Abstract:
A bias-field apparatus for a magneto-optical system having a magneto-optical recording element which includes a conductive substrate and which moves through a magnetic field created by the bias-field device so that information is selectively recorded on or erased from the recording element, the device comprises a support member; a magnet mounted for rotation in the support member such that, when the recording element rotates, the conductive substrate creates a magnetic coupling with the magnet for causing the magnet to rotate; and (c) a ferromagnetic element enclosing a portion of said magnet and having a plurality of spatially separated pole teeth, each of said pole teeth having an operably connected coil disposed thereon that releases said magnet for rotation caused by the conductive substrate when the coil is energized, and prevents rotation when said coil is de-energized for permitting said magnet to be selectively rotated.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is related to U.S. application Ser. No. 08/800,347 entitled &#34;A Motorless Bias-Field Device and Method having Magnetic Coupling Between A Magnet and the Recording Element for Inverting the Magnetic Field&#34; by Christopher Williams et. al. and U.S. application Ser. No. 09/045,410 entitled &#34;A Magnet Drive and Bias Field Apparatus for Inverting A Bias Field Magneto-Optic Recording and Erasing&#34; by Edward Furlani et. al. filed on Mar. 20, 1998. 
    
    
     FIELD OF THE INVENTION 
     The invention relates generally to the field of magneto-optic recording and erasing and, more particularly, to such recording and erasing utilizing a motorless bias-field device for selectively inverting a bias field for permitting such recording and erasure. 
     BACKGROUND OF THE INVENTION 
     In the magneto-optic recording process, a vertically magnetizable recording layer is initially sensitized by simultaneously subjecting it to a uniform magnetic field and a temperature which exceeds its Curie temperature. The magnetic field, being directed perpendicular to the recording layer, serves to uniformly align all of the magnetic domains therewith. Once all the magnetic domains are facing in the same direction, the recording layer is ready to record information. Such recording is effected by subjecting the recording layer to a magnetic field of reverse polarity while scanning the layer with an intensity-modulated laser beam. 
     During the recording process, a laser beam intensity is switched between high and low levels, representing the digital (binary) information being recorded. Only the high level is sufficiently intense to raise the temperature of the irradiated portion of the recording layer to above its Curie temperature; thus, digital information is recorded at the point of incidence of the laser as the more intensely irradiated magnetic domains flip in orientation to align themselves with the magnetic bias field. Playback of the recorded information is commonly achieved by scanning the information tracks with a plane-polarized beam of radiation and monitoring the reflected beam for shifts in the plane of polarization, as produced by the well known Kerr effect. To erase the recorded information, the polarity of the applied external magnetic field is reversed, and the recording layer is scanned with a beam of sufficient intensity to again heat the recording layer to above its Curie temperature. After this erasure step, all of the irradiated magnetic domains will again face in the same direction. 
     Various schemes have been proposed to achieve the magnetic field inversions required in switching between the record and erase modes of the magneto-optic recording process. In the disclosures of U.S. Pat. Nos. 5,020,042 and 5,291,345, for example, the field inversion apparatus consists of a magnetic field producing coil surrounding a cylindrical bipolar magnet. One pole of the magnet is placed adjacent the recording medium for inducing its particular magnetic field to the recording medium, and when the coil is energized, the field that the coil creates imparts a torque to the magnet forcing it to rotate for causing the other pole of the magnet to be adjacent the recording medium. 
     Although the presently known and utilized device is satisfactory, it is not without drawbacks. The coils are not energy efficient because they consume a substantial amount of energy. 
     Consequently, a need exists in the construction and mode of operating the bias-field device for overcoming the above-described drawbacks. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to overcoming one or more of the problems set forth above. Briefly summarized, according to one aspect of the present invention, the invention resides in a bias-field apparatus for a magneto-optical system having a magneto-optical recording element which includes a conductive substrate and which moves through a magnetic field created by the bias-field device so that information is selectively recorded on or erased from the recording element, the device comprising: (a) a support member; (b) a magnet mounted for rotation in said support member such that,. when the recording element rotates, the conductive substrate creates a magnetic coupling with the magnet for causing said magnet to rotate; and (c) a ferromagnetic element enclosing a portion of said magnet and having a plurality of spatially separated pole teeth, each of said pole teeth having an operably connected coil disposed thereon that releases said magnet for rotation caused by the conductive substrate when the coil is energized, and prevents rotation when said coil is de-energized for permitting said magnet to be selectively rotated. 
     It is an object of the present invention to provide a bias-field device for overcoming the above-described drawbacks. 
     It is also an object of the present invention to provide an energy efficient and motorless bias field device. 
     It is an advantage of the present invention to provide a cost efficient bias-field device. 
     It is a further advantage of the present invention to provide a bi-directional bias-field device for selectively inverting the magnetic bias field. 
     These and other aspects, objects, features and advantages of the present invention will be more clearly understood and appreciated from a review of the following detailed description of the preferred embodiments and appended claims, and by reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1A is a perspective view of a magneto-optic recording element and a bias magnet of the present invention; 
     FIG. 1B is a cross-sectional view taken along line A--A of FIG. 1A; 
     FIG. 2A is a perspective view of the bias field device of the present invention; 
     FIG. 2B is a perspective view of the bias magnet; 
     FIG. 3 is a perspective view of the bias field device with end support members; 
     FIG. 4A is a cross-sectional view of the bias field apparatus taken along line A--A of FIG. 1A illustrating the bias magnet in a first bias position; 
     FIG. 4B is a cross-sectional view of the bias field device taken along line A--A of FIG. 1A illustrating the bias magnet undergoing rotation from a first to a second bias position; and 
     FIG. 4C is a cross-sectional view of the bias field apparatus taken along line A--A of FIG. 1A illustrating the bias magnet in the second bias position. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIGS. 1A and 1B, there is illustrated a magneto-optic recording element as shown in the form of a disk 10 which is adapted to be rotated about its central axis x. The essential features of the recording element are a vertically magnetizable recording layer 20 and a supporting substrate 30. During the recording step, the recording layer 20 is selectively heated by a beam of radiation, as provided by a laser source 35. The beam intensity is modulated by an information source (not shown) representing certain digital information which is to be recorded. The information source functions to switch the beam intensity of the laser source 35 between high and low levels, only the high level being sufficient to heat the recording layer 20 to at least its Curie temperature. An elongated permanent magnet 50 is radially disposed with respect to the rotating disk 10, and serves to produce a magnetic bias field. The magnet 50 includes a plurality of poles and, for purposes of example, illustrates four poles 52a, 52b, 52c, and 52d in the preferred embodiment (FIG. 1B). Magnet 50 is mounted for rotation about its longitude axis so that the direction of the bias field may be switched from the upward direction indicated by field vector 54 in FIG. 1B with pole 52a directly beneath the disk to a downward direction in which the magnet 50 is rotated so that either pole 52b or 52d is directly beneath the disk 10. The amount of rotation depends on the number of poles in magnet 50. For the 4 pole magnet 50 shown in FIG. 1, a 90 degree rotation is required. A magnetic drive mechanism 90 (not shown, see FIGS. 2A and 2B) is in operative relation to the magnet 50 for causing rotation as will be described. The magnet 50 will cause magnetic domains in the recording element to flip orientations, from upward to downward, whenever the laser beam intensity is at its high level. In this manner, the digital information provided by the laser source 35 is magnetically recorded in the recording layer 20. To erase the previously recorded information in the recording layer 20, the direction of the magnetic bias field is inverted, or in other words, the magnet 50 is rotated 90 degrees in this embodiment. After the field inversion, the laser source 35 scans the recording element while its intensity is maintained at its high level and, while so radiated, all of the magnetic domains align themselves with the bias field thereby providing a uniformly sensitized disk or track which is again ready to record information. 
     Referring to FIGS. 2A, and 2B, a bias field device 90 and bias magnet 50 of the present invention are illustrated, respectively. The bias field device 90 comprises a magnet 50, a ferromagnetic stator member 102 with stator pole teeth 104A, 104B, and 104C, and coils 106A, 106B, 106C. The magnet 50 is polarized with a plurality of radially disposed surface poles (denoted by N &amp; S) of alternating north and south polarity around its circumference. In the preferred embodiment, the magnet 50 has four poles 52a, 52b, 52c and 52d as shown in FIG. 1B. The magnet 50 further comprises axle members 58 which are fixedly attached as shown in FIG. 2B. 
     The ferromagnetic stator member 102 has coils 106A, 106B and 106C wrapped around the stator pole teeth 104A, 104B, and 104C, respectively. The power source 110 supplies power to the coils 106A, 106B and 106C through conductors 108A, 108B and 108C, respectively as shown. The ferromagnetic stator member 102 is preferably formed from a soft magnetic material including either permalloy, supermalloy, sendust, iron, nickel, nickel-iron or alloys thereof. 
     Referring to FIG. 3, a perspective is shown of a support structure 112 comprising the ferromagnetic element 102, end support members 114 and bias magnet 50. The end support members 114 are fixedly attached to the ferromagnetic element 102 as shown. The end support members 114 have bearings 116 for receiving axle members 58 of bias magnet 50 (FIG. 2B). Thus the bias magnet 50 is mounted for rotation about it longitudinal axis with its axle members free to rotate in bearings 116. 
     FIGS. 4A, 4B and 4C are cross-sectional views of the bias field device taken along line A--A of FIG. 1 illustrating the operation and field inversion of the bias field device 90. For purposes of illustration, the disk 10 is rotating in the direction illustrated by the arrow 120. In FIG. 4A, the magnet 50 is in a first bias position with its north pole 52a adjacent to the disk 10 thereby producing an upward directed magnetic field across recording layer 20 as indicated by field vector 122. The poles 52b, 52c and 52d are adjacent stator pole teeth 104A, 104B, and 104C, respectively as shown. The poles 52b, 52c and 52d of magnet 50 induce a north pole 124A, a south pole 124B, and a north pole 124C in stator pole teeth 104A, 104B, and 104C, respectively as shown. The rotation of disk 10 causes the conductive substrate 30 to impart a first torque on the magnet 50 which tends to rotate the magnet 50 clockwise as illustrated by rotation arrow 130. The magnet 50 is held stationary in the first bias position by the presence of the north pole 124A ,the south pole 124B, and the north pole 124C in stator pole teeth 104A, 104B, and 104C, respectively which impart a second torque that tends to rotate the magnet counterclockwise as indicated by the rotation arrow 140. The first and second torques are of equal magnitude and therefore there is no rotation of magnet 50. It is instructive to note that the coils 106A, 106B and 106C which are wrapped around the stator pole teeth 104A, 104B, and 104C, respectively of ferromagnetic element 102, are de-energized when it is desired to have the magnet 50 stationary. 
     When rotation of the magnet 50 is desired, the coils 106A, 106B and 106C are energized by power source 110 (FIG. 2A) as shown in FIG. 4B. The current flows through the coils 106A, 106B and 106C as indicated by conventional current arrows 126 and 128 with current arrow 126 indicating that current is going into the page and current arrow 128 indicating that current is going out of the page. This current induces a south pole 134A, a north pole 134B, and a south pole 134C in the stator pole teeth 104A, 104B, and 104C, respectively. The south pole 134A, north pole 134B, and south pole 134C impart a clockwise torque to magnet 50 as does the rotation of conductive substrate 30. Therefore, the magnet 50 undergoes rotation as indicated by rotation arrow 130 from the first bias position with its north pole 52a adjacent to disk 10 to a second bias position with is south pole 52d adjacent to disk 10 as shown in FIG. 4C. The current is on just long enough to implement the rotation. 
     Referring to FIG. 4C, the magnet 50 is in a second bias position with its south pole 52d adjacent to the disk 10 thereby producing a downwardly directed magnetic field across recording layer 20 as indicated by field vector 142. The poles 52a, 52b and 52c are adjacent stator pole teeth 104A, 104B, and 104C, respectively as shown. The poles 52a, 52b and 52c of magnet 50 induce a south pole 144A, a north pole 144B, and a south pole 144C in stator pole teeth 104A, 104B, and 104C of ferromagnetic element 102, respectively as shown . The rotation of disk 10 causes the conductive substrate 30 to impart a first torque to the magnet 50 which tends to rotate the magnet 50 clockwise as illustrated by rotation arrow 130. The magnet 50 is held stationary in the first bias position by the presence of the south pole 144A , the north pole 144B, and the south pole 144C in stator pole teeth 104A, 104B, and 104C, respectively of ferromagnetic element 102 which impart a second torque that tends to rotate the magnet counterclockwise as indicated by the rotation arrow 140. The first and second torques are of equal magnitude and therefore there is no rotation of magnet 50. It is instructive to note that the coils 106A, 106B and 106C which are wrapped around the stator pole teeth 104A, 104B, and 104C, respectively, of ferromagnetic element 102, are de-energized when it is desired to have the magnet 50 stationary. It is obvious to those skilled in the art that the magnet can be rotated to its first bias position with its north pole adjacent to the disk 10 by energizing the coil as described in FIG. 4B with the direction of current opposite to that shown in FIG. 4B. 
     The invention has been described with reference to a preferred embodiment. However, it will be appreciated that variations and modifications can be effected by a person of ordinary skill in the art without departing from the scope of the invention. 
     Parts List: 
     10 disk 
     20 recording layer 
     30 substrate 
     35 laser source 
     50 magnet 
     52a north pole 
     52b south pole 
     52c north pole 
     52d south pole 
     54 field vector 
     58 axle member 
     90 bias field device 
     102 ferromagnetic stator member 
     104A stator pole tooth 
     104B stator pole tooth 
     104C stator pole tooth 
     106A coil 
     106B coil 
     106C coil 
     108A conductor 
     108B conductor 
     108C conductor 
     110 power source 
     112 support member 
     114 end support member 
     116 bearing 
     122 field vector 
     124A north pole 
     124B south pole 
     124C north pole 
     126 current arrow 
     128 current arrow 
     130 rotation arrow 
     134A south pole 
     134B north pole 
     134C south pole 
     140 rotation arrow 
     142 field vector 
     144A south pole 
     144B north pole 
     144C south pole