Abstract:
Heat assisted magnetic recording systems with composite recording films are disclosed. The magnetic recording films include superparamagnetic nanoparticles dispersed in an antiferromagnetic or ferrimagnetic matrix. The matrix provides antiferromagnetic coupling with the superparamagnetic nanoparticles, and minimizes exchange interactions between adjacent nanoparticles.

Description:
GOVERNMENT CONTRACT 
     This invention was made with United States Government support under Agreement No. 70NANB1H3056 awarded by the National Institute of Standards and Technology (NIST). 
     The United States Government has certain rights in the invention. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to heat assisted magnetic recording films, and more particularly relates to films having superparamagnetic nanoparticles dispersed in an antiferromagnetic or ferrimagnetic matrix. 
     BACKGROUND OF THE INVENTION 
     Longitudinal magnetic recording in its conventional form has been projected to suffer from superparamagnetic instabilities at high bit densities. As the grain size of the magnetic recording medium is decreased in order to increase the areal density, a threshold known as the superparamagnetic limit at which stable data storage is no longer feasible is reached for a given material and temperature. 
     An alternative to longitudinal recording is perpendicular magnetic recording. Perpendicular magnetic recording is believed to have the capability of extending recording densities well beyond the limits of longitudinal magnetic recording. Perpendicular magnetic recording heads for use with a perpendicular magnetic storage medium may include a pair of magnetically coupled poles, including a main write pole having a relatively small bottom surface area and a flux return pole having a larger bottom surface area. A coil having a plurality of turns is located around the main write pole or yoke for inducing a magnetic field from the write pole, through a hard magnetic recording layer of the storage medium, into a soft magnetic underlayer and back to the return pole. 
     Thermal stability of magnetic recording systems can be improved by employing a recording medium formed of a material with a very high magnetic anisotropy K u ). The energy barrier for a uniaxial magnetic grain to switch between two stabilized states is proportional to the product of the magnetic anisotropy K u ) of the magnetic material and the volume (V) of the magnetic grains. In order to provide adequate data storage, the product K u V should be as large as 60 kT, where k is the Boltzman constant and T is the absolute temperature, in order to provide 10 years of thermally stable data storage. Although it is desirable to use magnetic materials with high K u , very few of such hard magnetic materials exist. Furthermore, with currently available magnetic materials, recording heads are not able to provide a sufficient magnetic writing field to write on such materials. 
     Heat assisted magnetic recording (HAMR) refers to the concept of locally heating a magnetic recording medium to reduce the coercivity of the recording medium so that the applied magnetic writing field can more easily direct the magnetization of the recording medium during the temporary magnetic softening of the recording medium caused by the heat source. HAMR allows for the use of small grain media, which is desirable for recording at increased areal densities, with a larger magnetic anisotropy at room temperature assuring a sufficient thermal stability. 
     However, several major problems are associated with HAMR designs. There are limited types of hard magnetic materials having sufficiently high magnetic anisotropies, e.g., K u &gt;10 7 erg/cm 3 . The candidates are L 10 -phased materials such as FePt, CoPt, FePd and CoPd, NdFeB, and SmCo. All of these materials have chemically ordered structures, which are difficult to obtain while optimizing the desired microstructure. For example, annealing a FePt thin film at high temperature helps obtain the ordered L 10  phase, but promotes grain growth significantly. Another problem is that the Curie temperatures of such hard magnetic materials are generally very high, e.g., greater than 400° C. for FePt. Since magnetic recording discs rotate at very high speeds, it is difficult to raise the temperature of a surface spot of the media above its Curie temperature within such a short duration. Moreover, organic lubricant materials evaporate under such high temperature. 
     A need therefore exists for recording films that can effectively be used for heat-assisted magnetic recording. 
     SUMMARY OF THE INVENTION 
     The present invention provides heat assisted magnetic recording media including recording films having superparamagnetic nanoparticles embedded in an antiferromagnetic or ferrimagnetic matrix. As used herein, the term “superparamagnetic nanoparticles” means particles of ferromagnetic material having sufficiently small KuV, such that the magnetization of the material is unstable at ambient temperatures, i.e., the magnetization is capable of switching due to ambient thermal energy. The threshold at which a ferromagnetic particle becomes superparamagnetic depends on the composition of the material and its size. For some types of ferromagnetic materials, such as Co—Pt and other Co alloys, superparamagnetism occurs as the particle size is decreased below about 5 nm. The term “antiferromagnetic” refers to matrix materials in which adjacent spins align in opposite or antiparallel arrangements throughout the material at relatively low temperatures such that the material exhibits essentially no gross external magnetism. The term “ferrimagnetic” refers to matrix materials in which the magnetic spins of one class of atoms are opposed to the magnetic spins of another class of atoms within the material such that the material exhibits essentially very small gross external magnetism. 
     The individual superparamagnetic nanoparticles of the recording film are not thermally stable. However, since the nanoparticles are coupled with the antiferromagnetic or ferrimagnetic matrix, the particles are stable as long as the matrix is thermally stable. Due to antiferromagnetic coupling between the superparamagnetic nanoparticles and the matrix, the materials of the nanoparticles do not have to be chemically ordered, such as conventional L 10 -phased FePt and SmCo which are difficult to fabricate. The exchange interactions between adjacent nanoparticles are small because the superparamagnetic nanoparticles are separated by the matrix. Therefore, the signal-to-noise ratio level is high even at small bit sizes. The thermal stability of the composite recording film is excellent because it takes an extremely high magnetic field to switch the anisotropy direction of the matrix material. Furthermore, the antiferromagnetic or ferrimagnetic matrix has essentially no (or very small) magnetic moment and contributes essentially no extra signal to the reader. 
     An aspect of the present invention is to provide a heat assisted magnetic recording film comprising an antiferromagnetic or ferrimagnetic matrix and superparamagnetic nanoparticles dispersed in the matrix. 
     Another aspect of the present invention is to provide a heat assisted magnetic recording system comprising a recording medium and a heat assisted magnetic recording head. The recording medium includes a magnetic recording film comprising an antiferromagnetic or ferrimagnetic matrix and superparamagnetic nanoparticles dispersed in the matrix. The heat assisted magnetic recording head is positioned adjacent to the recording medium and comprises a write pole for applying a magnetic write field to the recording medium, and a heat source for heating the recording medium proximate to where the write pole applies the magnetic write field to the recording medium. The magnetic recording film includes an antiferromagnetic or ferrimagnetic matrix, and superparamagnetic nanoparticles dispersed in the matrix. 
     A further aspect of the present invention is to provide a heat assisted method of recording data on a magnetic recording medium. The method comprises the steps of heating a magnetic recording film, and magnetically writing on the heated magnetic recording film, wherein the magnetic recording film comprises an antiferromagnetic or ferrimagnetic matrix, and superparamagnetic nanoparticles dispersed in the matrix. 
     These and other aspects of the present invention will be more apparent from the following description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a pictorial representation of a disc drive storage system that can include a heat-assisted magnetic recording head and recording medium having a magnetic recording film in accordance with an embodiment of the present invention. 
         FIG. 2  is a partially schematic illustration of a heat-assisted magnetic recording head and recording medium which may incorporate a magnetic recording film in accordance with an embodiment of the present invention. 
         FIG. 3  is a partially schematic isometric sectional view of a heat-assisted magnetic recording medium including a magnetic recording film in accordance with an embodiment of the present invention. 
         FIG. 4  is a partially schematic isometric view of a portion of a heat-assisted magnetic recording film of the present invention representing a single recording bit. 
         FIG. 5  is a top view of the recording bit of  FIG. 4 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  is a pictorial representation of a disc drive  10  that can utilize a heat assisted magnetic recording head constructed in accordance with this invention. The disc drive  10  includes a housing  12  (with the upper portion removed and the lower portion visible in this view) sized and configured to contain the various components of the disc drive. The disc drive  10  includes a spindle motor  14  for rotating at least one magnetic storage medium  16 , which may be a perpendicular magnetic recording medium, within the housing. At least one arm  18  is contained within the housing  12 , with each arm  18  having a first end  20  with a recording head or slider  22 , and a second end  24  pivotally mounted on a shaft by a bearing  26 . An actuator motor  28  is located at the arm&#39;s second end  24  for pivoting the arm  18  to position the recording head  22  over a desired sector or track  27  of the disc  16 . The actuator motor  28  is regulated by a controller, which is not shown in this view and is well known in the art. 
       FIG. 2  is a partially schematic side view of a HAMR head  22  and a magnetic recording medium  16 . Although an embodiment of the invention is described herein with reference to recording head  22  as a perpendicular magnetic recording head and the medium  16  as a perpendicular magnetic recording medium, it will be appreciated that aspects of the invention may also be used in conjunction with other type recording heads and/or recording mediums where it may be desirable to employ heat assisted recording. Specifically, the HAMR head  22  may include a writer section comprising a main write pole  30  and a return or opposing pole  32  that are magnetically coupled by a yoke or pedestal  35 . It will be appreciated that the HAMR head  22  may be constructed with a write pole  30  only and no return pole  32  or yoke  35 . A magnetization coil  33  may surround the yoke or pedestal  35  for energizing the HAMR head  22 . The HAMR head  22  also may include a read head, not shown, which may be any conventional type read head as is generally known in the art. The recording medium  16  is positioned adjacent to or under the recording head  22  for movement, for example, in the direction of arrow A. 
     As illustrated in  FIG. 2 , the recording head  22  also includes structure for HAMR to heat the magnetic recording medium  16  proximate to where the write pole  30  applies the magnetic write field H to the recording medium  16 . Specifically, such structure for HAMR may include, for example, a planar optical waveguide schematically represented by reference number  50 . The waveguide  50  is in optical communication with a light source  52 . The light source  52  may be, for example, a laser diode, or other suitable laser light sources for coupling a light beam  54  into the waveguide  50 . Various techniques that are known for coupling light beam  54  into the waveguide  50  may be used in conjunction with the invention, such as, for example, the light source  52  may work in association with an optical fiber and external optics, such as an integrated spherical lens, for collimating the light beam  54  from the optical fiber toward a diffraction grating (not shown). Alternatively, for example, a laser may be mounted on the waveguide  50  and the light beam  54  may be directly coupled into the waveguide  50  without the need for external optical configurations. Once the light beam  54  is coupled into the waveguide  50 , the light may propagate through the optical waveguide  50  toward a truncated end  56  of the waveguide  50  that is formed adjacent the air-bearing surface (ABS) of the recording head  22 . 
     As shown in  FIG. 3 , the heat-assisted magnetic recording medium  16  includes a composite magnetic recording film  42  of the present invention. The recording medium  16  also includes a substrate  38 , an optional soft underlayer  40 , an optional seed layer  41  and a protective overcoat  43 . The substrate  38  may be made of any suitable material such as ceramic glass, amorphous glass, aluminum or NiP coated AlMg. The soft underlayer  40  has a typical thickness of from about 50 to about 1,000 nm, and may be made of any suitable material such as CoFe, FeCoB, FeAlN, FeAlSi, NiFe, CoZrNb or FeTaN. The soft underlayer  40  may also comprise laminated structures such as (FeCoB/Ta)·n where n is from 2 to 10, or (FeAlSi/C)·n where n is from 2 to 10. The soft underlayer  40  may further comprise exchange biased structures such as Cu/(IrMn/FeCo)·n where n is from 1 to 5. The seed layer  41  has a typical thickness of from about 1 to about 50 nm and may be used to control properties such as orientation and grain size of the subsequently deposited layers. For example, the seed layer  41  may be a face centered cubic material such as Pt which controls the orientation of the subsequently deposited film  42 , may be a material such as Ru or Rh which controls grain size and facilitates epitaxial growth of the subsequently deposited layers, or a combination thereof. The seed layer may be made of one or more layers of material such as CoCr, CoCrRu, Ru, Pt, Pd, Rh, Ta, TiC, indium tin oxide (ITO), AIN or ZnO. The protective layer  43  may be made of any suitable material such as diamond-like carbon. 
       FIG. 3  illustrates a magnetic recording bit  60  (not drawn to scale) in the recording track  27  of the recording film  42 . The recording bit  60  has a width W measured across the direction of the recording track  27 , and a thickness T measured through the thickness of the film  42 . The recording bit  60  also has a length L measured in the direction of the recording track  27 , as most clearly shown in  FIG. 4 . The width W of the bit  60  typically ranges from about 10 to about 200 nm, for example, from about 30 to about 100 nm. The length L of the bit  60  typically ranges from about 5 to about 50 nm, for example, from about 7 to about 10 nm. The thickness T of the bit  60  typically ranges from about 2 to about 50 nm, for example, from about 5 to about 10 nm. 
       FIG. 4  is an isometric view of the recording bit  60 .  FIG. 5  is a top view of the recording bit  60  of  FIG. 4 . In accordance with the present invention, the recording bit  60  of the recording film  42  comprises superparamagnetic nanoparticles  62  dispersed in an antiferromagnetic or ferrimagnetic matrix  64 . As shown in the embodiment of  FIG. 4 , the superparamagnetic nanoparticles  62  have easy axes of magnetizations aligned in a vertical direction perpendicular to the plane of the recording film  42 , or the magnetic easy axis could be in the film plane as in longitudinal recording. As shown in  FIG. 5 , each superparamagnetic nanoparticle  62  has a diameter D which typically ranges from about 0.5 to about 5 nm. For example, the average diameter D of the nanoparticles  62  may be from about 0.7 to about 3 nm, or from about 1 to about 2 nm. As shown in  FIG. 5 , adjacent nanoparticles  62  are separated by an interparticle spacing I. The interparticle spacing I typically ranges from about 1 to about 10 nm. For example, the interparticle spacing I may be from about 2 to about 6 nm. 
     The nanoparticles  62  of the recording film  42  typically comprise from about 20 to about 70 volume percent of the recording film, for example, from about 25 to about 60 volume percent. The matrix  64  of the recording film  42  typically comprises from about 30 to about 80 volume percent of the recording film  42 , for example, from about 40 to about 75 volume percent. Each recording bit  60  typically includes at least 40 or 50 of the nanoparticles  62 . 
     The superparamagnetic nanoparticles  62  may be made of material such as a Co alloy. For example, the nanoparticles  62  may comprise a Co—Pt alloy comprising from about 10 to about 30 atomic percent Pt. The antiferromagnetic or ferrimagnetic matrix  64  may be made of any suitable material such as NiO, CoO, CoF 3 , FeF 3 , LaFeO 3 , NdFeO 3 , HoFeO 3 , ErFeO 3 , Al—Mn alloy and/or Pt—Mn alloy. 
     The material of the matrix  64  may have a Neel temperature T N  above 373K (100° C.). One example of a matrix material is NiO (T N =523K). It can be replaced by CoF 3  (T N =460K), FeF 3  (T N =394K), LaFeO 3  (T N =740K), NdFeO 3  (T N =760K), HoFeO 3  (T N =700K), ErFeO 3  (T N =620K), etc. 
     During heat assisted magnetic recording a laser  52  as shown in  FIG. 2  or any other suitable heat source may be used to heat the matrix above or close to the Neel temperature TN of the matrix  64  of the recording film  42 . A moderate magnetic write field H generated by the head  22 , is synchronized with the heat source. In accordance with the present invention, the antiferromagnetic or ferrimagnetic matrix  64  of the recording film  42  will lose its thermal stability below but close to the T N , or totally lose its antiferromagnetic ordering. Since the Curie temperature of the superparamagnetic particles is much higher than the T N , the superparamagnetic particles still have very large magnetization, which enables them to be aligned with the writing field. Immediately, the matrix is cooled down in the field. When the antiferromagnetic (ferrimagnetic) matrix is cooled down far below the T N , it regains thermal stability and couples with the superparamagnetic particles which are aligned in the field direction. Then the field is removed. Since the nanoparticles  62  are superparamagnetic and coupled with the antiferromagnetic or ferrimagnetic matrix  64 , their orientation of magnetization will be maintained as soon as the matrix  64  switching process is complete. A conventional GMR reader or the like may be used to read back the magnetic signal generated by the ferromagnetic grains of the superparamagnetic nanoparticles. 
     EXAMPLE 1 
     In this example, NiO is used as an antiferromagnetic matrix material, and a Co—Pt alloy is used as the ferromagnetic material of the superparamagnetic nanoparticles. NiO has a moderate Neel temperature of about 250° C., superior corrosion resistance, and a relatively high blocking temperature of 200° C. The Co—Pt alloy has a hexagonal close packed (hcp) structure and its magnetic easy axis is the c-axis. A Ru seed layer is sputter deposited on top of a glass substrate, then the Co—Pt alloy and NiO are cosputtered by conventional sputtering techniques onto the Ru layer. The film is then covered with a protective carbon overcoat. The magnetic easy axis of the ferromagnetic grains of the Co—Pt nanoparticles are perpendicular to the film. 
     EXAMPLE 2 
     In this example, NiO is used as an antiferromagnetic matrix material and a Co—Pt alloy is used as the ferromagnetic material of the superparamagnetic nanoparticles. A MgO layer is deposited onto a glass substrate. A layer of NiO is deposited onto the MgO with the NiO grains oriented in the (100) direction. Co—Pt and NiO are then cosputtered onto the NiO prelayer, followed by deposition of a protective carbon overcoat. The magnetic grains of the Co—Pt nanoparticles have easy axes randomly oriented in the film plane. 
     Whereas particular embodiments of this invention have been described above for purposes of illustration, it will be evident to those skilled in the art that numerous variations of the details of the present invention may be made without departing from the invention as defined in the appended claims.