Patent Publication Number: US-2005136291-A1

Title: Magnetic recording medium, recording method and magnetic storage apparatus

Description:
BACKGROUND OF THE INVENTION  
      This application claims the benefit of a Japanese Patent Application No. 2003-422800 filed Dec. 19, 2003, in the Japanese Patent Office, the disclosure of which is hereby incorporated by reference.  
      1. Field of the Invention  
      The present invention generally relates to magnetic recording media, recording methods and magnetic storage apparatuses, and more particularly to a magnetic recording medium provided with a magnetic layer which is exchange-coupled to another layer and is suited for high-density recording, a recording method for recording information on such a magnetic recording medium, and a magnetic storage apparatus which employs such a recording method.  
      2. Description of the Related Art  
      Recently, the recording densities of magnetic recording media have increased rapidly, even at a rate reaching 100% per year. However, in the popularly employed longitudinal (or in-plane) recording system, it is expected that a limit of the longitudinal recording density will be on the order of 100 Gb/in 2 , because of problems associated with thermal stability of the magnetic recording medium. In order to reduce the medium noise in the high-density recording region, the size of crystal grain forming the magnetization unit is reduced, so as to reduce the zigzag of the boundary between the magnetization units, that is, the magnetization transition region. However, when the size of the crystal grain is reduced, the volume forming the magnetization unit decreases, to thereby cause the magnetization to decrease due to thermal instability. Accordingly, in order to achieve a high recording density exceeding 100 Gb/in 2 , it is necessary to simultaneously reduce the medium noise and improve the thermal stability.  
      Magnetic recording media which simultaneously reduce the medium noise and improve the thermal stability have been proposed in Japanese Laid-Open Patent Applications No. 2001-056921 and No. 2001-056924, for example.  FIG. 1  is a cross sectional view showing a part of a proposed magnetic recording medium  100 . The proposed magnetic recording medium  100  shown in  FIG. 1  includes an exchange layer structure provided on a substrate  105 , and a magnetic layer  102  provided on the exchange layer structure. The exchange layer structure is made up of a ferromagnetic layer  101  provided on the substrate  105 , and a nonmagnetic coupling layer  103  provided on the ferromagnetic layer  101 . The ferromagnetic layer  101  and the magnetic layer  102  are exchange-coupled anti-ferromagnetically via the nonmagnetic coupling layer  103 . The effective crystal grain volume becomes the sum of crystal grain volumes of the ferromagnetic layer  101  and the magnetic layer  102  which are exchange-coupled. Consequently, the thermal stability is greatly improved, and the medium noise can be reduced because the crystal grain size can further be reduced. By using the proposed magnetic recording medium  100 , the thermal stability of the recorded (written) bits improve, and the medium noise is reduced, thereby enabling a highly reliable high-density recording.  
      In the proposed magnetic recording medium  100 , the reproduced output is approximately proportional to a difference between the remanent magnetizations of the magnetic layer  102  and the ferromagnetic layer  101 , because the magnetization directions of the magnetic layer  102  and the ferromagnetic layer  101  are mutually antiparallel. Hence, in order to obtain a reproduced output comparable to that obtained by the conventional magnetic recording medium having the magnetic layer with the single-layer structure, the magnetic layer  102  closer to a recording and/or reproducing magnetic head is set thicker than the ferromagnetic layer  101  which is further away from the magnetic head, and also thicker than the conventional magnetic layer having the single-layer structure, if materials having the same composition are used for the magnetic layer  102  and the ferromagnetic layer  101 . However, when the proposed magnetic recording medium  100  has the magnetic layer  102  with such a thickness, there is a possibility of deteriorating the write performances, such as the overwrite performance and the Non-Linear-Transition-Shift (NLTS) performance, due to the increased thickness of the magnetic layer  102 .  
      On the other hand, when a recording magnetic field is applied to the proposed magnetic recording medium  100  from the magnetic head at the time of the recording, the magnetization directions of the magnetic layer  102  and the ferromagnetic layer  101  align in the direction of the recording magnetic field and become mutually parallel. Thereafter, when the magnetic head moves and the recording magnetic field weakens, the magnetization direction of the ferromagnetic layer  101  switches in response to an exchange field of the magnetic layer  102  and the magnetization directions of the ferromagnetic layer  101  and the magnetic layer  102  become mutually antiparallel. However, in a vicinity of a magnetic pole of the magnetic head at a trailing edge along the moving direction of the magnetic head, the behaviors of the magnetic layer  102  and the ferromagnetic layer  101 , such as the switching of the magnetization directions, immediately after switching the direction of the recording magnetic field, become complex due to the exchange field and the demagnetization field of each of the magnetic layer  102  and the ferromagnetic layer  101 . With respect to the magnetic layer  102 , the position, inclination and the like of the magnetization transition region may change and the NLTS performance may deteriorate, particularly due to the magnetic characteristics and the like of the ferromagnetic layer  101 .  
      But if the thickness of the ferromagnetic layer  101  is simply increased to increase the effective crystal grain volume for the purposes of improving the thermal stability, the overwrite performance may deteriorate.  
     SUMMARY OF THE INVENTION  
      Accordingly, it is a general object of the present invention to provide a novel and useful magnetic recording medium, recording method and magnetic storage apparatus, in which the problems described above are suppressed.  
      Another and more specific object of the present invention is to provide a magnetic recording medium, a recording method and a magnetic storage apparatus, which can realize satisfactory thermal stability of written bits, low medium noise and satisfactory write performances.  
      Still another object of the present invention is to provide a magnetic recording medium comprising a first ferromagnetic layer; a first nonmagnetic coupling layer disposed on the first ferromagnetic layer; a second ferromagnetic layer disposed on the first nonmagnetic coupling layer; a second nonmagnetic coupling layer disposed on the second ferromagnetic layer; and a magnetic layer disposed on the second nonmagnetic coupling layer, the first and second ferromagnetic layers being exchange-coupled, the second ferromagnetic layer and the magnetic layer being exchange-coupled, the first and second ferromagnetic layers having magnetizations which are mutually parallel in a state where no external magnetic field is applied thereto, the second ferromagnetic layer and the magnetic layer having magnetizations which are mutually antiparallel in the state where no external magnetic field is applied thereto, the first and second ferromagnetic layers and the magnetic layer respectively having dynamic coercivities Hc 1 ′, Hc 2 ′ and Hc 3 ′ which satisfy a relationship Hc 1 ′&lt;Hc 3 ′≦Hc 2 ′ in a switching time region of a recording magnetic field. According to the magnetic recording medium of the present invention, it is possible to realize satisfactory thermal stability of written bits, low medium noise and satisfactory write performances.  
      A further object of the present invention is to provide a magnetic recording medium comprising a first ferromagnetic layer; a first nonmagnetic coupling layer disposed on the first ferromagnetic layer; a second ferromagnetic layer disposed on the first nonmagnetic coupling layer; a second nonmagnetic coupling layer disposed on the second ferromagnetic layer; and a magnetic layer disposed on the second nonmagnetic coupling layer, the first and second ferromagnetic layers being exchange-coupled, the second ferromagnetic layer and the magnetic layer being exchange-coupled, the first and second ferromagnetic layers having magnetizations which are mutually parallel in a state where no external magnetic field is applied thereto, the second ferromagnetic layer and the magnetic layer having magnetizations which are mutually antiparallel in the state where no external magnetic field is applied thereto, the first ferromagnetic layer being made of a CoCr alloy or a CoCrPt alloy, the second ferromagnetic layer and the magnetic layer being made of a CoCrPt alloy, the first and second ferromagnetic layers and the magnetic layer respectively having Pt contents Pt 1 , Pt 2  and Pt 3  satisfying a relationship Pt 1 &lt;Pt 3 ≦Pt 2 . According to the magnetic recording medium of the present invention, it is possible to realize satisfactory thermal stability of written bits, low medium noise and satisfactory write performances.  
      Another object of the present invention is to provide a magnetic recording medium comprising a first ferromagnetic layer; a first nonmagnetic coupling layer disposed on the first ferromagnetic layer; a second ferromagnetic layer disposed on the first nonmagnetic coupling layer; a second nonmagnetic coupling layer disposed on the second ferromagnetic layer; and a magnetic layer disposed on the second nonmagnetic coupling layer, the first and second ferromagnetic layers being exchange-coupled, the second ferromagnetic layer and the magnetic layer being exchange-coupled, the first and second ferromagnetic layers having magnetizations which are mutually parallel in a state where no external magnetic field is applied thereto, the second ferromagnetic layer and the magnetic layer having magnetizations which are mutually antiparallel in the state where no external magnetic field is applied thereto, a magnetization direction of the magnetic layer switching before a magnetization direction of the second ferromagnetic layer when a recording magnetic field for switching the magnetization direction of the magnetic field is applied to the magnetic recording medium. According to the magnetic recording medium of the present invention, it is possible to realize satisfactory thermal stability of written bits, low medium noise and satisfactory write performances.  
      Still another object of the present invention is to provide a magnetic storage apparatus comprising at least one magnetic recording medium comprising a first ferromagnetic layer, a first nonmagnetic coupling layer disposed on the first ferromagnetic layer, a second ferromagnetic layer disposed on the first nonmagnetic coupling layer, a second nonmagnetic coupling layer disposed on the second ferromagnetic layer, and a magnetic layer disposed on the second nonmagnetic coupling layer, the first and second ferromagnetic layers being exchange-coupled, the second ferromagnetic layer and the magnetic layer being exchange-coupled, the first and second ferromagnetic layers having magnetizations which are mutually parallel in a state where no external magnetic field is applied thereto, the second ferromagnetic layer and the magnetic layer having magnetizations which are mutually antiparallel in the state where no external magnetic field is applied thereto, the first and second ferromagnetic layers and the magnetic layer respectively having dynamic coercivities Hc 1 ′, Hc 2 ′ and Hc 3 ′ which satisfy a relationship Hc 1 ′&lt;Hc 3 ′≦Hc 2 ′ in a switching time region of a recording magnetic field; and a head to record information on and/or reproduce information from the magnetic recording medium. According to the magnetic storage apparatus of the present invention, it is possible to realize satisfactory thermal stability of written bits, low medium noise and satisfactory write performances.  
      Still another object of the present invention is to provide a magnetic storage apparatus comprising at least one magnetic recording medium comprising a first ferromagnetic layer, a first nonmagnetic coupling layer disposed on the first ferromagnetic layer, a second ferromagnetic layer disposed on the first nonmagnetic coupling layer, a second nonmagnetic coupling layer disposed on the second ferromagnetic layer, and a magnetic layer disposed on the second nonmagnetic coupling layer, the first and second ferromagnetic layers being exchange-coupled, the second ferromagnetic layer and the magnetic layer being exchange-coupled, the first and second ferromagnetic layers having magnetizations which are mutually parallel in a state where no external magnetic field is applied thereto, the second ferromagnetic layer and the magnetic layer having magnetizations which are mutually antiparallel in the state where no external magnetic field is applied thereto, the first ferromagnetic layer being made of a CoCr alloy or a CoCrPt alloy, the second ferromagnetic layer and the magnetic layer being made of a CoCrPt alloy, the first and second ferromagnetic layers and the magnetic layer respectively having Pt contents Pt 1 , Pt 2  and Pt 3  satisfying a relationship Pt 1 &lt;Pt 3 ≦Pt 2 ; and a head to record information on and/or reproduce information from the magnetic recording medium. According to the magnetic storage apparatus of the present invention, it is possible to realize satisfactory thermal stability of written bits, low medium noise and satisfactory write performances.  
      A further object of the present invention is to provide a magnetic storage apparatus comprising at least one magnetic recording medium comprising a first ferromagnetic layer, a first nonmagnetic coupling layer disposed on the first ferromagnetic layer, a second ferromagnetic layer disposed on the first nonmagnetic coupling layer, a second nonmagnetic coupling layer disposed on the second ferromagnetic layer, and a magnetic layer disposed on the second nonmagnetic coupling layer, the first and second ferromagnetic layers being exchange-coupled, the second ferromagnetic layer and the magnetic layer being exchange-coupled, the first and second ferromagnetic layers having magnetizations which are mutually parallel in a state where no external magnetic field is applied thereto, the second ferromagnetic layer and the magnetic layer having magnetizations which are mutually antiparallel in the state where no external magnetic field is applied thereto, a magnetization direction of the magnetic layer switching before a magnetization direction of the second ferromagnetic layer when a recording magnetic field for switching the magnetization direction of the magnetic field is applied to the magnetic recording medium; and a head to record information on and/or reproduce information from the magnetic recording medium. According to the magnetic storage apparatus of the present invention, it is possible to realize satisfactory thermal stability of written bits, low medium noise and satisfactory write performances.  
      Another object of the present invention is to provide a recording method for magnetically recording information on a magnetic recording medium by applying a recording magnetic field on the magnetic recording medium, the magnetic recording medium comprising a first ferromagnetic layer, a first nonmagnetic coupling layer disposed on the first ferromagnetic layer, a second ferromagnetic layer disposed on the first nonmagnetic coupling layer, a second nonmagnetic coupling layer disposed on the second ferromagnetic layer, and a magnetic layer disposed on the second nonmagnetic coupling layer, the first and second ferromagnetic layers being exchange-coupled, the second ferromagnetic layer and the magnetic layer being exchange-coupled, the first and second ferromagnetic layers having magnetizations which are mutually parallel in a state where no external magnetic field is applied thereto, the second ferromagnetic layer and the magnetic layer having magnetizations which are mutually antiparallel in the state where no external magnetic field is applied thereto, the recording method comprising the steps of switching a magnetization direction of the magnetic layer; and switching magnetization directions of the first and second ferromagnetic layers by applying the recording magnetic field to make the magnetizations of the first and second ferromagnetic layers mutually parallel and thereafter removing the recording magnetic field. According to the recording method of the present invention, it is possible to realize satisfactory thermal stability of written bits, low medium noise and satisfactory write performances.  
      Still another object of the present invention is to provide a recording method for magnetically recording information on a magnetic recording medium by applying a recording magnetic field on the magnetic recording medium, the magnetic recording medium comprising a first ferromagnetic layer, a first nonmagnetic coupling layer disposed on the first ferromagnetic layer, a second ferromagnetic layer disposed on the first nonmagnetic coupling layer, a second nonmagnetic coupling layer disposed on the second ferromagnetic layer, and a magnetic layer disposed on the second nonmagnetic coupling layer, the first and second ferromagnetic layers being exchange-coupled, the second ferromagnetic layer and the magnetic layer being exchange-coupled, the first and second ferromagnetic layers having magnetizations which are mutually parallel in a state where no external magnetic field is applied thereto, the second ferromagnetic layer and the magnetic layer having magnetizations which are mutually antiparallel in the state where no external magnetic field is applied thereto, the recording method comprising the steps of switching a magnetization direction of the magnetic layer; and satisfying a relationship Hh 3 +HE  3 −Hc 3 ′&gt;Hh 2 +HE 2 −Hc 2 ′&gt;0 when switching a direction of the recording magnetic field, where Hc 2 ′ denotes a dynamic coercivity of the second ferromagnetic layer, Hc 3 ′ denotes a dynamic coercivity of the magnetic layer, HE 2  denotes an exchange field applied to the second ferromagnetic layer due to exchange fields of the first ferromagnetic layer and the magnetic layer, HE 3  denotes an exchange field of the second ferromagnetic layer applied to the magnetic layer, Hh 2  denotes a recording magnetic field at the second ferromagnetic layer, and Hh 3  denotes a recording magnetic field at the magnetic layer. According to the recording method of the present invention, it is possible to realize satisfactory thermal stability of written bits, low medium noise and satisfactory write performances.  
      A further object of the present invention is to provide a magnetic recording medium comprising a first ferromagnetic layer; a first nonmagnetic coupling layer disposed on the first ferromagnetic layer; a second ferromagnetic layer disposed on the first nonmagnetic coupling layer; a second nonmagnetic coupling layer disposed on the second ferromagnetic layer; and a magnetic layer disposed on the second nonmagnetic coupling layer, the first and second ferromagnetic layers being exchange-coupled and having mutually parallel magnetizations, the second ferromagnetic layer and the magnetic layer being exchange-coupled and having magnetizations which are mutually antiparallel, the first and second ferromagnetic layers and the magnetic layer respectively having dynamic coercivities Hc 1 ′, Hc 2 ′ and Hc 3 ′ which satisfy a relationship Hc 1 ′&lt;Hc 3 ′≦Hc 2 ′ in a switching time region of a recording magnetic field. According to the magnetic recording medium of the present invention, it is possible to realize satisfactory thermal stability of written bits, low medium noise and satisfactory write performances.  
      Other objects and further features of the present invention will be apparent from the following detailed description when read in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a cross sectional view showing a part of a proposed magnetic recording medium;  
       FIG. 2  is a cross sectional view showing a part of a first embodiment of a magnetic recording medium according to the present invention;  
       FIG. 3  is a diagram showing a static magnetic characteristic and magnetization states of the first embodiment of the magnetic recording medium;  
       FIG. 4  is a diagram showing relationships of a dynamic coercivity and a static coercivity, and a magnetic field switching time and a magnetization switching time of the first embodiment of the magnetic recording medium;  
       FIGS. 5A through 5F  are diagrams for explaining a first embodiment of a recording method according to the present invention;  
       FIG. 6  is a cross sectional view showing a part of a first embodiment of a magnetic storage apparatus according to the present invention; and  
       FIG. 7  is a plan view showing a part of the first embodiment of the magnetic storage apparatus. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      A description will be given of embodiments of a magnetic recording medium, a recording method and a magnetic storage apparatus according to the present invention, by referring to  FIGS. 2 through 7 .  
       FIG. 2  is a cross sectional view showing a part of a first embodiment of the magnetic recording medium according to the present invention. A magnetic recording medium  10  shown in  FIG. 2  includes a substrate  11 , and a stacked structure provided on the substrate  11 . The stacked structure includes a first seed layer  12 , a second seed layer  13 , an underlayer  14 , a nonmagnetic intermediate layer  15 , a first ferromagnetic layer  16 , a first nonmagnetic coupling layer  18 , a second ferromagnetic layer  19 , a second nonmagnetic coupling layer  20 , a magnetic layer  21 , a protection layer  22 , and a lubricant layer  23  which are successively stacked in this order. The second ferromagnetic layer  19  and the magnetic layer  21  are antiferromagnetically exchange-coupled via the second nonmagnetic coupling layer  20 . In addition, the first ferromagnetic layer  16  and the second ferromagnetic layer  19  are ferromagnetically exchange-coupled via the first nonmagnetic coupling layer  18 .  
      The substrate  11  may be formed by a disk-shaped plastic substrate, glass substrate, NiP-plated Al alloy substrate, Si substrate and the like, for example. The substrate  11  may also be formed by tape-shaped plastic films made of PET, PEN, polyimide and the like. The substrate  11  may or may not be textured. In a case where the magnetic recording medium  10  is a magnetic disk, a texturing process is carried out in a circumferential direction of the magnetic disk, that is, in a direction in which a track on the magnetic disk extends.  
      The first seed layer  12  may be made of a nonmagnetic material such as NiP, CoW and CrTi. The first seed layer  12  may or may not be textured. In a case where the first seed layer  12  is made of an amorphous material such as NiP, the first seed layer  12  is preferably oxidized, so that the in-plane orientation of the c-axis improves for the first ferromagnetic layer  16 , the second ferromagnetic layer  19  and the magnetic layer  21 . Of course, a known material which improves the c-axis orientation may be used for the first seed layer  12  in place of NiP.  
      The second seed layer  13  may be made of an amorphous material such as NiP, CoW and CrTi, or an alloy having a B2 structure such as AlRu, NiAl and FeAl. In a case where the second seed layer  13  is made of the amorphous material and the underlayer  14  is made of an alloy having the B2 structure, the orientation of the (001) face or (112) face of the underlayer  14  is improved. The second seed layer  13  may or may not be textured. In a case where the magnetic recording medium  10  is the magnetic disk, the texturing process is carried out in the circumferential direction of the magnetic disk, that is, in the direction in which the track on the magnetic disk extends.  
      The underlayer  14  may be made of Cr or a Cr alloy such as CrMo, CrW, CrV, CrB and CrMoB, or an alloy having a B2 structure such as AlRu, NiAl and FeAl. When the underlayer  14  is epitaxially grown on the second seed layer  13 , the underlayer  14  shows a good orientation of the (001) face or the (112) face in the growth direction if the alloy having the B2 structure is used for the underlayer  14 , and shows a good orientation of the (002) face in the growth direction if the Cr or Cr alloy is used for the underlayer  14 . The underlayer  14  may have a multi-layer structure made up of a plurality of stacked layers formed by the Cr or Cr alloy and the alloy having the B2 structure. The orientation of the underlayer  14  itself is improved by employing the multi-layer structure for the underlayer  14 . In addition, by employing the multi-layer structure for the underlayer  14 , a good epitaxial growth of the nonmagnetic intermediate layer  15  can be achieved, and the orientations of the first ferromagnetic layer  16 , the second ferromagnetic layer  19  and the magnetic layer  21  can further be improved.  
      The nonmagnetic intermediate layer  15  may be made of a nonmagnetic alloy having an hcp structure and obtained by adding M to a CoCr alloy, where M denotes an element selected from Pt, B, Mo, Nb, Ta, W and Cu or an alloy thereof. The nonmagnetic intermediate layer  15  has a thickness in a range of 1 nm to 5 nm. The nonmagnetic intermediate layer  15  is epitaxially grown to inherit the crystal properties and crystal grain sizes of the underlayer  14 . Hence, the nonmagnetic intermediate layer  15  improves the crystal properties of the first ferromagnetic layer  16 , the second ferromagnetic layer  19  and the magnetic layer  21  which are epitaxially grown afterwards, reduces a distribution width of the crystal grain (magnetic grain) sizes, and promotes the in-plane orientation of the c-axis. The in-plane orientation refers to the orientation in a direction parallel to the substrate surface. The nonmagnetic intermediate layer  15  may have a multi-layer structure which is made up of a plurality of layers which are made of the above described alloys and stacked. Therefore, the nonmagnetic intermediate layer  15  improves the orientation of the first ferromagnetic layer  16 , the second ferromagnetic layer  19  and the magnetic layer  21 .  
      The lattice constant of the nonmagnetic intermediate layer  15  may be made slightly different, that is, a several % different, from the lattice constant of the first ferromagnetic layer  16 , so as to generate an internal stress in the in-plane direction at an interface of the nonmagnetic intermediate layer  15  and the first ferromagnetic layer  16  or within the first ferromagnetic layer  16 . In this case, it is possible to increase the static coercivity of the first ferromagnetic layer  16 .  
      As will be described later, a static magnetic characteristic of the magnetic recording medium  10  may be measured by a Vibration Sample Magnetometer (VSM) or the like, and the measuring time of one loop is on the order of approximately several minutes. A time required to switch the direction of the external magnetic field is on the order of approximately several seconds. Such a time required to switch the direction of the external magnetic field will hereinafter be referred to as a “magnetic field switching time”, and the coercivity Hc for a case where the magnetic field switching time is on the order of seconds or greater is referred to as a static coercivity Hc.  
      On the other hand, the magnetic field switching time at the time of the recording when the magnetic head applies the magnetic field on the magnetic recording medium  10  is on the sub-nano-second to approximately one nano-second order. When switching the magnetic field in such a short magnetic field switching time, a force (for example, a viscous force) acts in a direction interfering with the magnetization motion, and a large magnetic field needs to be applied in order to switch the magnetization direction. In other words, the coercivity Hc increases, and this coercivity Hc which increases in such a manner is referred to as a dynamic coercivity Hc′.  
      The first ferromagnetic layer  16  may be made of Co, Ni, Fe, Co alloy, Ni alloy, Fe alloy and the like. It is particularly preferable to use CoCrTa, CoCrPt or alloys thereof for the first ferromagnetic layer  16 . The preferable CoCrPt alloy may be obtained by adding an element selected from B, Mo, Nb, Ta, W and Cu or an alloy thereof. The first ferromagnetic layer  16  has a thickness in a range of 1 nm to 10 nm. The first ferromagnetic layer  16  is epitaxially grown in a (11-20) direction on the nonmagnetic intermediate layer  15 , where “(11-20)” denotes (“1” “1” “2 bar” “0”), and the c-axis is orientated in the in-plane direction and the axis of easy magnetization matches the in-plane direction.  
      The first ferromagnetic layer  16  has a dynamic coercivity lower than that of the second ferromagnetic layer  19 . Hence, the magnetization direction of the first ferromagnetic layer  16  is more easily switched than that of the second ferromagnetic layer  19  in response to a small magnetic field which is applied thereto, such as a recording magnetic field and an exchange field. On the other hand, by providing the first ferromagnetic layer  16 , this first ferromagnetic layer  16  becomes indirectly exchange-coupled to the magnetic layer  21  via the second ferromagnetic layer  19 , to thereby increase the exchange-coupling volume and improve the thermal stability. Accordingly, compared to a conventional magnetic recording medium provided with a magnetic layer having the single-layer structure with a thickness equal to a total thickness of the first ferromagnetic layer  16  in place of the exchange-coupled structure, the second ferromagnetic layer  19  and the magnetic layer  21 , the magnetic recording medium  10  can obtain approximately the same thermal stability. Moreover, since the dynamic coercivity of the first ferromagnetic layer  16  which is further away from a magnetic head than the second ferromagnetic layer  19  and the magnetic layer  21  is set lower than the dynamic coercivities of the second ferromagnetic layer  19  and the magnetic layer  21 , the magnetic recording medium  10  can improve the overwrite performance compared to the above conventional magnetic recording medium.  
      The first nonmagnetic coupling layer  18  may be made of Ru, Rh, Ir, Ru alloy, Rh alloy, Ir alloy and the like, for example. Rh and Ir have an fcc structure, while Ru has the hcp structure. The lattice constant a=0.25 nm for the CoCrPt alloy used for the first ferromagnetic layer  16 , while the lattice constant a=0.27 nm for the Ru used for the first nonmagnetic coupling layer  18 . Hence, it is preferable to use Ru or Ru alloy for the first nonmagnetic coupling layer  18  so as to have the lattice parameter a close to that of the first ferromagnetic layer  16 . The Ru alloy used for the first nonmagnetic coupling layer  18  may preferably be an alloy of Ru and an element selected from Ti, V, Cr, Mn, Fe, Co, Ni, Nb, Mo, Rh, Pd, Ta, W, Re, Os, Ir and Pt or an alloy thereof. It is preferable to use a Ru alloy Ru 100-x Co x  for the first nonmagnetic coupling layer  18 , where x is greater than 0 at. % and less than or equal to 60 at. %, and x is more preferably greater than 0 at. % and less than or equal to 40 at. %. By using such a Ru alloy for the first nonmagnetic coupling layer  18 , it is possible to expand a thickness range of the first nonmagnetic coupling layer  18  which ferromagnetically couples the first ferromagnetic layer  16  and the second ferromagnetic layer  19  towards the thicker side.  
      The Rh alloy used for the first nonmagnetic coupling layer  18  may preferably be an alloy of Rh and an element selected from Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zr, Nb, Mo, Ru, Pd, Ag, Sb, Hf, Ta, W, Re, Os, Ir and Pt or an alloy thereof. The Ir alloy used for the first nonmagnetic coupling layer  18  may preferably be an alloy of Ir and an element selected from Al, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Nb, Mo, Tc, Ru, Rh, Pd, Ta, W, Re and Os or an alloy thereof.  
      In a case where the Ru is used for the first nonmagnetic coupling layer  18 , the first nonmagnetic coupling layer  18  preferably has a thickness in a range of 0.1 nm to 0.45 nm. In a case where a Ru alloy such as RuCo is used for the first nonmagnetic coupling layer  18 , the first nonmagnetic coupling layer  18  preferably has a thickness in a range of 0.1 nm to 0.95 nm. Hence, it is preferable to appropriately select the thickness range of the first nonmagnetic coupling layer  18  depending on the alloy material and the content of the added element within the alloy material used for the first nonmagnetic coupling layer  18 , so that the first ferromagnetic layer  16  and the second ferromagnetic layer  19  become ferromagnetically coupled. By setting such a thickness range for the first nonmagnetic coupling layer  18 , it is possible to ferromagnetically exchange-couple the first ferromagnetic layer  16  and the second ferromagnetic layer  19 , and make the magnetization directions of the first ferromagnetic layer  16  and the second ferromagnetic layer  19  mutually parallel in a state where no external magnetic field is applied thereto. In addition, from the point of view of expanding the thickness range of the first nonmagnetic coupling layer  18  which ferromagnetically couples the first ferromagnetic layer  16  and the second ferromagnetic layer  19 , the first nonmagnetic coupling layer  18  is preferably made of RuCo to enable such an expansion of the thickness range. From the point of view of obtaining a large exchange-coupling strength between the first ferromagnetic layer  16  and the second ferromagnetic layer  19 , the nonmagnetic coupling layer  18  preferably has a thickness in a range of 0.2 nm to 0.8 nm. Moreover, in a case where Ru 80 CO 20  is used for the nonmagnetic coupling layer  18 , the nonmagnetic coupling layer  18  preferably has a thickness in a range of 0.2 nm to 0.7 nm.  
      The first nonmagnetic coupling layer  18  may be formed by sputtering, vacuum deposition, Chemical Vapor Deposition (CVD) and the like. It is possible to employ an Ion Cluster Beam (ICB) to form the first nonmagnetic coupling layer  18  in order to suppress the thickness inconsistency for the entire substrate area. When the ICB is employed, the thickness inconsistency of the first nonmagnetic coupling layer  18  is suppressed because the kinetic energy reaching the surface and the amount of deposition can be controlled satisfactorily. Furthermore, by employing the ICB to form the first nonmagnetic coupling layer  18 , it may be regarded that the minimum thickness of the Ru layer or Ru alloy layer forming the first nonmagnetic coupling layer  18  can be controlled to approximately 0.2 nm.  
      The second ferromagnetic layer  19  may be made of Co, Ni, Fe, Co alloy, Ni alloy, Fe alloy and the like, similarly as in the case of the first ferromagnetic layer  16 . It is particularly preferable to use CoCrTa, CoCrPt or alloys thereof for the second ferromagnetic layer  19 . The preferable CoCrPt alloy may be obtained by adding an element selected from B, Mo, Nb, Ta, W and Cu or an alloy thereof.  
      In this embodiment, the Pt content of the material or alloy used for the second ferromagnetic layer  19  is larger than those of the first ferromagnetic layer  16  and the magnetic layer  21 . In other words, a Pt content Pt 1  of the first ferromagnetic layer  16 , a Pt content Pt 2  of the second ferromagnetic layer  19  and a Pt content Pt 3  of the magnetic layer  21  satisfy a relationship Pt 1 &lt;Pt 3 ≦Pt 2 . In addition, a dynamic coercivity Hc 1 ′of the first ferromagnetic layer  16 , a dynamic coercivity Hc 2 ′ of the second ferromagnetic layer  19  and a dynamic coercivity Hc 3 ′ of the magnetic layer  21  satisfy a relationship Hc 1 ′&lt;Hc 3 ′≦Hc 2 ′. For example, the Pt content Pt 3  of the CoCrPtB alloy used for the magnetic layer  21  is 9 at. %, the Pt content Pt 1  of the CoCrPtB alloy used for the first ferromagnetic layer  16  is 2 at. %, and the Pt content Pt 2  of the CoCrPtB alloy used for the second ferromagnetic layer  19  is 15 at. %. In this case, the dynamic coercivity Hc 2 ′ of the second ferromagnetic layer  19  may be increased.  
      It is not essential for the layer forming the first ferromagnetic layer  16  to include Pt. By satisfying at least one of the above described relationships, it is possible to reduce the dynamic coercivity Hc 1 ′ of the first ferromagnetic layer  16 , and to easily and quickly switch the magnetization direction of the first ferromagnetic layer  16  in the same direction as the magnetization direction of the second ferromagnetic layer  19  by the exchange field of the second ferromagnetic layer  19  after the recording magnetic field is removed. The quick switching of the magnetization direction of the first ferromagnetic layer  16  occurs within a time on the order of approximately 1 msec. This time in which the magnetization direction of the first ferromagnetic layer  16  is switched is shorter than a time required for the magnetic disk to undergo 1 revolution, in the case where the magnetic recording medium  10  is the magnetic disk. Therefore, by satisfying at least one of the above described relationships, the magnetization state stabilizes while the magnetic disk undergoes 1 revolution after the recording, and the reproduced output of the magnetic head can be stabilized. Of course, the above described relationship of the dynamic coercivities Hc 1 ′, Hc 2 ′ and Hc 3 ′ of the first ferromagnetic layer  16 , the second ferromagnetic layer  19  and the magnetic layer  21  may be satisfied by appropriately adjusting contents of an element other than Pt in the alloys forming the first ferromagnetic layer  16 , the second ferromagnetic layer  19  and the magnetic layer  21 .  
      In order to satisfy the relationship Hc 1 ′&lt;Hc 3 ′≦Hc 2 ′ among the dynamic coercivity Hc 1 ′ of the first ferromagnetic layer  16 , the dynamic coercivity Hc 2 ′ of the second ferromagnetic layer  19  and the dynamic coercivity Hc 3 ′ of the magnetic layer  21 , an anisotropic field Hk 1  of the first ferromagnetic layer  16 , an anisotropic field Hk 2  of the second ferromagnetic layer  19  and an anisotropic field Hk 3  of the magnetic layer  21  may be set to satisfy a relationship Hk 1 &lt;Hk 3 ≦Hk 2 . The following relationship between the dynamic coercivity Hc′ and the anisotropic field Hk is described in H. N. Bertram et al., J. Appl. Phys., vol. 85, No. 8, pp. 4991 (1999), where f o  denotes an attempt frequency, K u  denotes an anisotropy constant, V denotes a volume of a magnetic unit, k B  denotes the Boltzmann&#39;s constant, and T denotes the absolute temperature. 
 
Hc′=0.474 Hk[1-1.55{(k B T/K U V)−ln (f o t/ln2)/2}] 2/3  
 
      Hence, it may be regarded that the magnetic field switching time t=10 −9 /ln2 seconds and the anisotropic field Hk and the dynamic coercivity Hc′ are proportional. For this reason, by setting the first ferromagnetic layer  16 , the second ferromagnetic layer  19  and the magnetic layer  21  to satisfy the relationship Hk 1 &lt;Hk 3 ≦Hk 2 , the first ferromagnetic layer  16 , the second ferromagnetic layer  19  and the magnetic layer  21  can be set to satisfy the relationship Hc 1 ′&lt;Hc 3 ′≦Hc 2 ′.  
      The second ferromagnetic layer  19  has a thickness in a range of 0.2 nm to 3.0 nm. By setting the thickness of the second ferromagnetic layer  19  within such a range, it is possible to reduce a static coercivity Hc 2  of the second ferromagnetic layer  19 , and a relationship Hc 3 &gt;Hc 2  can be satisfied between the static coercivity Hc 3  of the magnetic layer  21  and the static coercivity of the second ferromagnetic layer  19 . From the point of view of obtaining a satisfactory overwrite performance, it is preferable for the thickness of the second ferromagnetic layer  19  to be within a range of 0.5 nm to 2.0 nm as long as a relationship Hc 3 ′≦Hc 2 ′ is satisfied between the dynamic coercivity Hc 3 ′ of the magnetic layer  21  and the dynamic coercivity Hc 2 ′ of the second ferromagnetic layer  19 . Since the second ferromagnetic layer  19  is thin, the overwrite performance is maintained approximately the same or only a slight deterioration will occur if any, even if the dynamic coercivity Hc 2 ′ is greatly increased, compared to a case where the dynamic coercivity Hc 2 ′ is relatively small.  
      The second nonmagnetic coupling layer  20  may be made of a material similar to that used for the first nonmagnetic coupling layer  18  described above. The second nonmagnetic coupling layer  20  has a thickness in a range of 0.5 nm to 1.4 nm, and the thickness may be appropriately selected depending on the material used for the second nonmagnetic coupling layer  20 . By setting such a thickness range for the second nonmagnetic coupling layer  20 , it is possible to antiferromagnetically exchange-couple the second ferromagnetic layer  19  and the magnetic layer  21 , and make the magnetization directions of the second ferromagnetic layer  19  and the magnetic layer  21  mutually antiparallel in a state where no external magnetic field is applied thereto. In a case where the Ru is used for the second nonmagnetic coupling layer  20 , the second nonmagnetic coupling layer  20  preferably has a thickness in a range of 0.5 nm to 0.9 nm. In a case where a Ru alloy such as RuCo is used for the second nonmagnetic coupling layer  20 , the second nonmagnetic coupling layer  20  preferably has a thickness in a range of 1.0 nm to 1.4 nm. From the point of view of expanding the thickness range of the second nonmagnetic coupling layer  20  which antiferromagnetically couples the second ferromagnetic layer  18  and the magnetic layer  21 , the second nonmagnetic coupling layer  20  is preferably made of RuCo to enable such an expansion of the thickness range.  
      The magnetic layer  21  may be made of Co, Ni, Fe, Co alloy, Ni alloy, Fe alloy and the like, similarly as in the case of the first ferromagnetic layer  16  and the second ferromagnetic layer  19 . It is particularly preferable to use CoCrTa, CoCrPt or alloys thereof for the magnetic layer  21 . The preferable CoCrPt alloy may be obtained by adding an element selected from B, Mo, Nb, Ta, W and Cu or an alloy thereof. The magnetic layer  21  has a thickness in a range of 5 nm to 30 nm. Since the layers of the stacked structure, from the first seed layer  12  to the magnetic layer  21 , are grown epitaxially, the magnetic layer  21  has satisfactory crystal properties and finely controlled crystal grain diameters. For this reason, the medium noise of the magnetic recording medium  10  can be reduced.  
      It is preferable that a saturation magnetization Ms 1  and a thickness t 1  of the first ferromagnetic layer  16 , a saturation magnetization Ms 2  and a thickness t 2  of the second ferromagnetic layer  19 , and a saturation magnetization Ms 3  and a thickness t 3  of the magnetic layer  21  satisfy the following relationship. 
 
(Ms 1 ×t 1 +Ms 2 ×t 2 )&lt;(Ms 3 ×t 3 ) 
 
      By satisfying this relationship of the saturation magnetizations Ms 1 , Ms 2  and Ms 3  and the thicknesses t 1 , t 2  and t 3  of the first ferromagnetic layer  16 , the second ferromagnetic layer  19  and the magnetic layer  21 , the magnetic layer  21  closest to the magnetic head bears the net remanent magnetization and thickness product, and it becomes possible to more accurately record the information in the magnetic layer  21  in correspondence with the switching positions of the recording magnetic field of the magnetic head. It is possible to set the relationship to (Ms 1 ×t 1 +Ms 2 ×t 2 )&gt;(Ms 3 ×t 3 ), but in this case, the first ferromagnetic layer  16  and the second ferromagnetic layer  19  which are further away from the magnetic head than the magnetic layer  21  bear the remanent magnetization and thickness product. Consequently, it becomes more difficult to accurately record the information in the magnetic layer  21  in correspondence with the switching positions of the recording magnetic field of the magnetic head, and the reproduced output of the magnetic head decreases because the first ferromagnetic layer  16  and the second ferromagnetic layer  19  are further away from the magnetic head than the magnetic layer  21 .  
      As described above, the dynamic coercivities Hc 2 ′ and Hc 3 ′ of the second ferromagnetic layer  19  and the magnetic layer  21  satisfy the relationship Hc 2 ′≧Hc 3 ′. By satisfying this relationship Hc 2 ′≧Hc 3 ′, the magnetization direction of the magnetic layer  21  switches before the magnetization direction of the second ferromagnetic layer  19 , in response to the switching of the recording magnetic field of the magnetic head. Accordingly, a magnetization transition region which matches the switching timing of the recording magnetic field is formed in the magnetic layer  21 , and the NLTS can be reduced.  
      The protection layer  22  may be made of diamond-like carbon, carbon nitride, amorphous carbon and the like. The protection layer  22  has a thickness in a range of 0.5 nm to 10 nm, and preferably in a range of 0.5 nm to 5 nm.  
      The lubricant layer  23  may be made of an organic liquid lubricant having perfluoropolyether as a main chain and —OH, benzene ring or the like as the terminal functional group. More particularly, ZDol manufactured by Monte Fluos (terminal functional group: —OH), AM3001 manufactured by Ausimonoto (terminal functional group: benzene ring), Z25 manufactured by Monte Fluos, and the like, with a thickness in a range of 0.5 nm to 3.0 nm, may be used for the lubricant layer  23 . The lubricant may be appropriately selected depending on the material used for the protection layer  22 .  
      The layers  12  through  16  and  18  through  22  may be successively formed on the substrate  11  by sputtering, vacuum deposition and the like. On the other hand, the lubricant layer  23  may be formed by dipping, spin-coating and the like. In a case where the magnetic recording medium  10  has a tape-shape, the lubricant layer  23  may be formed by die-coating, dipping and the like.  
      Next, a description will be given of a case where this embodiment is applied to the magnetic disk. First, a NiP first seed layer  12  having a thickness of 25 nm was formed on a glass substrate  11 , and exposed to the atmosphere to oxidize the NiP first seed layer  12 . A CrMoW alloy second seed layer  13  having a thickness of 5 nm, a CrMo alloy underlayer  14  having a thickness of 3 nm, and a CoCr alloy nonmagnetic intermediate layer  15  having a thickness of 1 nm were successively formed on the NiP first seed layer  12 . A CoCrPt 2 Ta alloy first ferromagnetic layer  16  having a thickness of 4 nm, a Ru first nonmagnetic coupling layer  18  having a thickness of 0.3 nm, a CoCrPt 16 B alloy second ferromagnetic layer  19  having a thickness of 1.5 nm, a Ru second nonmagnetic coupling layer  20  having a thickness of 0.8 nm, a CoCrPt 12 B alloy magnetic layer  21  having a thickness of 17 nm, and a diamond-like carbon protection layer  22  having a thickness of 4.5 nm were successively formed on the CoCr alloy nonmagnetic intermediate layer  15 . The layers  12  through  16  and  18  through  22  were formed by use of a DC magnetron sputtering apparatus. A lubricant layer  23  was formed by AM3001 manufactured by Ausimonoto (terminal functional group: benzene ring) to a thickness of 1.0 nm on the diamond-like carbon protection layer  22  by dipping.  
       FIG. 3  is a diagram showing a static magnetic characteristic and magnetization states of the first embodiment of the magnetic recording medium. In  FIG. 3 , the ordinate indicates the magnetization M in arbitrary units, and the abscissa indicates the external magnetic field H in arbitrary units.  
      As shown in  FIG. 3 , when the external magnetic field H is increased from the remanent magnetization state, from a state STB to a state STC or, from a state STD to a state STA, the magnetization directions of the first ferromagnetic layer  16 , the second ferromagnetic layer  19  and the magnetic layer  21  become aligned in the direction of the applied external magnetic field H and become mutually parallel. Then, when the external magnetic field H is decreased, the magnetization direction of the second ferromagnetic layer  19  switches due to the exchange field of the magnetic layer  21 , and the magnetization direction of the first ferromagnetic layer  16  switches due to the exchange field of the second ferromagnetic layer  19 . In a state STB or STD where no external magnetic field H is applied, the magnetization directions of the magnetic layer  21  and the second ferromagnetic layer  19  become mutually antiparallel, and the magnetization directions of the first ferromagnetic layer  16  and the second ferromagnetic layer  19  become mutually parallel. Furthermore, when the direction of the external magnetic field H is switched and the external magnetic field H is increased, the magnetization direction of the magnetic layer  21  starts to switch, and the net magnetization M of the 3 layers  16 ,  19  and  21  becomes 0, thereby making the value of the external magnetic field H the coercivity Hc of the 3 layers  16 ,  19  and  21 . The static magnetic characteristic is measured by a Vibration Sample Magnetometer (VSM) or the like, and the measuring time of one loop is on the order of approximately several minutes. The time required to switch the direction of the external magnetic field H is on the order of approximately several seconds. As mentioned above, such a time required to switch the direction of the external magnetic field H will hereinafter be referred to as the “magnetic field switching time”, and the coercivity Hc for the case where the magnetic field switching time is on the order of seconds or greater is referred to as the static coercivity Hc.  
      The static magnetic characteristic for the case where the time required to switch the direction of the external magnetic field H is on the order of approximately several seconds is approximately the same for the magnetic recording medium  10  of this embodiment and the conventional magnetic recording medium having the magnetic layer with the single-layer structure in place of the exchange-coupled structure.  
       FIG. 4  is a diagram showing relationships of the dynamic coercivity and the static coercivity, and the magnetic field switching time and the magnetization switching time of the first embodiment of the magnetic recording medium. The magnetization switching time refers to a time required to switch the magnetization direction. In  FIG. 4 , the left ordinate indicates the dynamic coercivity in arbitrary units, the right ordinate indicates the static coercivity in arbitrary units, and the abscissa indicates the magnetic field switching time and the magnetization switching time in arbitrary units.  
      On the other hand, the magnetic field switching time at the time of the recording when the magnetic head applies the magnetic field on the magnetic recording medium  10  is on the sub-nano-second to approximately one nano-second order. When switching the magnetic field in such a short magnetic field switching time, a force (for example, a viscous force) acts in the direction interfering with the magnetization motion, and a large magnetic field needs to be applied in order to switch the magnetization direction. In other words, the coercivity Hc increases, and this coercivity Hc which increases in such a manner is referred to as the dynamic coercivity Hc′.  
       FIG. 4  shows a coercivity characteristic curve CV 1  of the first ferromagnetic layer  16 , a coercivity characteristic curve CV 2  of the second ferromagnetic layer  19 , and a coercivity characteristic curve CV 3  of the magnetic layer  21 . As shown in  FIG. 4 , in the magnetic recording medium  10  of this embodiment, the static coercivities Hc 1 , Hc 2  and Hc 3  of the first ferromagnetic layer  16 , the second ferromagnetic layer  19  and the magnetic layer  21  are small during a magnetic field switching time tA which is on the order of approximately several seconds. On the other hand, the dynamic coercivities Hc 1 ′, Hc 2 ′ and Hc 3 ′ of the first ferromagnetic layer  16 , the second ferromagnetic layer  19  and the magnetic layer  21  are large during a magnetic field switching time tB which is on the sub-nano-second to approximately one nano-second order.  
      It is preferable for the static coercivities Hc 1 , Hc 2  and Hc 3  of the first ferromagnetic layer  16 , the second ferromagnetic layer  19  and the magnetic layer  21  to satisfy relationships Hc 3 &gt;Hc 2  and Hc 3 &gt;Hc 1  during the magnetic field switching time tA, because the magnetic layer  21  can bear the net remanent magnetization and thickness product. That is, the magnetization transition region can be formed in the magnetic layer  21  in correspondence with the switching position of the recording magnetic field of the magnetic head.  
      On the other hand, it is preferable for the dynamic coercivities Hc 1 ′, Hc 2 ′ and Hc 3 ′ of the first ferromagnetic layer  16 , the second ferromagnetic layer  19  and the magnetic layer  21  to satisfy a relationship Hc 1 ′&lt;Hc 3 ′≦Hc 2 ′ during the magnetic field switching time tB. For example, in a case where the applied recording magnetic field and exchange field (including the directions) are approximately the same for each of the first ferromagnetic layer  16 , the second ferromagnetic layer  19  and the magnetic layer  21 , the magnetization switching time is shortest for the first ferromagnetic layer  16 , second shortest for the magnetic layer  21 , and longest for the second ferromagnetic layer  19 . Accordingly, when the recording magnetic field is switched, the magnetization direction of the first ferromagnetic layer  16  switches first, the magnetization direction of the magnetic layer  21  switches second, and the magnetization direction of the second ferromagnetic layer  19  switches last. In the case of the magnetic recording medium  10  of this embodiment, the magnetization direction of the magnetic layer  21  is easily switched due to the action of the exchange field.  
      Next, a description will be given of a first embodiment of a recording method according to the present invention, by referring to  FIGS. 5A through 5F .  FIGS. 5A through 5F  are diagrams for explaining this first embodiment of the recording method according to the present invention. For the sake of convenience,  FIGS. 5A through 5F  show only a part of the magnetic recording medium  10 , and it is also assumed that the magnetic head and the magnetic recording medium  10  are stationary relative to each other. Further, the illustration of the magnetic head is omitted in  FIGS. 5A through 5F .  
       FIG. 5A  shows a state where a recording magnetic field HAP from the magnetic head is applied in a rightward direction as indicated by a large arrow, and a magnetization M 1  of the first ferromagnetic layer  16 , a magnetization M 2  of the second ferromagnetic layer  19  and a magnetization M 3  of the magnetic layer  21  are all magnetized in the rightward direction, as indicated by solid arrows.  
       FIG. 5B  shows a state where the recording magnetic field HAP is switched in a leftward direction as indicated by a large arrow, and the magnetization M 1  of the first ferromagnetic layer  16  having the lowest dynamic coercivity Hc 1 ′ is switched in the leftward direction by the recording magnetic field HAP before the magnetizations M 2  and M 3  of the second ferromagnetic layer  19  and the magnetic layer  21 , as indicated by solid arrows. In this state, the first ferromagnetic layer  16  and the magnetic layer  21  are respectively exchange-coupled to the second ferromagnetic layer  19 . For this reason, an exchange field HE 21  of the first ferromagnetic layer  16  and an exchange field HE 23  of the magnetic layer  21  are applied to the second ferromagnetic layer  19  in parallel to the recording magnetic field HAP, as indicated by dotted arrows. In addition, an exchange field HE 3  of the second ferromagnetic layer  19  is applied to the magnetic layer  21  in parallel to the recording magnetic field HAP, as indicated by a dotted arrow. The magnetization M 3  of the magnetic layer  21  switches in the direction of the recording magnetic field HAP before the magnetization M 2  of the second ferromagnetic layer  19  when the following relationship is satisfied, where Hh 2  denotes a magnitude of the recording magnetic field HAP at the second ferromagnetic layer  19  and Hh 3  denotes a magnitude of the recording magnetic field HAP at the magnetic layer  21 . 
 Hh 3 +HE 3 −Hc 3 ′&gt;Hh 2 +HE 2 −Hc 2 ′&gt;0  
       FIG. 5C  shows a state after a slight time has elapsed from the state shown in  FIG. 5B . In this state, the magnetization M 3  of the magnetic layer  21  has switched in the direction of the recording magnetic field HAP. By increasing the recording magnetic field HAP, the magnetization M 2  of the second ferromagnetic layer  19  switches as shown in  FIG. 5D . As a result, in the state shown in  FIG. 5D , the magnetizations M 1 , M 2  and M 3  of the first ferromagnetic layer  16 , the second ferromagnetic layer  19  and the magnetic layer  21  become mutually parallel. Further, in the magnetic layer  21 , a magnetization transition region, which is a boundary between the previously formed magnetization in the rightward direction, is formed at the timing with which the magnetization M 3  of the magnetic layer  21  is switched in  FIG. 5C .  
       FIG. 5E  shows a state after the recording magnetic field HAP is removed. The direction of the magnetization M 2  of the second ferromagnetic layer  19  is switched by magnetization attempt due to the exchange fields HE 21  and HE 23  applied thereto. A time required for the magnetization M 2  of the second ferromagnetic layer  19  to switch direction is indicated as a magnetization switching time tRL 2  shown in  FIG. 4 , which satisfies the following relationship with respect to the exchange field HE 2  (=|HE 23 −HE 21 |) and the dynamic coercivity Hc 2 ′. 
 |HE 23 −HE 21 |≧Hc 2 ′ 
      The magnetization switching time tRL 2  is a time region between the magnetic field switching times tA and tB, and it is preferable that the magnetization switching time tRL 2  is as short as possible. Due to an exchange field HE 1  of the second ferromagnetic layer  19  which is applied to the first ferromagnetic layer  16 , the magnetization M 1  of the first ferromagnetic layer  16  requires a magnetization switching time tRL 1  shown in  FIG. 4  to switch direction.  
       FIG. 5F  shows a remanent magnetization state after the direction of the magnetization M 1  of the first ferromagnetic layer  16  is switched. In this state, the magnetizations M 1  and M 2  of the first and second ferromagnetic layers  16  and  19  are antiparallel with respect to the magnetization M 3  of the magnetic layer  21 .  
      Accordingly, the magnetization M 3  of the magnetic layer  21  switches direction before the magnetization M 2  of the second ferromagnetic layer  19 , due to the recording magnetic field HAP which is intensified by the exchange field HE 3  of the second ferromagnetic layer  19 . Moreover, it is possible to positively switch the directions of the magnetizations M 1  and M 2  of the first and second ferromagnetic layers  16  and  19  to become antiparallel with respect to the direction of the magnetization M 3  of the magnetic layer  21  after the recording magnetic field HAP is removed.  
       FIG. 6  is a cross sectional view showing a part of a first embodiment of a magnetic storage apparatus according to the present invention, and  FIG. 7  is a plan view showing a part of the first embodiment of the magnetic storage apparatus. This embodiment of the magnetic storage apparatus employs an embodiment of a recording method according to the present invention, to record information on the embodiment of the magnetic recording medium described above.  
      As shown in  FIGS. 6 and 7 , a magnetic storage apparatus  40  generally includes a housing  43 . A motor  44 , a hub  45 , a plurality of magnetic recording media  46 , a plurality of recording and reproducing heads (composite heads)  47 , a plurality of suspensions  48 , a plurality of arms  49 , and an actuator unit  41  are provided within the housing  43 . The magnetic recording media  46  are mounted on the hub  45  which is rotated by the motor  44 . The recording and reproducing head  47  is made up of a reproducing head  47 A and a recording head  47 B. For example an Magneto-Resistive (MR) element, a Giant Magneto-Resistive (GMR) element, a Tunneling Magneto-Resistive (TMR) element, a Current Perpendicular to Plane (CPP) element and the like may be used as the reproducing head  47 A. On the other hand, an inductive head such as a thin film head may be used for the recording head  47 B. Each recording and reproducing head  47  is mounted on the tip end of a corresponding part  49  via the suspension  48 . The arms  49  are moved by the actuator unit  41 . The basic construction of this magnetic storage apparatus is known, and a detailed description thereof will be omitted in this specification.  
      The magnetic storage apparatus  40  is characterized by the magnetic recording media  46 . Each of the magnetic recording media  46  has the stacked structure of the embodiment of the magnetic recording medium described above in conjunction with  FIGS. 2 through 5 . In other words, each of the magnetic recording media  46  may have the structure of the magnetic recording medium  10  shown in  FIG. 2 . Of course, the number of magnetic recording media  46  is not limited to 3, and only 1, 2 or 4 or more magnetic recording media  46  may be provided.  
      The basic construction of the magnetic storage apparatus is not limited to that shown in  FIGS. 6 and 7 . In addition, the magnetic recording medium  46  used in the present invention is not limited to a magnetic disk. For example, the magnetic recording medium  46  may be a magnetic tape. When using the magnetic tape as the magnetic recording medium  46 , the magnetic storage apparatus may be formed by a helical scan type video tape recording and/or reproducing apparatus or, a magnetic tape apparatus for computers which forms a plurality of tracks in a direction taken along the width of the magnetic tape.  
      According to the magnetic storage apparatus  40 , it is possible to carry out a highly reliable high-density recording, because each magnetic recording medium  46  has satisfactory write performances, a satisfactory thermal stability of written bits and low medium noise.  
      Further, the present invention is not limited to these embodiments, but various variations and modifications may be made without departing from the scope of the present invention.