Abstract:
A coil structure apparatus comprising a yoke portion and a coil portion. The yoke portion substantially surrounds the coil portion. In one aspect, the yoke portion is configured to increase the bias field strength produced by the coil structure while minimizing the self-inductance of the coil structure.

Description:
CROSS-REFERENCE TO A RELATED APPLICATION 
     This invention is based on U.S. Provisional Patent Application Serial No. 60/191,942 filed Mar. 24, 2000, entitled PLANAR WRITING COIL FOR OPTICAL ANTENNA ELECTRICAL AND MAGNETIC DESIGN filed in the name of Terry McDaniel. The priority of this provisional application is hereby claimed. 
    
    
     BACKGROUND OF THE DISCLOSURE 
     1. Field of the Invention 
     The invention relates to magneto-optical heads and, more particularly, the invention relates to coil structures that are used in magneto-optical heads. 
     2. Description of the Background Art 
     Disk drives are a known memory storage device in the computer industry. Magnetic disk drives are the most common type of disk drive, and use magnetic heads to write data to, and read data from, magnetic disks. Magneto-optical (MO) drives are another type of disk-based storage device. MO drives include MO heads that write data to, and read data from, an MO disk. Of great importance in MO disk drives is the ability to accurately read data into, and write data from, minute tracks on the MO disk using a laser. 
     MO heads typically include a microcoil for the purpose of magnetic field generation in thermomagnetic writing of an MO data storage medium. An example of such a structure is shown in U.S. Pat. No. 5,903,525 entitled “Coil for Use With Magneto-Optical Head” by McDaniel and Wang, incorporated herein by reference. Fabrication of an MO head, including a working planar microcoil, is challenging for mass production considering the precision and relatively small dimensions of the components of the MO head and disk drive. 
     Therefore, a need exists in the art for a planar coil structure of a magneto-optic (MO) head. 
     SUMMARY OF THE INVENTION 
     The present invention comprises a microcoil mounted to a read/write head in a magneto-optical (MO) data storage device. More specifically, the microcoil mounted to a read/write head in a magneto-optical head that utilizes a sub-micron optical antenna as a read/write transducer. The microcoil is affixed to the focusing lens of the MO head proximate a region where the light exits the focusing lens. The microcoil creates a source of a magnetic bias field that provides efficient generation of a dynamic writing field consistent with a driving current magnitude and frequency. The microcoil is also compatible with known low-cost batch fabrication processes. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The teachings of the present invention can be readily understood by considering the following detailed description in conjunction with the accompanying drawings, in which: 
     FIG. 1 shows a top view of one embodiment of a magneto-optical (MO) data storage system employing a MO head; 
     FIG. 2 shows a perspective view of one embodiment of MO head; 
     FIG. 3 shows a side cross-sectional elevational view of the MO head of FIG. 2; 
     FIG. 4 shows an exploded view of one embodiment of the optical antenna of FIG. 3; 
     FIG. 5 shows a partial sectional perspective of one embodiment of an optical antenna of the type shown in the MO head of FIG. 3; 
     FIG. 6 shows a perspective view of a circular current loop for B-field generation; and 
     FIG. 7 is a vertical section view of the yoke and storage medium used in an exemplary embodiment of the invention. 
    
    
     To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 depicts a schematic diagram of a magneto-optical (MO) data storage system  100 , such as a MO disk drive. One embodiment of the MO data storage system  100  comprises a laser optics assembly  101 , on optical switch  104 , an actuator arm  105 , a suspension  130 , a set of double-sided MO disks  107 , a rotary actuator magnet/coil assembly  120  and a MO head  106 . The laser optics assembly  101  is optically coupled to an optical switch  104 . One embodiment of the MO head  106  is fashioned as a set of Winchester-type flying MO heads. Each MO head  106  is configured and can be positioned to interact with one side of one MO disk  107 . Each MO head  106  is coupled to a rotary actuator magnet/coil assembly  120  by a distinct respective actuator arm  105  and suspension  130  to position the MO head  106  over the surface of the MO disk  107  associated with that MO head. 
     In operation, the MO disk  107  is rotated by a spindle motor  195 . Aerodynamic lift forces are generated between the MO head  106  and the MO disk  107  as the MO disk rotates. These aerodynamic lift forces are opposed by substantially equal and opposite spring forces applied by the suspension  130  on the MO head  106  toward the MO disk. The substantial equality in forces between the aerodynamic and the spring forces maintain the MO head  106  in a so-called flying state that is closely adjacent, e.g., approximately 1-20 micro-inches from, a surface of the MO disk  107 . The MO head  106  is maintained between a minimum and maximum flying height over the surface of the MO disk  107  over a full radial stroke of the actuator arm  105  in a manner to limit contact between the MO head  106  and the MO disk  107 . When not engaged in a reading or writing operation, the MO head  106  is statically disposed in a “storage” position remote from the surface of the MO disk  107 . 
     As shown in FIG. 1, one embodiment of the laser optics assembly  101  controls the laser beam transmitted to, and receives the laser beam transmitted by the MO head  106 . In this disclosure, the term “laser beam” is intended to describe the path of any radiation originating at a laser and being transmitted through passing through either fiber or air. The laser beam actually includes two laser beam portions: an incident laser beam portion and a return (reflected) laser beam portion. The laser optics assembly  101  generates the incident laser beam  191 . The incident laser beam portion is generally linearly-polarized and originates from, e.g., a Fabry-Perot (FP) diode laser source and is directed toward the MO disk  107  via an optical fiber. The laser optics assembly  101  also receives return laser beams in a rotated polarization state (compared to the incident laser beam) over respective return optical fibers  110 ,  112  that travels from the MO disk  107  through the MO head  106 . The particular polarization state of the return laser beam signal can be controlled in a manner to transmit information stored on the MO disk to the laser optics assembly during a reading operation. A single-mode polarization maintaining (PM) or low birefringence optical fiber  102  (FIG. 1) optically couples the optical switch  104  to each MO head  106 . The return optical fibers  110 ,  112  that receive the respective return laser beams optically couple each head  106  to the laser optics assembly  101 . 
     A perspective view of one embodiment of MO head  106  is shown in FIG. 2. A side cross-sectional view of the MO head  106  taken along line  4 — 4  of FIG. 2 is shown in FIG.  3 . The MO head  106  includes an optical fiber  401  (that transmits the incident laser beam  191  and the return laser beams) a plurality of wafers  402 ,  404 , and  406 , a mirror  408 , a collimating lens  410 , a focusing lens  412 , an actuator  414 , a planar micro coil structure  416 , and an optical antenna  418 . The wafers  402 ,  404  and  406  are affixed to one another to form a stack  403 . The collimating lens  410  and the focusing lens  412  are referred to as a lens pair  422 , and the lens pair  422  is configured to respectively collimate and focus light of the prescribed wavelength as produced by the laser optics assembly  101 , e.g., having a laser wavelength of 1.55 μm. Light is applied to the MO head  106  via the optical fiber  401 . In one embodiment, the optical fiber  401  is a 1.55 μm, low-birefringence, single mode optical fiber. One embodiment of the mirror  408  includes a mirror body  409  formed of a silicon body with the silicon body coated with a gold coating  411 . The mirror  408  reflects the incident laser beam applied by the optical fiber  401  toward the lens pair  422 , and also reflects the return laser beam from the lens pair  422  toward the optical fiber  401 . 
     In one embodiment, the effective numerical aperture (NA) of the lens pair  422  is about 0.8. This NA corresponds to a focal spot size of about 1.0 μm Full-Width Half Maximum (FWHM) in silicon at the 1.55 μm laser wavelength. The collimating lens  410  collimates the laser beam applied by the fiber  401 . The focusing lens  412  focuses the incident laser beam  191  passing through the collimating lens  410  onto the optical antenna  418 . The optical antenna  418  is proximate and substantially coplanar with an air bearing surface  424 . As such, the antenna  418  will be spaced from the disk by the gap formed between the surface  424  and the disk  107 . 
     The planar micro coil structure  416  is integrated in the bottom of the focusing lens  412  proximate the optical antenna  418 . The focusing lens  412  focuses the light beam onto the optical antenna  418  that is formed in the center of a bottom surface of the planar micro coil structure  416 . The micro coil structure  416  defines an aperture  480  having a diameter of about 10 μm. 
     The collimating lens  410  has an aspheric surface  460  and a planar surface  462 . The focusing lens  412  has an aspheric surface  464  and a planar surface  466 . The planar surface  466  of the focusing lens supports the optical antenna  418 . The planar surface  466  of the focusing lens is substantially co-planar with the air bearing surface  424  of the MO head  106 . 
     The collimating lens  410  and the focusing lens  412  can both be fabricated using current silicon processing methods. The aspheric surfaces  460  and  464  in the respective collimating lens  410  and focusing lens  412  can be patterned in the silicon using reactive ion etching. The lenses  410 ,  412  are each polished as desired to decrease the height of the lens to the necessary lens thickness. This direct wafer-scale patterning allows thousands of lenses to be formed on a silicon wafer. 
     Spot size is chosen to maximize coupling of the laser beam to the optical antenna  418 , and thereby also to maximize the output power re-radiated by the optical antenna  418 . 
     The optical fiber  401  is actively aligned to the lens path during head assembly. In this alignment procedure, the distance between the fiber  401  and the collimating lens  410  is adjusted to maintain maximum coupling efficiency of back-reflected light from the focal plane returning through the single mode fiber. The x-y position of the fiber  401  is also adjusted to optimize the spatial overlap of the beam on the optical antenna. 
     The MO head  106  of FIG. 3 can both write data to, or read data from, the MO disk  107 . The MO head  106  writes data on the MO disk  107  when the radiated field from the optical antenna  418  heats the magnetic layer on the MO disk  107 . Heating the magnetic layer on the MO disk acts to lower the coercivity of a small region of the surface. The writing action allows the magnetic field generated by the planar micro coil structure  416  to reverse the magnetic moment of the magnetic layer in the heated region of the MO disk  107 . 
     The MO head  106  reads data from the MO disk  107  when the radiated near field from the optical antenna  418  is reflected from the MO disk  107 . During the reading action, the reversed magnetic moment can be differentiated from the non-reversed magnetic moment. The reflected, near field radiation propagates through the lens pair  422  towards the optical fiber  401 . A standard differential detection technique is used to determine the sign of the Kerr rotation of the return light upon its receipt in the laser optics assembly  101 . 
     In one embodiment, a unitary focusing lens/coil/optical antenna assembly  432  is provided including the focusing lens  412 , the planar coil structure  416 , and the optical antenna  418 . The focusing lens/coil/optical antenna assembly  432  can be controllably displaced by the actuator  414 . The actuator  414  preferably comprises, e.g., a servo motor that can precisely displace the focusing lens/coil/optical antenna assembly  432  in a sinusoidal motion. The accurate sinusoidal motion provided by displacement of the actuator  414  to effect focusing lens/coil/optical antenna assembly  432  can be utilized for track following at track densities of up to 250 ktpi (kilo-tracks-per-inch). One exemplary sinusoidal motion has a displacement of ±1.5 μm and a frequency of 18 kHz. Since the incident laser beam  191  is collimated after passing through the collimating lens  410 , the focal spot of the incident laser beam can be controllably directed by the microactuator to remain overlapped with the optical antenna  418 . Providing the actuator  414  to displace the focusing lens/coil/optical antenna assembly  433  eliminates the need for a complicated second stage servo to maintain overlap of the incident laser beam  191  relative to the optical antenna  418 . 
     One embodiment of optical antenna  418 , shown in FIG. 4, consists of a thin gold film structure  438  of submicron dimensions with a small aperture  440  formed at the center. The aperture has a width of about 50 nm, but may be as narrow as about 10 nm. Although one embodiment of the MO head uses an incident laser beam that has a focal spot diameter of about 1 μm, the optical antenna resolution is governed by the near field radiation  442  generated by the aperture  440 . The pattern of the near field radiation generated by the aperture  440  can be configured to effect accurate writing and reading marks on the MO disk  107  with less than 100 nm spatial resolution that can result in mark densities on the MO disk  107  exceeding 100 Gb/in 2 . The spatial resolution applied to the MO disk  107  is governed by the width of the aperture  440 . The aperture  440  can be formed as small as 10 nm using currently available electron beam technology. Minimizing the aperture dimension enhances the resolution produced by the optical antenna  418  on the MO disk  107  to a level below the diffraction limit of the focussed incident laser beam. 
     The coil  416  is formed such that its structure is that of a toroid with a center aperture. FIG. 5 is a perspective view of the coil  416  showing all of its elements. The coil structure  416  is comprised of wires  1106  wound into coils  1102  concentrically about a central aperture in a single plane and radiates outward. The wires  1106  are housed in a magnetically permeable housing (yoke) that concentrates the magnetic B field in the central aperture. The yoke is formed having an upper  1148  and lower  1152  section that encases the wires  1106 . The upper  1148  and lower  1152  sections are joined about the exterior circumference of the toroid and are separated by an annular gap  1155  formed about the aperture&#39;s internal circumference. The annular gap  1155 , which is contoured and unbroken, spans the entire center aperture. The wires  1106  are supported in coil form by a dielectric material  1105 , preferably photo resist. 
     Using a 1.55 μm wavelength for the laser source  231  allows matching the laser excitation wavelength to the resonance of the optical antenna  418 . This matching of wavelength results in an optical antenna being formed of features having dimensions that can be produced using current manufacturing technology. In one embodiment, both the collimating lens  410  and the focusing lens  412  are formed from silicon. Selecting the 1.55 μm laser wavelength is desired because this wavelength is within the transparency region of silicon. The lens pair  422  can thus be integrated with the actuator  414 . 
     The planar micro coil structure  416  is integrated in the MO head  106 , with the planar coil structure placed near the air bearing surface (ABS) of a flying slider (head). The purpose of the planar micro coil structure  416  is to generate a writing and erasing magnetic field at the position of the storage films on the disk recording medium  107 . The position of the planar micro coil structure relative to the medium is to be determined by the placement of the planar micro coil structure  416  on the slider and the flying attitude of the MO head  106 . 
     The embodiment of planar micro coil structure  416  shown in FIG. 5 comprises a coil  1102  and a yoke  1104 . The yoke  1104  is formed of a magnetic material. The coil  1102  is configured to be nearly axisymmetric. The details of the shapes of the coil  1102  (spiral versus concentric “winding”; oval due to via connections versus circular) in one embodiment, displays second-order effects relative to meeting basic design requirements. 
     One embodiment of the planar micro coil structure  416  is configured in a right circular cylindrical volume whose outer radius is less than 75 μm and whose height is less than 15 μm. Furthermore, the micro planar coil structure  416  has a center aperture  430  to accommodate the passage of a converging focused light beam whose focal point lay within about one micrometer of the base plane of the planar micro coil structure  416 . One embodiment of the conical light beam passing through the center aperture exhibits a half-angle of from 12 to 15 degrees, as indicated by lines  1170  in FIG.  7 . This geometry, combined with anticipated tolerances on beam alignment and centration with respect to the aperture, defines a minimum allowable inner profile of the aperture. 
     The MO head  106  is configured so the outer surface of the focusing lens  412  is secured to the inner surface of the actuator  414 . The planar micro coil structure  416  is integrated in, or attached to the outer surface of, the focusing lens  412 . Therefore, any vertical displacement of the actuator  414  is thus transferred to the focusing lens  412  and the planar coil structure  416 . 
     The conductor size of the coil  1102  is compatible with the anticipated current-carrying capacity of the planar coil structure  416 . The yoke  1104  is configured with inner pole tips  1150 ,  1151 . The thickness and placement accuracy of the inner pole tips  1150 ,  1151  of the magnetic yoke  1104  are configured to provide the desired B-field strength at the disk medium  107 . 
     Two components of the magnetic field produced by the planar micro coil  416  are (a) a field from the externally-supplied current applied to the conductor coil  1102 , and (b) the magnetic material of the magnetic yoke  1104 . To illustrate the magnetic field generated by the planar micro coil structure  416 , consider FIG. 6 that illustrates a circular conductor (coil) of radius a. The cross-section of the conductor coil is considered for analysis purposes to be essentially point-like, i.e., a filamentary wire. The sources of the planar coil structure  416  lie in the half-plane Z less than 0. The magnetic field required in the half-plane Z is greater than 0. Typically, a minimum separation (along the Z-axis) is provided between the bounding plane of the source assembly and the field point P. This minimum separation is referred to as Z 0 . The textbook expression for the magnetic induction on the axis of symmetry of the circular coil is:            B   z          (       r   =   0     ,   z     )       =         μ   0        I                   a   2         2          (       a   2     +     z   2       )       3   /   2                                  
     The optimal coil radius maximizes the field B z  at (r=0, Z=Zo). Setting the partial derivative with respect to a equal to zero and solving the resulting equation for a gives a α opt   ={square root over (2z O )}. Putting this optimal radius into the expression for B   z  gives            B   z   opt          (       r   =   0     ,     z   0       )       =           μ   0        I                      3     3   /   2            z   0                                
     The optimum coil position of the planar coil structure subtends a half-angle of tan −1  (a/z 0 )=tan −1 {square root over (2)}=54.7° from the field point P. If conducting wire is the only source of field available, we should bunch the conductor positions along the 54.7° line, and not displace Z far below the Z o  location. 
     Electromagnetic principles indicate that magnetic material is a much stronger source of external field than a macroscopic current source. This principle reflects the remarkable strength of cooperative atomic currents. Consequently, amplifying the flux generation from a conductor using a well-shaped magnetic yoke (magnetic flux conductor) is more effective for external field generation purposes than magnet designs that rely on optimally placed conductive wires alone. 
     Use of a commercially-available finite-element magnetostatics simulation software (e.g., Ansoft Maxwell 2D and 3D) is helpful for optimizing the position of the wires  1106  of the coil  1102  relative to the size and shape of the yoke  1104 , especially the pole tips  1150  and  1151 . The software program simulates non-linear material magnetic properties. The cross-sectional area of each conductor turn  1106  was modeled as 2.5×3.5 μm 2 . The main changes explored were in the geometry of the yoke, while the conductor array may have been occasionally rigidly shifted by increasing or decreasing the innermost radius of the array slightly. The changes in the yoke  1104  included first the addition of a flux return layer forming the bottom of the planar coil structure at the Z approximately equals 0 position. 
     The net result of optimizing the planar coil structure  416  results in total self-inductance approaching approximately 78 nH, including the leads, and a field output in the media at nominal operating current (50 mA) of 570 G. This estimate does not involve a soft magnetic underlayer in the medium  107 , but only a single magnetic storage film with non-magnetic thin film over- and under-layers resting atop the disk substrate. 
     FIG. 7 is a vertical section of the antenna of FIG. 5 shown in combination with a preferred recording medium. Thus the figure shows yoke  1104  with upper  1148  and lower  1152  housing sections surrounding wires  1106  forming coil  1102 . The yoke focuses light beam rays  1170  on medium film structure  710 . The medium comprises, in sequence built up on substrate  722 , a soft underlayer  724 ; a buffer layer  726 ; a magnetic film  728 ; and finally an overcoat  730 . 
     A soft magnetic underlayer  724  of thickness of approximately 300 nm having the properties of Ni 0.81 Fe 0.19  permalloy is a further embodiment. Such a soft underlayer provides an improvement in the magnetic circuit flux closure of the coil portion efficiency across the air gap region  760  between the pole tips  1150 ,  1151  resulting in a boost of the achievable B Z  in the media to about 1000 G, again using 50 mA current. 
     The use of a soft underlayer in the medium can nearly double the bias field strength in the storage layer without affecting the self-inductance of the planar coil structure. An enhanced efficiency of the planar coil structure results in a significant reduction in structure complexity and performance. The magnetic, thermal, and possibly optical behavior of the medium would need to be re-optimized if the soft underlayer is selected. 
     The desired nominal peak field magnitude at the recording layer on the disk medium on the center axis of the planar coil structure is 500 Oersteds at the nominal operating current of the planar coil structure. The field direction is within ±15° of perpendicular to the disk plane. These recording conditions should be achievable with either positive or negative field polarity. 
     The planar coil structure must not interfere in any way with the propagating light on its passage from the objective lens past the optical antenna to the disk, or on its return path from the disk to the objective. 
     While foregoing is directed to the preferred embodiment of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.