribution of such materials is not carefully controlled, very large fringing fields can introduce substantial amounts of noise in the read element. Not only can the reader sense the steady-state distribution of magnetization in the soft underlayer, but it can also affect the distribution of magnetization in the soft underlayer, thus generating time-dependent noise. Both types of noise should be minimized. 
     The present invention has been developed in view of the foregoing. 
     SUMMARY OF THE INVENTION 
     The present invention provides a perpendicular magnetic recording head which magnetically biases the soft underlayer of the magnetic recording media. By forcing substantial magnetic flux into the body of the soft underlayer, the magnetic domain walls are effectively driven out of the soft underlayer, particularly in the region underneath the read-part of the recording head. The reduction or elimination of magnetic domain walls suppresses unwanted noise that would otherwise be caused by the domain walls. 
     An aspect of the present invention is to provide a perpendicular magnetic recording head including at least one magnetic recording element, and means for generating a magnetic field which reduces noise from a soft magnetic underlayer of a recording medium during operation of the magnetic recording element. 
     Another aspect of the present invention is to provide a perpendicular magnetic recording head including at least one magnetic recording element, and at least one noise-suppressing magnetic field generating element spaced apart from the magnetic recording element. 
     A further aspect of the present invention is to provide a perpendicular magnetic recording system. The system includes a perpendicular magnetic recording medium having a hard magnetic recording layer and a soft magnetic underlayer, and a perpendicular magnetic recording head positionable over the medium having at least one magnetic recording element and at least one magnetic field generating element positioned to generate a noise-suppressing magnetic field in the soft magnetic underlayer in a region of the medium under the recording element. 
     These and other aspects of the present invention will be more apparent from the following description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a partially schematic side view of a perpendicular magnetic recording head and recording disk illustrating magnetic flux paths through the soft underlayer of the disk during recording operations. 
     FIG. 2 is a partially schematic bottom view showing the air bearing surface of a perpendicular magnetic recording head which includes both writing and reading elements. 
     FIG. 3 is a top view of a magnetic recording disk illustrating two different noise-reducing magnetic fields that may be generated in the soft magnetic underlayer of the disk in accordance with embodiments of the present invention. 
     FIG. 4 is a partially schematic side sectional view of the reader portion of a perpendicular magnetic recording head including noise-suppressing magnets in accordance with an embodiment of the present invention. 
     FIG. 5 is a partially schematic side sectional view of the reader portion of a perpendicular magnetic recording head including noise-suppressing magnets in accordance with another embodiment of the present invention. 
     FIG. 6 is a partially schematic side sectional view of the reader portion of a perpendicular magnetic recording head including a noise-suppressing coil and yoke assembly in accordance with a further embodiment of the present invention. 
     FIG. 7 is a partially schematic side sectional view of the reader portion of a perpendicular magnetic recording head including a noise-suppressing conductive wire and yoke assembly in accordance with another embodiment of the present invention. 
     FIG. 8 is a partially schematic sectional view of the reading and writing portions of a perpendicular magnetic recording head including a noise-suppressing conductive wire and yoke assembly in accordance with a further embodiment of the present invention. 
     FIG. 9 is a partially schematic bottom view of the air-bearing surface of the perpendicular magnetic recording head of FIG.  8 . 
     FIG. 10 is a partially schematic bottom view of the air-bearing surface of a perpendicular magnetic recording head including noise-suppressing magnets in accordance with another embodiment of the present invention. 
     FIG. 11 is a partially schematic view taken through section  11 — 11  of FIG.  10 . 
     FIG. 12 is a partially schematic bottom view of the air-bearing surface of a perpendicular magnetic recording head including noise-suppressing magnets in accordance with another embodiment of the present invention. 
     FIG. 13 is a partially schematic view taken through section  13 — 13  of FIG.  12 . 
     FIG. 14 is a partially schematic side view of a test arrangement evaluating the effectiveness of using magnets for noise suppression in accordance with the present invention. 
     FIG. 15 is a graph of playback versus time for the test arrangement of FIG. 14, showing the difference in playback characteristics when the soft magnetic underlayer of the recording disk is biased and unbiased. 
     FIG. 16 is a graph of magnetic field versus distance across the width of a write pole of a perpendicular magnetic recording head. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 is a partially schematic side view of a perpendicular magnetic recording head  10 . The recording head  10  includes a writer section comprising a trailing main pole  12  and a return or opposing pole  14 . A magnetization coil  15  surrounds the main pole  12 . The recording head  10  also includes a reader section comprising a read element  16  positioned between a reader pole  18  and the opposing pole  14 . The read element  16  may be a conventional GMR reader, MR reader, inductive reader or the like. In the embodiment shown in FIG. 1, the reader section shares the opposing pole  14  of the writer section. 
     A perpendicular magnetic recording medium  20  is positioned under the recording head  10 . The recording medium  20  travels in the direction of the arrow shown in FIG. 1 during recording. The recording medium  20  includes a substrate  22 , which may be made of any suitable material such as ceramic glass, amorphous glass or NiP plated AlMg. A magnetically soft underlayer  24  is deposited on the substrate  22 . Suitable soft magnetic materials for the underlayer  24  include CoFe and alloys thereof, Fe and alloys thereof, FeAlN, NiFe, CoZrNb and FeTaN, with CoFe and FeAlN being particularly suitable soft materials. A thin exchange decouple layer  25  made of a non-magnetic material such as CoCr may be deposited on the soft underlayer  24 . A magnetically hard perpendicular recording layer  26  is deposited on the exchange decouple layer  25 . Suitable hard magnetic materials for the recording layer  26  include multi-layers of Co/Pd or Co/Pt, L10 phases of CoPt, FePt, CoPd and FePd and hcp Co alloys, with such multi layers and L10 phases being particularly suitable hard materials. A protective overcoat  28  such as diamond-like carbon may be applied over the recording layer  26 . 
     As shown in FIG. 1, during writing operations, magnetic flux is directed along a path M W  from the main pole  12  perpendicularly through the recording layer  26 , then in the plane of the soft underlayer  24  back to the opposing pole  14 . During reading operations, magnetic flux M R  is directed along the paths M R  from the recording layer  26  into the read element  16 , through the poles  18  and  14 , into the soft underlayer  24 , and back through the recording layer  26 . 
     FIG. 2 is a partially schematic bottom view of the air bearing surface of the recording head  10 , which may be modified in accordance with the present invention. The perpendicular read/write head  10  includes an air bearing surface  30  which may be flush with the ends of the main pole  12  and opposing pole  14 . 
     FIG. 3 is a top view illustrating the soft underlayer  24  of the magnetic recording disk  20 . The magnetic field M W  generated by the perpendicular recording head travels through the soft magnetic underlayer  24  along the direction of the recording tracks of the disk  20  during writing operations. FIG. 3 also illustrates noise-suppressing magnetic fields that may be generated in the soft underlayer  24 . As used herein, the term “noise-suppressing magnetic field” means a magnetic field generated in the soft underlayer that reduces or eliminates magnetic domain walls from the underlayer. In one embodiment, the noise-suppressing magnetic field H R  generated in the soft underlayer is substantially perpendicular to the magnetic field M W . In another embodiment, the noise-suppressing magnetic field H L  generated in the soft underlayer  24  is substantially parallel with the magnetic field M W . Although a radial noise-suppressing field H R  and a circumferential noise-suppressing field H L  are shown in FIG. 3, other orientations of the noise-suppressing magnetic field(s) may be used in accordance with the present invention. 
     The strength of the noise-suppressing magnetic field may be controlled in order to sufficiently reduce noise caused by domain walls in the underlayer. Anisotropic underlayer materials have an easy axis and a hard axis. If the noise-suppressing magnetic field is applied along the easy axis, the magnetic field may be relatively low, e.g., from about 0.5 to about 10 Oe. If the noise-suppressing magnetic field is applied along the hard axis, the magnetic field may be higher, e.g., from about 50 to about 100 Oe. For orientations between the easy axis and hard axis, the noise-suppressing magnetic field may be an intermediate strength. For isotropic underlayer materials, typical strengths of the noise-suppressing magnetic field range from about 1 Oe to about 100 Oe. The strengths of the noise-suppressing magnetic fields, e.g., H R  and H L , in the soft underlayer  24  may be substantially less than the strength of the recording magnetic field M W . For example, the noise-suppressing magnetic field may be at least 100 times smaller than the recording field M W . As a particular example, the noise-suppressing magnetic field may range from about 1 to about 100 Oe, while the recording magnetic field Mw may range from about 5,000 to about 20,000 Oe. 
     FIG. 4 is a partially schematic side sectional view of a reader portion of a perpendicular magnetic recording head  34  including noise-suppressing magnets  35  and  36  located on opposite sides of the reader  16 . The magnets  35  and  36  may be made of any suitable material such as hep Co alloys, CoSm, NdFeB or L10 phases of CoPt, CoPd, FePt and FePd. As shown in FIG. 4, the magnets  35  and  36  generate a noise-suppressing magnetic field H L  in the soft underlayer  24  which extends in a direction parallel with the recording tracks of the recording layer  26 . 
     FIG. 5 is a partially schematic side sectional view of a reader portion of a perpendicular magnetic recording head  40  including permanent magnets  42  and  44  located on opposite sides of the read element  16 . A magnetically permeable yoke  46  extends between the upper portions of the magnets  42  and  44 . The yoke  46  may be made of any suitable material such as Permalloy, Ni45Fe45, CoZrNb, CoZrTa, FeAlN, FeTaN, CoFe and CoFeB. The magnets  42  and  44  generate the noise-suppressing magnetic field H L  in the soft underlayer  24 . 
     FIG. 6 is a partially schematic side sectional view of a reader portion of a perpendicular magnetic recording head  50  in accordance with a further embodiment of the present invention. The recording head  50  includes soft magnetic poles  52  and  54  on opposite sides of the read element  16 . The magnetic poles  52  and  54  are connected by a magnetically permeable yoke  56 . The poles  52 ,  54  and yoke  56  may be made of a material such as Permalloy, Ni45Fe45, CoZrNb, CoZrTa, FeAlN, FeTaN, CoFe and CoFeB. A magnetization coil  58  surrounds the yoke  56 . When electric current is supplied to the coil  58 , the noise-suppressing magnetic field H L  is generated between the poles  52  and  54  in the soft underlayer  24 . As opposed to the embodiments shown in FIGS. 4 and 5, the recording head  50  of FIG. 6 is capable of switching the direction of the noise-suppressing magnetic field H L  depending upon the current direction in the coil  58 . 
     FIG. 7 is a partially schematic side sectional view of a portion of a perpendicular magnetic recording head similar to the embodiment shown in FIG. 6, except the magnetization coil  58  is replaced by an electrically conductive wire or line  62 , which is shown in cross section in FIG.  7 . The conductive line  62  may be made of any suitable electrically conductive material such as Cu, Ag or Ta. The noise-suppressing magnetic field H L  can be reversed by switching the direction of current flow in the conductive line  62 . 
     FIGS. 8 and 9 schematically illustrate read and write portions of a perpendicular magnetic recording head  70  in accordance with another embodiment of the present invention. In this embodiment, a magnetically permeable read pole  19  is located on one side of the reader element  16 , while the opposing pole  14  is located on the opposite side of the reader element  16 . A magnetically permeable yoke  17  extends from the reader pole  19  to the opposing pole  14 . Another magnetically permeable yoke  13  extends from the opposing pole  14  to the main write pole  12 . A first electrically conductive wire or line  72  is located above the reader element  16  between the reader pole  19  and the opposing pole  14 . A second conductive line  74  is located between the opposing pole  14  and the main write pole  12 . As most clearly shown in the air bearing surface view of FIG. 9, the first and second conductive lines  72  and  74  are electrically connected by a termination  76 . The conductive lines  72 ,  74  and termination  76  may be made of any suitable electrically conductive material such as Cu, Ag or Ta. 
     When current is applied across the conductive lines  72  and  74 , the noise-suppressing magnetic field H L  is generated between the writer pole  19  and opposing pole  14  through the soft underlayer  24 . Due to the smaller cross sectional size of the main write pole  12 , current flowing through the conductive lines  72  and  74  generates the magnetic field H L  shown in FIG. 8 between the reader pole  19  and opposing pole  14 , rather than generating any significant amount of magnetic flux between the opposing pole  14  and main write pole  12 . In this manner, the noise-suppressing magnetic field H L  is generated in the region of the soft magnetic underlayer  24  located under the reader element  16 . 
     The yoke width of the poles  14  and  18  should be significantly greater than the read element  16  width so that a sufficiently wide region of the soft underlayer is saturated. Also, the reluctance of the magnetic circuit defined by the yoke and the soft underlayer will decrease as the width of the yoke structure is increased. If a soft underlayer of 0.3 μm thickness is used, a yoke structure having widths of at least 2 μm may be adequate to efficiently saturate the soft underlayer with the currents less than 50 mA. 
     In the embodiments shown in FIGS. 4-9, the noise-suppressing magnetic field H L  is generated in a direction parallel with the recording tracks of the recording layer  26 , i.e., in the circumferential travel direction of the disk. In contrast, in the embodiments of FIGS. 10-13 described below, the noise-suppressing magnetic field H R  is generated in the soft underlayer  24  in the radial direction of the disk, i.e., perpendicular to the recording tracks and circumferential travel direction of the disk. Alternatively, the noise-suppressing magnetic field may be generated in any other orientation in the soft underlayer which effectively reduces the underlayer noise. 
     FIG. 10 is a partially-schematic bottom view of the air bearing surface  30  of a perpendicular magnetic recording head  80  in accordance with an embodiment of the present invention. FIG. 11 is a sectional view taken through section  11 — 11  of FIG.  10 . The recording head  80  includes the main write pole  12 , opposing pole  14 , reader element  16  and reader pole  18 , as previously described. In this embodiment, magnets  82  and  84  are located on opposite sides of the recording head  80 . The reader element  16  is positioned between the magnets  82  and  84 . The magnets  82  and  84  generate the noise-suppressing magnetic field H R  in the soft underlayer  24 . In FIG. 11, the direction of travel of the recording disk is perpendicular to the plane of the drawing. The noise-suppressing magnetic field H R  is oriented in the radial direction of the disk, perpendicular to the recording tracks of the disk. 
     FIGS. 12 and 13 schematically illustrate a recording head  90  similar to the embodiment shown in FIGS. 10 and 11. In this embodiment, magnets  92  and  94  are located on opposite sides of the recording head  90 , with their trailing edges even with the trailing edge of the reader element  16 . As shown in FIG. 13, a magnetically permeable yoke  96  connects the magnets  92  and  94 . By positioning the magnets  92  and  94  along the sides of the recording head  90  as shown in FIG. 12, the noise-suppressing magnetic field H R  is generated in the soft underlayer  24  in a region located below both the reader pole  18  and reader element  16 . This arrangement maximizes the noise-suppressing magnetic field H R  in the most beneficial region of the soft underlayer  24  as the disk travels below the reader of the head  90 . 
     By generating a noise-suppressing magnetic field H R  in a radial direction in the soft underlayer  24 , e.g., as shown in the embodiments of FIGS. 10-13, the magnetization switching characteristics of the recording media may be improved. This results from rotation of magnetization being faster than domain wall motion. By biasing the noise-suppressing magnetic field perpendicular to the recording tracks of the media, the magnetization in the soft underlayer will be rotated during the reading process. Furthermore, radial biasing of the soft underlayer may also reduce Barkhausen noise. 
     The present recording head structure may be fabricated using standard deposition techniques. First, a relatively hard magnetic material is deposited. A conventional longitudinal media combination to orient magnetization in the plane of the substrate, such as NiP/Cr/CoCr, can be utilized. During deposition, the magnetic field can be applied to define the in-plane orientation perpendicular to the air bearing surface. Then, isolated by the alumina layers from both sides a read element is deposited. Lithography can be used to define an interconnect made of a soft NiFe to create a closed magnetic path. As the last step, a soft shield, e.g., Permalloy, may be deposited. As a result, a reader is surrounded by a closed magnetic flux path. A part of this closed path is the region in the soft underlayer underneath the head. This, in turn, provides automatic biasing of the soft underlayer near the reader, thus eliminating domain walls in the vicinity of the reader. 
     A perpendicular write head similar to the recording head  10  shown in FIGS. 1 and 2 was made by a standard focused ion beam trimming process from the air bearing surface. Then, using a spin-stand, a single transition was written by this head on a perpendicular recording disk with a 1,000 nm thick soft underlayer made of Permalloy. The signal was read by a conventional read element. During reading two permanent magnets  101  and  102  were put at the opposite edges of the disk, as schematically shown in FIG.  14 . Due to the soft underlayer  24 , there was a well-defined magnetic path H for the flux generated by the magnets  101  and  102  (the path is through a soft underlayer, thus the reader was not affected by stray fields from the magnets). Acting as a partially closed magnetic flux path H between the two magnets  101  and  102 , the soft underlayer  24  was automatically biased. 
     The signals recorded for the cases of a non-biased and biased soft underlayer are shown in FIG.  15 . It can be seen that the noise significantly decreases when the biasing is generated. At least a 3-4 dB change can be seen. 
     Commercially available boundary element solver software, Amperes, was used to calculate magnetic field distribution. Both the yoke structure and the soft underlayer are assumed to be made of a soft magnetic material with 4πM S =19.8 kGauss and H k =6.06 Oe. A 50 mA current through the current lead is sufficient to completely saturate the soft underlayer region underneath the read element. The biasing field direction corresponds to the direction either along or across the track. The choice for the orientation of the proposed yoke structure may depend on the recorded bit pattern and on a particular biasing of the soft underlayer. In one embodiment, the magnetization switching in the soft underlayer is driven by the magnetization rotation. For example, if a cross-talk between the tracks is negligible, the orientation of the yoke may be such that the biasing field direction is perpendicular to the direction of the track. FIG. 16 shows components of magnetization M inside the soft underlayer underneath the yoke structure. The region of the soft underlayer located between the two poles is completely saturated. 
     Whereas particular embodiments of this invention have been described above for purposes of illustration, it will be evident to those skilled in the art that numerous variations of the details of the present invention may be made without departing from the invention as defined in the appended claims.