Patent Publication Number: US-6985318-B2

Title: Method and apparatus for precessional switching of the magnetization of storage medium using a transverse write field

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of provisional patent application Ser. No. 60/386,774 entitled “Precessional Switching of the Magnetization of a Storage Medium with a Transverse Write Field”, filed on Jun. 6, 2002, the entire disclosure of which is incorporated by reference herein. 
    
    
     FIELD OF THE INVENTION 
     The present invention is directed toward magnetic recording processes and, more particularly, toward a magnetic recording process utilizing a write field applied transverse to the magnetization of the recording medium. 
     BACKGROUND OF THE INVENTION 
     The ability to increase the storage capacity of magnetic recording media is an on going concern. As the bit areal densities of magnetic recording media continue to progress in an effort to increase the storage capacity of hard disc drives, the physical size of the sensors and writers designed to read and write data from and to the magnetic recording media must correspondingly decrease. As a result of this push to increase the storage capacity of hard disc drives, magnetic transition, i.e., bit, dimensions and, concomitantly, recording head critical features are being pushed below the 100 nm scale. In a parallel effort, in order to make the magnetic recording medium stable at higher areal densities, magnetically harder recording medium materials having a high coercivity are required. The high coercivity of the recording medium helps to ensure the thermal stability of the data recorded thereon. However, a problem with using high coercivity recording media is that the magnetic field from the small recording pole needs to be sufficient to overcome the coercivity of the magnetic recording medium in the disc in order to define the recorded bits along the recording track in the recording medium. 
     Traditionally, writing to a harder recording medium has been achieved by increasing the saturation magnetic flux density, i.e., 4πM s  value, of the magnetic material which makes up the inductive write head, thus bolstering the magnetic field applied to the recording medium. Although there has been some success in the field of materials research to increase the saturation magnetization M s  of write heads, the rate of increase that has been achieved is not significant enough to sustain the annual growth rate of bit areal densities in disc drive storage applications. Further, continued increases in the saturation magnetization of write heads is likely unsustainable as the materials typically used for write heads reach their fundamental limitations. 
     A consequence of higher areal densities in magnetic recording has been an increase in the data rates at which the data is magnetically recorded. Data rates are advancing toward a point where they will reach a giga-hertz (GHz) and beyond. At these high data rates, it becomes increasingly difficult to switch the magnetization of the recording medium using a conventional write field applied anti-parallel to the magnetization direction of the recording medium, i.e., to the recording medium&#39;s easy axis of magnetization. Thus, there is a need in the field of magnetic recording for a recording process capable of switching higher coercivity recording media at increasingly higher data rates. 
     The present invention is directed toward overcoming one or more of the above-mentioned problems. 
     SUMMARY OF THE INVENTION 
     A magnetic recording process is provided according to the present invention whereby the write field is applied perpendicular to the recording medium magnetization direction (easy axis of magnetization) in order to write a bit (magnetic transition) in the recording medium. Specifically, a transverse write field, with a magnitude exceeding a predetermined minimum value, is applied to the recording medium for a duration of time less than a magnetic time scale of the medium, typically on a nanosecond timescale, such that the magnetization of the recording medium switches precessionally to its opposite state. The transverse write field applies the maximum torque to the recording medium magnetization, thus minimizing the energy required to write a magnetic transition (bit). The short time scale of the applied magnetic field makes it possible to extend data rates well beyond present recording technology. The inventive magnetic recording process may be utilized on both longitudinal and perpendicular oriented recording media. 
     The inventive magnetic recording process generally includes the steps of determining an initial magnetization direction of the magnetic recording medium, and selectively applying a magnetic field to the magnetic recording medium along an axis substantially perpendicular to an axis of the initial magnetization direction of the recording medium. The magnetic field is selectively applied for a period of time sufficient to switch the magnetization of the magnetic recording medium from its initial magnetization direction to a final magnetization direction substantially anti-parallel to the initial magnetization direction. Typically, the initial and final magnetization directions will be along an easy axis of magnetization of the magnetic recording medium. 
     In one form, the initial magnetization direction of the magnetic recording medium is compared with the magnetization direction of a bit to be recorded and, if the compared magnetization directions are different, the magnetic field is applied to the magnetic recording medium to precessionally switch the magnetization of the magnetic recording medium from its initial magnetization direction to the final, anti-parallel magnetization direction of the bit to be recorded. If, on the other hand, the compared magnetization directions are the same, no magnetic field will be applied to the magnetic recording medium, such that the magnetic recording medium is left in its initial magnetization direction which is the magnetization direction of the bit to be recorded. Thus, when the compared magnetization directions are the same, no magnetic field is required by the inventive recording process to record a bit. 
     In another form, the magnetic recording media is DC erased prior to magnetically recording information thereon. DC erasing the recording medium ensures that the medium is uniformly magnetized along the data path to be written, thus allowing the initial magnetization direction of the magnetic recording medium to be determined. A selectively applied magnetic field reverses the magnetization of the recording medium where appropriate, and where the DC erased magnetization direction is desired, no magnetic field is applied so that no magnetization switching occurs. 
     A magnetic recording device for magnetically recording information on a magnetic recording medium is also provided according to the present invention. The magnetic recording device includes a main magnetic pole positionable adjacent the magnetic recording medium, and a coil magnetically coupled to the main magnetic pole for developing a magnetic field in the main magnetic pole in a first magnetization direction. In accordance with the present invention, the magnetic recording medium has an easy axis of magnetization along which magnetic transitions, or bits, are recorded. The first magnetization direction of the magnetic field is substantially perpendicular to the magnetic recording medium&#39;s easy axis of magnetization. The magnetic field developed in the main magnetic pole is selectively applied to the magnetic recording medium in the first magnetization direction for a select period of time sufficient to switch the magnetization of the magnetic recording medium from an initial magnetization direction to a final magnetization direction substantially anti-parallel to the initial magnetization direction. The magnetic recording device may further include a controller operatively connected to the coil for selectively energizing the coil to selectively develop the magnetic field in the main magnetic pole. 
     In one form, the controller includes a magnetic read head for determining the initial magnetization direction of the magnetic recording medium, and a comparison circuit receiving the determined initial magnetization direction and the magnetization direction of a bit to be recorded. Based on a comparison of the magnetization directions, the comparison circuits generates an output signal to selectively energize the coil to selectively develop the magnetic field in the main magnetic pole to switch the initial magnetization direction of the magnetic recording medium where appropriate. The output signal, by selectively energizing the coil, generates an appropriate sequence of magnetic field pulses in the main magnetic pole to reverse the initial magnetization direction of the magnetic recording medium where appropriate and, where the initial magnetization direction is desired, the main magnetic pole is left in its quiescent state so that no magnetic switching of the magnetic recording medium occurs. 
     It is an aspect of the present invention to increase the data rate of magnetic recording processes. 
     It is a further aspect of the present invention to increase the storage capacity of hard disc drives. 
     It is yet a further aspect of the present invention to utilize materials having high coercivities as magnetic recording media in magnetic recording processes. 
     It is still a further aspect of the present invention to develop a magnetic recording process capable of switching higher coercivity recording media at increasingly higher data rates. 
     Other aspects and advantages of the present invention can be obtained from a study of the specification, the drawings, and the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic illustration of the precessional switching process according to the present invention; 
         FIG. 2  is a perspective view of a magnetic recording head according to a first embodiment of the present invention; 
         FIG. 3  is a timing diagram of current pulses in accordance with the precessional switching process of the present invention; 
         FIG. 4  is a perspective view of a magnetic recording head according to a second embodiment of the present invention; 
         FIG. 5  is a perspective view of a magnetic recording head according to a third embodiment of the present invention; 
         FIG. 6  is a perspective view of a magnetic recording head according to a fourth embodiment of the present invention; 
         FIG. 7  is a perspective view of a magnetic recording head according to a fifth embodiment of the present invention; 
         FIG. 8  is a perspective view of a magnetic recording head according to a sixth embodiment of the present invention; and 
         FIG. 9  is a perspective view of a magnetic recording head according to a seventh embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention demonstrates that magnetization reversal can be achieved in lithographically defined magnetic elements using sub-nanosecond magnetic field pulses applied along the magnetization hard axis at right angles to the initial magnetization direction of the magnetic elements. In fact, the present invention reveals that the magnetization can be reversed from either of its bi-stable states with a unidirectional, transverse magnetic field pulse. The field pulse need only be applied with enough field strength that the precessional trajectory of the magnetization overshoots the magnetic hard axis (goes beyond 90° from the easy axis of magnetization), while the pulse duration should be short enough that the field turns off just before the magnetization reaches the anti-parallel direction, i.e., Δt&lt;τ π , where τ π  is the time required to precessionally switch the magnetization 180°. The underlying physics of the present invention are expressed by the following Landau-Lifshitz equation, which provides a simple model to describe the dynamics of a single-domain magnetization {right arrow over (M)} in the presence of a magnetic field {right arrow over (H)}. 
                 ⅆ     M   →         ⅆ   t       =         -     μ   o       ⁢   γ   ⁢           ⁢     M   →     ×     H   →       -       α   ⁡     (         μ   o     ⁢   γ            M   →            )       ⁢     M   →     ×       (       M   →     ×     H   →       )     .                 (     Eq   .           ⁢   1     )             
 
     The constants in Eq. 1 are as follows: μ o —the permeability of free space; γ—the gyromagnetic ratio of the media; α—the damping constant of the media. The first term of Eq. 1 describes the precessional motion of the magnetization {right arrow over (M)} about the field {right arrow over (H)}, while the second term of Eq. 1 represents the damping of the precessional motion and ultimately will force the magnetization {right arrow over (M)} to relax along the magnetic field {right arrow over (H)}. For timescales short enough, the precessional motion term of Eq. 1 describes most of the dynamics, as there is no time for significant damping to occur. In magnetic recording processes, the conventional write process is quasi-static and damping term of Eq. 1 will describe the relevant dynamics of the magnetization of the storage medium, where {right arrow over (M)} ultimately relaxes along the effective direction of the write field, i.e., {right arrow over (M)}∥{right arrow over (H)} write , parallel to the easy axis of magnetization of the storage medium. However, it has been found herein that a writing process using a transverse magnetic field has the benefit of applying the field with a maximum torque, T, applied to the magnetization where T=|{right arrow over (M)}∥{right arrow over (H)}|sin θ=(|{right arrow over (M)}×{right arrow over (H)}|), and θ is the angle between {right arrow over (M)} and {right arrow over (H)}. Additionally, it has been found herein that if the magnetic field is applied on a short timescale, such that the magnetization reverses precessionally, then the switching speed will exceed current state-of-the-art data rates. Both of these aspects associated with the precessional switching method described herein minimize the energy needed to reverse the magnetization, and are conducive to extending areal densities and data rates in the field of magnetic recording. 
     The precessional switching process of the present invention is schematically depicted in  FIG. 1 . The magnetization of the recording medium is in an initial state {right arrow over (M)} o  along the recording medium&#39;s easy axis of magnetization, which is shown along the y-axis in  FIG. 1 . A magnetic field pulse {right arrow over (H)}(Δt) is applied perpendicular to the initial magnetization {right arrow over (M)} o  with a sufficient magnitude that the initial magnetization {right arrow over (M)} o  overshoots its hard axis (goes beyond 90° from the easy axis of magnetization). If the damping constant, α, of the recording media is small enough, the precessional overshoot helps to reduce the transverse magnetic field required for switching. If the magnetic field is turned off just before the magnetization precessional trajectory {right arrow over (M)}(t) passes the anti-parallel direction, the final magnetization {right arrow over (M)} f  will be reversed from the initial magnetization {right arrow over (M)} o . While the field pulse {right arrow over (H)}(Δt) is shown in  FIG. 1  as applied along the x-axis, which is a magnetic hard axis of the recording media, the magnetic filed pulse {right arrow over (H)}(Δt) may also be applied along the z-axis, which is another magnetic hard axis of the media, without departing from the spirit and scope of the present invention. The inventive switching process, as outlined above, requires knowing the initial magnetization state to achieve a particular, final magnetization state. 
     The above-outlined inventive method is particularly useful in disc storage recording processes, where the magnetization is that of the magnetic recording medium and the write head delivers the transverse magnetic field pulse. The write field is a spatial and temporal coordination of both a transverse (switching) field and a field parallel to the recording medium&#39;s easy axis of magnetization (set field). An inventive writing process is described herein whereby a transverse magnetic field can be used exclusively to record data to a magnetic storage medium. The duration of the transverse magnetic field pulse, Δt, has a similar role in determining the final magnetization direction as that of the set field. The magnetic pulse duration is a function of the storage medium used, its physical parameters, as well as a function of the intensity of magnetic field pulse from the write head. For exemplary purposes only, it is contemplated herein that a pulse duration Δt on the order of 1 nanosecond may be sufficient to precessionally the magnetization, however, other pulse durations are contemplated in accordance with the parameters previously set forth. Described below are several detailed realizations of the present invention that are by no means exhaustive, but are intended to convey the general idea of the present invention to one of ordinary skill in the art. 
       FIG. 2  illustrates a single-pole inductive writer shown generally at  10 , which incorporates the inventive precessional writing process. The writer  10  includes a main magnetic pole  12 , a magnetic return pole  14 , and a magnetic via  16  connecting the main  12  and return  14  magnetic poles. An electrically conductive magnetizing coil(s)  18  is provided about the magnetic via  16  and is magnetically coupled to the main pole  12  to generate a write flux  20  through the main pole  12 . The write flux  20  flows into a recording medium  22  disposed adjacent the writer  10  at an air bearing surface thereof to write information onto the recording medium  22 . The return pole  14  and magnetic via  16  provide a return path for the flux  20 . 
     The writer  10  shown in  FIG. 2  can deliver a largely perpendicular field to the recording medium  22 , which is a longitudinal media having an easy axis of magnetization  24  parallel to a plane of the media  22 . A magnetically soft underlayer (SUL)  26  is provided underneath the recording medium  22  which has the effect of “pulling” magnetic field  20  through the recording medium  22 , such that the magnetic field  20  is largely perpendicular as it passes through the recording medium  22 . 
     The dashed arrow  28  shown in  FIG. 2  represents the initial magnetization direction {right arrow over (M)} o  associated with a data bit previously recorded in the media  22 . As shown in  FIG. 2 , the magnetic field  20  is applied to the medium  22  along a magnetic hard axis perpendicular to the easy axis of magnetization  24 . The perpendicular field  20  is applied with a magnitude and duration appropriate to reverse the initial magnetization direction  28  of the recorded data bit to the desired final state {right arrow over (M)} f  represented by the solid arrow  30 . As shown in  FIG. 2 , the final magnetization direction  30  is substantially anti-parallel to the initial magnetization direction  28 , with both magnetization directions  28 ,  30  lying along the medium&#39;s easy axis of magnetization  24 . 
     Although the magnetic field  20  generated by the writer  10  can be unidirectional for magnetization reversal, since either field polarity can be generated by such a writer design it is proposed to utilize the write field orientation depicted in  FIG. 2 , where the small, but non-zero, longitudinal field component of the magnetic field  20  is parallel (as opposed to anti-parallel) with the final magnetization direction  30  to further minimize the energy required to write a magnetic transition (bit). For example, if the situation shown in  FIG. 2  were reversed and the initial magnetization direction of the medium  22  was shown at the solid arrow  30  with the final, desired magnetization direction shown at the dotted arrow  28 , the field polarity of the magnetic field  20  would be reversed from that shown in  FIG. 2  (travel counter-clockwise), to ensure that the small, non-zero longitudinal field component of the field  20  is parallel with the final magnetization direction. In order for the magnetization switching to be precessional, the write field  20  is required to be applied on a short timescale, energized by a short timescale current pulse I(Δt), shown at  32 , to effectively create a magnetic footprint in the media  22 . Additionally, the media  22  should be properly engineered to have a small damping constant, α, and to rotate coherently upon application of the transverse switching field  20 . In other words, the individual magnetic grains which make up a recorded bit should all rotate along basically the same path upon application of the transverse switching field  20 . 
     The time dependence of the current pulses required to generate the switching magnetic field is shown in  FIG. 3 . As shown in  FIG. 3 , the approximate time dependence of the current pulses are realizable on a sub-nanosecond timescale with the inventive technology. The current pulses, shown at  34 , should not exceed the duration Δt&gt;τ π , where τ π  is the maximum time to precessionally switch the magnetization of the medium to a substantially anti-parallel direction. As an example, a scenario of writing to a DC erased medium is considered. In a DC erased medium, the initial magnetization of all bits is known and is the same. The clock cycle time, τ clock , which is the inverse of the data rate (GHz), needs to be at least as long as the current pulse duration Δt, as two current pulses  34  of opposite current polarity will be generated every two clock cycles, and thus τ clock ≧Δt. The zero-current time τ o  is given by the equation τ 0 =τ clock −Δt, and the zero-current time To will be dictated by the media switching speeds and data rate of the magnetic recording. Modeling results have indicated that there is no lower bound to the magnetic field duration for precessional switching according to the present invention, only an upper bound. It can thus be assumed that the current pulse  34  duration Δt is at least a fraction of a nanosecond and, likely, considerably less than the clock cycle (τ clock &gt;&gt;Δt). In this case, the magnetic write head would write by making a magnetic footprint in the recording medium, where a recorded bit in the medium would be basically a “snapshot” of the field distribution of the whole magnetic recording head where the field exceeds the coercivity of the recording medium. 
     The inventive writing process described herein has the potential for very high data rates, well in excess of a giga-hertz (GHz) as discussed previously. With this in mind, a writer designed in accordance with the present invention must have a high bandwidth capability. Presently, it is not known to what frequencies the inductive writers shown and described herein can be extended and, thus, it is proposed to use a writer that has the high frequency characteristics appropriate for the inventive recording process described herein. There are various writer designs for either longitudinal or perpendicular magnetic recording that have been proposed and designed to have a very high bandwidth for writing, and would be appropriate to use for the inventive precessional recording concept described herein at frequencies in excess of a giga-hertz. However, for pedagogical purposes only, the present invention described herein is illustrated as utilized in connection with inductive writers, since their operation is well recognized in the field. However, by no means is the present invention intended to be limited to only conventional writer designs, and other writer designs may be utilized without departing from the spirit and scope of the present invention. 
       FIG. 4  illustrates a longitudinal inductive writer, shown generally at  36 , which incorporates the inventive precessional writing process. The writer  36  includes a main magnetic pole  38 , a magnetic return pole  40 , and a magnetic yoke, or via,  42  connecting the main  38  and return  40  magnetic poles. An electrically conductive magnetizing coil  44  is provided about the magnetic via  42  and is magnetically coupled to the main pole  38  to generate a write flux  46  through the main pole  38 . The write flux  46  flows into the magnetic recording medium  48  disposed adjacent the writer  36  at an air bearing surface thereof to write information onto the recording medium  48 . The return pole  40  and magnetic via  42  provide a return path for the flux  46 . 
     The recording medium  48  is a longitudinal recording media having an easy axis of magnetization  50  which lies parallel to a plane of the recording medium  48 . The soft underlayer shown in  FIG. 2  is not provided, and the magnetic field  46  flowing through the recording medium  48  to write a magnetic transition (bit) includes both longitudinal  52  and perpendicular  54  field components. The peak magnitudes of the perpendicular  54  and longitudinal  52  field components are comparable, but the perpendicular field component  54  applies the largest torque to the media  48 . If the magnetic field pulse duration is short enough, the longitudinal field component  52  will not effect the magnetization of the media  48  significantly, and the writing will be precessional as the perpendicular field component  54  will dominate the process. As shown in  FIG. 4 , the perpendicular write field is the perpendicular field component  54  at the trailing edge  56  of the main magnetic pole  38 . The dashed arrow  58  represents the initial magnetization direction associated with a data bit recorded in the medium  48 , and the perpendicular field component  54  is applied with a magnitude and duration appropriate to reverse its direction to the desired final magnetization state represented by the solid arrow  60 . It is proposed herein to use a write field  46  orientation as depicted in  FIG. 4 , where the longitudinal field component  52  is parallel (as opposed to anti-parallel) with the final magnetization direction  60  to further minimize the energy required to write a magnetic transition (bit). For example, if the situation shown in  FIG. 4  were reversed and the initial magnetization direction of the medium  48  was shown at the solid arrow  60  with the final, desired magnetization direction shown at the dotted arrow  58 , the field polarity of the magnetic field  46  would be reversed from that shown in  FIG. 4  (travel counter-clockwise), to ensure that the longitudinal field component  52  is parallel with the final magnetization direction. However, other write field orientations may be utilized without departing from the spirit and scope of the present invention. Additionally, in order for the magnetization switching to be precessional, the write field  46  is required to be applied on a short timescale, energized by a short timescale current pulse I(Δt), shown at  61 , to effectively create a magnetic footprint in the media  48 . 
       FIG. 5  illustrates a single-plane yoke (SPY) inductive writer, shown generally at  62 , for applying a transverse field to a longitudinal recording media in accordance with the precessional recording method according to the present invention. The SPY writer  62  includes a main magnetic pole  64 , a magnetic return pole  66 , and a magnetic via  68  connecting the main  64  and return  66  magnetic poles. An electrically conductive magnetizing coil  70  is provided about the magnetic via  68  and is magnetically coupled to the main pole  64  to generate a write flux  72  through the main pole  64 . The write flux  72  flows into the recording medium  74  disposed adjacent the SPY writer  62  at an air bearing surface thereof to write information onto the recording medium  74 . The return pole  66  and magnetic via  68  provide a return path for the flux  72 . 
     The magnetic recording medium  74  is longitudinal recording media having an easy axis of magnetization  76  which is parallel with the plane of the longitudinal media  74 . The SPY writer  62  has the benefit of applying a largely transverse magnetic field  72  to the magnetization of the media  74  using a low complexity writer design. The magnetic field  72  is applied perpendicular to the magnetization direction of the magnetic transitions recorded along the medium&#39;s easy axis  76 , but with a magnetic field  72  that is largely in the plane of the medium  74 . The dashed arrow  78  represents the initial magnetization direction associated with a data bit previously recorded in the medium  74 . The magnetic field  72  is applied with a magnitude and duration appropriate to reverse the initial magnetization direction  78  to the desired final state magnetization direction represented by the solid arrow  80 , which is substantially anti-parallel to the initial magnetization direction  78 . In order for the media switching to be precessional, the perpendicular write field  72  is applied on a short timescale, energized by a short timescale current pulse I(Δt), shown at  82 , effectively creating a magnetic footprint in the media  74 . 
       FIG. 6  illustrates the SPY writer  62  shown in  FIG. 5  utilized for precessional recording in accordance with the present invention to a perpendicular magnetic recording medium  84 . The perpendicular medium  84  includes an easy axis of magnetization  85  which is substantially perpendicular to the plane of the medium  84 . As shown in  FIG. 6 , the SPY writer  62  has the benefit of applying a largely transverse magnetic field  72  to the magnetization of the media  84  using a low complexity writer design. The magnetic field  72  applied by the SPY writer  62  is applied transverse to the magnetization direction of the magnetic transitions recorded in the media  84 , but with a magnetic field that is largely in the plane of the media  84 . The dashed arrow  86  represents the initial magnetization direction associated with a data bit previously recorded in the medium  84 . The perpendicular magnetic field  72  is applied with a magnitude and duration appropriate to reverse the initial magnetization direction  86  to the desired final state magnetization direction represented by the solid arrow  88 . In order for the media switching to be precessional, the write field  72  is applied on a short timescale, energized by the short timescale current pulse I(Δt), shown at  82 , effectively creating a magnetic footprint in the media  84 . 
     In using the SPY writer  62  to record magnetic transitions in a perpendicular media  84 , there is a field component applied to the initial magnetization direction  86  that is parallel to the magnetization easy axis  85  of the media  84 . The peak magnitudes of the transverse and parallel field components are comparable, but the transverse field component applies the largest torque to the media  84 . If the field pulse duration is short enough, the parallel field component will not effect the magnetization significantly and the writing will be precessional as the transverse field component dominates the process. 
       FIG. 7  illustrates the longitudinal inductive writer  36  shown in  FIG. 4  utilized to record magnetic transitions to a perpendicular magnetic recording medium  90  in accordance with the precessional recording method of the present invention. The perpendicular magnetic medium  90  has an easy axis of magnetization  92  which is perpendicular to the plane of the medium  90 . The magnetic field  46  is applied transverse to the magnetization direction of the magnetic transitions recorded in the media  90 , but with a field that is largely in the plane of the media  90 . The dashed arrow  94  represents the initial magnetization direction associated with a data bit previously recorded in the medium  90 . The perpendicular magnetic field  46  is applied with a magnitude and duration appropriate to reverse the initial magnetization direction  94  to the desired final state magnetization direction represented by the solid arrow  96 . The magnetic field  46  is applied transverse to the magnetization direction of the recorded bits, but with a field that is largely in the plane of the media  90 . In order for the media switching to be precessional, the write field  46  is applied on a short timescale, energized by the short timescale current pulse I(Δt), shown at  61 , effectively creating a magnetic footprint in the media  90 . 
     It should be noted that there is a field component applied parallel to the magnetization easy axis  92  of the media  90 , as well. The peak magnitudes of the transverse and parallel field components are comparable, but the transverse field component applies the largest torque to the media  90 . If the field pulse duration is short enough, the parallel field component will not effect the magnetization significantly, and the writing will be precessional as the transverse field component will dominate the process. It is proposed to use the write field orientation depicted in  FIG. 7 , where the parallel component of the field at the trailing edge  56  is aligned, as opposed to anti-parallel, with the final magnetization direction  96  to further minimize the energy required to write magnetic transitions (bits). For example, if the situation shown in  FIG. 7  were reversed and the initial magnetization direction of the medium  90  was shown at the solid arrow  96  with the final, desired magnetization direction shown at the dotted arrow  94 , the field polarity of the magnetic field  46  would be reversed from that shown in  FIG. 7  (travel counter-clockwise), to ensure that the small, non-zero longitudinal field component of the field  46  is parallel with the final magnetization direction. However, any write field orientation may be utilized without departing from the spirit and scope of the present invention. 
     As previously discussed, the present invention for precessional writing requires knowledge of the initial magnetization orientation of the recording medium to achieve the desired final magnetization direction. This is unlike traditional magnetic recording where an overwrite process is essentially independent of the initial magnetization condition.  FIGS. 8 and 9  illustrate two ways to precessionally write according to the present invention when the initial magnetization conditions need to be established. 
       FIG. 8  illustrates the single-pole inductive writer  10  shown in  FIG. 2  utilized with a controller, shown generally at  98 , for determining the initial magnetization direction of the recording medium  22  and selectively energizing the coil  18  to selectively develop the magnetic field  20  in the main magnetic pole  12 . While the controller  98  is depicted in  FIG. 8  as utilized with the single-pole inductive writer  10 , the controller  98  may be utilized with any writer design for precessionally recording magnetic transitions according to the present invention. 
     As shown in  FIG. 8 , the controller  98  includes a magnetic read head  100  for sensing the initial magnetization orientation direction of the recording medium  22  prior to writing to it. The determined magnetization direction of a previously recorded bit in the magnetic recording medium  22  is sensed by the read head  100  and fed back into the writing process. The reader  100  is positioned at the leading edge of the writer  10  to sense the magnetization orientation of the bit. The reader output, shown at  102 , is fed to a comparison circuit  104  which also receives the data  106  to be recorded in the magnetic recording medium  22 . The reader output data  102  and the to-be-written data  106  are compared by the comparison circuit  104 , which in turn generates an output signal  108  which selectively energizes the coil  18  using current pulses  32  to selectively develop the magnetic field  20  based on the comparison of the reader output  102  and to-be-written data  106 . The recording medium  22  may be magnetized in either of two bi-stable states along the easy axis of magnetization  24 . The two bi-stable states of magnetization represent either logic “1” or logic “0” recorded bits. 
     Basically, three unique outcomes are possible based on the possible initial and final magnetization orientations of the magnetic recording media  22  (±M), where +M represents logic “1” and −M represents logic “0”. If the initial and final magnetizations are determined to be the same, no magnetic field is applied and the sensed magnetization orientation of the previously recorded bit becomes the to-be-written data (initial (+M)=final (+M) or initial (−M)=final (−M), no magnetic field applied). If the initial magnetization is determined to be positive, and the final magnetization is required to be negative, a positive magnetic field pulse is applied as shown in  FIG. 8  (initial (+M)≠final (−M), positive magnetic field pulse applied). Finally, if the initial magnetization is determined to be negative and the final magnetization is required to be positive, a negative field pulse is applied which would be in the direction oppose that shown in  FIG. 8  (initial (−M)≠final (+M), negative magnetic field pulse applied). 
     Since the reader on a conventional head is inactive during the write process, the reader is available during writing to function as the above-described read sensor  100 . Thus, this embodiment of the present invention does not require an additional field sensor, and the level of complexity of the magnetic recording head for precessional recording according to the present invention is simplified. It should be noted, however, that the reader  100  should be properly shielded from the write head  10  so that it can continue to perform during the entire writing process. 
     A further embodiment of the present invention is to precessionally write to a DC erased media.  FIG. 9  illustrates the longitudinal inductive writer  36  shown in  FIGS. 4 and 7  utilized for precessionally writing to a DC erased longitudinal medium  110 . The DC erased longitudinal medium  110  includes an easy axis of magnetization  112  which is parallel with the plane of the medium  110 . As shown in  FIG. 9 , the medium  110  is initially uniformly magnetized along the data path to be written, i.e., DC erased. The initial magnetization states are shown by the dotted arrows  114 . An appropriate sequence of magnetic pulses provided by the writer  36  will reverse the magnetization where appropriate, and where the DC erased orientation  114  is desired, the writer  36  will be left in its quiescent state so that no switching occurs and the initial magnetization  114  becomes the final magnetization, as shown at arrow  116 . It should be noted that the embodiment shown in  FIG. 9  requires a magnetic recording head which can generate a large enough field parallel to the media magnetization to DC erase it. 
     The present describes a method and apparatus for magnetic recording based on precessional switching of the magnetization of the media, which is in contrast to the quasi-static switching employed in conventional magnetic recording. The magnetization of the storage medium can be reserved using a transverse magnetic field applied for a duration of time that is short compared to the clock cycle. A transverse magnetic field applies the maximum torque to the medium magnetization, minimizing the energy required to write a magnetic transition (bit), while the short timescale makes it possible to extend data rates well beyond present recording technology. Additionally, the inventive precessional writing technique and apparatus described herein should make it possible to extend areal densities of hard disc drives well beyond the present state-of-the-art technology. 
     Both the magnitude of the applied transverse magnetic field and the pulse duration Δt can be determined, or calculated, theoretically using the equations provided herein. Alternately, they can be determined using a trial and error approach which will be readily appreciated by one of ordinary skill in the art. For example, the pulse duration Δt may be determined by bringing the write head in contact with the recording media and initially applying a magnetic field to the media for the shortest duration possible. The magnetic field should be at a fixed magnetic field strength starting with the maximum field available from the write head. The duration of the applied field is then increased until the write head writes to the recording media. The pulse duration Δt is then continually increased until the write process is no longer optimum (the write head stops writing or writes the wrong bit, or the writing process takes too long to be consistent with the desired data rate, etc.). This will give a pulse window (minimum and maximum field-pulse time duration) in which to work. The optimum pulse duration Δt should be within this pulse window. 
     Similarly, and for exemplary purposes only, the magnitude of the transverse magnetic field can be determined using the experimental process previously described at different magnetic field strengths (different write currents, different write head designs, different write head materials, etc.). In this manner, both the pulse duration Δt and the magnetic field strength can be optimized for a given recording system. 
     While the present invention has been described with particular reference to the drawings, it should be understood that various modifications can be made without departing from the spirit and scope of the present invention. For example, the current pulse duration to develop the magnetic field pulses may vary depending upon the particular physical parameters of the recording media utilized and the magnetic field intensity from the magnetic recording head. Additionally, the recording medium should be chosen to have a small damping constant, α, and rotate coherently upon application of the transverse magnetic field. However, based on the teachings herein, these particular variables and materials are readily ascertainable by those of ordinary skill in the art.