The Effect of High Deposition Energy of Carbon Overcoats on Perpendicular Magnetic Recording Media using a Novel Approach M. Shakerzadeh1,a, S.N. Piramanayagam1, R. Ji1, B. Varghese1, H.K. Tan1, M. Bosman2 1
Data Storage Institute, (A*STAR) Agency for Science, Technology and Research, 117608
Singapore 2
Institute for Materials Research and Engineering, (A*STAR) Agency for Science,
Technology and Research, 3 Research Link, 1176027 Singapore
High-energy carbon deposition techniques provide thin overcoats with high corrosion and wear protection for magnetic recording media applications. This letter describes the effect of high-energy deposition on the implantation induced changes in magnetic and structural properties of granular perpendicular magnetic recording media. To control the deposition energy a negative substrate bias of 0-300V was applied to the substrate during the deposition. In order to observe subtle changes in a thin region of recording media, a novel antiferromagnetically coupled layer structure was used. Clear changes in the magnetic properties, observed as a function of the carbon deposition energy, correlate with other measurements such as X-ray Photoelectron Spectroscopy, indicating the need to consider such effects when designing media and overcoat.
a
Author to whom correspondence should be addressed. Electronic mail:
[email protected]
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Hard disk drives have played an immense role in the era of information technology. The areal density of stored bits has increased by 2000 times in the last two decades, which enabled many applications (video, games, online community etc.) that require large information storage capacity.1 Such a progress in the capacity of hard disk required improvement in all key aspects of magnetic recording. Magnetic spacing, which is the spacing between the recording surface and the read/write head, is one such factor which limits the ultimate possible density of information. A critical parameter that contributes to the magnetic spacing is the thickness of the carbon overcoat on the magnetic layers of the recording media.2 Although the carbon overcoat is essential to protect the recording media from corrosion and wear as a result of head-disk contacts, its thickness leads to a loss of resolution and achievable density.3 Therefore, the design of carbon overcoats is a crucial aspect of recording media fabrication.4 Amorphous carbon thin film with high fraction of sp3 bonding hybridization is an ideal choice as media and head overcoat in hard disk drive (HDD) technology due to the high density, outstanding mechanical properties and chemical inertness.5 It is known that the mechanical properties and density of carbon thin films are direct functions of the carbon bonded sp3 hybridization.6 According to both subplantation7 and stress-induced mechanisms8 for the stability of sp3 hybridization, the sp3 content of the carbon thin films is determined by controlling the deposition energy of the carbon ions (in ion based deposition techniques such as filtered-cathodic vacuum arc (FCVA) or mass selected ion beam (MSIB) deposition) or other carbonaceous precursors (such as hydrocarbon in plasma enhanced chemical vapour deposition (PECVD)).9 Techniques such as facing-targets sputtering10 and hybrid facingtargets sputtering11 have also been employed to take advantage of the deposition energy in improving the properties of carbon overcoat. Depending on the deposition techniques, there is 2
an optimum deposition energy at which the highest sp3 content and density can be achieved. It should also be noted that besides excellent mechanical properties, tetrahedrally amorphous carbon (ta-C) films with high sp3 content have outstanding thermal stability suitable for overcoat applications in heat-assisted magnetic recording (HAMR) media, which is emerging as the next generation technology.12 Although energetic deposition results in a dense carbon film, it may also affect the structure and composition and hence the magnetic properties of the underlying media layer.13 This issue has not been investigated systematically in recent CoCrPt-oxide based thin films which are commonly used as recording media layer in perpendicular magnetic recording technology. However, as the current generation recording media have a high coercivity, the changes associated with the overcoat deposition may not be observed clearly, especially if the affected layer is thin. Therefore, in this paper, we have employed the overcoat deposition on an inverse antiferromagnetically coupled (AFC) media structure, in order to understand the changes in the magnetic properties associated with high energy deposition. AFC structures were commonly used in longitudinal magnetic recording to enhance the magnetization thermal stability.14,15 In this letter, the effects of the energy of the impinging carbon ion on the composition and magnetic properties of inverse AFC CoCrPt based media are reported. Since shallow carbon implantation may show more significant effects in the thinner stabilizing layer (SL) rather than the thicker recording layer (RL), inverse AFC structures were used as shown in Figure 1. In an inverted AFC structure, two CoCrPt layers are antiferromagnetically coupled through a thin (0.8 nm) Ruthenium spacer layer which induces antiparallel coupling. We will denote the thinner magnetic layer at the top as SL and the thicker bottom layer as RL throughout this paper.
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The AFC media samples were prepared on glass disk substrates by dc magnetron sputtering using an Intevac Lean 200 tool for making hard disk media. Typical seedlayers to induce a hcp-Co(002) texture were deposited prior to the deposition of the recording layer. The recording layer has oxide segregants in an otherwise CoCrPt matrix. The stabilizing layer and the recording layer were deposited in an identical manner except for the thickness. Alternating gradient force magnetometry (AGM) was used to study the magnetic properties of the films. X-ray photoelectron spectroscopy (XPS) was used to monitor the changes in the composition depth-profile of the media layers due to energetic carbon deposition. Figure 2 shows the magnetization vs. applied magnetic field (M-H) hysteresis loop of the asdeposited inverse AFC structure shown in Figure 1. In the as-deposited structure, the magnetization reversal of the SL starts in the first quadrant and extends to the second quadrant of the hysteresis loop. Therefore, it reduces the remanent moment (Mr)-thickness(t) product Mrt at the remanence slightly. The coercivity (Hc) and exchange field (Hex) of the SL can be determined from the minor loop.16 It is obvious from the minor loop of figure 2 that the antiparallel configuration of magnetic moments in as-deposited samples is achieved only for a negative applied field. Such a design was intentional, as the change of Hc and Hex can be more easily observed in samples when the carbon deposition changes the magnetic properties.
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Figure 1: (A) Schematics and (B) HRTEM image of the Carbon deposited inverse AFC media
Figure 2: Hysteresis loop and minor loop of the inverted AFC structure shown in Figure1. The major loop depicts the reversal of the two layers and the minor loop depicts the reversal of the stabilizing layer only during the application of a narrow range of field. In order to study the effect of energetic carbon deposition, as-deposited inverse AFC media samples were exposed to carbon plasma using an off-plane double bend FCVA. Compared to other conventional techniques such as PECVD and sputtering, FCVA possesses highly ionized plasma. Therefore, by applying a negative DC bias on the substrate, the ion energy can be very well controlled.17 Although the glass substrate is an insulator, the conductive 5
metallic magnetic layer transfers the bias to the top surface of the sample. In addition, a conducting path was made by sticking an aluminium foil to the substrate which also makes a contact with the substrate holder. Figure 3(A) shows the hysteresis loop of the as-deposited and carbon irradiated inverse AFC structure (for a bias voltage of 250 V). It can be noticed that the hysteresis loop, particularly the portion that corresponds to the magnetization reversal of SL, changes shape due to the modification of the SL by the carbon implantation. The kink of the hysteresis loop shifts from the second quadrant to the first quadrant, expected to be due to a change in the coercivity of the SL. In order to obtain a clear trend on the changes caused by the energetic carbon deposition on the anisotropy constant or the interface coupling, the coercivity of the SL and the exchange field were estimated from the minor hysteresis loop (as shown by the equations in Figure 2). The values, thus extracted for all the samples are shown in Figure 3. The Hc and Hex for the uncoated inverted AFC sample (without any carbon deposition) is also shown—indicated by the black ellipse—for the sake of comparison. It can be noticed that the Hc shows a steady decrease as a function of the magnitude of bias voltage. Such a change is expected if carbon atoms are implanted into the SL. The value of Hex, on the other hand, does not show a systematic change beyond the experimental errors.
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Figure 3: (A) Hysteresis loop of the as deposited inverted AFC structure and the same structure irradiated with carbon plasma while applying 250V substrate bias, (B): Variation of the exchange field (Hex) and the coercivity of the stabilizing layer (HC) as a function of deposition ion energies
In order to gather more information about the carbon implantation into SL, depth profile XPS was used to measure the composition of the as-deposited and carbon irradiated media. Figure 4 shows the typical XPS composition depth profile of the media post treated by carbon plasma. As can be observed, carbon implants into the top SL while its concentration decreases with the implantation depth. No trace of carbon can be observed at the Ru interlayer, as reported before.18 The variation of the carbon concentration as a function of sputtering depth is shown in Figure 4-B for various values of carbon deposition energy. For the sake of comparison, the x-axis values were normalized with respect to the maximum Ru peak of the samples (sputtering time at which Ru has its highest content is considered as x=1) and C concentration for different deposition energies are compared. Figure 4-B shows that by increasing the carbon deposition energy, the atomic percentage of the carbon increases at a constant depth inside the SL. This is better clarified in Figure 5 which shows the carbon atomic concentration at the vicinity of 7
the Ru interlayer (x=0.9). As can be observed, by using 300V substrate bias, the carbon concentration at x=0.9 is 50% which shows a significant dilution of the magnetic elements in the media layer due to carbon implantation. This can have a considerable effect in decreasing the Hc of SL as observed in Figure 3.
Figure 4: XPS analysis of the as deposited and carbon plasma irradiated AFC structure. (A): Typical depth profile of the carbon irradiated media. (B): Variation of carbon depth profile as a function of carbon ion energy. C1s profile for different deposition bias voltages are shown with different colours as illustrated by the figure inset. For comparison, the profile is normalized based on the Ru peak (x=1).
Figure 5: Carbon atomic percentage at x=0.9 (approximately 2 nm from Ru layer) as a function of energy of the deposited carbon
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In conclusion, it was observed that energetic carbon on recording media will result in the dilution of the magnetic layer composition and a reduction in the coercivity; the extent of which is directly related to the deposition ion energy. This study clearly highlights the implantation of carbon overcoat at high energy deposition, even in the case of media deposited at room temperature. This point needs to be taken into consideration while designing media and the overcoat.
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