Influence of gas pressures on the magnetic properties and recording performance of CoCrPt–SiO2 perpendicular media

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Journal of Magnetism and Magnetic Materials 303 (2006) e145–e151 www.elsevier.com/locate/jmmm

Influence of gas pressures on the magnetic properties and recording performance of CoCrPt–SiO2 perpendicular media J.Z. Shi, S.N. Piramanayagam, C.S. Mah, J.M. Zhao Data Storage Institute, 5 Engineering Drive 1, Singapore 117608, Singapore Available online 20 March 2006

Abstract CoCrPt–SiO2 -based granular perpendicular media with dual-Ru intermediate layers were studied in the paper. The effects of gas pressures, such as argon pressure for the top Ru layer, oxygen partial pressure ratio and the total gas pressure for the magnetic layer, on the magnetic properties and recording performance of the media were systematically investigated. The results show that all these gas pressure parameters have significant effects on the magnetic properties and recording performance of the media to different extents. Combined with the results by X-ray diffraction, transmission electron microscopy, magnetic force microscopy, magneto-optical polar Kerr magnetometer and Guzik spin-stand tester, the growth mechanism of the perpendicular media and the functions of the gas pressure parameters for certain individual layer were demonstrated. r 2006 Elsevier B.V. All rights reserved. Keywords: Perpendicular media; Pressure; Magnetic properties; Recording performance

1. Introduction High-density perpendicular recording medium requires a well-isolated grain structure to reduce the intergranular exchange effect so as to get low noise and achieve high signal-to-noise ratio (SNR) [1–3]. It has been reported that the oxygen/silicon ratio of 2 is critical to achieve high Hc in CoCrPt–SiO2 layer [4]. It has also been reported that there is an optimized argon pressure for intermediate layer (Ru) to achieve a high Hc in Co–Pt–Ta2 O5 film [5]. In our previous study, we used dual-Ru layers as intermediate layer for CoCrPt–SiO2 -based granular media. The bottom Ru layer ðRub Þ deposited under a high-mobility condition, i.e. low working gas pressure and bias on substrate, was helpful to provide a narrow c-axis orientation dispersion, while the top Ru layer ðRut Þ deposited under a lowmobility deposition condition, i.e. high working pressure, provided an isolated template for the magnetic layer [6]. In this paper, we systematically study the effects of gas pressures, such as argon pressure for the top Ru layer ðPRut Þ, oxygen partial pressure ratio ðRO Þ and total pressure for the magnetic layer ðPm Þ on the magnetic Corresponding author.

E-mail address: [email protected] (J.Z. Shi). 0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2006.01.244

properties and the recording performance of CoCrPt–SiO2 based granular media. The aim of this investigation is to get an understanding on the effect of these parameters, which will help to get an optimized preparation condition for the best recording performance. 2. Experimental details The samples used in this study were prepared on 95 mm polished NiP-plated AlMg substrates by DC magnetron sputtering with a commercial BPS Circulus M12 tool at room temperature. The layer structure was Ta=Rub =Rut = CoCrPt2SiO2 =C for the media. The working gas for Ru layers was pure argon. In this study, the pressure for Rub layer was 0.5 Pa for all samples, while the pressure for Rut layer was changed by adjusting the flow rate. The CoCrPt target contained 6 mol% of SiO2 . The working gas for this magnetic layer was a mixture of argon and oxygen. Two mass-flow controllers were used to carry out the sputtering of CoCrPt–SiO2 . Through one controller, a gas mixture of 95% Ar and 5% O2 was flown into the sputtering chamber, and through another controller pure Ar (99.999%) gas was introduced. The flow rates of these two controllers were adjusted to control the

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oxygen concentration at minute levels, which could help to control the grain size and grain isolation of the magnetic layer. The oxygen partial pressure ratio RO is calculated as, 5x=ðx þ yÞ, where x is the flow rate of argon–oxygen (95–5%) gas mixture and y is the flow rate of pure Ar gas. The magnetic properties were characterized by a magnetooptical polar Kerr magnetometer (MOKE). The microstructure was investigated by X-ray diffraction (XRD), and transmission electron microscopy (TEM). The magnetic domains were observed by magnetic force microscopy (MFM). The recording performance was studied employing a Guzik spin-stand tester with a ring-head writer and a MR reader. 3. Results and discussion Fig. 1(a) and (b) shows the TEM images of plane-view and cross-sectional view of Tað3 nmÞ=Rub ð9 nmÞ= Rut ð9 nmÞ=CoCrPt2SiO2 ð14 nmÞ=Cð3:5 nmÞ, respectively. Rub and Rut layers were deposited at 0.5 and 7 Pa, respectively. The plane-view exhibits a grain growth image with grain size of 6–15 nm in diameter. The white-contrast portion located in the grain boundaries was thought to be SiO2 by many authors [7–9], which serves as the barrier to decouple the neighbor magnetic grains. As shown in Fig. 1(b), the carbon layer cannot be seen because of the poor contrast and the presence of glue used during TEM sample preparation. However, we can observe that the Rub layer has a continuous structure. This is because Rub was deposited under high-mobility deposition conditions. During the initial growth stage of the Rut layer, the columns are packed closely and in the late growth stage the columns are well-isolated one another. This can be explained by Thornton’s film growth model: in sputtering deposition a low-mobility deposition condition (high Ar gas pressure) tends to create polycrystalline film, which corresponds to the ‘‘zone 1’’ structure in the well-known Thornton microstructure zone diagram, and leads to fine columnar grains with voided grain boundaries [10].

On the other hand, a high-mobility deposition condition (low Ar gas pressure) tends to promote a continuous structure of morphology. In CoCrPt–SiO2 layer, the columns of Co-alloy stand on those of Rut layer, and the segregation between the columns in the magnetic layer remains. In order to study the effect of PRut and RO , samples with a layer structure of Tað2:5 nmÞ=Rub ð8 nmÞ=Rut ð5 nmÞ= CoCrPt2SiO2 ð14 nmÞ=Cð3:5 nmÞ were used. Only PRut and RO were changed independently with the rest deposition conditions for the media kept the same. For example, the total pressure for magnetic layer Pm was kept to be 8.5 Pa. Fig. 2(a) shows Hc as a function of PRut and RO . Here, PRut was varied from 2.5 to 5.5 Pa, and RO from 0.00% to 2.22%. With the increase of PRut , Hc increases rapidly at lower PRut (2.5–3.5 Pa) and slowly at higher PRut (3.5–5.5 Pa). When RO increases from 0.00–1.67%, Hc increases monotonously. However, when RO further increases from 1.67% to 2.22%, Hc reduces. The phenomenon of Hc increase with both PRut and RO increasing could be attributed to the improved segregation between the magnetic grains. The reduction in Hc with RO increasing from 1.67% to 2.22% could arise from two factors: (i) part of oxygen was incorporated into the magnetic grains and reduced the magnetic anisotropy and accordingly reduced the Hc [5]; (ii) the addition of oxygen has reduced the grain size, which led to a reduction in the K u V =K b T. Recently, Inaba et al. observed that the grain size decreased significantly from 8.8 to 5.4 nm and K u decreased from 9:5
106 to 4:0
106 erg=cm3 as the SiO2 content increases from 0 to 14.4 at % [7]. This indicates that a suitable combination of PRut and RO is critical to achieve a high Hc. It is interesting to note that the full width at half-maximum (FWHM, or Dy50 ) of Co(0 0 0 2) and Ru(0 0 0 2) was almost kept unchanged, i.e. around 4 . This implies that the perpendicular orientation of the magnetic grain is mainly determined by the deposition conditions of Rub layer of the dual-Ru media.

Fig. 1. TEM images of (a) plane-view and (b) cross-sectional view of Tað3 nmÞ=Rub ð9 nmÞ=Rut ð9 nmÞ=CoCrPt2SiO2 ð14 nmÞ=Cð3:5 nmÞ.

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Fig. 2. (a) Hc as a function of PRut layer and RO ; (b) Hn as a function of PRut layer and RO .

A good perpendicular magnetic medium needs a negative nucleation field (Hn). Hn is defined as the applied field, where the magnetization reaches 95% of the saturation magnetization. Medium with a large negative Hn is more stable in its remanent magnetic state since a larger reversing field is needed to alter the magnetization. Fig. 2(b) shows Hn as a function of PRut and RO . We observe that except for the case of RO ¼ 2:22% with PRut no less than 3.5 Pa, all samples have a negative Hn. The lower the RO , the higher the negative nucleation field. The changes in Hn with PRut are divided into three groups: (1) for lower RO cases (0.00%, 0.56%), Hn increases slightly as PRut increasing; (2) for medium RO case (1.11%), Hn almost keeps constant as PRut increasing; (3) for higher RO case (1.67%, 2.22%), Hn reduces as PRut increasing, particularly Hn changes its sign from negative to positive for the case of RO ¼ 2:22%. Hn is also correlated to the K u V of the grains. The decrease of Hn with oxygen indicates that the anisotropy constant, grain size or both are decreasing with

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Fig. 3. (a) S as a function of PRut layer and RO ; (b) SFD as a function of PRut layer and RO .

the increase of RO . In comparison of the effects of PRut and RO on Hn, the later is much more obvious. Fig. 3(a) and (b) show S and switching field distribution (SFD) as functions of PRut and RO , respectively. One can find that S  reduces and SFD increases as PRut and RO increasing. S  and SFD are related to intergranular exchange coupling strength. The decrease of S  and increase of SFD indicate that the grains become well decoupled magnetically from each other with the increase of PRut and RO . As the magnetic grains are decoupled from each other, the smaller grains would switch at a lower field and larger grains would switch at a larger field. Therefore, SFD would increase with PRut and RO . Fig. 4 shows the MFM images for four kinds of gas parameter combinations, i.e. (a) PRut : 4.5 Pa, RO : 0.56%; (b) PRut : 4.5 Pa, RO : 1.1%; (c) PRut : 5.5 Pa, RO : 0.56%; (d) PRut : 5.5 Pa, RO : 1.1%. The magnetic domains can be observed for the cases of lower PRut (a) and lower RO (c). The domain size reduces with PRut and RO increasing,

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Fig. 4. MFM images for different gas pressure combinations: (a) PRut : 4.5 Pa, RO : 0.56%; (b) PRut : 4.5 Pa, RO : 1.1%; (c) PRut : 5.5 Pa, RO : 0.56%; (d) PRut : 5.5 Pa, RO : 1.1%.

which implies that medium noise reduces with PRut and RO increasing. To further study the noise dependence on PRut and RO , Guzik testing was taken. Fig. 5(a) shows the medium noise as a function of PRut and RO . It is clearly seen that with PRut increasing the medium noise reduces rapidly at lower PRut range (2.5–3.5 Pa) and slowly at higher PRut (3.5–5.5 Pa). This trend of medium noise with PRut and RO is opposite to that of Hc with PRut and RO , as

shown in Fig. 2(a). The medium noise also reduces with RO increasing. Particularly, with RO increasing the medium noise reduces rapidly at lower PRut range (2.5–3.5 Pa) and slowly at higher PRut (3.5–5.5 Pa). Fig. 5(b) shows the medium SNR as a function of PRut and RO . The data exhibit a considerably large variation which may be because ring-head was used for the test. Nevertheless, the trend that SNR increases as PRut

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Fig. 5. (a) Medium noise as a function of PRut layer and RO ; (b) SNR a function of PRut layer and RO .

increases is clear. At a certain PRut , the trend for SNR with RO needs to be tested by single-pole head in the future. However, the results from Fig. 5(a) and (b) give an important clue for the production of CoCrPt–SiO2 media. At low PRut , the noise and SNR are very sensitive to RO . However, at high PRut , it is less sensitive to oxygen partial pressure. In the production, a larger sputter process window of parameters (say, oxygen flow rate, or argon–oxygen mixture flow rate) is desirable. If the SNR or noise is too sensitive to oxygen flow, any drift in the oxygen flow could affect the performance of the disks drastically. Therefore, higher PRut may be suitable for production from the process-control point of view. Based on the above discussions, PRut and RO play important roles on the magnetic properties and recording performance. Both high PRut and high RO make contribution on the isolation between the magnetic grains, and affect Hc, Hn, SFD, S and subsequently reduce the medium noise

and increase the SNR. Here, the effects of PRut and RO are main effects. In a standard design-of-experiment (DOE) of Six-Sigma tools for process design, it is usually to explore possible interaction between the two factors on a response [11]. Interaction effect is the extent to which the influence of a factor on the response depends upon the level of another factor. In above experiments, the factors are PRut and RO , and the responses are Hc, Hn, and so on. In Figs. 2, 3, and 5, the presence of unparallel guide-lines indicates interaction effect available between the PRut and RO . In Fig. 5(b), part of the guide-lines are cross, imply that interaction effect by PRut and RO on media SNR is the strongest if ring-head used is not considered as the main effect. Apart from PRut and RO ; the total pressure for the magnetic layer Pm also has significant effect on the magnetic properties and recording performance of the media. This will be discussed in Figs. 6–8, where samples with layer structure of Tað2:5 nmÞ=Rub ð8 nmÞ=Rut ð5 nmÞ= CoCrPt2SiO2 ð14 nmÞ=Cð3:5 nmÞ were used. Only the total pressure for the magnetic layer Pm were varied with the rest deposition conditions for the media kept the same. For example, the RO was maintained to be 1.1% and PRut at 4.5 Pa for all five samples. Fig. 6(a) shows Hc and Hn as functions of Pm . Hc increases slightly as the Pm increases

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Fig. 7. MFM images of samples with magnetic layer prepared at different Pm .

from 5.5 to 7.0 Pa, and then almost keeps constant. The effect of Pm on Hc is different from that of PRut on Hc as shown in Fig. 2(a). Hc is more sensitive to PRut than Pm in the discussed range of pressures. On the contrary, Hn reduces slightly as Pm increases from 5.5 to 7.0 Pa, and then reduces more rapidly as Pm increases from 7.0 Pa to 11.5 Pa. Fig. 6(b) shows S  and SFD as functions of the Pm . S  almost linearly reduces and SFD increases as Pm increasing. These results indicate that, as the total pressure

is increased, the exchange coupling decreased. This may be because wider grain boundaries are formed at high Pm . As Thornton pointed out that high-pressure sputter deposition easily gives rise to voided grain boundaries [10]. In our experiment, since O2 was used high gas pressure provided a possibility of more collisions in the sputtering chamber and led to an increasing possibility of Si atoms to react with the reactive oxygen gas, and hence enlarged the grain boundaries.

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4. Summary

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In summary, we have systematically studied the effects of gas pressures, such as argon pressure for the top Ru layer, oxygen partial pressure ratio and the total gas pressure for the magnetic layer, on the magnetic properties and the recording performance of CoCrPt–SiO2 -based granular perpendicular media with dual-Ru intermediate layers. The grain physical isolation, which is helpful to reduce medium noise and increase SNR, arises from the contributions from all these parameters of pressures based on different mechanism. The results related to magnetic properties and recording performance include:

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Fig. 7 shows the MFM images for the samples with magnetic layer prepared at different Pm . Fig. 7(a)–(d) corresponds Pm of 7, 8.5, 10, and 11.5 Pa, respectively. It can be observed that with Pm increasing, the magnetic domain size reduces. Especially, the magnetic domain size reduces rapidly when Pm increases from 8.5 to 10 Pa. This implies that the medium noise reduces. The medium noise as a function of Pm is shown in Fig. 8. One can find that with Pm increasing the medium noise reduces. We observe that when Pm increases from 8.5 to 10 Pa, the medium noise reduces rapidly. This observation result is consistent with the MFM results. We also observed that SNR increases with Pm increasing. This further demonstrates that high Pm favors to achieve good segregation between the magnetic grains and reduce the intergranular interaction. High PRut and high Pm are helpful to get high Hc, low noise and high SNR based on Thornton’s film growth model; while high but not excessive RO gives rise to high Hc and other benefits based on reactive sputtering. The oxygen is supposed to be ironed in the plasma and make chemical reactions with Si and Cr in the magnetic layer. The formed SiO2 and Cr oxides are located on the grain boundary. This grain physical isolation is a very critical factor to the magnetic properties and recording performance of magnetic media. Because this isolation was improved in dual-Ru media, subsequently the magnetic intergranular exchange coupling between the adjacent CoCrPt grains became weak. As a result, the magnetic grains switched their moments independently instead of collectively and hence the Hc increased. More importantly, the noise reduction and SNR increase were the direct result of the reduced exchange coupling due to the improved segregation of the magnetic grains.

(i) A suitable combination of PRut and RO is needed to achieve high Hc; Pm has less effect on Hc; (ii) Hn is sensitive to Pm and RO ; but is less sensitive to PRut when it is not less than 3.5 Pa; (iii) High of PRut , high RO , and high Pm tend to give rise to small S  , and large SFD; (iv) High of PRut , high RO , and high Pm tend to lead to a small domain size, low noise; (v) High PRut is helpful to get high SNR and obtain stable process-control since at low PRut , the noise and SNR are very sensitive to RO . However, at high PRut , it is less sensitive to oxygen partial pressure; (vi) Interaction effect by PRut and RO on media SNR is the strongest. Interaction effects by PRut and RO on media noise, S , SFD, Hn are also observed.

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