Abstract:
The embodiments disclosed generally relate to an STO structure for a magnetic head. The STO structure has an FGL having a greater thickness than the SPL. The SPL may have multiple layers. In one embodiment, a MAMR head comprises a main pole; a trailing shield; and an STO coupled between the main pole and the trailing shield. The STO includes: a first magnetic layer having a first thickness; a non-magnetic spacer layer coupled to the first magnetic layer; and a second magnetic layer having a second thickness and coupled to the non-magnetic spacer layer, wherein the first thickness is greater than the second thickness, wherein a current is charged from the first magnetic layer to the second magnetic layer, and wherein a vertical magnetic anisotropy field of the second magnetic film is less than 0 kOe.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    Embodiments disclosed herein generally relate to the structure of a vertical recording head used in a magnetic disk device. 
         [0003]    2. Description of the Related Art 
         [0004]    Microwave-assisted magnetic recording (MAMR) has been studied in recent years as a recording method for improving surface density. In MAMR, exerting a magnetic field by a main pole applies an AC field from a spin-torque oscillator (STO) to a medium. Applying an AC field to a medium reduces the coercivity of the medium, which facilitates high-quality recording. Therefore, an important issue for MAMR is to develop an STO which generates a sufficiently large AC field. 
         [0005]    With the STO structure  100  shown in  FIG. 1 , the STO  100  comprises a field generation layer (FGL)  102  for generating an AC field, a spacer  104 , and a spin polarization layer (SPL)  106  for transmitting spin-polarized torque. The STO  100  is disposed between the trailing shield  108  and main pole  110  with a cap layer  112  and underlayer  114  present as well. A material having strong vertical anisotropy energy is used for the SPL  106 . The STO  100  is also charged by a current from the SPL  106  toward the FGL  102 . During this charging, a spin torque oriented in the same direction as the magnetization of the FGL  102  acts on the magnetization of the SPL  106 , and a spin torque oriented in the antiparallel direction to the magnetization of the SPL  106  acts on the magnetization of the FGL  102 . Because a perpendicular field is applied to the STO  100 , the magnetization of the SPL  106  is stable vertically. The magnetization of the FGL  102 , however, oscillates in a state having a large in-plane component. Oscillation of the STO  100  in this structure is called T-mode oscillation because the SPL  106  and the FGL  102  oscillate in a T-shape. 
         [0006]    A different STO structure  200  is shown in  FIG. 2  where the STO  200  comprises an FGL  102  for generating an AC field, a spacer  104 , and an SPL  106  for transmitting a spin-polarized torque. The STO  200  is disposed between the trailing shield  108  and main pole  110  with a cap layer  112  and underlayer  114  present as well. The points of difference from  FIGS. 1 and 2  otherwise are that the magnetization of the SPL  106  is effectively oriented in the in-plane direction of the film, and both the FGL  102  and the SPL  106  oscillate. Specifically, a current is charged from the FGL  102  toward the SPL  106 , and a structure is used in which the SPL  106  has a thin film thickness and a vertical anisotropy field of about several kOe such that the anisotropy field of the SPL  106  is effectively zero. Because the SPL  106  receives a spin torque in the antiparallel direction to the FGL  102  and the FGL  102  receives a spin torque in the parallel direction to the SPL  106  when a current is charged from the FGL  102  to the SPL  106  in this structure, the SPL  106  and the FGL  102  readily oscillate together in-plane, which can generate a high AC field. This structure has the useful feature for high-speed transfer recording that the FGL  102  inverts quickly because inversion of the magnetization of the SPL  106  is not delayed by switching the polarity of the write head field. Oscillation of the STO  200  in this structure is called AF-mode oscillation because the SPL  106  and the FGL  102  oscillate while maintaining an antiparallel state. 
         [0007]    The most important feature demanded of an STO is to generate a high AC field. For this purpose, increasing the spin torque acting on the FGL is effective. Since the size of the spin torque is inversely proportional to the density of the current to the STO, increasing the application current obtains higher AC field strength. Too high a charging current, however, increases the temperature of the STO, which increases the probability of failure. Therefore, there is a demand for development of an STO film capable of generating a high AC field by as low a current as possible. 
       SUMMARY OF THE INVENTION 
       [0008]    The embodiments disclosed herein generally relate to an STO structure for a magnetic head. The STO structure has an FGL having a greater thickness than the SPL. The SPL may have multiple layers. 
         [0009]    In one embodiment, a MAMR head comprises a main pole; a trailing shield; and an STO coupled between the main pole and the trailing shield. The STO includes: a first magnetic layer having a first thickness; a non-magnetic spacer layer coupled to the first magnetic layer; and a second magnetic layer having a second thickness and coupled to the non-magnetic spacer layer, wherein the first thickness is greater than the second thickness, wherein a current is charged from the first magnetic layer to the second magnetic layer, and wherein a vertical magnetic anisotropy field of the second magnetic film is less than 0 kOe. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    So that the manner in which the above recited features can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments. 
           [0011]      FIG. 1  is a schematic illustration of a prior art T-mode STO structure. 
           [0012]      FIG. 2  is a schematic illustration of a prior art AF-mode STO structure. 
           [0013]      FIG. 3  is a schematic illustration of a AF-mode STO structure according to one embodiment. 
           [0014]      FIG. 4  is a graph illustrating the AC field vs. STO current. 
           [0015]      FIG. 5  is a schematic illustration of the magnetization configuration for a conventional STO. 
           [0016]      FIG. 6  is a schematic illustration of the magnetization configuration for an STO according to one embodiment. 
           [0017]      FIG. 7  is a graph illustrating the FGL angle vs. STO current. 
           [0018]      FIG. 8  is a graph illustrating the SPL angle vs. STO current. 
           [0019]      FIG. 9  is a graph illustrating a single domain ratio of SPL vs. STO current. 
           [0020]      FIG. 10  is a schematic illustration of a MAMR head structure according to one embodiment. 
           [0021]      FIG. 11  is a schematic illustration of a MAMR head structure according to another embodiment. 
           [0022]      FIG. 12  is a graph illustrating the AC field vs. Hk of SPL. 
           [0023]      FIG. 13  is a schematic illustration of a MAMR head structure according to another embodiment. 
       
    
    
       [0024]    To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. It is contemplated that elements disclosed in one embodiment may be beneficially utilized on other embodiments without specific recitation. 
       DETAILED DESCRIPTION 
       [0025]    In the following, reference is made to embodiments of the invention. However, it should be understood that the invention is not limited to specific described embodiments. Instead, any combination of the following features and elements, whether related to different embodiments or not, is contemplated to implement and practice the invention. Furthermore, although embodiments of the invention may achieve advantages over other possible solutions and/or over the prior art, whether or not a particular advantage is achieved by a given embodiment is not limiting of the invention. Thus, the following aspects, features, embodiments and advantages are merely illustrative and are not considered elements or limitations of the appended claims except where explicitly recited in a claim(s). Likewise, reference to “the invention” shall not be construed as a generalization of any inventive subject matter disclosed herein and shall not be considered to be an element or limitation of the appended claims except where explicitly recited in a claim(s). 
         [0026]      FIG. 3  shows a schematic view of an STO structure  300  according to one embodiment. The STO  300  is arranged between the main pole  110  and the trailing shield  108  of a magnetic head, but structural parts other than the STO  300  have been omitted. The STO  300  comprises a first magnetic layer (SPL  106 ) having a negative magnetic anisotropy axis vertical to the film surface, a nonmagnetic spacer  104  (anti-ferro coupling spacer), and a magnetic layer (FGL  102 ) effectively having a plane of easy magnetization in the film surface. The structure of the embodiments disclosed is an STO  300  which produces AF-mode oscillation, in which a current charges the STO  300  from the FGL  102  to the SPL  106 , and the SPL  106  has a thinner film thickness as shown by arrow “B” than the FGL  102  as shown by arrow “A”. The greatest feature of the embodiments disclosed is that the vertical magnetic anisotropy field of the SPL  106  is less than 0 kOe. High AC field strength is also obtained by the following structure. The vertical magnetic anisotropy field of the SPL is −16 kOe to less than 0 kOe. The SPL comprises the following materials: [Fe a /Co b ] n  multilayer film (a/b=0.25-4, a+b=1.5-40 A); Co 1-x Ir x  (x=8-38 at %); and Fe 1-x C x  (x=1.5-8 at %). Additionally, the materials described in Tables 1, 2, 3, and 4 may be used. The film thickness of the SPL is between 0.6 nm to 4 nm. The material of the FGL is a single layer or a laminate structure containing a CoFe alloy. 
         [0027]      FIG. 4  shows the relationship between AC field strength and application current in an AF-mode STO structure using CoFe in the FGL, and  FIG. 5  shows the state of magnetization of the FGL and the SPL, as viewed from the side opposite the medium, when the charging current is 4, 8, and 18 mA. The state of magnetization is the result of a micromagnetic simulation numerical calculation.  FIG. 4  reveals that increasing the current charging the STO increases AC field strength, but too high a charging current attenuates AC field strength. Although a charging current to the STO of 4 mA generates hardly any AC field because the in-plane component of magnetization of the FGL and the SPL is small, a charging current of 8 mA increases the in-plane component and generates an AC field. A charging current of 18 mA, however, reduces the AC field strength because the state of magnetization exhibits multi domains due to supplying too much spin torque to the FGL and the SPL on the spacer boundary side. Therefore, it is demanded that an STO having an AF-mode structure have a capacity to generate an AC field by as low an electrodynamic force as possible, and minimize multi domains of the FGL and the SPL even when a high current is applied. The disclosed embodiments solve these problems by providing an STO capable of generating a high AC field strength by a low current. Therefore, one key to improving the strength of the AC field generated by the STO is to equalize the size of the spin torque inside the FGL. Making the size of the spin torque acting in the FGL of an STO uniform can provide an STO obtaining high AC field strength. 
         [0028]      FIG. 4  shows results of assessing the AC field of an MAMR head on which the STO has been mounted. The STO  300  is the structure shown in  FIG. 3 , and the SPL  106  has a Bs of 2.35 T and an Hk of −7 kOe. The SPL material is the [Co 3 /Fe 7 ] 3  described in Table 1 below. The Co/Fe ratio is a/b=0.25˜0.4 and the period is a+b=1.5˜40 (A). 
         [0000]    
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Material 
                 Bs (T) 
                 Ms (emu/cc) 
                 Hk (kOe) 
                 Ku (10 −6  erg/cc) 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 [Co 2 /Fe 3 ] 3   
                 2.32 
                 1846 
                 −3.0 
                 −2.8 
               
               
                 [Co          /Fe          ] 3   
                 2.35 
                 1868 
                 7.0 
                 6.5 
               
               
                 [Co 4 /Fe 6 ] 3   
                 2.35 
                 1868 
                 −10.0 
                 −9.3 
               
               
                 [Co 6 /Fe 4 ] 3   
                 2.34 
                 1865 
                 −12.0 
                 −11.2 
               
               
                 [Co          /Fe 3 ] 3   
                 2.30 
                 1829 
                 −9.0 
                 −8.2 
               
               
                 [Co          /Fe 2 ] 3   
                 2.22 
                 1770 
                 −7.0 
                 −6.2 
               
               
                   
               
               
                             indicates data missing or illegible when filed 
               
             
          
         
       
     
         [0029]    This material is a laminated structure of seven periods of 3-Å Co and 8-Å Fe, and has a film thickness of 3 nm. As is clear from  FIG. 4 , the structure disclosed, in which a negative vertical anisotropy material is applied for the SPL  106 , reduces the current required to generate AC field strength to 2 mA from the 4 mA of a structure using a positive vertical magnetic anisotropy material having an Hk of 5 kOe. Under the conditions that a large current of about 10 mA has been applied, the AC field strength achievable by the structure is about 320 Oe, or greatly improved over about 230 Oe by a conventional structure. 
         [0030]    The reason that applying a negative vertical anisotropy field material for the SPL  106  improves AC field strength will be indicated hereinafter.  FIG. 6  shows the state of magnetization of the SPL  106  and the FGL  102  when the charging current to the STO  300  of the example is 4, 8, and 14 mA, and  FIGS. 7 and 8  show the dependency of the angle of magnetization of the FGL  102  and the SPL  106  on the STO current in the example of the structure and an example of a conventional structure. The angle of magnetization of the FGL  102  and the SPL  106  is 90 deg vertically to the plane and 0 deg in the in-plane direction. Therefore, AC field strength becomes stronger as the angle of magnetization of the FGL approaches 0 deg. First, when the example of  FIG. 6  is compared to the state of magnetization of the SPL  106  in the conventional example of  FIG. 5 , the example has a larger in-plane component of magnetization for both the FGL  102  and the SPL  106 . This means that the example of the disclosed embodiments can generate an AC field by a low current. Because AF-mode oscillation occurs, in which the FGL  102  and the SPL  106  oscillate in an antiparallel state, the magnetization of the SPL  106  must first collapse in-plane. As is clear from the angle of magnetization of the SPL  106  in  FIG. 8 , the angle of magnetization of the SPL  106  is small even when the STO  300  current is small because the SPL  106  in the example disclosed has a negative vertical anisotropy field. As shown in  FIGS. 7 and 8 , as the angle of magnetization of the SPL  106  becomes smaller, the angle of magnetization of the FGL  102  also becomes smaller, and an AC field is generated. Therefore, the example of the disclosed embodiments can generate an AC field even by a low application current to the STO  106 . 
         [0031]    Another advantage of applying a negative vertical anisotropy field material for the SPL  106  is that magnetization of the SPL  106  resists become multi domains because the SPL  106  is bound in-plane by a negative vertical anisotropy field even when the charging current to the STO  300  is large and the SPL  106  receives a spin torque.  FIG. 9  shows the dependency of the single domain ratio of the SPL  106  on the STO  300  current in an example of the structure and an example of a conventional structure. The single domain ratio of the SPL  106  is an indicator of uniformity of magnetization. The FGL  102  becomes more stable as the single domain ratio of the SPL  106  increases, and therefore a higher AC field strength can be generated. As is clear from  FIG. 9 , where the single domain ratio of the SPL  106  in the conventional example is significantly decreased by increasing the charging current to the STO, increasing the STO current in the example disclosed produces little reduction in the single domain ratio. Therefore, the example of the disclosed embodiments can minimize the FGL  102  becoming multi domains, which causes the SPL  106  to become multi domains, even when the STO current is high, and as a result, can realize high AC field strength. 
         [0032]      FIG. 10  shows a detail view of an example of the structure according to one embodiment. A magnetic recording and reproducing head  1000  comprises a recording head (writer)  1002  and a reproducing head (reader)  1004 . The reproducing head  1004  must be able to reproduce information recorded on a magnetic recording medium  1006 . The recording head  1002  comprises an STO  1008  for generating an AC field, a main pole  1010  for generating a recording head field, a coil  1012  for exciting a magnetic field in the main pole  1010 , and a trailing shield  1014 . Although not shown in  FIG. 10 , a side shield may be disposed on the outside of the main pole  1010  in the track width direction. Although not a part of the structure of the disclosed embodiment, a magnetic recording medium  1006  is shown in the drawing for reference. The structure of the STO  1008  is the structure shown in  FIG. 3 , and the materials of the SPL  106  may be selected arbitrarily from among the materials described in Tables 1, 2, 3, and 4. Table 2 has a Co/Fe ratio of a/b=0.25˜0.4 and a period of a+b=1.5˜40 (A). Table 3 shows Co 1-x Ir x  where x is 8˜38 atomic percent. Table 4 shows Fe 1-x C x  where x=1.5˜8 atomic percent. 
         [0000]    
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                 Material 
                 Bs (T) 
                 Ms (emu/cc) 
                 Hk (kOe) 
                 Ku (10 −6  erg/cc) 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 [Co          /Fe          ] 20   
                 2.40 
                 1910 
                 −2.6 
                 −2.5 
               
               
                 [Co 7.5 Fe 2.5 ] 4   
                 2.33 
                 1854 
                 −7.0 
                 −6.5 
               
               
                 [Co          /Fe          ] 4   
                 2.35 
                 1870 
                 −10.0 
                 −9.4 
               
               
                 [Co          /Fe          ] 2   
                 2.26 
                 1798 
                 −4.0 
                 −3.6 
               
               
                 [Co 10 /Fe 10 ] 2   
                 2.20 
                 1751 
                 −2.5 
                 −2.2 
               
               
                 [Co 15 /Fe 15 ] 2   
                 2.08 
                 1655 
                 −1.2 
                 −1.0 
               
               
                 [Co 20 /Fe 20 ] 1   
                 2.04 
                 1623 
                 −0.8 
                 −0.6 
               
               
                   
               
               
                             indicates data missing or illegible when filed 
               
             
          
         
       
     
         [0000]    
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 3 
               
               
                   
               
               
                 Material 
                 Bs (T) 
                 Ms (emu/cc) 
                 Hk (kOe) 
                 Ku (10 −6  erg/cc) 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Co 3 Ir          (at %) 
                 1.51 
                 1200 
                 0.0 
                 0.0 
               
               
                 Co 15 Ir 25 (at %) 
                 1.38 
                 1100 
                 −7.3 
                 −4.0 
               
               
                 Co 22 Ir          (at %) 
                 1.13 
                 900 
                 −13.3 
                 −6.0 
               
               
                 Co          Ir 66 (at %) 
                 0.88 
                 700 
                 −10.0 
                 −3.5 
               
               
                 Co          Ir 62 (at %) 
                 0.63 
                 500 
                 −8.0 
                 −2.0 
               
               
                   
               
               
                             indicates data missing or illegible when filed 
               
             
          
         
       
     
         [0000]    
       
         
               
               
               
               
               
             
           
               
                 TABLE 4 
               
               
                   
               
               
                 Material 
                 Bs (T) 
                 Ms (emu/cc) 
                 Hk (kOe) 
                 Ku (10 −6  erg/cc) 
               
               
                   
               
             
             
               
                 Fe 93.5 C 1.5  (at %) 
                 2.10 
                 1671 
                 −1.6 
                 −1.3 
               
               
                 Fe 96 C 4  (at %) 
                 2.10 
                 1671 
                 −2.2 
                 −1.8 
               
               
                 Fe 94 C 4  (at %) 
                 2.10 
                 1671 
                 −3.1 
                 −2.6 
               
               
                 Fe 92 C 8  (at %) 
                 2.10 
                 1671 
                 −2.8 
                 −2.3 
               
               
                   
               
             
          
         
       
     
         [0033]    The recording head in the example of a structure will be described in detail hereinafter. The following structural example is one example, and the effects of the disclosed embodiments are not specifically limited with respect to features other than the SPL  106 . The STO  1008  comprises an under layer  114 , an SPL  106 , a nonmagnetic spacer  104 , an FGL  102 , and a cap layer  112  in this order from the main pole  1010  side. In the present structural example, the under layer  114  is 2-nm Ta, the cap layer  112  is 2-nm Cr, and the nonmagnetic spacer  104  is 3-nm Cu. The under layer  114 , the cap layer  112 , and the nonmagnetic spacer  104  may be conductive nonmagnetic materials, which may be single metals such as Ta, Cr, Cu, Pt, Ag, Rh, or Ru, or laminated structures. The film thicknesses may be set arbitrarily so as to obtain high characteristics of the magnetic recording head. The FGL  102  in the example is Co 50 Fe 50  and has a film thickness of 10 nm. A material having high saturation magnetization is preferably used for the FGL  102 ; for example, a CoFe alloy or the like may be used. A so-called Heusler material having high spin polarizability may be used, and Co/Fe multilayer film or Co/Ni, Co/Pd, Co/Pt, Fe/Pt, or the like having positive and negative vertical magnetic anisotropy fields may be used. A combination of these materials may also be used. The film thickness of the FGL  102  is preferably about 4 nm or greater from the standpoint of obtaining high AC field strength. The track width and the element height of the STO  1008  are both 40 nm. The main pole  1010  is a CoFe alloy having an Ms of 2.4 T, a track width of 60 nm, and a film thickness of 300 nm. The trailing shield  1014  is an NiFe alloy having an Ms of 1.2 T. The geometrical dimensions of these STO  1008  and recording head parts are not specifically limited, and may be designed arbitrarily so as to obtain high field strength and a high field gradient from the STO  1008  and the recording head  1002 . 
         [0034]    Specifically, the SPL  106  of the present structural example may have the structure shown in  FIG. 11 . The SPL  106  is [Co 3 /Fe 7 ] 3  described in Table 1. This material is a laminated structure of seven periods of 3-Å Co and 8-Å Fe, and has a film thickness of 3 nm. The Bs is 2.35 T, and the Hk is −7 kOe. Using such a structure obtains effects such as described above. 
         [0035]    The optimum range of Hk of the SPL  106 , which is a feature of the structure, will be described using the relationship between AC field strength and the Hk of the SPL  106  shown in  FIG. 12 . The AC field strength starts to increase as the Hk of the SPL  106  becomes about 0 kOe or lower and reaches a peak at about −7 kOe, after which, the AC field strength drops as the Hk of the SPL  106  decreases further.  FIG. 12  reveals that keeping the Hk of the SPL  106  in a range of less than 0 kOe and −17 kOe or greater obtains an effect improving the AC field. The reason that the AC field strength drops when the Hk of the SPL  106  is too low is that too low an Hk produces too strong a force binding the magnetization of the SPL  106  in-plane, which tends to stop oscillation of the SPL  106  and effectively reduces the spin torque acting on the FGL  102 . The optimum range of the Hk of the SPL  106  does not greatly vary even when conditions differ, such as the material and film thickness of the FGL  102 , and the film thicknesses of the nonmagnetic spacer and the SPL  106 . Therefore, keeping the Hk of the SPL  106  in a range of less than 0 kOe and −17 kOe or greater obtains an AC field improving effect. 
         [0036]    The laminated film of [Co a /Fe b ], Co 1-x Ir x  alloy, and Fe 1-x C x  alloy described in Tables 1, 2, 3, and 4 can be used as materials for realizing this optimum Hk of the SPL  106 . The composition ratio a/b of Co and Fe in the laminated film of [Co a /Fe b ] must be in a range of 0.25 to 0.4, and the period (a+b) must be in a range of 15 to 40 Å. Tables 1 and 2 show examples of structures in which a Co/Fe multilayer film obtains the required Hk. The composition ratio of Ir in the Co 1-x Ir x  alloy must be 8-38%, and the composition ratio of C in the Fe 1-x C x  alloy must be 1.5-8%. Applying these materials for the SPL can keep the Hk of the SPL in a range of less than 0 kOe and −17 kOe, which improves AC field strength. 
         [0037]      FIG. 13  shows another example of the structure according to one embodiment. The only difference between the present structural example and the structure shown in  FIGS. 3 and 8  is that the lamination order of the SPL  106 , the spacer  104 , and the FGL  102  of the STO  1302  differs. Where the lamination order of the STO  1302  in the structure shown in  FIGS. 3 and 8  was SPL  106 , Cu, and FGL  102  in this order from the main pole  1010  side, the lamination order in the structure shown in  FIG. 13  is FGL  102 , spacer  104 , and SPL  106  in this order from the main pole  1010  side. Both structures have an equivalent effect improving the AC field, by making the Hk of the SPL  106  negative. The FGL  102  can be brought near the main pole  1010  in the present structural example, which has the characteristic of obtaining a high effect improving the effective recording field by applying an AC field in a location having high recording field strength. 
         [0038]    By utilizing an STO having an SPL with a smaller thickness than the FGL, the STO is capable of generating a high AC field by as low a current as possible. 
         [0039]    While the foregoing is directed to embodiments, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.