Magnetic Properties and Corrosion Resistance Studies ... - IEEE Xplore

IEEE TRANSACTIONS ON MAGNETICS, VOL. 46, NO. 4, APRIL 2010

1069

Magnetic Properties and Corrosion Resistance Studies on Hybrid Magnetic Overcoats for Perpendicular Recording Media Wei Choong Poh1;2 , S. N. Piramanayagam1 , and T. Liew1;2 Data Storage Institute, A*STAR, Singapore 117608 Department of Electrical and Computer Engineering, National University of Singapore, Singapore 119260 Magnetic properties and corrosion inhibition properties of hybrid magnetic overcoats (Hy-MOC) that are suitable for ultra-highdensity recording are investigated. Two types of Hy-MOC using different types of Co-based alloys were investigated for the magnetic overcoat (MOC). Hy-MOC( ) used CoCrPt : SiO2 (14% SiO2 ) while Hy-MOC( ) used CoCr22 as the sputtering targets for the reactive sputtering with N2 . Magnetic hysteresis loops indicated that both types of Hy-MOC—using various percentages of N2 flow rate—are coupled with the recording layers of the hard disk media. Hy-MOC also increased the coercivity of the perpendicular media which is attributed to the enhanced thermal stability. It was also found that the remanent squareness (S = Mr Ms ) of the media using the MOC was closer to 1. The negative nucleation field (Hn ) of the media using Hy-MOC( ) also showed a slight increase as compared to the medium without Hy-MOC but it is not seen for the media using Hy-MOC( ). Potentiodynamic polarization analysis of Hy-MOC samples also indicated that the Hy-MOC are able to provide sufficient corrosion protection for the hard disk media; and they are as good or even better than 2 nm amorphous carbon overcoat. Index Terms—Corrosion, magnetic spacing, overcoat, perpendicular magnetic recording, recording media.

I. INTRODUCTION

N the last few years, perpendicular recording has emerged as the dominant recording technology for the hard disk drives (HDD). It is expected that perpendicular recording technology may help to achieve up to 1000 Gb/in using the existing CoCrPt-Oxide media technology. Increasing the areal density from the recording media perspective would involve reducing the grain size, grain size distribution, improving the writability, etc. [1]–[5]. Another important consideration for recording media, which is mostly addressed by tribology researchers, is the reduction of overcoat thickness. Although thicker overcoat layers might ensure the absence of pinholes and show enhanced corrosion protection, they add to the magnetic spacing, which is the spacing between the GMR head and the magnetic layer. It has been reported that for ultra-high recording density (e.g., 500 Gbit/in and above), the magnetic spacing should be below 6.5 nm and the corresponding thickness of the disk overcoat would be in the sub-2 nm range [6]. Therefore, it is essential to find out methods to reduce the overcoat thickness, which will in turn, help to achieve a lower magnetic spacing. Two approaches have generally been used by the researchers to reduce the thickness of the disk overcoat: 1) To improve deposition techniques to fabricate thin carbon films with enhanced properties [7]–[11]. Techniques such as facing targets sputtering (FTS) and filtered cathodic vacuum arc (FCVA) belong to this category. 2) To find out alternative overcoat materials that can provide better corrosion than carbon [6], [13]. Materials such as Zirconia, silicon dioxide, silicon nitride SiN , etc. have been

I

Manuscript received August 14, 2008; revised October 13, 2009; accepted November 14, 2009. First published December 11, 2009; current version published March 19, 2010. Corresponding author: S. N. Piramanayagam (e-mail: [email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TMAG.2009.2037430

attempted [6] in this approach. A deviation from some of the more common approaches mentioned above, is the proposal of the magnetic overcoat [13], which utilizes an oxide based material for the overcoat. But this solely oxide based magnetic overcoat has a limitation, that the oxide material is not compatible with the presently used lubricants. Although finding lubricants for the oxide based magnetic overcoat is a probable solution, a simpler but effective approach that does not involve too many changes from the existing processes in recording media is more desirable. Such an approach is the hybrid magnetic overcoat (Hy-MOC) containing two layers with different purposes [14]; an amorphous carbon overcoat at the top and a magnetic overcoat at the bottom. In this work, two types of Hy-MOC—reactively sputtered using CoCrPt-SiO (with 14 mol% SiO content) and CoCr —in different nitrogen N flow rates were prepared and their magnetic and corrosion resistant behavior were investigated. II. EXPERIMENTAL PROCEDURE The samples used in this study were prepared using 95 mm polished AlMg substrates by dc magnetron sputtering at room temperature using a BPS Circulus M12 tool. For recording measurements using spin stand, antiferromagnetically coupled soft underlayers (SULs) of the type CoTaZr/Ru/CoTaZr (about 100 nm thick) were deposited on Ti seed-layers. Over the SUL, 5 nm Ta seedlayer, 15 nm Ru intermediate layer, 14 nm CoCrPt-SiO recording layer and the overcoats were deposited sequentially. The working gas for all the layers was argon with a purity of Torr while 99.999%. The base pressure was kept at around and Torr. Oxygen the sputter pressure was between was used as a reactive gas in the sputtering of the magnetic layer to obtain better exchange decoupling. Two types of Hy-MOC were deposited by reactive sputtering in argon/nitrogen gas atmosphere with an applied substrate bias of 100 V. CoCrPt-SiO (with 14 mol% SiO content) (which is called as Hy-MOC henceforth) was reactive sputtered with nitrogen in order to form SiN SiO or SiN O at the grain boundary. For comparison,

0018-9464/$26.00 © 2010 IEEE

1070


another type of Co-based alloy with a composition of CoCr (Hy-MOC ) was reactive sputtered to form CrN grain boundaries. It is desirable to have nitrides in the grain boundaries as they are known for their good corrosion resistance and mechanical hardness. The 1 nm ultra-thin carbon of Hy-MOC and the carbon overcoat used for comparison was deposited using hybrid facing targets sputtering (HyFTS) technique [15]. The corrosion analysis was done using a three electrodes setup (consisting of the sample surface as the working electrode, Ag/AgCl reference electrode and the Pt counter electrode) as shown in Fig. 1. The electrolyte used in this is 0.2 M NaCl (aq) solution. In this work, all potentials were measured with respect to the Ag/AgCl reference electrode. The corrosion resistance analysis was carried out with an Autolab with potentiostat/galvanostat PGSTAT 30 using potentiodynamic polarization technique. The induced corrosion test according to “The International Disk Drive Equipment and Materials Association” (IDEMA) standard was also carried out to investigate the corrosion resistance of the Hy-MOC samples. The induced corrosion test procedure used is as follows. 1) Hold at ambient (25 C, 40% RH) conditions for 1 h to stabilize test chamber. 2) Ramp from 25 C to 60 C, at 40% RH in 2 h. 3) At 60 C, ramp from 40% RH to 80% RH in 2 h. 4) Hold at 60 C, 80% RH for 96 h. 5) At 60 C, ramp from 80% RH to 40% RH in 2 h. 6) Ramp from 60 C to 25 C, at 40% RH in 2 h. 7) Ambient (25 C, 40% RH) soak until removal, minimum of 2 h. 8) Total cycle time is 105 h (4.5 days). After the corrosion studies the samples underwent scanning electron microscopy (SEM) and Auger electron spectroscopy (AES) using A PHI SMART 200 Field Emission Scanning Nanoprobe. X-ray photoelectron spectroscopy (XPS) analysis was carried out using PHI Quantera SXM (Scanning X-ray X-ray source. The Microprobe) equipped with an Al transmission electron micrographs were taken using a high-resolution TEM (Philips CM300). The magnetic properties were characterized by a vibrating sample magnetometer (VSM) or alternating gradient magnetometer (AGM). The recording performance was studied employing a Guzik spin-stand tester with a ring-head writer and a MR reader. III. RESULTS AND DISCUSSION Fig. 2 shows the media with conventional overcoats and with the Hy-MOC. As can be seen from Fig. 2(b), the Hy-MOC is a dual layer overcoat consisting of an amorphous carbon overcoat at the top and a magnetic overcoat at the bottom. The purpose of the magnetic overcoat is to provide both corrosion protection and mechanical property. At the same time, the magnetic overcoat should have permeability as high as that of recording layer in order to reduce the magnetic spacing. It should also couple with the magnetic layer without degrading the magnetic property of the hard disk media. On the other hand, the main function of ultra-thin carbon is to provide good overcoat-lubricant interaction. In addition, the ultra-thin carbon may also act as a re-enforcement for corrosion protection and enhancement of the mechanical property of the whole Hy-MOC system. As

Fig. 1. Schematic diagram of the electrochemical setup used in this investigation.

Fig. 2. Schematic diagram of the structures of media with (a) carbon overcoat and (b) hybrid magnetic overcoat (Hy-MOC).

shown in Fig. 2, the magnetic spacing is the distance between the magnetic layer and the head. At a given flying height nm and a constant lubricant thickness, the magnetic spacing for the Hy-MOC media [Fig. 2(b)] can be greatly reduced as a result of using a 4 nm magnetic overcoat. However, the magnetic spacing for the conventional carbon overcoat media [Fig. 2(a)] is comparatively larger although it is using a typical overcoat thickness of about 3 nm. Therefore, improved performance is expected in the media with Hy-MOC. As mentioned earlier, CoCrPt-SiO (with 14 mol% SiO content) and CoCr are selected as target materials for the reactive sputtering of Hy-MOC systems. It is because, as most of the corrosion takes place through the grain boundary, having corrosion resistive materials such as SiN SiO or SiN O in and CrN in Hy-MOC at the grain boundaries Hy-MOC is expected to improve the corrosion resistance and mechanical hardness. In particular, having high SiO in the target material

POH et al.: MAGNETIC PROPERTIES AND CORROSION RESISTANCE STUDIES ON HYBRID MAGNETIC OVERCOATS

1071

TABLE I THE ESTIMATED PERCENTAGE COMPOSITIONS OF VARIOUS SPECIES IN THE GRAIN BOUNDARIES HY-MOC SYSTEMS AFTER PEAK FITTING (ASSUMING THAT THE FOLLOWING SPECIES ARE MAINLY FOUND IN THE GRAIN BOUNDARIES OF HY-MOC SYSTEMS

Fig. 3. Typical fitted XPS spectra of various elements in the grain boundaries Hy-MOC() and Hy-MOC( ).

of Hy-MOC than that used in the sputtering of the recording layer ensured a higher concentration of SiN SiO or SiN O in the grain boundary. Likewise, a Co-alloy target with a high Cr content is also desirable for similar purpose. However, in order to serve as a magnetic overcoat, these materials have to maintain a significant magnetic permeability after reactive sputtering with N . Hence, different percentage flow rate of N was used in this study to understand how a variation in the amount of N during sputtering will affect magnetic properties and corrosion protection ability. XPS analysis was carried out to verify that nitrides species and MOC . The typical fitted are formed in both MOC XPS results shown in Fig. 3 show that the nitrides of Si [shown specin Si 2s spectrum in Fig. 3(a)] and Cr [shown in Cr trum in Fig. 3(b)] are indeed formed in MOC ; with peaks

at 152.2 eV and 575.3 eV, respectively. In addition, elemental Si [shown in Si 2s spectrum in Fig. 3(a)], SiO [shown in Si 2s specspectrum in Fig. 3(a)], and Cr oxides [shown in Cr trum in Fig. 3(b)] are also deposited during the sputtering of MOC ; with peaks at 150.2 eV, 154.1 eV, and 576.9 eV, respectively. These observations are confirmed by the N 1s and O 1s spectra; with N 1s spectrum [shown in N 1s spectrum in Fig. 3(c)] showing peaks of SiN at 398.8 eV and Cr at 397.6 eV and O 1s spectrum [shown in O 1s spectrum in Fig. 3(d)] showing peaks of SiO at 532.0 eV and Cr oxides at 531.0 eV. It is also observed in the fitted N 1s spectrum that there is peak at 400.3 eV which is attributed to X-N where X may be H, O, or adsorbed N [16]. Upon further analysis of the XPS results (Table I), it was found that as N flow rate increased from 5% to 30% during the deposition of MOC led to a mere 9% increase in the SiN content; from about 21.7% to 30.6%. On the other hand, both the Si and SiO decreased from 24.0% to 16.7% and 4.3% to 2.7%, respectively. However, for Cr N and Cr oxides the variation was much smaller; from 31.5% to 34.2% and 15.77% to 18.5%, respectively. It was also confirmed by the XPS analysis that there is a presence of Cr nitrides and Cr in MOC . It was observed from the fitted Cr peak in Fig. 3(e) that there are both elemental Cr and Cr nitrides, with binding energies at 574.3 eV and 575.6 eV, respectively. This presence of Cr nitrides is confirmed by the N 1s spectrum (in Fig. 3(f)), which also further indicates the presence of both CrN at 397.5 eV and Cr N at 398.5 eV. Upon further analysis of the XPS results (Table I), it was found that the increase of N flow rate from 5% to 30% during led to a mere 5% increase in the CrN the deposition of MOC content; from about 24% to 29%. This increase in CrN corresponds to a decrease in Cr N (from 16% to 10%) which is attributed to Cr N reacting with more N (as percentage of N flow rate increased) to form the saturated species of CrN. However, the variation in the elemental Cr is minimal; ranging from 60% to 62%. In addition, atomic force microscopy (AFM) analysis was also performed to investigate the morphology of the Hy-MOC samples. The results (not shown here) indicate that the average of the various samples are less than 3 RMS roughness (based on m m scan area; scan rate at 1 Hz) which is within the required roughness.

1072


Fig. 5. The coercivity (H ) and thermal stability vs. percentage N flow rate.

Fig. 6. The remanent squareness (S) and nucleation field (H ) vs. percentage N flow rate. Fig. 4. Plots of hysteresis loop of (a) Hy-MOC() and (b) Hy-MOC( ) at different percentage N flow rate.

Fig. 4 shows the hysteresis loops of medium with a-C overcoat and that with Hy-MOC measured in the out-of-plane direction. Based on the “squareness” of the M-H curves, it is confirmed that the media using both types of Hy-MOC have their magnetic anisotropies orientated in the perpendicular direction. Although not shown here, it was also confirmed that the MOC layers exhibit hysteresis loops in the absence of recording layer, indicating a soft magnetic property for the MOC. The M-H loops for all the media with Hy-MOC show no observable “kink.” The absence of the “kink” indicates that the Hy-MOC couples seamlessly with the recording layer. This means that the magnetic moments of the media grain and the magnetic overcoat grain will switch their magnetization together in an applied magnetic field. Looking at Fig. 4(a), it was noted there is an for the samples using obvious increase in the coercivity but this is not that significant in the samples using Hy-MOC as shown in Fig. 4(b). To determine if the increase Hy-MOC in the grain volume was the cause of increase in , time-dependent coercivity was measured. Sharrock’s equation was used to of the determine the thermal stability factor samples [18]. The results are depicted in Fig. 5. As can be noticed from Fig. 5, the increase in the coercivity for the samples using Hy-MOC and Hy-MOC is indeed due to an increase in thermal stability factor , which arose from an increase in volume due to the thicker effective magnetic layer when the Hy-MOC

is coupled with the recording layer. The trend is less noticeable in Hy-MOC than in Hy-MOC . It is speculated that the segregation mechanism could be the reason for this different trend. The MOC layers show soft magnetic properties in continuous films. Therefore, sputtering of a continuous MOC layer (in the absence of nitrogen) on top of the recording layer could bring down the coercivity. In the presence of nitrogen, the grains are expected to decouple due to the formation of nitrides in the grain boundary. In Hy-MOC , the grains are decoupled with a lower nitrogen than that is needed in (as will be seen from the TEM images in a later Hy-MOC shows a faster increase of part). Therefore, Hy-MOC with nitrogen content. Fig. 6 shows the squareness (S) and nucleation field of the samples with two different types of Hy-MOC. It can be noted that all the samples using Hy-MOC have S very near to unity. This is desirable, as higher S means larger remanence and lower DC noise. It can also be observed that the samples using had an initial increase in the negative nucleation Hy-MOC decreases even field up to 5% N flow rate and then the below the value of the medium without using Hy-MOC . On the other hand, all the media using Hy-MOC have lower than the medium without Hy-MOC . Grain size, grain size distribution and the exchange coupling between grains are important issues in determining the signal-to-noise ratio (SNR) of a medium. In order to address this concern, a high resolution TEM analysis was utilized in this investigation to obtain plane view of the CoCrPt SiO magnetic layer, Hy-MOC and Hy-MOC . The results are


Fig. 7. TEM micrographs of (a) magnetic layer without Hy-MOC, (b) magnetic layer with Hy-MOC(), (c) magnetic layer with Hy-MOC( ), and (d) schematic diagram of desired magnetic layer with Hy-MOC system.

Fig. 8. Typical grain size distribution based on 300 measurements for each sample; magnetic layer without Hy-MOC, magnetic layer with Hy-MOC() and magnetic layer with Hy-MOC( ).

shown in Figs. 7 and 8, respectively. Looking at TEM micrographs in Fig. 7(a), it can be noticed that the recording layer of the perpendicular media without MOC has very well-defined grain boundaries, with an average grain pitch (center-to-center 1.5 nm. The distance between the grains) of about 7.6 in Fig. 7(b) shows an average sample using the Hy-MOC grain size of about 7.5 1.2 nm, which is very close to that of the recording layer without Hy-MOC. This is similar to the requirement depicted by the schematic diagram shown in in Fig. 7(c), Fig. 7(d). But, for the media using the Hy-MOC the average grain size has increased to about 8.1 1.2 nm and the grain boundaries were not as well-defined as the hard disk media using the Hy-MOC .

1073

Other than the grain size, it is also important to determine the grain size distribution of the samples using both Hy-MOC and Hy-MOC . The result of this investigation is depicted in Fig. 8. Based on the plot of frequency vs. grain size, it was observed that the grain size distribution curve of the hard seemed to envelop that of the disk media using Hy-MOC recording layer almost perfectly, which means that hard disk has similar grain size distribution as media using Hy-MOC that of the recording layer. On the hand, it was observed that has a distribution centered hard disk media using Hy-MOC at around 7.5 nm to 8 nm shifting to higher grain size. In addition, the grain size distribution curve had also broadened indicating a larger range of grain size, which is not preferred due to SNR considerations. Hence, the above results indicate is not likely to degrade that hard disk media using Hy-MOC SNR of the recording layer, as it has similar grain size and grain size distribution as the recording layer. But due to larger grain size, and larger grain size distribution, hard disk media may decrease the overall SNR of the hard using Hy-MOC disk media. In order to confirm the above point, as well as to check if it is possible to make disks that can support flying heads on the surface, disks were made with 2 nm a-C:N and 4 nm MOC. For this study, the carbon layer was fixed at 2 nm (instead of 1 nm), as the buffing process used in our lab was not suitable for 1 nm thick carbon layer. A spin stand test was also conducted to compare the recording performance of the media with and without the two types of Hy-MOC. The read-back signals shown in Fig. 9 suggested that all the samples have smooth surface to support flying heads. It was also noticed that the media with Hy-MOC showed at least values. Based on theoretical calcula10% reduction in the tions and considering only carbon overcoat of 1 nm (represents 1 nm in the Hy-MOC) to 4 nm (represents 4 nm carbon overcoat layer(s) in current media overcoat) thickness; a 75% reduction in carbon overcoat thickness from 4 nm to 1 nm should , which suggested that the Hy-MOC give a 10% reduction in has the potential to achieve high linear density, when studied in the state-of-the art industry setting. It must be noted that the noisy and lower intensity read-back signal of the hard disk using compared to that of the medium without Hy-MOC Hy-MOC may be due to unstable “flyand the medium using Hy-MOC ability” of head and the less distinct grains segregation in media using Hy-MOC . The main purpose of the overcoat is to protect the magnetic media against corrosion. In this work, the corrosion rates of the media with 2 nm and 1 nm a-C overcoat and also 5 nm Hy-MOC were determined using potentiodynamic polarization and the as reported by Tomcik et al. [19]. In this work, the here is defined as the corrosion rate were investigated. The resistance to electron transfer across the film-electrolyte interface over a narrow potential range about the corrosion potential. It must be also noted that the corrosion rate expression reported by [19] is more suited for pitting corrosion process instead of a uniform process shown in this work. However, the use of this expression in this work provides a useful comparison between the different Hy-MOC samples. Hence, the analysis of the derived corrosion rates should not be taken quantitatively and should be

1074


Fig. 9. Read-back signals of media with 2 nm a-C overcoat, Hy-MOC() and Hy-MOC( ) at 38 kfci.

Fig. 11. The R and corrosion rate of hard disk media with 5 nm Hy-MOC( ) deposited using varying percentage N flow rate and varying thicknesses of carbon overcoats.

Fig. 10. The R and corrosion rate of of hard disk media with 5 nm Hy-MOC() deposited using varying percentage N flow rate and varying thicknesses of carbon overcoats.

limited to comparison between the different types of Hy-MOC samples. Looking at both Fig. 10(a) and Fig. 11(a), it was found that exhibits in the 0.2 M NaCl solution, media with Hy-MOC

(Hy-MOC : slightly higher Rp than media with Hy-MOC cm to cm ; comranging from cm to pared to Hy-MOC : ranging from cm ). As a high polarization resistance value indicates the ability to protect the magnetic layer from corrosion, has better corrosion prothis result indicates that Hy-MOC values, the tection ability than Hy-MOC . Other than the corrosion rate determined also confirmed that the Hy-MOC is more corrosion resistant than Hy-MOC as shown by the lower corrosion rate of the hard disk using Hy-MOC . By comparing the two types of Hy-MOC with the 1 nm, 2 nm and 4 nm a-C overcoat, it was observed that both Hy-MOC have lower values than 4 nm a-C overcoat cm but they values as the 2 nm a-C overcoat of about have very similar cm and they are much higher than the value of cm . Based on an earlier the 1 nm a-C overcoat report [15], a-C overcoat sputtered using hybrid facing target sputtering (HyFTS) is able to inhibit corrosion down to a thickness of 2 nm, hence, the above results suggested that Hy-MOC should be able to inhibit corrosion like the 2 nm a-C overcoat by HyFTS (which is used as a reference). Based on Fig. 10(b) and Fig. 11(b), the corrosion rates of various overcoats determined also confirmed observation shown by values above. Similar to the above the determination of the observations, the 4 nm a-C overcoat has the lowest corrosion rate of 0.19 m/year; which indicates the ability to offer highest


corrosion protection among all the samples. The 2 nm a-C overcoat has a corrosion rate of 1.67 m/year, which are at least two magnitudes lower than that of the 1 nm a-C overcoat of 176 m/year. This trend also suggests that a-C overcoats that has thickness of 2 nm or more is able to minimize exposure of the magnetic layer from the external environment which limit of the migration of the Co ion (from the magnetic layer) to the surface of the media as corrosion product Co OH . Hence, this accounts for the low corrosion rates observed. Likewise, the corrosion rates of Hy-MOC samples were compared with those of (0.41 the a-C overcoats. The corrosion rate of the Hy-MOC m/year to 1.29 m/year) was generally slightly higher than that (0.49 m/year to 0.78 m/year). Nevertheof the Hy-MOC less, all the media samples using Hy-MOC have lower corrosion rate than that of the 2 nm a-C overcoat reference. This means that it takes least 20% more time for media using Hy-MOC to be corroded to the same extent as that of the reference 2 nm a-C overcoat. These observations also suggest Hy-MOC have good corrosion inhibition property. and corrosion rate There are several possible reasons for to show no clear dependence on the N flow rate during the deposition of both types of Hy-MOC samples. They are; (a) potentiodynamic polarization is insensitive to small change in the composition of corrosion resistant compounds (including SiN , and SiO , Cr N and Cr oxides) of about 7% in Hy-MOC (b) the amount of nitrides generated during deposition at 5% N flow rate (using the given sputtering parameter) is sufficient enough to give high and low corrosion rate; however increasing the N flow rate from 5% to 30% will only have minand corrosion rate. Using higher N flow imal effect on the rate may improve the corrosion resistance but this may cause the film to become nonmagnetic, which defeats the purpose of using a Hy-MOC. AES was employed to further confirm the results shown by the electrochemical analysis. Upon analysis with SEM and AES after electrochemical analysis, it was observed from the SEM micrographs that sample with 1 nm a-C overcoat [Fig. 12(b)] had undergone severe corrosion with the whole surface scattered with sub-micron size corrosion products (cobalt oxides/ hydroxides) which resulted in a higher amount of oxygen and cobalt in the AES analysis [Fig. 12(f)]. On the other hand, no corroand sion product was observed for the media with Hy-MOC [Fig. 12(c) and Fig. 12(d)] and with 2 nm of a-C Hy-MOC overcoat [Fig. 12(a)], which was confirmed by the AES analysis showing no noticeable increase in amount of oxygen and cobalt in the samples [Fig. 12(e), Fig. 12(g), and Fig. 12(h), respectively). This indicated that although Hy-MOC has 1 nm of ultra-thin a-C layer, the presence of the magnetic overcoat at the bottom of the Hy-MOC is able to provide sufficient corrosion protection required for the magnetic media which is not adequately provided if only 1 nm a-C layer was used. Work is still on the way to verify the mechanism of corrosion improvement achieved by Hy-MOC layers. However, it is believed that having corrosion resistant Si nitride/ oxynitride and Cr nitrides in the grain boundary of the MOC reinforce the corrosion of the Hy-MOC layers. Hence, when MOC is combined with the 1 nm HyFTS a-C (HyFTS is known to deposit carbon that is denser, and more corrosion resistant [15]), it is expected that the use

1075

Fig. 12. SEM micrographs of media with (a) 2 nm a-C overcoat, (b) 1 nm a-C overcoat, (c) Hy-MOC() and (d) Hy-MOC( ); AES spectra of media with (e) 2 nm a-C overcoat, (f) 1 nm a-C overcoat (Points 1–3 are analysis spots in areas where electrochemical analysis was done), (g) Hy-MOC() and (h) Hy-MOC( ).

Fig 13. Typical SEM micrograph (a to c) and AES spectra (d to f) of the commercial sample, Hy-MOC() and Hy-MOC( ) sputtered using 20% N2 flow rate, after IDEMA induced corrosion test.

of these two films in the Hy-MOC layers can reinforce corrosion resistance by greatly reducing the probability of corrosion agents reaching the magnetic layer; hence achieving good corrosion protection and a reduction in magnetic spacing. In addition, induced corrosion test according to the IDEMA standard humidity test at elevated temperature was also done. The SEM micrograph and AES spectra of the commercial sample after the IDEMA standard induced corrosion test (the IDEMA test) is shown in Fig. 13(a) and Fig. 13(d), respectively. The SEM micrograph of the commercial sample after undergoing the IDEMA test showed a very “clean” surface, free from any observable corrosion spots. But upon closer examination using AES, a smaller amount of Co was observed in the AES spectrum after the IDEMA test. However, the Co peak was absent in AES spectrum before the IDEMA test. This indicated that very slight corrosion may have occurred after the IDEMA test even for the commercial sample. is shown in A typical SEM micrograph of Hy-MOC Fig. 13(b). It was observed that the surfaces of all Hy-MOC samples after the IDEMA test remained very “clean” and no corrosion particles were observable. The dark spots are due to burning of adsorption of atmospheric carbon by the electron beam during analysis. The typical AES spectrum shown in Fig. 12(e) also showed that the Co and O concentrations remain almost similar before and after the IDEMA test. In fact, the

1076


Co and O intensities actually decreased by about 5%, possibly due to adsorption of atmospheric carbon and Cl contamination (the Cl contamination (from previous use) was still observed despite thorough cleaning of the humidity chamber). Similar observations were also noted for all the Hy-MOC samples after the IDEMA test. Like Hy-MOC , from the typsample (Fig. 13(c)), it can ical SEM micrograph of Hy-MOC be observed that all the samples surfaces remained very “clean” and there is no observable corrosion particles. Upon analysis with AES (in Fig. 13(f), it was also observed that the Co and O concentrations remain almost the similar before and after the IDEMA test. Preliminary hardness investigation also indicated promising mechanical property of the Hy-MOC. Research is still in progress to find a suitable technique to determine the mechanical hardness of the Hy-MOC reliably. IV. CONCLUSION A comparison is made between two magnetic overcoat layers for application as hybrid magnetic overcoat. In both cases, the MOC is coupled with the recording layer. This research has better have found that hard disk media using Hy-MOC magnetic property than those using Hy-MOC , giving more well-defined read-back signals. On the other hand, Hy-MOC is more corrosion resistant than Hy-MOC , although both types of Hy-MOC are able to provide adequate corrosion protection for the recording layer. The novel Hy-MOC system may be used to protect the magnetic media from corrosion (using only an ultra-thin carbon layer) without having the problem of interfering with the good magnetic properties of the magnetic layer. In addition, the use of Hy-MOC may also enhance thermal stability of the magnetic media. Several possibilities, and Hy-MOC , are options to such as stacking Hy-MOC improve the performance of Hy-MOC even further. The results of this study indicate that the use of the Hy-MOC may provide a possible solution for further reduction in the magnetic spacing while at the same time providing sufficient protection against corrosion. REFERENCES [1] S. N. Piramanayagam, “Perpendicular recording media for hard disk drives,” J. Appl. Phys., vol. 102, pp. 011301–011322, Jul. 2007.

[2] S. N. Piramanayagam, C. K. Pock, L. Lu, C. Y. Ong, J. Z. Shi, and C. S. Mah, “Grain size reduction in CoCrPt : SiO perpendicular recording media with oxide-based intermediate layers,” Appl. Phys. Lett., vol. 89, pp. 162504–162506, Oct. 2006. [3] S. N. Piramanayagam and K. Srinivasan, “Sub-6-nm grain size control in polycrystalline thin films using synthetic nucleation layer,” Appl. Phys. Lett., vol. 91, pp. 142508–142510, Oct. 2007. [4] R. H. Victora and X. Shen, “Magnetic anisotropy of perpendicular media: Measurement and intermediate layer effect,” IEEE Trans. Magn., vol. 43, no. 2, pp. 621–626, Feb. 2007. [5] H. Yuan, D. E. Laughlin, X. Zhu, and B. Lu, “Ru+oxide interlayer for perpendicular magnetic recording media,” J. Appl. Phys., vol. 103, pp. 07F513–07F516, Jan. 2008. [6] B. K. Yen, R. L. White, R. J. Waltman, C. M. Mate, Y. Sonobe, and B. Marchon, “Coverage and properties of a- SiN hard disk overcoat,” J. Appl. Phys., vol. 93, pp. 8704–8706, May 2003. [7] M. U. Guruz, V. P. Dravid, Y. W. Chung, M. M. Lacerda, C. S. Bhatia, Y. H. Yu, and S. C. Lee, “Corrosion performance of ultrathin carbon nitride overcoats synthesized by magnetron sputtering,” Thin Solid Films, vol. 381, pp. 6–9, Jan. 2001. [8] J. R. Shi and J. P. Wang, “Diamond-like carbon films prepared by facing-target sputtering,” Thin Solid Films, vol. 420, pp. 172–176, Dec. 2002. [9] J. R. Shi, Y. J. Xu, and J. Zhang, “Corrosion resistance of nitrogenated amorphous carbon films prepared by facing target sputtering,” Surf. Coat. Technol., vol. 198, pp. 437–440, Aug. 2005. [10] S. A. Pirzada, J. J. Liu, D. W. Park, Z. F. Li, C. Y. Chen, B. Demczyk, K. E. Johnson, P. S. Wang, and J. Xie, “Ultrathin carbon overcoats: Processing, characterization and tribological performance,” IEEE Trans. Magn., vol. 39, no. 2, pp. 759–764, Mar. 2003. [11] D. J. Li, M. U. Guruz, C. S. Bhatia, and Y. W. Chung, “Ultrathin CNx overcoats for 1 Tb/in hard disk drive systems,” Appl. Phys. Lett., vol. 81, pp. 1113–1115, Aug. 2002. [12] T. Yamashita, G. L. Chen, J. Shir, and T. Chen, “Sputtered ZrO overcoat with superior corrosion protection and mechanical performance in thin film rigid disk application,” IEEE Trans. Magn., vol. 24, no. 6, pp. 2629–2634, Nov. 1988. [13] S. N. Piramanayagam and J. P. Wang, “Magnetic Recording Media With a Magnetic Overcoat Layer,” Patent No.: SP2002005123-3, (Publication No.: 118153). [14] W. C. Poh, S. N. Piramanayagam, and T. Liew, “Novel hybrid magnetic overcoats: A prospective solution for low magnetic spacing,” J. Appl. Phys., vol. 103, pp. 07F523–07F525, Feb. 2008. [15] W. C. Poh, S. N. Piramanayagam, J. R. Shi, and T. Liew, “Novel hybrid facing targets sputtered amorphous carbon overcoat for ultra-high density hard disk media,” Diamond Rel. Mater., vol. 16, pp. 379–387, Feb. 2007. [16] G. R. Yang, Y.-P. Zhao, Y. Z. Hu, T. P. Chow, and R. J. Gutmann, “XPS and AFM study of chemical mechanical polishing of silicon nitride,” Thin Solid Films, vol. 333, pp. 219–223, Apr. 1998. [17] S. N. Piramanayagam, J. P. Wang, Z. S. Shan, W. C. Ye, and T. C. Chong, “A method to determine dynamic remnant coercivity over a larger time-scale,” IEEE Trans. Magn., vol. 37, no. 4, pp. 1950–1952, Jul. 2001. [18] R. J. T. Morris and B. J. Truskowski, “The evolution of storage systems,” IBM Syst. J., vol. 42, pp. 205–217, Mar. 2003. [19] B. Tomcik, S. C. Seng, B. Balakrishnan, and J. Y. Lee, “Electrochemical tests on the carbon protective layer of a hard disk,” Diamond Rel. Mater., vol. 11, pp. 1409–1415, Jul. 2002.