Abstract:
A dual-stage actuation assembly for a disc drive includes a movable actuator arm controlled by an actuator motor. A suspension assembly is supported by the actuator arm, and includes a flexure. A microactuator is also provided, including a stator attached to the flexure and a rotor supporting the slider, the rotor being movable with respect to the stator in a first horizontal plane generally parallel to a surface of the disc. A vertically arranged magnetic circuit is formed, which in an exemplary embodiment includes a bottom ferromagnetic keeper, a plurality of magnets, a patterned conductive coil and atop ferromagnetic keeper, those elements being vertically arranged in different horizontal planes. The magnets are operable to move laterally and thereby cause movement of the microactuator rotor in the first horizontal plane generally parallel to the surface of the disc in response to circulation of a current through the patterned conductive coil.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     This application claims priority from Provisional Application No. 60/089,010 filed Jun. 11, 1998, for “Moving Magnet Micro-Actuator With Coil on Flex Circuit” by P. Crane, W. Bonin and B. Zhang. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to a disc drive microactuator, and more particularly to a magnetic microactuator utilizing a vertical magnetic circuit contained on a substrate and a flex circuit to provide microactuation force. 
     The density of concentric data tracks on magnetic discs continues to increase (that is, the size of data tracks and radial spacing between data tracks are decreasing), requiring more precise radial positioning of the head. Conventionally, head positioning is accomplished by operating an actuator arm with a large-scale actuation motor, such as a voice coil motor, to radially position a head on a flexure at the end of the actuator arm. The large-scale motor lacks sufficient resolution to effectively accommodate high track-density discs. Thus, a high resolution head positioning mechanism, or microactuator, is necessary to accommodate the more densely spaced tracks. 
     One promising approach for high resolution head positioning involves employing a high resolution microactuator in addition to the conventional lower resolution actuator motor, thereby effecting head positioning through dual-stage actuation. Various microactuator designs have been considered to accomplish high resolution head positioning. One design involves inserting a silicon-based thin film structure between the suspension and the slider in a disc drive assembly. A major technical challenge in implementing such a microactuator is to provide sufficiently large actuation force to overcome spring bias forces to drive the head at a speed high enough to accommodate the required bandwidth. Such a design must be realized in a relatively small wafer area, to keep costs reasonable and to allow easy integration into the disc drive design. 
     Therefore, there is a need in the art for a microactuator design providing large actuation force with reasonable power consumption and within a reasonable wafer area to microposition a transducing head at a speed that accommodates the high bandwidth required by high performance disc drives. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention is a dual-stage actuation assembly for a disc drive having a recording disc rotatable about an axis and a slider supporting a transducing head for transducing data with the disc. The dual-stage actuation assembly includes a movable actuator arm controlled by an actuator motor. A suspension assembly is supported by the actuator arm, and includes a flexure. A microactuator is also provided, including a stator attached to the flexure and a rotor supporting the slider, the rotor being movable with respect to the stator in a first horizontal plane generally parallel to a surface of the disc A magnetic circuit is arranged vertically in a plurality of planes substantially parallel to the first horizontal plane to move the microactuator rotor in the first horizontal plane generally parallel to the surface of the disc in response to a current provided to the magnetic circuit. In an exemplary embodiment, the magnetic circuit includes a bottom ferromagnetic keeper, a plurality of magnets, a patterned conductive coil and a top ferromagnetic keeper, vertically arranged in different horizontal planes. The magnets are operable to move laterally and thereby cause movement of the microactuator rotor in the first horizontal plane generally parallel to the surface of the disc in response to circulation of a current through the patterned conductive coil. 
     Another aspect of the present invention is a method of forming a microactuator in a disc drive having a recording disc rotatable about an axis, a slider supporting a transducing head for transducing data with the disc, and an actuation system supporting the slider to coarsely position the transducing head adjacent a selected radial track of the disc. A microactuator substrate is formed with a plurality of outer preloading bars and a plurality of inner alignment clips that are movable with respect to the plurality of outer preloading bars. The alignment clips are suspended from the preloading bars by flexible beam springs. A tub is etched in the microactuator substrate, and a first ferromagnetic keeper is plated on a bottom surface of the tub. A plurality of magnets are formed on the first ferromagnetic keeper in the tub. A flexure is formed with a second ferromagnetic keeper and a patterned conductor on an underside of the flexure below the second ferromagnetic keeper. The flexure is attached to the microactuator substrate with the patterned conductor positioned between the plurality of magnets and the second ferromagnetic keeper. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a plan view of a disc drive actuation system for positioning a slider over tracks of a disc. 
     FIG. 2 is an exploded perspective view of a portion of a disc drive including a microactuator according to the present invention. 
     FIG. 3 is a perspective view of a microactuator system for positioning a slider over tracks of a disc. 
     FIG. 4 is a top view of the microactuator system shown in FIG.  3 . 
     FIG. 5 is a perspective view of the underside of a flex circuit for use with the microactuator of the present invention. 
     FIG. 6 is a perspective view of the microactuator of the present invention attached to the flex circuit for use in a disc drive. 
     FIG. 7 is a layer diagram illustrating the vertical magnetic circuit formed by the microactuator and flex circuit of the present invention. 
     FIGS. 8-10 are perspective views of various phases of formation of the microactuator according to the present invention. 
     FIG. 11 is a perspective view of an alternate microactuator according to the present invention implementing four magnets. 
     FIG. 12 is a perspective view of the underside of an alternate flex circuit for use with the microactuator utilizing four magnets as shown in FIG.  9 . 
     FIG. 13 is a perspective view of a first layer of a dual-layer coil according to an alternate embodiment of the present invention. 
     FIG. 14 is a perspective view of a second layer of a dual-layer coil according to an alternate embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 is a plan view of a disc drive actuation system  10  for positioning slider  24  over a track  29  of disc  27 . Actuation system  10  includes voice coil motor (VCM)  12  arranged to rotate actuator arm  16  around axis  14  on spindle  15 . Head suspension  18  is connected to actuator arm  16  at head mounting block  20 . Flexure  22  is connected to an end of head suspension  18 , and carries slider  24 . Slider  24  carries a transducing head (not shown in FIG. 1) for reading and/or writing data on concentric tracks of disc  27 . Disc  27  rotates around axis  28 , so that windage is encountered by slider  24  to keep it aloft a small distance above the surface of disc  27 . 
     VCM  12  is selectively operated to move actuator arm  16  around axis  14 , thereby moving slider  24  between tracks  29  of disc  27 . However, for disc drive systems with high track density, VCM  12  lacks sufficient resolution and frequency response to position a transducing head on slider  24  precisely over a selected track  29  of disc  27 . Therefore, a higher resolution actuation device is necessary. 
     FIG. 2 is an exploded perspective view of a portion of a disc drive including microactuator  30  according to the present invention. Flexure  22  is attached to load beam  18 , and microactuator  30  is attached to flexure  22  to carry slider  24  above a surface of disc  27  (FIG.  1 ). Transducing head  26  is carried by slider  24  to transduce data with the disc. 
     In operation of the disc drive, load beam  18 , flexure  22  and microactuator  30  carrying slider  24  are all moved together as coarse positioning is performed by VCM  12  (FIG. 1) moving actuator arm  16  (FIG.  1 ). To achieve fine positioning of transducing head  26 , microactuator  30  generates a force which causes bending of the beam springs of the microactuator. As a result, the portion of microactuator  30  carrying slider  24  moves slightly with respect to flexure  22  in the direction of arrows  31 , displacing transducing head  26  with high resolution for precise positioning over a selected track of the disc. 
     FIG. 3 is a perspective view, and FIG. 4 is a top view, of microactuator  30  according to the present invention. Microactuator  30  includes outer preloading bars  32  and  34  and inner alignment clips  36  and  38 , with inner alignment clips  36  and  38  clamping against the sides of slider  24  at a frontal portion (to the right in FIGS. 3 and 4) thereof. Flexible beam springs  33  and  35  extend between the proximal ends of preloading bars  32  and  34  and the distal ends of alignment clips  36  and  38 . A tub  40  having a bottom surface lined with a ferromagnetic keeper material is formed in the substrate of microactuator  30  opposite transducing head  26  of slider  24 , and structural bond pad  42  is provided for attachment to the top surface (opposite the air-bearing surface) of slider  24 . Magnets  52  and  53  are located in tub  40 , attached to the ferromagnetic lining on the bottom surface of tub  40 . Standoffs  54 ,  56 ,  58  and  60  are formed on respective standoff bases  44 ,  46 ,  48  and  50  on outer preloading bars  32  and  34 , to be borne upon so as to apply pre-load force to microactuator  30  as it supports slider  24  over the surface of the disc. 
     FIG. 5 is a perspective view of the underside of flexure  22  for use with microactuator  30  of the present invention. Flexure  22  is formed of a non-conductive polyimide material such as Kapton, for example, and forms the substrate of a flex circuit including conductive coil  62  connected to current-providing source  63 , and standoffs  64 ,  66 ,  68  and  70 . Standoffs  64 ,  66 ,  68  and  70  are aligned with respective standoffs  54 ,  56 ,  58  and  60  of microactuator  30  to apply pre-load force to microactuator  30  as it carries slider  24  (FIG.  3 ). In addition, conductive leads  72 ,  74 ,  76  and  78  are provided to electrically contact transducing head  26  carried by slider  24  (FIG.  3 ). Coil  62  and data leads  72 ,  74 ,  76  and  78  are formed of a conductive material such as copper. Standoffs  64 ,  66 ,  68  and  70  are preferably formed of copper, or alternatively of a material such as nickel. In some embodiments, standoffs  64 ,  66 ,  68  and  70  may be formed with sufficient thickness so as to render standoffs  54 ,  56 ,  58  and  60  on microactuator  30  (FIG. 3) unnecessary. The total standoff height between magnets  52  and  53  and coil  62  is typically about 10 to 100 micro-meters (μm), with an exemplary height of 50 μm in one embodiment. Flexure  22  is typically about 50 μm thick, and may be made as thin as about 25 μm with some trade-off in durability. Coil  62  is typically about 20 μm thick. 
     FIG. 5 is a perspective view of microactuator  30  of the present invention attached to the underside of flexure  22  for use in a disc drive. Flexure  22  includes ferromagnetic keeper  82  formed on its top surface opposite coil  62  (FIG. 5) above magnets  52  and  53  located in tub  40  of microactuator  30  (FIG.  3 ). A vertically arranged magnetic circuit is thereby formed from the bottom keeper (ferromagnetic lining of tub  40 ), magnets  52  and  53 , an airgap formed by the spacing between magnets  52  and  53  and coil  62 , the windings of coil  62  itself, flexure  22  and top keeper  82 . This vertically arranged magnetic circuit is shown schematically in FIG. 7, and its magnetic characteristics are discussed in more detail below in the discussion of FIG.  7 . Keeper  82  may be plated on the polyimide substrate of flexure  22  or adhesively bonded thereto. In an alternate embodiment, keeper  82  may be located on the bottom surface of flexure  22 , with an additional insulating polyimide layer on the bottom of keeper  82  to insulate it from coil  62 . 
     Flexure  22  is shaped to include arms  84  and  86  and cross beams  88  and  90  for supporting data leads  72 ,  74 ,  76  and  78 , and central portion  92  for supporting coil  62  and standoffs  64 ,  66 ,  68  and  70 . Pre-load force is applied in a preferred embodiment to central portion  92  of flexure  22  by load beam  18  (FIG.  2 ). The arrangement of flexure  22  and the design of microactuator  30  are such that operation of microactuator  30  by circulating current through the windings of coil  62  results in cross-track movement of slider  24  in a horizontal plane generally parallel to the surface of the disc, in the direction of arrows  31 , with the movement being permitted by the flexibility of beam springs  33  and  35  (FIG.  3 ). Outer preloading bars  32  and  34  having standoffs  54 ,  56 ,  58  and  60  thereon effectively form the stator of the motor, with magnets  52  and  53 , slider bond pad  42 , slider  24 , flexible beams  33  and  35  and inner alignment clips  36  and  38  effectively forming the rotor of the motor (FIG.  3 ). Thus, lateral movement of magnets  52  and  53  affects the cross-track (horizontal) position of slider  24  with respect to outer preloading bars  32  and  34  and standoffs  54 ,  56 ,  58  and  60 , and also with respect to flexure  22  and to the tracks of the disc. 
     FIG. 7 is a layer diagram illustrating the vertical magnetic circuit formed by bottom ferromagnetic keeper  41 , magnets  52  and  53 , coil windings  62 , flexure  22  and keeper  82  of the present invention. A current flowing through coil windings  62  interacts with the magnetic field in the airgap between magnets  52  and  53  and top keeper  82 , so that magnetic flux flows in a path illustrated by the arrows in FIG. 7 referred to as B g  (for flux in the airgap) and B k  (for flux in the keepers). This interaction causes a force to be exerted on magnets  52  and  53  in a lateral direction (across the tracks of the disc), the force being controllable to achieve high resolution movement for fine positioning of the microactuator rotor with respect to the tracks of the disc. 
     The actuation force achievable by the magnetic circuit shown in FIG. 7 is governed by the Lorentz equation, with the current through coil  62  flowing at a 90° angle to the airgap flux (B g ), given as: 
     
       
         
           F 
           y 
           =NIB 
           g 
           L 
           m 
         
       
     
     where F y  is the actuation force, N is the number of coil traverses across the face of magnets  52  and  53 , I is the current in the coil, B g  is the flux density in the airgap, and L m  is the length of the faces of magnets  52  and  53  (into the paper in FIG.  7 ). 
     In addition to the actuation force, there is a force of attraction between magnets  52  and  53  and top keeper  82 , given as:          F   z     =       AB   g   2       2                   μ   0                                
     where F z  is the attraction force, A is the area of the faces of magnets  52  and  53 , B g  is the airgap flux density, and μ 0  is the permeability of free space. 
     An actuation stroke can be determined from the designed resonant frequency of the microactuator and the actuation force achieved. The microactuator resonant frequency is generally controlled by the servo system of the disc drive, and is also affected by the mass of the microactuator rotor and the offtrack stiffness of the beams. In an exemplary embodiment, a resonance target frequency is 1000 Hz±150 Hz, with a rotor mass of 1.8 milli-grams and a beam offtrack stiffness at 1150 Hz of 93.9 Newtons per meter. A table of actuation stroke and attraction force for the 1000 Hz±150 Hz resonance frequency, where the magnet faces have a length of 527 μm and a width of 800 μm, is shown below: 
     
       
         
               
               
               
             
           
               
                   
               
               
                 Gap Flux Density 
                 Actuation 
                 Attraction 
               
               
                 (B g ) 
                 Stroke 
                 Force 
               
               
                   
               
             
             
               
                 0.2 Tesla 
                 3.95 μm 
                 0.68 gmf 
               
               
                 0.4 Tesla 
                 7.90 μm 
                 2.73 gmf 
               
               
                 0.6 Tesla 
                 11.9 μm 
                 6.14 gmf 
               
               
                 0.8 Tesla 
                 15.8 μm 
                 10.9 gmf 
               
               
                   
               
             
          
         
       
     
     As can be seen from the table above, large actuation strokes are achieved with rather significant vertical attraction forces between the magnets and the top keeper. The beam springs of the microactuator must be designed to support both this vertical attraction force and the vertical pre-load force applied to the slider with minimal vertical deflection. To accomplish this, the middle of the span of the beam springs is made to coincide with the net loading point of the microactuator. The net loading point is the point where the bending moments from the air-bearing pre-load force and the magnetic attraction force sum to zero. For example, where a layout distance is defined as the distance from the center of the magnets to the center of application of pre-load force to the slider, a microactuator design for 2.73 gmf pre-load force and 0.4 Tesla flux density places the net loading point halfway along the layout distance. A microactuator design for 3.07 gmf pre-load force and 0.6 Tesla flux density places the net loading point at one-third of the layout distance (closer to the magnets than the slider). A microactuator design for 2.73 gmf pre-load force and 0.8 Tesla flux density places the net loading point at one-fifth of the layout distance (closer to the magnets than the slider). Beam springs for other combinations of air-bearing pre-load and magnetic attraction forces maybe laid out in a similar manner. 
     In the force equations given above, the airgap flux density (B g ) is given as:          B   g     =       B   r       (     1   +       T   g       T   m         )                              
     where B r  is the remanant magnetization of the magnetic material of magnets  52  and  53 , T g  is the thickness of the airgap between magnets  52  and  53  and top keeper  82 , including open space, coil windings  62  and flexure  22 , and T m  is the thickness of magnets  52  and  53 . 
     The thickness of the airgap is affected by several factors, such as the thickness of flexure  22 , the placement of top keeper  82  on the top or bottom of flexure  22 , and the number of coil layers (that is, whether a single-layer coil or a dual-layer coil such as is shown in FIGS. 13 and 14 is used). The airgap thickness in various embodiments is as follows: 
     
       
         
               
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 Substrate 
                 Keeper 
                 Coil 
                 Airgap 
               
               
                   
                 Thickness 
                 Placement 
                 Layers 
                 Thickness 
               
               
                   
                   
               
             
             
               
                   
                 50 μm 
                 Top 
                 2 
                 110 μm  
               
               
                   
                 25 μm 
                 Top 
                 2 
                 85 μm 
               
               
                   
                 50 μm 
                 Top 
                 1 
                 80 μm 
               
               
                   
                 Any 
                 Bottom 
                 2 
                 70 μm 
               
               
                   
                 25 μm 
                 Top 
                 1 
                 55 μm 
               
               
                   
                 Any 
                 Bottom 
                 1 
                 40 μm 
               
               
                   
                   
               
             
          
         
       
     
     The thickness of keepers  41  and  82  are determined by the requirement of carrying the flux linking the magnetic circuit without saturating. This relationship is expressed as:          T   k     =       W   m            B   g       B   s                                
     where T k  is the thickness of keepers  41  and  82 , W m  is the width of magnets  52  and  53  (across the page in FIG.  7 ), and B s  is the saturation moment of the material used to form keepers  41  and  82 . Thus, it can be appreciated that a design with a set of four magnets requires only half the keeper thickness of a design with two magnets, since each magnet face is half as wide. Since the wafer package thickness of the microactuator is limited to about 200 μm, and a typical airgap thickness is about 80 μm (see table above), a reduction in keeper thickness is advantageous in some embodiments. This alternate design is shown in FIGS. 11 and 12 and is discussed in more detail below. In addition, the thickness of magnets  52  and  53  may be reduced while maintaining a high actuation stroke by utilizing magnetic materials with high remanence characteristics. In general, an increase in magnetic remanence of magnets  52  and  53  enables a decrease in the thickness of magnets  52  and  53  while maintaining a constant airgap flux density and actuation force. The table shown below illustrates the remanence of several potential magnetic materials: 
     
       
         
               
               
               
             
           
               
                   
                   
               
               
                   
                 Material 
                 Remanence 
               
               
                   
                   
               
             
             
               
                   
                 Ceramic 
                 0.40 Tesla 
               
               
                   
                 Neodymium Iron Boron 
                 1.27 Tesla 
               
               
                   
                 Samarium Cobalt 
                 1.09 Tesla 
               
               
                   
                   
               
             
          
         
       
     
     Ceramic magnets are formed by particles of strontium ferrite embedded in a ceramic matrix. Ceramic magnets and Samarium Cobalt magnets are able to withstand process temperatures of 220° C. typically experienced in solder reflow ovens, enabling soldered attachment to be utilized to connect microactuator substrate  30  to flexure  22 . Neodymium iron boron is limited to process temperatures below 150° C., requiring flexure  22  to be attached to microactuator substrate  30  by mechanical bonding with epoxy, for example. 
     FIGS. 8-10 are perspective views of various phases of formation of microactuator  30 , illustrating the novel formation process of microactuator  30  according to the present invention. The substrate of microactuator  30  is composed of a high-strength wafer substrate material such as molybdenum or cold-rolled titanium, or alternatively of a conventional wafer substrate material such as silicon. As shown in FIG. 8, the outline of microactuator  30  and the shapes of outer preloading bars  32  and  34 , flexible beam springs  33  and  35  and inner alignment clips  36  and  38  are etched into the wafer substrate by a method such as deep trench reactive ion etching (DTRIE) to achieve the desired feature resolution. The substrate of microactuator  30  is then coated with a release layer which is etched to form a wet etching mask. Further wet etching is then performed to form features as described below. 
     As shown in FIG. 9, tub  40  is formed in the substrate of microactuator  30 , and is plated with a ferromagnetic keeper  41  composed of a material such as cobalt-iron, for example. In an exemplary embodiment, tub  40  is etched to a depth of about 100 μm. Structural bond pad  42  is formed on keeper  41  to extend from the edge of tub  40 , for attachment to slider  24  (FIG. 3) to support slider  24  between inner alignment clips  36  and  38 . Standoff bases  44 ,  46 ,  48  and  50  are plated on outer preloading bars  32  and  34 . 
     As shown in FIG. 10, magnets  52  and  53  are installed in tub  40  on keeper  41 . Magnets  52  and  53  are attached to keeper  41  by known methods such as adhesive or glass bonding. Standoffs  54 ,  56 ,  58  and  60  are plated on respective standoff bases  44 ,  46 ,  48  and  50  to be borne upon so as to apply pre-load force to microactuator  30  as it supports slider  24 . Standoffs  54 ,  56 ,  58  and  60  may be formed of copper in an exemplary embodiment, and are designed to be attached to corresponding standoffs on the underside of flexure  22  (FIG. 5) in a preferred arrangement of microactuator  30 , by an attachment method such as soldering or another method known in the art. Attachment between flexure  22  and standoffs  54 ,  56 ,  58  and  60  on outer preloading bars  32  and  34  forms the stator of the microactuator motor. 
     In an alternative preferred embodiment, tub  40  is initially formed in the wafer substrate by a method such as DTRIE etching, coincident with the process of forming the outline of microactuator  30  and the shapes of outer preloading bars  32  and  34 , flexible beam springs  33  and  35  and inner alignment clips  36  and  38 . Keeper  41  is cut from a sheet of ferromagnetic alloy such as cobalt-iron, and keeper  41  and magnets  52  and  53  are formed as a separate assembly and bonded to the wafer substrate in tub  40 . In one embodiment, keeper  41  is bonded around its edges to the side walls of tub  40 . 
     FIG. 11 is a perspective view of an alternate microactuator design implementing four magnets  100 ,  102 ,  104  and  106  in tub  40 . Other than the provision of four magnets rather than two, the microactuator design pictured in FIG. 11 is identical to microactuator  30  shown in FIG.  3 . As discussed above, providing four magnets  100 ,  102 ,  104  and  106  enables the keeper thickness to be reduced, which may be advantageous in some embodiments of the invention, particularly if vertical space between discs is at a premium. 
     FIG. 12 is a perspective view of the underside of an alternate flexure  22  for use with the microactuator utilizing magnets  100 ,  102 ,  104  and  106  as shown in FIG.  11 . Flexure  22  pictured in FIG. 11 is identical to the flexure of FIG. 5 except that the coils are realized by serpentine coils  110 , which traverse the length of magnets  100 ,  102 ,  104  and  106  four times. The additional coil traverses keep the actuation force at a desired level for the increased number of magnets; that is, the number of coil traverses per magnet is maintained. 
     FIG. 13 is a perspective view of a first layer  120 , and FIG. 14 is a perspective view of a second layer  130 , of an alternate dual-layer coil pattern for use with the microactuator of the present invention. The dual-layer coil pattern is realized by spiraling conductor patterns vertically spaced from one another by an insulating layer, which is not shown in FIGS. 13 and 14 for the sake of clarity. First layer  120  has a center point  122  for attachment to center point  132  of second layer  130 . Similarly, second layer  130  has side point  134  for attachment to side point  124  of first layer  120 . These attachment points are the only places where first layer  120  and second layer  130  are connected through the insulating layer between them. The insulating layer may be provided by flexure  22  (FIG. 5) itself, or by an additional layer of insulating material between first layer  120  and second layer  130 . The multiple layers of the coils allow a significantly lower operating current through the coils, which could be advantageous and save on the cost of components in some embodiments. This concept is illustrated in the table of current ratings for different coil types shown below: 
     
       
         
               
               
               
               
             
           
               
                   
               
               
                 Coil Type 
                 Resistance 
                 Current 
                 NI Product 
               
               
                   
               
             
             
               
                 Single layer, one turn 
                 8.15 milli- 
                 1919 milli-Amps  
                 3.84 Amp- 
               
               
                 (FIG. 7) 
                 Ohms 
                   
                 Lengths 
               
               
                 Single layer, serpentine 
                 38.8 milli- 
                 879 milli-Amps 
                 3.52 Amp- 
               
               
                 (FIG. 11) 
                 Ohms 
                   
                 Lengths 
               
               
                 Dual layer, seven turns 
                  462 milli- 
                 265 milli-Amps 
                 3.71 Amp- 
               
               
                 (FIGS. 12 &amp; 13) 
                 Ohms 
                   
                 Lengths 
               
               
                   
               
             
          
         
       
     
     In view of the microactuator design options explained above, it will be apparent to one skied in the art that a number of magnetic microactuator designs may be implemented according to the present invention. Performance characteristics for three exemplary embodiments are shown in the table below: 
     
       
         
               
               
               
               
               
             
           
               
                   
               
               
                   
                   
                   
                 Bottom 
                 Stroke 
               
               
                 Magnetic 
                 Number of 
                 Top keeper 
                 keeper 
                 (1000 Hz 
               
               
                 Material 
                 Magnets 
                 thickness 
                 thickness 
                 resonance) 
               
               
                   
               
             
             
               
                 Ceramic 
                 2 
                 148 μm  
                 37 μm 
                  4.8 μm 
               
               
                 Samarium 
                 2 
                 40 μm 
                 100 μm  
                 13.0 μm 
               
               
                 Cobalt 
               
               
                 Samarium 
                 4 
                 201 μm 
                 50 μm 
                 11.9 μm 
               
               
                 Cobalt 
               
               
                   
               
             
          
         
       
     
     In the above table, the top keeper is formed of nickel, with a saturation moment of 0.6 Tesla, and the bottom keeper is formed of cobalt-iron, with a saturation moment of 2.4 Tesla. In an alternative embodiment, the t op keeper may also be formed of cobalt-iron, which would reduce its thickness significantly—the above description has assumed a nickel top keeper due to material s limitations imposed by flex circuit vendors. As can b e seen, the microactuation stroke increases significantly when a high remanence magnetic material is used. The four magnet design trades off some microactuation stroke for a reduction in keeper thickness. 
     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.