Abstract:
Magnetic tape onto which information may be recorded on either side exhibits the risk of having information recorded on one side affect the opposite side when the two sides are adjacent, such as when the tape is wound in a tape pack. The chance that information recorded onto one surface of a double sided magnetic tape will affect information recorded on the other surface of the magnetic tape is reduced by keeping fields emanating from a recorded region less than about one-half the coercivity of the magnetic medium onto which the information is recorded.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to magnetic tape onto which information may be recorded on both sides.  
           [0003]    2. Background Art  
           [0004]    Magnetic tape continues to be a popular means for recording and storing information. Magnetic tape storage systems may hold vast quantities of data at a relatively low cost per bit stored. Magnetic tapes are easily manipulated by automated storage systems. Data may be added to information already stored on a magnetic tape. In addition, magnetic tapes may be erased and rerecorded. Finally, magnetic tape has a long shelf life under proper storage conditions.  
           [0005]    Magnetic tape and tape storage systems may be made increasingly more efficient and cost-effective by increasing the data storage density. Traditionally, increases in storage density have resulted from narrower data tracks, increasing the number of data tracks per tape width, increasing the density of data recorded along the length of each track, and the like. These increases have resulted from improvements in one or more of magnetic media, tape thickness, read and record electronics, tape head positioning, data encoding and decoding schemes, and the like. However, all of these improvements have focused on the traditional tape configuration of a supportive web onto which one side is coated with magnetic material.  
           [0006]    A doubling of information density can be achieved if both sides of the supporting web are coated with magnetic material onto which information may be recorded and from which information can be retrieved. One difficulty with such double sided recording occurs when the tape is wound into a cassette or cartridge for storage. Unlike traditional, one-sided tape where magnetic media contacts the non-magnetic backside of tape as the tape is wound around a hub, a double sided tape places magnetically coated tape sides face-to-face. This may result in magnetic regions recorded on one side affecting the information stored on an adjacently facing side.  
           [0007]    What is needed is a double sided magnetic tape which greatly reduces the risk that magnetic regions recorded onto the tape will affect facing regions when the tape is wound into a cartridge or cassette.  
         SUMMARY OF THE INVENTION  
         [0008]    The present invention reduces the possibility that data recorded onto one surface of a double sided magnetic tape will affect data recorded on the other surface of the magnetic tape when the tape surfaces are adjacent by keeping fields emanating from a recorded region less than about one-half the coercivity of the magnetic medium onto which the data is recorded.  
           [0009]    A magnetic tape for storing digital data is provided. A first side of the magnetic tape is coated with a first side magnetic coating operative to have data recorded thereon. A second side of the magnetic tape opposite of the first side is coated with a second side magnetic coating operative to have data recorded thereon. Each of the first side magnetic coating and the second side magnetic coating has a set coercivity. Data is recorded on the magnetic tape first side to have a fringe field strength no greater than one-half the set coercivity of the second side. Data is also recorded on the magnetic tape second side to have a fringe field strength no greater than one-half the set coercivity of the first side.  
           [0010]    In an embodiment of the present invention, the set coercivity is at least 1800 Oe.  
           [0011]    In another embodiment of the present invention, each of the first side magnetic coating and the second side magnetic coating has a remanence of not more than 600 memu/cm 3 .  
           [0012]    In yet another embodiment of the present invention, each of the first side magnetic coating and the second side magnetic coating is not greater than 120 nm in thickness.  
           [0013]    In still another embodiment of the present invention, each of the first side magnetic coating and the second side magnetic coating has a magnetic strength not greater than 6 memu/cm 2 .  
           [0014]    In a further embodiment of the present invention, each of the first side magnetic coating and the second side magnetic coating has a coercive squareness of at least 0.7.  
           [0015]    In a still further embodiment of the present invention, each of the first side magnetic coating and the second side magnetic coating has a remanent squareness of at least 0.7.  
           [0016]    A system for recording data is also provided. The system includes a magnetic tape recordable on both the first side and the second side. Each of the first side and the second side is coated with a material having a set coercivity. The system includes a first record module for recording data on the first side and a second record module for recording data on the second side. At least one hub receives the magnetic tape in a tape pack with the first side in contact with the second side. Data is recorded on the first side and on the second side to have a field strength no greater than one-half the set coercivity.  
           [0017]    The above objects and other objects, features, and advantages of the present invention are readily apparent from the following detailed description of the preferred embodiments when taken in connection with the accompanying drawings.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]    [0018]FIGS. 1 a  and  1   b  are schematic diagrams illustrating magnetic tape according to an embodiment of the present invention;  
         [0019]    [0019]FIG. 2 is an M-H graph illustrating hysteresis curves for two magnetic tape media in accordance with an embodiment of the present invention;  
         [0020]    [0020]FIG. 3 is an M-H graph illustrating calculation of squareness according to an embodiment of the present invention;  
         [0021]    [0021]FIG. 4 is a schematic diagram illustrating manufacture of a double sided tape according to an embodiment of the present invention; and  
         [0022]    [0022]FIG. 5 is a schematic diagram illustrating an alternative manufacture of a double sided tape according to an embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0023]    Referring to FIGS. 1 a  and  1   b,  diagrams illustrating magnetic tape according to an embodiment of the present invention are shown. FIG. 1 a  illustrates a perspective view and FIG. 1 b  a cross-sectional view of magnetic tape  20 . Magnetic tape  20  is wound on hub or spool  22  within cartridge  24  to form tape pack  26 . One end of tape  20  is affixed to leader block  28 . When tape  20  is fully wound within cartridge  24 , leader block  28  is positioned near opening  30  in one corner of cartridge  24 . In operation, leader block  28  is pulled away from cartridge  24  and around components in a tape deck so that tape  20  may be recorded to and/or read from. Such components include first module  32  for recording data onto tape first side  34  and second module  36  for recording data onto tape second side  38 . Modules  34 ,  36  convert electrical signals into magnetic fields which produce residual effects on regions of tape sides  34 ,  38 , respectively.  
         [0024]    Magnetic tape  20  consists of flexible substrate  40  on which has been deposited two recording surfaces. The two recording surfaces comprise first magnetic coating  42  on tape first side  34  and second magnetic coating  44  on tape second side  38 . During the record process, regions of magnetization  46 ,  48  are formed in magnetic tape  20  by record modules  32 ,  36 , respectively. Regions of magnetization  46 ,  48  are recorded to have field strength no greater than one-half the coercivity of magnetic coatings  42 ,  44 . Thus, if tape first side  34  is in contact with tape second side  38  such that region of magnetization  46  is adjacent to region of magnetization  48 , region of magnetization  46  will not have a significant effect on region of magnetization  48  and region of magnetization  48  will not have a significant effect on region of magnetization  46 .  
         [0025]    Referring now to FIG. 2, an M-H graph illustrating hysteresis curves for two magnetic tape media in accordance with an embodiment of the present invention are shown. A first hysteresis loop, indicated by  60 , has a coercivity, indicated by  64 , of approximately 1700 Oe and a remanent magnetic strength, indicated by  66 , of approximately 6.5 memu/cm 2 . A second hysteresis loop, indicated by  62 , is from a magnetic material exhibiting a flatter curve. Second hysteresis loop  62  has a coercivity, indicated by  68 , of approximately 2600 Oe and a remanent magnetic strength, indicated by  70 , of approximately 3.5 memu/cm 2 .  
         [0026]    The effects of having a field strength no greater than one-half the coercivity  64 ,  68  can be readily examined from hysteresis curves  60 ,  62 . Considering first hysteresis curve  60 , a field having half the strength of coercivity  64  creates a movement along hysteresis loop  60  from remanent magnetic strength  66  to point  72 . Since point  72  is still high on the knee of hysteresis curve  60 , there will be very little effect on information recorded onto media exhibiting characteristics represented by hysteresis loop  60 .  
         [0027]    The effect of having a first tape side with a recorded region emitting a magnetic field placed adjacent to a second tape side susceptible to that field is reduced by magnetic material exhibiting characteristics resulting in hysteresis loop  62 . A field strength of one-half coercivity  68  results in movement along hysteresis curve  62  from remanent magnetic strength  70  to point  74  well before the knee in hysteresis loop  62 . Thus, virtually no change will occur to data recorded on a tape surface exhibiting characteristics represented by hysteresis loop  62 .  
         [0028]    One or more of several parameters may be limited to improve the characteristics of a magnetic coating so as to result in a hysteresis loop more similar to plot  62  than to plot  60 . First, the coercivity may be set at at least 1800 Oe. Second, the magnetic coating may be chosen to have a magnetic strength not greater than 6 memu/cm 2 . Remanence is related to remanent magnetic strength by the magnetic coating thickness. Limiting the magnetic coating to a thickness not greater than 120 nm lowers the magnetic strength to that shown in hysteresis loop  62 . Improvements in performance may also be achieved by limiting the remanence to not more than 600 memu/cm 3 .  
         [0029]    Referring now to FIG. 3, an M-H graph illustrating calculation of squareness according to an embodiment of the present invention is shown. Improvements in performance are also achieved by increasing the squareness of the hysteresis loop. As illustrated in generalized hysteresis plot  80 , modifying characteristics of a magnetic media such that knee  82  moves closer in the horizontal direction to coercivity  84  has the effect of bringing angle  86  closer to 90°. Increasing the squareness results in decreasing the effect of a field strength at one-half coercivity  84 , indicated by  88 , by shifting point  88  higher on hysteresis loop  80 .  
         [0030]    One measure of squareness is the coercive squareness CS, as expressed in Equation (1).  
             CS   =         aH   c     -     M   r         aH   c               (   1   )                               
 
         [0031]    The value a is the slope of hysteresis curve  80  at coercivity  84 . The value H c  is coercivity  84 . The value M r  is the remanence of the magnetic coating at zero applied field, indicated by  90 . Preferably, the coercive squareness should be at least 0.7.  
         [0032]    Another measure of squareness is the remanent squareness, RS, as expressed in Equation (2).  
             RS   =       M   r       M   s               (   2   )                               
 
         [0033]    The value M r  is remanence  90 . Saturation magnetization, M s , is the maximum magnetization attained at a very high applied magnetic field (H&gt;H c ). Measurement may be read by running a horizontal (zero slope) line back to the M axis. Preferably, the remanent squareness should be at least 0.7.  
         [0034]    Referring now to FIG. 4, a schematic diagram illustrating manufacture of a double sided tape according to an embodiment of the present invention is shown. Substrate  40  may comprise any flexible material or coating such as polyethylene teraphalate (PET), polyethylene napthalate (PEN), ARAMID, PbO, and the like. Substrate  40  is pulled past first coating head  100  in direction  102 . First coating head  100  injects under-layer  104  onto substrate  40  and magnetic coating  106  onto under-layer  104 . Under-layer  104  provides a smooth surface onto which magnetic coating  106  may be deposited. Magnetic coating  106  comprises magnetic particles mixed with a polymeric binder. Preferably, the magnetic particles are filtered to include particles of similar size. Similarly sized particles improve the squareness of hysteresis loop  80 . Substrate  40  is also pulled past second coating head  108  which deposits under-layer  104  and magnetic coating  106  on second side  38 . A coating process disclosing suitable coating heads for manufacturing double sided magnetic tape  20  is described in U.S. Pat. No. 5,069,934 to Chino et al., which is incorporated by reference herein.  
         [0035]    Referring now to FIG. 5, a schematic diagram illustrating an alternative manufacture of a double sided tape according to an embodiment of the present invention is shown. Substrate  40  is pulled in direction  120  through a vacuum environment past magnetic film material sources  122 ,  124 . Sources  122 ,  124  expel magnetic material through a vapor deposition method such as sputtering, evaporation, or the like. First magnetic film material source  122  deposits magnetic film  126  on tape first side  34 . Similarly, second magnetic film material source  124  deposits magnetic film  128  on tape second side  38 . Magnetic properties of films  126 ,  128  depend on grain size distribution and film thickness. These parameters may be controlled by magnetic material composition, the speed of substrate  40  past sources  122 ,  124 , the use of seed layers such as chromium on tape first side  34  and tape second side  38 , deposition parameters, and the like.  
         [0036]    While embodiments of the invention have been illustrated and described, it is not intended that these embodiments illustrate and describe all possible forms of the invention. Rather, the words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the invention.