Abstract:
A laminate structure is disclosed comprising multiple ferromagnetic layers achieving incoherent reversal while maintaining good SNR. A high magnetic moment density, low anisotropy field material may form a thin overlayer deposited over a high-anisotropy media layer. The media layer may have a lower magnetic moment density than the overlayer and have decoupled magnetic grains. A coupling layer may be interposed between the overlayer and the media layer to modulate the exchange there between, thereby reducing the pass-through of noise while still promoting incoherent reversal to achieve reduced write energy requirements.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to laminate magnetic thin films for data recording and more particularly to magnetic thin films having multiple ferromagnetic layers. 
     2. Description of the Related Art 
       FIG. 1  illustrates a typical head and disk system  100  including a magnetic transducer  102  supported by a suspension  104  as it flies above the disk  106 . The magnetic transducer  102 , usually called a “read/write head” or “slider,” may include elements that perform the task of writing magnetic transitions (the write head  108 ) and reading the magnetic transitions (the read head  110 ). The electrical signals to and from the read and write heads  110 ,  108  travel along conductive paths (leads), which are attached to or embedded in the suspension  104 . The magnetic transducer  102  is positioned over points at varying radial distances from the center of the disk  106  to read and write circular tracks. The disk  106  comprises a substrate on which a laminate  120  having multiple layers is deposited. The laminate  120  may include ferromagnetic layers in which the write head  108  records the magnetic transitions in which information is encoded. 
     Extremely small regions, or bits, on the ferromagnetic layers are selectively magnetized in chosen directions in order to store data on the disk  106 . The orientation of the magnetic moments of the magnetized regions is typically longitudinal. That is, the magnetic moments typically point along the plane of the laminate, rather than out of the plane. To increase the amount of data that can be stored on the disks  106  the number of bits per unit area, or storage density, must be increased. 
     As the storage density of magnetic recording disks has increased, the product of the remanent magnetic moment density (M r ) (the amount of magnetic moments per unit volume of ferromagnetic materials) and the magnetic layer thickness t has decreased. Similarly, values of coercivity (Kc) and anisotropy (Ku) have also increased. However, the extent to which M r t may be decreased and K c  and K u  may be increased is limited. 
     To achieve the reduction in M r t, the thickness t of the magnetic layer has been reduced. However, as t is reduced, the magnetic layer exhibits increasing magnetic decay, attributed to thermal activation of small magnetic grains (the superparamagnetic effect). The thermal stability of a magnetic grain is to a large extent determined by K u V, where K u  is the magnetic anisotropy constant of the layer and V is the volume of the magnetic grain. As the layer thickness is decreased, V decreases. If the layer thickness is too thin, the stored magnetic information will no longer be stable at normal disk drive operating conditions. One possible solution to these limitations is to increase the intergranular exchange, so that the effective magnetic volume V of the magnetic grains is increased. However, this approach has been shown to be deleterious to the intrinsic signal-to-noise ratio (SNR) of the magnetic layer. 
     Increasing the values of K c  and K u  increases the amount of energy required to write data to the recording disk. However, write-energy requirements may not exceed the capacity of currently available write heads  108 . The amount of write field required to write to a magnetic film is given by the coercive field H C  which is proportional to the anisotropy field H K  (approximately equal to K u /M S  for longitudinal recording media where M S  is the saturation magnetization). 
     It is known that the write-energy requirements of a high anisotropy field and high coercivity field magnetic layers  130  may be decreased by depositing a layer  132  of thin, high magnetic moment density material with a lower H K . This high moment material is closer to the write head and more effectively couples the write field and for the proper thicknesses and materials choices can achieve “incoherent reversal.” Incoherent reversal results where the high-moment layer changes its orientation in response to an applied field and is no longer collinear with the higher anisotropy layers ( 130 ) and in turn amplifies the “torque,” or reverse field, exerted on the high-anisotropy field layer, causing it to change orientation in response to a weaker applied field than would suffice in the absence of the high-moment layer. 
     The high-moment layer is magnetically “soft” and can more readily change the orientation of its magnetic moment when a write-field is applied compared to the high anisotropy layer. The change in orientation of the high moment layer causes the magnetic moment of the high-anisotropy field layer to change its orientation slightly, due to the direct exchange between the two layers. It is known that for high-anisotropy field materials, the energy required to cause a change in orientation of the magnetic moments is greatest where the applied field is exactly opposite current orientation. Accordingly, the high-moment layer, by inducing the magnetic moment of the high-anisotropy field layer to shift from a direction directly opposed to the write field, reduces the amount of energy required to cause the high-anisotropy layer to reverse. 
     The high-moment layer also enables more effective reading and writing to the laminate by concentrating large number of magnetic moments at the top of the media. It is known that reading and writing performance increases with proximity of the transducer  102  to the media. Accordingly, the high-moment layer, due to its direct exchange coupling with the high-anisotropy layer, effectively places the signal, or stored information, in the uppermost layer of the laminate increasing the read back signal and resolution. 
     Prior systems attempting to achieve the benefits of a high-moment overlayer have significant drawbacks. The high-moment layers tend to have a great deal of intergranular exchange which leads to increased noise and reduced storage density. When the write head  108  applies a field causing the grains in a region  134  to transition, intergranular exchange, will cause the adjoining grains to transition. Accordingly, a larger region  136  will be affected by the write field, thereby increasing the media noise and reducing storage density. SNR is also reduced as writing one bit causes unwanted changes in adjoining bits. 
     In prior systems, this intergranular exchange in the high-moment layer also affects the high-anisotropy layer. The high-anisotropy layer typically also has a low M r t and decoupled grains, which tend to reduce noise due to intergranular exchange. However, imposing a high-moment layer on the high-anisotropy field layer results in the noise of the high-moment layer being passed to the high-anisotropy layer. 
     In view of the foregoing, it would be an advancement in the art to provide a thin film magnetic laminate achieving incoherent reversal while avoiding a reduction in SNR or storage density. It would be an advancement in the art to provide such a film for use in commonly used longitudinal recording media. 
     SUMMARY OF THE INVENTION 
     The present invention has been developed in response to the present state of the art, and in particular achieves incoherent reversal of multilayer magnetic laminates while maintaining or increasing SNR. In some embodiments, a laminate  120  may include a thin overlayer located closest to the magnetic transducer  102 , a media layer located beneath the upper magnetic layer, and an antiferromagnetic slave layer beneath the media layer. A coupling layer may be disposed between the overlayer and the media layer and serve to modulate the magnetic exchange between the upper and lower ferromagnetic layers. An antiferromagnetic coupling layer may be disposed between the lower ferromagnetic layer and the antiferromagnetic slave layer and serve to antiferromagnetically couple the lower ferromagnetic layer and the antiferromagnetic slave layer. 
     The media layer may comprise a magnetic alloy having either a high anisotropy field, decoupled grains, or both. The overlayer may comprise a material having either a high magnetic moment density, a relatively low anisotropy field, or both. The coupling layer may comprise either a ferromagnetically coupling material, a weakly ferromagnetic material, or a paramagnetic material. The coupling layer permits exchange between the overlayer and media layer, however, the exchange may be weaker than direct exchange as when the overlayer is deposited directly onto the media layer. 
     In the illustrated embodiment, the coupling layer is a CoCr alloys or CoRu alloy. CoCr alloys may have the composition s Co 100-x Cr x , where 26&lt;x&lt;40. CoRu alloys may have the composition Co 100-x Ru x , where 25&lt;x&lt;70. The overlayer may be embodied as a CoCrB or CoCr alloy. CoCrB alloys may have the composition CoCr x B y  where 0&lt;x&lt;20 and 0&lt;y&lt;15. CoCr alloys may have the composition Co 100-x Cr x  where 0&lt;x&lt;20. 
     Reference throughout this specification to features, advantages, or similar language does not imply that all of the features and advantages that may be realized with the present invention should be or are in any single embodiment of the invention. Rather, language referring to the features and advantages is understood to mean that a specific feature, advantage, or characteristic described in connection with an embodiment is included in at least one embodiment of the present invention. Thus, discussion of the features and advantages, and similar language, throughout this specification may, but do not necessarily, refer to the same embodiment. 
     Furthermore, the described features, advantages, and characteristics of the invention may be combined in any suitable manner in one or more embodiments. One skilled in the relevant art will recognize that the invention can be practiced without one or more of the specific features or advantages of a particular embodiment. In other instances, additional features and advantages may be recognized in certain embodiments that may not be present in all embodiments of the invention. 
     These features and advantages of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In order that the advantages of the invention will be readily understood, a more particular description of the invention briefly described above will be rendered by reference to specific embodiments that are illustrated in the appended drawings. Understanding that these drawings depict only typical embodiments of the invention and are not therefore to be considered to be limiting of its scope, the invention will be described and explained with additional specificity and detail through the use of the accompanying drawings, in which: 
         FIG. 1  is a schematic representation of one embodiment of a read/write head and recording medium comprising a laminate magnetic thin film; 
         FIG. 2  is an illustration of one embodiment of the layer structure of a laminate magnetic thin film medium, in accordance with the present invention; 
         FIG. 3  is a hysteresis loop of a laminated magnetic thin film medium, in accordance with the present invention; 
         FIGS. 4A-4D  are schematic representations of the various layers of a laminated magnetic thin film, in accordance with the present invention, having the orientation of the magnetic moments of the various layers illustrated for various points on the hysteresis loop; and 
         FIGS. 5A-5D  are plots representing measurements of indicia used to evaluate the performance of a recording medium. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Reference throughout this specification to “one embodiment,” “an embodiment,” or similar language means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases “in one embodiment,” “in an embodiment,” and similar language throughout this specification may, but do not necessarily, all refer to the same embodiment. 
     Furthermore, the described features, advantages, and characteristics of the invention may be combined in any suitable manner in one or more embodiments. One skilled in the relevant art will recognize that the invention can be practiced without one or more of the specific features or advantages of a particular embodiment. In other instances, additional features and advantages may be recognized in certain embodiments that may not be present in all embodiments of the invention. 
     Referring to  FIG. 2 , a laminate  120  may include an overlayer  200 , a media layer  202 , and a coupling layer  204 . In some embodiments, the media layer  202  may be antiferromagnetically coupled to an antiferromagnetic slave layer  206  by means of an antiferromagnetically coupling layer  208 . 
     The overlayer  200  may have a higher magnetic moment density (M r ) than the lower magnetic layer  202 . In some embodiments, the overlayer  200  has a magnetic moment density of approximately 500-1500 emu/cm 3 , whereas the media layer has a magnetic moment density of 100-500 emu/cm 3 . 
     The overlayer  200  may also have a lower magnetic anisotropy field (H k ) than the media layer  202 . In some embodiments, the anisotropy field of the overlayer  200  is less than half the anisotropy field of the media layer  202 . The anisotropies (K u ) of the overlayer  200  and media layer  201  may be identical. However, the anisotropy field H k  (approximately equal to K u /M r ) of the overlayer  200  may be lower than that of the media layer  202  due to its higher M r . In terms of writability, the value H k  typically determines the strength of the magnetic field required to cause a change in orientation. 
     The overlayer may have various embodiments. In one embodiment a CoCrB alloys having the composition CoCr x B y , where 0&lt;x&lt;20 and 0&lt;y&lt;15 is used. In others, a CoCr alloy having the composition Co 100-x Cr x , where 0&lt;x&lt;20 may be used. Various other metals and metal alloys having high magnetic moments and low anisotropies may be used. In the illustrated embodiment the overlayer  200  has a thickness of less than five nanometers. 
     Due to its lower anisotropy field (H k ), the overlayer  200  is magnetically “soft” and the orientation of its magnetic moments is readily changed by an applied field from the write head  108 . The change in orientation of the overlayer  200  causes the magnetic moment of the media layer  202  to change its orientation slightly, due to coupling between the two layers  200 , 202  via the coupling layer  204 . For high-anisotropy materials, the energy required to cause a change in orientation of the magnetic moments is greatest where the applied field is exactly opposite the current orientation. Accordingly, the overlayer  200 , by inducing the magnetic moment of the high-anisotropy media layer  202  to shift from a direction directly opposed to the write field, reduces the amount of energy required to cause the media layer  202  to transition. 
     The overlayer  200  also enables more effective reading and writing to the laminate by concentrating large number of magnetic moments at the top of the laminate  120 . Reading and writing performance increases with the proximity of the transducer  102  to the stored signal. Accordingly, the overlayer  200 , due to its coupling with the media layer  202 , effectively places the signal, or stored information, in the uppermost layer of the laminate. Since the overlayer is a higher moment alloy, the signal originated from an effectively thinner layer than if it were distributed over a much thicker recoding alloy. 
     The media layer  202  is typically made of a high anisotropy field material that may also be chosen for decoupling of the magnetic grains within the media. In the illustrated embodiment, the media layer  202  is a CoPtCrB alloy. Other ferromagnetic alloys having suitable anisotropy fields, decoupling, and magnetic moment densities may also be used for the media layer  202 . 
     The coupling layer  204  may be used to reduce the deleterious effects of a high-moment overlayer  200 . As discussed hereinabove, a high-moment, low anisotropy field material is subject to lateral exchange between magnetic grains leading to increased noise and decreased storage density. A coupling layer  204  may modulate the exchange between the overlayer  200  and the media layer  202  to inhibit the transfer of noise to the media layer  202 . The coupling of the layers  200 , 202  may be weaker than for direct exchange, thereby reducing the transfer of noise from the overlayer  200  to the media layer  202 . Various types of coupling means may be used. For example, the coupling layer  204  may be a nonmagnetic material having a thickness tuned to achieve ferromagnetic coupling. The coupling layer  204  may also be a paramagnetic material or a weakly ferromagnetic layer. 
     The coupling layer  204  may be formed of various materials known to effectively ferromagnetically couple magnetic layers. For example, a ruthenium layer having a thickness tuned to cause ferromagnetic coupling may be used. In the illustrated embodiment, a CoRu alloy or CoCr alloy having a thickness and composition chosen to achieve ferromagnetic coupling are used. CoRu alloys having the composition Co 100-x Ru x , where 25&lt;x&lt;70 may be used. CoCr alloys having the composition Co 100-x Cr x , where 26&lt;x&lt;40 may also be used. The coupling layer may have a thickness of less than four nanometers (nm). 
     The antiferromagnetic slave layer  206  maybe composed of a material suitable for use in antiferromagnetic media, such as CoCr 11 . The antiferromagnetically coupling layer  208  is typically formed of ruthenium having a thickness chosen to achieve antiferromagnetic coupling. 
     The hysteresis loop of  FIG. 3  demonstrates the embodiment of the laminate of  FIG. 2 . The measurements shown in  FIG. 3  reflect a laminate having a coupling layer  204  formed of a CoRu alloy and is measured from large positive fields to large negative fields. The loop reflects the change in the number of magnetic moments having a particular orientation, also referred to as saturation, as a reversing field of increasing magnitude is applied to the laminate. The curve  300  represents the major loop, indicating the magnetization in the presence of an applied field. The curve  302  represents the remanent magnetization of the laminate  120  after the applied field is removed.  FIGS. 4A through 4D  illustrate the orientation of the magnetic moments within each layer  200 , 202 , 206  at points A through D on the hysteresis loop. 
     At point A the magnetic moments of each layer  200 , 202 , 206  are at positive saturation oriented in the same direction in an applied field of 4 kOe. As the applied field is reduced to zero, the antiferromagnetic slave layer  206  reverses as shown in  FIG. 4B  as a result of the antiferromagnetic coupling between itself and the media layer  202 . At point C, approximately 3.5 kOe, the overlayer  200  reverses direction due to the applied field as shown in  FIG. 4C . At point D, the media layer reaches negative saturation, as shown in  FIG. 4D . 
     The remanent curve does not reflect the reversal of the overlayer  200  after point C. To the right of point C, the remanent curve  302  has much higher positive saturation than the major loop  300  because the media layer  202  has not reversed at this point and therefore switches the orientation of the overlayer  200  as soon as the applied field is removed, due to the coupling between the layers  200 , 202 . The remanent curve also reflects the reversal of the antiferromagnetic slave layer  206  when the applied field is removed at point D: the negative saturation of the remanent curve  302  is less than the major loop because the antiferromagnetic slave layer  206  reverses direction in the absence of an applied field due to its antiferromagnetic coupling to the media layer  202 . 
     Referring to  FIGS. 5A-5E , measurements of indicia used to evaluate recording performance clearly indicate improved overall performance through the use of a coupling layer  204  modulating the exchange between the layers  200 , 202 . The values in column  600  represent measurements of a media layer  202  without an overlayer  200  or coupling layer  204 . The values in column  602  represent measurements of a media layer  202  with an overlayer  200  but without a coupling layer  204 . The values in columns  604 - 610  represent measurements of media layers  202  with an overlayer  200  and coupling layers  204  of increasing thicknesses. The thickness of the coupling layer  204  increases from right to left. 
     Referring to  FIG. 5A , it is clear that the M r t of columns  602 - 610  is greater than for the laminate of column  600 , which does not have an overlayer  202  or coupling layer  204 . This demonstrates the composite magnetization of the media increases as the high moment over is added. 
     Referring to  FIG. 5B , smaller values for H c  correspond to smaller write-energy requirements. It is clear from column  602  that the addition of an overlayer  200  reduces H c . It is also clear from columns  604 - 610  that adding a coupling layer  204  and increasing the thickness of the coupling layer  204  further reduce H c . It will be noted that the measurements of  FIG. 6B  are of the effective H c  of the entire laminate  120 . However, the composition of the media layer  202  and the H c  of the media layer  202  are unchanged. Accordingly, the thermal stability of the media layer  202  is not affected, whereas the writability of the laminate  120  improves with the addition of an overlayer  202  and a coupling layer  204 . 
     Referring to  FIG. 5C , use of an overlayer  200  and coupling layer  204  also increases the ability of the laminate  120  to be overwritten. As shown by column  602  an overlayer  200  increases overwrite (OW) performance. As shown by columns  604 - 610 , adding a coupling layer further increases OW with further gains being achieved with increased thickness of the coupling layer  204 . 
     The above mentioned gains in OW performance are all accomplished without a reduction in SNR where a coupling layer  204  is used as shown in  FIG. 5D . Column  602  indicates an increase in noise where an overlayer  202  is added without a coupling layer  204  to modulate the exchange between the overlayer  202  and the media layer  204 . Columns  604 - 610  clearly show that SNR is improved where a coupling layer  204  is interposed between the layers  200 , 202 . 
     The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.