Magnetic recording media with antiferromagnetically coupled ferromagnetic films as the recording layer

MAGNETIC RECORDING MEDIA WITH ANTIFERROMAGNETICALLY COUPLED.FERROMAGNETIC FILMS AS THE RECORDING LAYER

Technical Field.

f- This invention relates generally to magnetic recording media, and more 5 particularly to thermally stable high density media.!

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Background of the Invention

Conventional magnetic recording media, such as the magnetic recording disks in hard disk drives, typically use a granular ferromagnetic layer, such as a sputter-deposited cobalt-platinum (CoPt) alloy, as the recording medium. Each 10 magnetized domain in the magnetic layer is comprised of many small magnetic grains. The transitions between magnetized domains represent the bits of the recorded data. IBM's US patents 4,789,598 and 5,523,173 describe this type of conventional rigid disk.

As the storage density of magnetic recording disks has increased, the product of the remanent magnetization Mr (the magnetic moment per unit volume of ferromagnetic material) and the magnetic layer thickness t has decreased.

Similarly, the coercive field or coercivity (H_c) of the magnetic layer has increased.

This has led to a decrease in the ratio Mrt/H_c. To achieve the reduction in Mrt, the thickness t of the magnetic layer can be reduced, but only to a limit because the 20 layer will exhibit increasing magnetic decay, which has been attributed to thermal activation of small magnetic grains (the superparamagnetic effect). The thermal stability of a magnetic grain is to a large extent determined by K_UV, where K_u is the magnetic anisotropy constant of the layer and V is the volume of the magnetic grain. As the layer thickness is decreased, V decreases. If the layer thickness is 25 too thin, the stored magnetic information will no longer be stable at normaldisk drive operating conditions.

One approach to the solution of this problem is to move to a higher anisotropy material (higher K„). However, the increase in K_u is limited by the point ' .2 where the coercivity H_c; which is approximately equal to K_u/Mr, becomes too great to be written by a conventional recording head. A similar approach is to reduce the Mr of the magnetic layer for a fixed layer thickness, but this is also limited by the ^:coercivity that can be written. Another solution is to increase the intergranular exchange, so that the effective magnetic volume V ofthe magnetic grains is ’ increased. However, this approach has been shown to be deleterious to the. intrinsic signal-to-ndise'ratio (SNR) of the magnetic layer.

What is need edis magnetic recording media that will support very high density recording while retaining good thermal stability and SNR.

Summary of the Invention

The invention is a magnetic recording medium wherein the magnetic recording layer is at least two ferromagnetic films antiferromagnetically coupled together across a nonferromagnetic spacer film. Because the magnetic moments ofthe two antiferromagneticallyrcoupled films are oriented antiparallel, the net remanent magnetization-thickness product (Mrt) of the recording layer is the difference in the Mrt values ofthe two ferromagnetic films. This reduction in Mrt is accomplished without a reduction in the thermal stability of the recording medium because the volumes of the grains in the antiferromagnetically-coupled films add constructively. The medium also enables much sharper magnetic transitions to be achieved with reduced demagnetization fields, resulting in a higher linear bit density for the medium. In one embodiment the magnetic recording medium comprises two ferromagnetic films, each a granular film of a sputter deposited CoPtCrB alloy, separated by a Ru spacer film having a thickness to maximize the antiferromagnetic exchange coupling between the two CoPtCrB films. One of the ferromagnetic films is made thicker than the other, but the thicknesses are chosen so that the net., moment in zero applied magnetic field is low, but nonzero.

For a fuller understanding of the nature and advantages of the present invention, reference should be made to the following detailed description taken together wilh the accompanying figures.

-2- . ' ί 3 * Brief Description of the Drawing ,

Fig. 1 is a schematic sectional view of the antiferromagnetically (AF).coupled magnetic recording layer in a recording medium according to the present invention.

Fig. 2A is a schematic illustration of the AF-coupled layer illustrating the orientations of the moments of the ferromagnetic films at a recorded magnetic transition. . >

Fig. 2B is a graph of calculated magnetic field above the AF-coupled layer and a single layer (SLj medium as a function of downtrack position from a transition.

Fig. 3 is a schematic sectional view of the disk structure of the present invention illustrating the substrate, underlayer, the films in the AF-coupled layer, and the protective overcoat.

Fig. 4 is a magnetic hysteresis loop for the structure with the AF-coupled layer of Fig. 3.

Detailed Description of the Invention

The magnetic recording medium of the present invention has a recording layer formed of two or more ferromagnetic films that are exchange-coupled antiferromagnetically (AF) to their neighboring ferromagnetic films by one or more nonferromagnetic spacer films. This is shown schematically in Fig. 1 for a recording layer 10 made up of two ferromagnetic films 12, 14 separated by a nonferromagnetic spacer film 16. The nonferromagrietic spacer film 16 thickness and composition are chosen so that the magnetic moments 22, 24 of adjacent films 12, 14, respectively, are AF-coupled through the·nonferromagnetic spacer film 16 and are antiparallel in zero applied fields.

The AF coupling of ferromagnetic films via a nonferromagnetic transition· ' metal spacer film has been extensively studied and described in the' literature. In . general, the exchange coupling oscillates from ferromagnetic to antiferromagnetic with increasing spacer film thickness. This oscillatory coupling relationship for selected material combinations is described by Parkin et al. in Oscillations in

Exchange Coupling and Magnetoresistance in Metallic Superlattice Structures: Co/Ru, Co/Crand Fe/Cr, Phys. Rev. Lett., Vol. 64, p. 2034 (1990).' The material combinations include ferromagnetic films made of Co[ Fe, Ni, and their alloys, such as/NFFe, Ni-Co, and Fe-Co, and nonferromagnetic spacer films such as ruthenium (Ru), chromium(Cr), rhodiurn (Rh), iridium (lr), copper (Cu), and their alloys. For each such material combination, the oscillatory exchange coupling relationship has to be determined, if.not already known, so that the thickness of the nonferromagnetic spacer film is selected to assure antiferromagnetic coupling between the two ferromagnetic films. The period of oscillation depends on the rionferromagnetic spacer material, but the strength and phase of the oscillatory coupling also depends on the ferromagnetic material and interfacial quality. The oscillatory antiferromagnetic coupling of ferromagnetic films has been used in .spin-valve type giant magnetoresistance (GMR) recording heads to design continuous magnetized antiferromagnetically codpled films whose magnetic moments are rigidly coupled together antiparallel during operation of the head. These type of spin-valve structures are described, for example, in IBM patents 5,408,377 and 5,465,185. The '185 patent describes a structure used in many commercially available spin-valve GMR heads,.namely a laminated antiparallel pinned ferromagnetic layer having ferromagnetic films whose moments are rigidly coupled together arid remain stationary during operation of the head.

The films 12, 14 have magnetic moment values of Mr, t, and Mr₂t₂, respectively. (Because the remanent magnetization Mr is expressed as the magnetic moment per unit volume of ferromagnetic material, the product Mrt is the magnetic moment per unit area for a magnetic layer of thickness t.) For this AF-coupled structure the orientations of the magnetic moments 22, 24 of adjacent films 12, 14, respectively, are aligned antiparallel and thus add destructively to \ i ' reduce the magnetic moment of the composite layer 10. The arrows 22, 24 represent the moment orientations of individual magnetic domains that are directly above and below one another across the AF coupling film 16. In the absence of an applied magnetic field, when the’ferromagnetic film 14 is deposited onto the

r. -- - -4-.

medium substrate, it will have a granular structure with multiple adjacent grains being coupled together to form individual magnetic domains. In the absence of an applied magnetic field the moments of these domains in film 14 will be essentially, randomly oriented. The spacer film or AF-coupling film 16 is then deposited to the : i correct thickness directly on ferromagnetic film 14. Next , the second ferromagnetic · film 12 is deposited directly on the AF coupling film 16. As the grains of ferromagnetic film 12 grow they will form magnetic domains with moment orientations that are antiparallel to the moment orientations of ferromagnetic film 14 that are directly across the AF coupling film 16. j

The type of ferromagnetic material and the thickness values.t,, t₂ of the ferromagnetic films 12, 14 are chosen so that the net moment in zero applied field will be low, but nonzero. For the case shown in Fig. 1, the Mrt for the structure is. given by Mr, t,-Mr₂t₂. In the. preferred; embodiment, Mr, t, should be > Mr₂t₂. This may be accomplished by using the same ferromagnetic materials in'the two films 12, 14 and having t, be greater than t₂, or the magnetization (the magnetic moment per unit volume of material) of the two ferromagnetic films Vnay be made different by using different ferromagnetic materials for the two films. While Fig. 1 is shown for a two-film structure .with a single spacer film 16, the invention is extendible to structures with multiple spacer films and multiple ferromagnetic films.

The present invention has a number of advantages over a magnetic layer formed as a single layer of ferromagnetic material. Low remanent magnetization can be obtained without using ultra-thin magnetic layers or low-magnetization alloys. This avoids the problems of thermal instability and difficulty in writing discussed above. If the magnetic layer.in Fig. 1 is compared to a single-layer consisting of only film 12, for example, the addition of the AF-coupled ferromagnetic film 14 reduces the. net.magnetic moment of the composite structure without .,; .decreasing either the thickness or the magnetization of film 12.

The enhanced thermal stability of the composite structure compared to a single magnetic layer arises because the anisotropy of the grains in both films 12 ) and 14 are substantially uniaxial·, and thus can add constructively even if the . ’ -5- ' I r ,· ' ^A ‘ < 6f

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i.- magnetic moments of films 12, 14 are anliparallel. The resulting stability parameter' ' of the coupled system K_UV is givenby Ku,V, < KuV < (Ku,V,+Ku₂V₂). where Ku,V, and Ku₂V₂ are the anisotropy energies of typical grains in films 12, J4, respectively. The-tipper limit for the composite stability parameter KuV = Ku,V,+Ku₂V, will be achieved for the case when magnetic grains in film 12 and 14 are strongly coupled and share a common anisotropy axis direction. The magnetic volume V of the ί composite structure (layer 10) that determines the thermal stability will be approximately the sum of the volumes of the exchange-coupled grains in films 12 and 14, whereas the magnetic moment of layer 10 is the difference of the individual 70 moments of films 12, 14. The antiferromagnetic coupling between the two ferromagnetic films provides a mechanism to increase the effective film thickness while reducing the net Mrt value of the composite structure. Thus the ferromagnetic films can contain very.small diameter grains and maintain thermal stability.

The AF-coupled medium according to the present invention is shown schematically in Fig. 2A with a recorded or written magnetic transition. The plus (+) and minus (-) symbols.represent the magnetic poles arising from the transition. The calculated longitudinal field (H„) 1 x10‘^e m. (10 nm) above the surface of the AF-coupled medium is shown in Fig. 2B as a function of X direction or downtrack position from the transition. The moment and thickness values for the two films 12, 14 and the calculated Mrt for the AF-coupled layer are listed in Fig. 2B. For comparison, Fig 2B also shows model calculations of longitudinal magnetic field arising from transitions in a single-layer (SL) medium that has a similar Mrt. The thickness values (t, and t₂) were chosen such that the peak longitudinal field was the same for the AF-coupled medium compared to the SL medium. The total thickness of the .:25 ferromagnetic material in the AF-coupled medium is 2.7 times thicker. Therefore, ' the AF-coupled medium should be more thermally stable than the SL medium. The ^:longitudinal field profile in the dowritrack direction decays faster for the AF-coupled medium, resulting in a sharper transition. This indicates that the transitions can be spaced closer than in the SL medium, resulting in a higher linear bit density for.the medium. While not shown in Fig. 2B, calculations have also shown that the demagnetization field from a transition within the AF-coupled medium also decreases faster than in.the SL medium. In addition, the magnitude and sign of the demagnetization field depends on the Y position (see Fig. 2A) within the medium. Th-e^for certain Y positions within the medium, the demagnetization field is reduced 5 to zero. Small demagnetization fields are desired because they can affect other transitions and cause the transition to demagnetize itself.

The present invention has been demonstrated using conventionaTCoPtCrB longitudinal recording media.alloys for the ferromagnetic films. An example structure is shown in .Fig. 3. The structure was fabricated using conventional sputter deposition equipment and processes. The films forming the structure were grown onto a Cr underlayer deposited onto a substrate of a AIMg disk blank with a nickel-phosphorous (NiP) surface coating, with the substrate temperature at . approximately 200 °C. The ferromagnetic films are CoPtCrB, with the top film corresponding to film 12 in Fig, 1 being, thicker than the bottom ferromagnetic film corresponding to film 14 in Fig. 1 (1.2 x 10'm vs. 7 x 10'⁹m (12 nm vs. 7 nm)! The ' ' ;

nonferromagnetic spacer film is a 6 x IO¹⁰ m (6 nm) Ru film. As with single-layer media, it is advantageous to use a granularferromagnetic material with isolated magnetic grains lo lower the media noise. The Ru film thickness was chosen to beat the first antiferromagnetic peak in the oscillatory coupling relationship. For this example, each CoPlCi B ferromagnetic film included an interface film consisting essentially of 5 x 10'^,0m (0.5 nm) of Co at the interface with the Ru film. These, ultra-thin Co films increase the inteifacial moment between the ferromagnetic films and the Ru film, resulting in enhanced antiferromagnetic coupling. However, antiferromagnetic exchange coupling has been demonstrated without incoporating the Co interface films in the CoPtCrB ferromagnetic films. J

Fig. 4 shows the major hysteresis loop (solid line) and the remanent .

hysteresis loop (dashed line) measured at T- 120°C (350°K) for the structure of Fig. 3. - -- ,.

Referring first to the remarient hysteresis loop, it is obtained by saturating the AF-coupled layer in a,positive field and then applying an increasing reverse negative field and measuring the remanent moment in the layer after the neoafive · sp field is applied. .' The remanent loop is a plot of the remanent moment versus the magnitude of the reverse field. For this sample the remanent loop shows Mrt=0.21, the remanent/bercive field Η_ο=3.2.Κ0β, and S-0.92 at room temperature, where

S' is a measure of the slope of the remanent loop at Η_σ. For comparison, a similarly grown 1.5.x 10 ^em (15-nm) single layer of the same CoPtCrB alloy has properties of

Mrt=0.38, H_cr=2.4 kOe and S-0.76at room temperature. Thus, the AF-coupled :

medium allows a significantly lower Mrt to be achieved with a greater total magnetic layer thickness. · . .

Referring next to the major hysteresis loop of Fig. 4, the pairs of horizontal arrows indicate the orientation of the ferromagnetic films in the AF-coupled layer at ' . · . I different points in the hysteresis loop. The applied field is increased in the positive direction (arrows 30, 32). For large applied fields (>3000 Oe), the antiferromagnetic coupling is overcome and the moments of the two ferromagnetic films are both parallel to the applied field (arrows 42, 44). As the applied field is reduced (arrow 34) the moment of the thinner bottom ferromagnetic film reverses and is becomes antiparaliel to the moment of the thicker top ferromagnetic film .: (arrows 52, 54) and to the applied field with a drop in the net moment. This switch occurs roughly at the exchange field felt by the bottom film (H_ex2=20Q0 Oe) arising, from the coupling across the Ru film. The value of H_ex2 = J_ex/1v1₂t₂ , where J_e!I is the antiferromagnetic interface exchange energy density across the Ru spacer layer and M₂ and t₂ are the magnetization and thickness of the bottom ferromagnetic film, respectively. For a'n antiparallel alignment of the ferromagnetic films to be realized requires that H_ex2 exceed the coercive field required to reverse the bottom ferromagnetic film (H^). H_o is the coercive field of the bottom film, assuming no exchange interaction with the top ferromagnetic film. Thus, the magnetic properties and thickness of the bottom film, as well as the AF-coupling film, must be designed to maintain H_ex2>H_c2. |

The remanent state after saturation in a positive field is given by the moment of the top ferromagnetic film parallel to the field direction and the moment of the bottom ferromagnetic film antiparallel to the positive field direction (arrows 52, 54).

in a reverse applied field’jarrow 36), the'magnetic state is stable until the moment'

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. . 9 of the top ferromagnetic film reverses and the moments of both films are parallel^;. and aligned in the negative saturation state (arrows 62, 64). The switching of the ' moment of the top ferromagnetic film determines the coercive field of the AEr-coupled layer and is given by H_c = H_ex, + H_c1 where H_exl is the exchange field _Lacting on the top ferromagnetic film (H_e>1=J_eM/M,t,) and H_C1 is the coercive field of the top ferromagnetic film, assuming ho interaction with the bottom ferromagnetic ' film. Thus, the properties of the top ferromagnetic film and the AF-coupling film must be designed to maintain H_c of the composite structure below the expected f , write field of the head. For this example the pathway to go from one remanent state (arrows 52, .54) to the next remanent state (arrows 72, 74) goes through an intermediate state .where the moments of the two films are parallel (arrows 62, 64). Thus, in contrast to AF-coupled structures used in spin-valve GMR'recording heads, the moments of the ferromagnetic films in the medium according to the present invention are not rigidly coupled together across the AF-coupling film because the coupling must be overcome to write on the medium. The hysteresis * loop of Fig. 4 exhibits the desired feature of an AF-coupled layer, i.e/ a low remanent magnetization relative to the saturation magnetization.

Recording performance*tests on,the AF-coupled layer were performed using a conventional longitudinal recording head! Signal to noise ratio measurements determined a media S₀NR of 31.9 dB at 9500 flux changes per millimeter (fc/mm), where S_o is the isolated pulse amplitude and N is the integrated media noise at 9500 fc/mm recording density. These results demonstrate the viability of AF-codpled magnetic layers for data storage. !

The AF-coupled media according to the present invention has also been demonstrated for structures with and without one or both Co interface films, with and without one or both CoCr interface layers, and with CoCrPtTa ferromagnetic films.

While the present invention has been particularly shown and described with reference to the preferred.embodiments, it will be understood by those skilled in the art that various changes in form and detail may be made without departing from thei spirit, scope, and teaching of the invention. Accordingly, the disclosed invention is to be considered merely as illustrative and limited in scope only as specified in the appended claims. . . .