Ru intermediate layers for perpendicular recording media

Ru intermediate layers for perpendicular recording media

ARTICLE IN PRESS Journal of Magnetism and Magnetic Materials 312 (2007) 476–479 www.elsevier.com/locate/jmmm Effect of seed layers on the textured g...

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ARTICLE IN PRESS

Journal of Magnetism and Magnetic Materials 312 (2007) 476–479 www.elsevier.com/locate/jmmm

Effect of seed layers on the textured growth of Pd/Ru intermediate layers for perpendicular recording media S.N. Piramanayagam, H.B. Zhao Data Storage Institute, 5, Engineering Drive 1, Singapore 117 608, Singapore Received 8 June 2006; received in revised form 15 September 2006 Available online 7 November 2006

Abstract The effect of several different types of materials on the textured growth of Pd and Ru layers have been investigated for their suitability as seed layers in perpendicular recording media. CrZr, CrMoZr, CrW, CrTi, NiAl and RuAl layers were introduced as seed layers and their effects on perpendicular c-axis growth and the magnetic properties were studied. Lower values of Dy50 of Pd(1 1 1) and Ru (0 0 .2) peaks and larger values in perpendicular coercivity were observed for samples using CrW as seed layer. By using CrW as seed layer, better oriented growth of Pd (1 1 1) could be observed which in turn leads to better c-axis growth of the CoCrPt–SiO2 magnetic layer. Generally, it was observed that Cr-based alloys lead to better crystallographic growth of the media structure compared to Al-based intermetallic compounds. r 2006 Elsevier B.V. All rights reserved. Keywords: Perpendicular recording media; Seed layer; Textured growth

1. Introduction Perpendicular recording is considered as the nextgeneration technology for hard disk drives, having advantages over the current longitudinal recording technology. CoCrPt:Oxide-based alloys are currently viewed as the candidates for the recording layer [1–12]. Compared to the non-oxide CoCrPt alloys, the oxide-based materials show promise in terms of reasonably higher anisotropy constant that will make these media thermally stable up to about 500 Gb/in2. Moreover, the media based on these materials show small grain size, low noise and allow room temperature fabrication. In perpendicular recording media, intermediate layers such as Ru, Pt/Ru, Pd/Ru are used to obtain a HCP[0 0 0 2] orientation of the recording layer perpendicular to the film plane. In Ru-based intermediate layers, one of the popular designs is to use two Ru layers to obtain optimized magnetic and crystallographic properties [9,13]. The bottom Ru layer is sputtered at lower pressures, which lead to a very good HCP[0 0 0 2] orientation Corresponding author. Tel.: +65 68748550; fax: +65 67772406.

E-mail address: [email protected] (S.N. Piramanayagam). 0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2006.10.466

perpendicular to the film plane. The top layer is sputtered at higher pressures, which lead to segregated Ru grains. Since the top layer grows on the bottom layer epitaxially, a good HCP[0 0 0 2] orientation is still maintained. More importantly, the segregation in the top layer helps to isolate the grains in the recording layer from one another. This isolation helps to minimize the exchange coupling between the grains and reduce the noise. In addition to Ru-based intermediate layers, Pt/Ru and Pd/Ru-based intermediate layers have also been investigated. Recently, we have reported that the growth of Pd with a FCC[1 1 1] orientation perpendicular to the film plane depended strongly on the type of seed layer used below the Pd layer [14]. We pointed out that CrTi-alloy based seed layer was much superior to Ta or Ti seed layers that are commonly used. In this paper, we present our detailed investigations on the effect of seed layer on the growth of Pd and on the magnetic properties of CoCrPt:Oxide-based perpendicular recording media. The materials chosen for seed layer in this study were Cr96Zr4, Cr76Mo20Zr4, CrW, CrTi, NiAl and RuAl. The main objective of this study is to examine which alloy can give CoCrPt–SiO2 film a better crystallographic texture, which favors perpendicular recording. This will be

ARTICLE IN PRESS S.N. Piramanayagam, H.B. Zhao / Journal of Magnetism and Magnetic Materials 312 (2007) 476–479

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referenced by the Dy50 values obtained for the various crystallographic orientations present in different layers. In addition, other magnetic properties such as perpendicular coercivity and squareness ratio are compared to provide a guide to the magnetic properties of the different samples. 2. Experimental Various sets of thin films of the type X/Pd/Ru/Co-alloy were prepared at room temperatures by DC magnetron sputtering on glass substrates. A different material X was used as seed layer for each set. As mentioned, X was Cr, CrZr, CrMoZr, CrW, CrTi, RuAl and NiAl. Pd layers were 10 nm thick and were sputtered at a pressure of 3 mTorr. Ru layers were 10 nm thick and sputtered at a pressure of 12 mTorr. CoCrPt–SiO2 layers were 14 nm thick and sputtered at a pressure of 25 mTorr. Higher pressures during the deposition of Ru and CoCrPt–SiO2 layers were employed to obtain a segregated grain structure in Ru and the recording layer. The deposition conditions and thickness of all the layers were maintained constant except that of the seed layers. The sputtering power of the seed layers was varied to obtain seed layers with different thickness. The Dy50 values for each plane were measured using X-ray diffraction while the coercivity and squareness ratios were measured using an alternating gradient magnetometer and/or polar Kerr magnetometer. 3. Results and discussion

Fig. 2. Dy50 of Pd(1 1 1) peaks of Pd layers deposited on top of different Cr-alloy based seed layers.

Fig. 3. Hysteresis loops of CoCrPt:SiO2 films deposited on CrW/Pd/Ru layers. Thickness of CrW was varied from 0.35 to 5.6 nm.

3.1. Cr-alloy seed layers Fig. 1 shows the X-ray diffraction spectra of the film samples using Cr76Mo20Zr4 as seed layer. The diffraction patterns show the presence of Pd (1 1 1), Ru (0 0 .2) as well as Co (0 0 .2) peaks present in the samples. However, the Ru(0 0 .2) and Co(0 0 .2) peaks are at close proximity and overlapped. It can be noticed that the peak intensity of Pd as well as Ru/Co peaks increases with the increase of thickness of CrMoZr layer and show a maximum at a thickness of 2 nm. Beyond 2 nm, the peak intensity

Fig. 1. X-ray diffraction pattern of Pd/Ru/CoCrPt–SiO2 with CrMoZr as seed layer.

decreases and reaches low values at 6 nm. Such an optimum thickness at which there is a maximum crystallographic texture is observed for most of the seed layers studied by our group. Fig. 2 shows the Dy50 of Pd(1 1 1) peak for all the Crbased alloys studied, as a function of thickness. It can be noticed that Dy50 of Pd(1 1 1) peaks shows a decrease with the increase of seed layer thickness, such as that of CrMoZr. After showing a minimum value, Dy50 starts increasing again with the seed layer thickness. Dy50 of Ru(0 0 2) peaks also behave in a similar fashion. This behavior is similar to the results observed in our previous study involving Ti, Ta, Cr and CrTi, where also a critical thickness provided the lowest Dy50 [14]. Fig. 3 shows the hysteresis loops of films deposited on CrW-alloy based seed layers. The following points can be noticed from the figure: (i) The coercivity (Hc) increases with the increase of seed layer thickness. (ii) The nucleation field (Hn) is in the first quadrant for the films with the thinnest seed layer. However, with the increase of the seed layer thickness, nucleation field moves to the second quadrant. The increase of coercivity with the seed layer thickness could come from at least two possible reasons: (i) The increase of seed layer thickness leads to an increased roughness, which results in a better segregated

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S.N. Piramanayagam, H.B. Zhao / Journal of Magnetism and Magnetic Materials 312 (2007) 476–479

Fig. 5. X-ray diffraction patterns of Pd/Ru/CoCrPt–SiO2 with RuAl as seed layer. Thickness of seed layer was varied from 1 to 6 nm. Fig. 4. Dependence of coercivity (Hc) and nucleation field (Hn) on the c-axis distribution of Ru, as measured by Dy50 of Ru(0 0 0 2) peaks in Cralloy based seed layer films.

grain structure. This will result into exchange-decoupled grains and increased coercivity. (ii) The increase of seed layer thickness, which leads to a reduction in the c-axis dispersion, increases the number of grains with a perpendicular c-axis orientation. Since these grains would need a larger reversal field, the average reversal field (which is coercivity) would increase. The increase in the magnitude of nucleation field is expected because of the reduced Dy50 or because of increased anisotropy constant or both [15]. In order to understand this further, we have also plotted Hc, and Hn as a function of Dy50. It can be noticed from Fig. 4 that the Hc shows an increase with the decrease of Dy50 of Ru peak. Similarly, Hn decreases with the decrease of Dy50. Although there is a scattering, similar effects have also been in other systems studied by our group.

Fig. 6. Dy50 of Pd(1 1 1) peaks in Pd/Ru/CoCrPt:SiO2 films deposited on B2 structured seed layers for different seed layer thickness.

3.2. B2 structured seed layers Fig. 5 shows the XRD patterns of films deposited on RuAl seed layers. It can be observed that the intensities of the various diffraction peaks are significantly lower than those reported for Cr-alloy based seed layers. It can also be noticed that the maximum intensities are observed for the film with the smallest thickness studied (1.64 nm), beyond which the intensity reduces with the seed layer thickness. Fig. 6 shows the Dy50 of Pd(1 1 1) peaks in Pd/Ru/ CoCrPt:SiO2 films deposited on B2 structured seed layers. It can be noticed that unlike the films deposited on Crbased alloys, which show a decrease of Dy50 with thickness up to a certain value the general trend is an increase of Dy50 with thickness of the seed layer (within the thickness range studied by the authors). Fig. 7 shows the hysteresis loops of films deposited on B2 structured compounds. It can be noticed that the coercivity decreases with the increase of thickness in the case of B2 seed layers. The nucleation field also increases as the thickness increases. These results, within the thickness studied, are in contrast to those obtained in case of Cr-alloy based seed layers. Again, these results are correlated to the Dy50 of

Fig. 7. Hysteresis loops of Pd/Ru/CoCrPt:SiO2 films deposited on B2 structured compounds as the seed layer.

Ru(0 0 0 2). Those samples which showed a lower values of Dy50 of Ru(0 0 0 2) showed a higher coercivity and a negative nucleation field. Fig. 8 shows the Dy50 of Ru(0 0 2) peaks of Pd/Ru/ CoCrPt:SiO2 films deposited on all the seed layers, such as Cr-alloy based seed layers and B2-type seed layers. It can be noticed that while the films deposited on some Cr-based alloy seed layers show a decrease with thickness or a

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of seed layers provide a lower Dy50 than CrTi-alloy based seed layers. In contrary, NiAl and RuAl seed layers lead to larger Dy50 for Pd and Ru layers.

Acknowledgments The authors acknowledge the help of Mr. Tan C.H in this study. References Fig. 8. Dy50 of Ru(0 0 2) peaks of Pd/Ru/CoCrPt:SiO2 films deposited on Cr-alloy and B2-type seed layers.

minimum in the value of Dy50, those deposited on B2 structured seed layers show an increase with thickness. It can also be noticed that the films with Cr-alloy based seed layers offer lower values of Dy50 than those with B2 structured seed layers. In addition to the comparison of different seed layers, the current study also has helped to identify better seed layers than those reported so far. In our previous study, we have pointed out that CrTi-alloy based seed layer, provides a lower Dy50 and an improved recording performance than previously reported Ta seed layer. The current study points out that much lower Dy50 can be obtained with CrMoZr and CrW-alloy based seed layers than with CrTi seed layers. Therefore, better recording performance is expected with CrMoZr or CrW seed layers. 4. Conclusions

 

Magnetic and crystallographic texture properties of Pd/Ru/CoCrPt:SiO2 alloys deposited on various kinds of seed layer have been investigated. It was observed that Cr-alloy based seed layers showed a lower Dy50 for Pd and Ru layers. CrW and CrMoZr type

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