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Effect of Phosphor Addition on Intergranular Exchange Coupling of Co-Pt Thin Films
J. Mater. Sci. Technol., 2011, 27(5), 398-402.
Effect of Phosphor Addition on Intergranular Exchange Coupling of Co-Pt Thin Films Bo Yang1) , Gaowu Qin1)† , Wenli Pei1) , Song Li1) , Yuping Ren1) and Shunji Ishio2) 1) Key Laboratory for Anisotropy & Texture of Materials, Northeastern University, Shenyang 110004, China 2) Department of Material Science & Engineering, Akita University, Akita 0108502, Japan [Manuscript received September 19, 2010, in revised form October 26, 2010]
In this paper, ternary Co-Pt-P thin films were prepared by magnetron sputtering with platinum and home-made Co-P chips pasted on the cobalt target. The structure and magnetic properties were investigated by X-ray diffraction (XRD) and vibrating sample magnetometer (VSM), respectively. With increasing phosphor content, the coercivities of Co-12 at.% Pt-P films increase from 1034 to 1525 Oe owing to the exchange decoupling among magnetic grains. The decrease of inter-granular exchange coupling was confirmed by delta-M curve measurement and magnetic force microscopy (MFM). Transmission electronic microscopy (TEM) with nanobeam composition analysis shows that phosphor segregated at cobalt-based grain boundaries is responsible for the exchange decoupling. It is thus suggested that the phosphor addition is effective to tune the exchange coupling of magnetic grains, particularly for perpendicular recording media. KEY WORDS: Intergranular exchange coupling; Co-Pt-P; Delta-M ; Magnetic recording media
1. Introduction Hard disk drive (HDD) has obtained remarkable achievements in the past half century due to the harmonic development of both magnetic recording head and media. Especially for recording system changes from longitudinal to perpendicular one, an ultra-high areal density of 610 Gb/in2 was demonstrated by Hitachi Ltd. in 2008. Unfortunately, the superparamagnetic effect limits a further increase of areal density with conventional Co-Cr-Pt based recording media. It has been thus suggested that the introduction of higher magnetic anisotropy recording media can effectively postpone the super-paramagnetic effect[1] . Besides the thermal stability, grain size distribution[2–4] and optimization of inter-granular exchange coupling[5,6] , which relate to the signal to noise ratio (SNR), are also challenges in perpendicular recording media. Hexagonal close-packed (HCP) structured Co† Corresponding author. Prof.; Tel.: +86 24 83683772; E-mail address: [email protected] (G.W. Qin).
Pt alloys with high magnetic anisotropy (Ku: ∼1×107 erg/cc)[3,7–9] , is one of particular interest and promising candidates for 500−1000 Gb/in2 perpendicular recording media. However, a vital obstacle for Co-Pt media is their strong inter-granular exchange coupling, because in order to improve signal to noise ratio (SNR), high density perpendicular magnetic recording requires not only high magnetic anisotropy material and fined grains, but also moderate inter-granular exchange coupling[3–6,10–12] . Much effort has been paid on the decoupling of Co-Pt thin films. One effective approach to adjust the intergranular exchange coupling is to segregate the magnetic grains by adding a nonmagnetic element (Cr[13] , B[14] , C[15] and P[16–19] ) or oxides[20] such as SiO2 , B2 O3 , Ta2 O5 , TiO2 and so on. However, these elements, such as Cr and B could modify the intergranular coupling but the deposition process usually needs a high substrate temperature, which limits the minimum grain size and c-axis control. Oxides, especially SiO2 , are effective isolation agents to decouple the magnetic nano-grains in media but they are also incorporated into magnetic grains, which seriously de-
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teriorates the magnetic anisotropy energy density of media[20] . Previous researches show that the addition of phosphor into Co-Pt films can obtain well isolated grains and fined grains[16,17,19,21] at room temperature. These Co-Pt-P films are fabricated by electroless or electric deposition, which increases the surface roughness and hardly controls film thickness at ∼10 nm compared to magnetron sputtering. Therefore, it is indispensable to look for a way to prepare CoPt-P films with lower surface roughness and tunable inter-granular exchange decoupling. Moreover, there are few researches on structural and magnetic properties of the Co-Pt-P thin films with different phosphor contents. In this paper, a series of ternary Co-PtP thin films with different phosphorus contents were successfully fabricated by using DC magnetron sputtering. The crystalline structure and magnetic properties of single layer Co-Pt-P thin films were investigated to confirm the exchange decoupling by the addition of phosphor. The results indicate that the inter-granular exchange coupling among HCP structured Co-Pt magnetic grains were decreased by the addition of phosphor. 2. Experimental In order to obtain Co-Pt-P films by magnetron sputtering, platinum and home-made Co-P chips were pasted on cathode target. The phosphorus and platinum concentration were controlled through changing areas of Co-P and Pt chips. For the detailed preparation procedure of Co-P chips, please refer to our recent work[22] . A series of Co-Pt-P thin films with various phosphor contents were deposited on Si (100) substrates at room temperature by DC magnetron sputtering. All the thin films were deposited at a working Ar pressure of 0.6 Pa and the base pressure before depositing was better than 9.0×10−5 Pa. The sputtering rate was about 0.16 nm/s. The nominal thickness of Co-Pt-P films was fixed at 25 nm and confirmed by stylus profiler (DEKTAK 150). The composition of Co-Pt-P films was estimated by energy dispersive analysis of X-ray spectroscopy (SEM-EDAX, JSM-7001F). Vibrating sample magnetometer (VSM, Lakeshore 7407) was used to measure the magnetic prosperities of Co-Pt-P thin films. Magnetic ΔM curves were obtained based on the isotherm remanent magnetization (MIRM ) and direct current demagnetization remanence (MDCD ). The crystalline structure was investigated by X-ray diffraction (XRD, PW3040/60) with CuKα radiation. The structure of the thin films were also characterized by high resolution transmission electron microscopy (HR-TEM) performed on JEOL JEM-2100F with an operation voltage of 200 kV, and the composition analysis of a series of points across one grain boundary was measured with a beam size of ∼1 nm. Magnetic force microscopy (MFM, SPI3800N) was used to char-
Fig. 1 Effects of phosphorous content on coercivities of Co-12 at.% Pt-xP (x=0–3.7 at.%) thin films sputtered at room temperature (a) and the corresponding in-plane M -H loops (b)
acterize the magnetic domains of Co-Pt-P thin films. The samples were observed without external magnetic field and in a vacuum about 5×10−3 Pa. 3. Results and Discussion Figure 1 shows the dependence of in-plane coercivity on phosphor contents (Fig. 1(a)) and the in-plane hysteresis loops of Co-12 at.% Pt-xP(x=0–3.7at.%) films (Fig. 1(b)). The coercivities of Co-12 at.% PtxP(x=0–3.7 at.%) films considerably increase with the phosphor content. Only a small amount addition of phosphor (1.7 at.%P) results in a great increase in coercivity from 1034 to 1420 Oe. A further increase in coercivity (1525 Oe) is achieved as the phosphor content is 3.7 at.%. According to the micromagnetic simulations, it is well-established that the coercivity increase with the decrease of exchange coupling[23–25] . The effect of phosphor content on exchange decoupling will be analyzed and discussed in detail in the following part. The drastic increase in coercivity of Co-12at.%PtP thin films indicates that the inter-granular exchange coupling strength has been considerably reduced with the addition of phosphor, as shown in Fig. 1. In order to further confirm it, the ΔM (H) technique is used to determine the magnetic interaction among the adja-
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phor precipitating at Co-Pt grain boundaries[27,28] . It can be clearly seen that the magnetic domain size greatly reduces with increasing phosphor content. For the Co-12 at.% Pt thin film, the magnetic domains were complex and the domain walls are not clear, i.e. the magnetic domains are hardly distinguished. In contrast, the domain walls can be clearly observed only by the addition of 2.3 at.% phosphor, as shown in Fig. 3(b), in spite of the fact that there are still some areas not clear (marked by arrows). For Co12 at.% Pt-3.7 at.% P, the situation is better, multidomain structure and domain walls are clear as seen in Fig. 3(c). Fig. 2 ΔM plot of Co-12 at.% Pt (a) and Co-12 at.% Pt-3.7 at.% P (b) thin films
cent magnetic grains. In ΔM (H) technique, ΔM (H) is defined as[26] ΔM (H) = 2MIRM (H) + MDCD (H) − 1 where MIRM is the isotherm remanent magnetization and MDCD is the direct current demagnetization remanence. As shown in Fig. 2. The ΔM plot of the Co-12 at.% Pt film shows a positive peak and a negative peak, and the former implies a strong ferromagnetic exchange coupling among the Co-Pt magnetic grains while the later means a long period magnetostatic coupling. With the addition of 3.7 at.% P, however, no positive peak was found, only a negative peak illustrates some dipolar or magnetostatic coupling. These results of ΔM measurement show that the inter-granular exchange coupling has been significantly decreased by the addition of phosphor, which is well consistent with coercivity dependence on the phosphor content above. The second direct evidence of exchange decoupling by the addition of phosphor is the change of magnetic domain size, as shown in Fig. 3. It is attributed to the reduction in magnetic domains size, exchange decoupling and the pinning effect on domain walls by phos-
As depicted above, the inter-granular exchange decoupling of the Co-12 at.% Pt thin film by the addition of phosphor is confirmed by ΔM curves and MFM images. All these results indicate that the addition of phosphor is an effective way to decouple the magnetic grains in the recording media, which is instructive for both longitudinal and perpendicular recording media. Since, it needs a full exchange decoupled magnetic grains for longitudinal recording media while a moderate inter-granular exchange decoupling for perpendicular recording media, which is helpful to the thermal stability and SNR. In order to clarify the mechanism of exchange decoupling, TEM and composition analysis with nanobeam were employed to examine the sample. Figure 4(a) and (b) show the TEM image of Co-12 at.% Pt and Co-12 at.% Pt-3.7 at.% P. As shown in Fig. 4(a), the Co-12 at.% Pt thin film is of HCP structure and randomly oriented. Compared to Co-12 at.% Pt, the Co-12 at.% Pt-3.7 at.% P thin films show a smaller grain size. The composition distributions of Co, Pt and P elements in grains and at grain boundaries were analyzed by nano-beam composition analysis. For Co12 at.% Pt thin films, the content of Co and Pt is equably distributed in grains and at grain boundaries, as shown in Fig. 4(c). However, for Co-12 at.% Pt3.7 at.% P thin films, the phosphor content is almost negligible in grains, but enriched at grain boundaries,
Fig. 3 MFM images of Co-12 at.% Pt-xP films without external magnetic field: (a) x=0, (b) x=2.3 at.%, (c) x=3.7 at.%
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Fig. 4 TEM images of Co-12 at.% Pt (a) and Co-12 at.% Pt-3.7 at.% P (b), and the corresponding composition distribution in grains and at grain boundaries for the Co-12 at.% Pt (c) and Co-12 at.% Pt-3.7 at.% P (d) films. GB means Co-Pt grain boundary and the dash line means the nominal phosphor content (3.7 at.%)
as shown in Fig. 4(d). Meanwhile, both the Co and Pt contents at grain boundaries are smaller than that in grains. It indicated that the phosphor element segregates at grain boundaries. As is well known, Pt has a large solid solubility in cobalt. The addition of 12 at.% Pt will not change the HCP structure. Since there is no solid solubility of phosphor in HCP structured cobalt, it is inevitable that the phosphor will precipitate in cobalt matrix, i.e. there will be two-phase microstructure in Co-PtP system with a small amount of the phosphor addition. From the viewpoint of diffusion kinetics, there is a large atomic size difference between cobalt (135 pm in radius) and phosphor (100 pm in radius). The phosphor can diffuse rapidly in cobalt and precipitate at grain boundary, especially at the sputtering condition. Thus, the inter-granular exchange coupling can be effectively decreased. X-ray diffraction technique (XRD) was also used to examine the crystalline structure of Co-12 at.% PtP thin films. Figure 5 shows the XRD patterns of Co12 at.% Pt-xP(x=0, 2.3, 3.7 at.%) films. The HCP structured (00.2) peak considerably increases with increasing phosphor content. For Co-12 at.% Pt, both HCP (00.2) and (10.0) peaks can be observed, while
Fig. 5 X-ray diffraction pattern of Co-12 at.% Pt-xP (x=0, 2.3, 3.7 at.%) thin films sputtered at room temperature
the weak (10.0) peaks and strong (00.2) peaks appear with the addition of phosphor. On the other hand, with increasing the phosphor content, the peak intensity ratio of (00.2)/(10.0) of Co-12 at.% Pt-P thin films increases drastically, suggesting that the addition of phosphor can be helpful to obtain c-axis tex-
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ture Co-Pt thin films. From the XRD and TEM results, it can be known that the platinum content in grains is larger than that at grain boundaries, and thus leads to the (00.2) peak in XRD pattern shifting from 44.13 to 43.82 deg. Generally the Pt enriched in the grains by the addition of phosphor promotes the increase of magnetic anisotropy energy and thus contributes to the thermal stability of recorded bits. But the detailed magnetic anisotropy energy of the Co-Pt-P should be determined experimentally in the future to confirm the effect of phosphor. Even so, the evidence that phosphor is enriched at grain boundaries is clear in this study, contributing to exchange decoupling and increase of coercivity as well as c-axis control of the Co-Pt-P thin films, and all of these suggests the CoPt-P system be a potential candidate of perpendicular recording media by appropriate alloy composition design and introduction of underlayer in the future. 4. Conclusion In this paper, a series of ternary Co-Pt-P thin films with different phosphorus contents were successfully fabricated by using DC magnetron sputtering. The crystalline structure and magnetic properties of single layer Co-Pt-P thin films with different phosphor contents were investigated. The coercivities of Co-12 at.% Pt-xP(x=0–3.7 at.%) thin films increase with increasing phosphor contents, mainly due to the intergranular exchange decoupling. ΔM measurement and MFM images directly show that the inter-granular exchange coupling was significantly decreased by the addition of phosphor. TEM results show that phosphor precipitated at Co-Pt grain boundaries is contributed to the inter-granular exchange decoupling.
Acknowledgements This work was supported by the Program for New Century Excellent Talents in University (No. NCET-09-0272) and the Liaoning Program for Excellent Talents in University (No. 2009R23). One of authors, B. Yang appreciates Northeastern University Research Foundation for Excellent Doctor Candidates (No. 200904). REFERENCES [1 ] N. Honda, K. Ouchi and S.I. Iwasaki: IEEE. Trans. Magn., 2002, 38(4), 1615. [2 ] J.J. Miles: IEEE. Trans. Magn., 2007, 43(3), 955. [3 ] G.W. Qin, Y.P. Ren, N. Xiao, B. Yang, L. Zuo and K. Oikawa: Int. Mater. Rev., 2009, 54(3), 157.
[4 ] H.J. Richter: J. Magn. Magn. Mater., 2009, 321(6), 467. [5 ] J. Yasumori, K. Miura, H. Muraoka, Y. Sonobe and K. Wago: J. Magn. Magn. Mater., 2008, 320(22), 3079. [6 ] Z. Hong, H.N. Bertram and M.E. Schabes: IEEE. Trans. Magn., 2002, 38(2), 1422. [7 ] K.K.M. Pandey, J.S. Chen and G.M. Chow: J. Appl. Phys., 2006, 100(5), 054909. [8 ] T. Shimatsu, Y. Okazaki, H. Sato, O. Kitakami, S. Okamoto, H. Aoi, H. Muraoka and Y. Nakamura: IEEE. Trans. Magn., 2007, 43(6), 2995. [9 ] H. Sato, T. Shimatsu, Y. Okazaki, O. Kitakami, S. Okamoto, H. Aoi, H. Muraoka and Y. Nakamura: in 10th Joint Magn. Magn. Mater. Conf./Inter. Magn. Conf., Balitmore, MD, 2007, 2106. [10] O. Chubykalo, B. Lengsfield, J. Kaufman and B. Jones: J. Appl. Phys., 2002, 91(5), 3129. [11] S.J. Greaves, A.M. Goodman, H. Muraoka and Y. Nakamura: J. Magn. Magn. Mater., 2005, 287, 66. [12] Y. Nakatani, N. Hayashi, Y. Uesaka and H. Fukushima: J. Appl. Phys., 1999, 85(8), 4726. [13] L. Zhang, H.S. Lee, J.G. Peng, Z.Y. Zhong and X.T. Zu: J. Phys. D-Appl. Phys., 2008, 41(19), 1950009. [14] Y.F. Xu, J.P. Wang, Z.S. Shan, H. Jiang, Y. Su, C.T. Chong and L. Lu: J. Magn. Magn. Mater., 2001, 225(3), 359. [15] W. Zheng, A.L. Wang, H.W. Jiang, Y.S. Zhou and T. Li: Acta Phys. Sin., 2004, 53(8), 2761. (in Chinese) [16] M. Cortes, S. Matencio, E. Gomez and E. Valles: J. Electroanal. Chem., 2009, 627(1-2), 69. [17] S. Kulkarni and S. Roy: J. Appl. Phys., 2007, 101(9), 09K524. [18] K.H. Lee, G.H. Kim and W.Y. Jeung: Electrochem. Commun., 2004, 6(2), 115. [19] H.D. Park, K.H. Lee, G.H. Kim and W.Y. Jeung: J. Appl. Phys., 2006, 99(8), 08N305. [20] T.P. Nolan, J.D. Risner, S.D. Harkness, E. Girt, S.Z. Wu, G.P. Ju and R. Sinclair: IEEE. Trans. Magn., 2007, 43(2), 639. [21] K.H. Lee, S.W. Kang, G.H. Kim and W.Y. Jeung: J. Magn. Magn. Mater., 2004, 272, E925. [22] B. Yang, G.W. Qin, W.L. Pei, Y.P. Ren and N. Xiao: J. Magn. Magn. Mater., 2010, 322, 1854. [23] X.Z. Cheng and M.B.A. Jalil: IEEE. Trans. Magn., 2005, 41(10), 3115. [24] H. Zhou, H.N. Bertram and M.E. Schabes: IEEE. Trans. Magn., 2002, 38(2), 1422. [25] H. Zhou and H.N. Bertram: IEEE. Trans. Magn., 2002, 38(2), 1405. [26] P.I. Mayo, K. O grady, P.E. Kelly, J. Cambridge, I.L. Sanders, T. Yogi and R.W. Chantrell: J. Appl. Phys., 1991, 69(8), 4733. [27] S.C. Chen, P.C. Kuo, A.C. Sun, C.T. Lie and C.C. Chiang: IEEE. Trans. Magn., 2003, 39(1), 584. [28] S.C. Chen and T.H. Sun: Vacuum, 2010, 84(12), 1430.
Effect of Phosphor Addition on Intergranular Exchange Coupling of Co-Pt Thin Films Bo Yang1) , Gaowu Qin1)† , Wenli Pei1) , Song Li1) , Yuping Ren1) and Shunji Ishio2) 1) Key Laboratory for Anisotropy & Texture of Materials, Northeastern University, Shenyang 110004, China 2) Department of Material Science & Engineering, Akita University, Akita 0108502, Japan [Manuscript received September 19, 2010, in revised form October 26, 2010]
In this paper, ternary Co-Pt-P thin films were prepared by magnetron sputtering with platinum and home-made Co-P chips pasted on the cobalt target. The structure and magnetic properties were investigated by X-ray diffraction (XRD) and vibrating sample magnetometer (VSM), respectively. With increasing phosphor content, the coercivities of Co-12 at.% Pt-P films increase from 1034 to 1525 Oe owing to the exchange decoupling among magnetic grains. The decrease of inter-granular exchange coupling was confirmed by delta-M curve measurement and magnetic force microscopy (MFM). Transmission electronic microscopy (TEM) with nanobeam composition analysis shows that phosphor segregated at cobalt-based grain boundaries is responsible for the exchange decoupling. It is thus suggested that the phosphor addition is effective to tune the exchange coupling of magnetic grains, particularly for perpendicular recording media. KEY WORDS: Intergranular exchange coupling; Co-Pt-P; Delta-M ; Magnetic recording media
1. Introduction Hard disk drive (HDD) has obtained remarkable achievements in the past half century due to the harmonic development of both magnetic recording head and media. Especially for recording system changes from longitudinal to perpendicular one, an ultra-high areal density of 610 Gb/in2 was demonstrated by Hitachi Ltd. in 2008. Unfortunately, the superparamagnetic effect limits a further increase of areal density with conventional Co-Cr-Pt based recording media. It has been thus suggested that the introduction of higher magnetic anisotropy recording media can effectively postpone the super-paramagnetic effect[1] . Besides the thermal stability, grain size distribution[2–4] and optimization of inter-granular exchange coupling[5,6] , which relate to the signal to noise ratio (SNR), are also challenges in perpendicular recording media. Hexagonal close-packed (HCP) structured Co† Corresponding author. Prof.; Tel.: +86 24 83683772; E-mail address: [email protected] (G.W. Qin).
Pt alloys with high magnetic anisotropy (Ku: ∼1×107 erg/cc)[3,7–9] , is one of particular interest and promising candidates for 500−1000 Gb/in2 perpendicular recording media. However, a vital obstacle for Co-Pt media is their strong inter-granular exchange coupling, because in order to improve signal to noise ratio (SNR), high density perpendicular magnetic recording requires not only high magnetic anisotropy material and fined grains, but also moderate inter-granular exchange coupling[3–6,10–12] . Much effort has been paid on the decoupling of Co-Pt thin films. One effective approach to adjust the intergranular exchange coupling is to segregate the magnetic grains by adding a nonmagnetic element (Cr[13] , B[14] , C[15] and P[16–19] ) or oxides[20] such as SiO2 , B2 O3 , Ta2 O5 , TiO2 and so on. However, these elements, such as Cr and B could modify the intergranular coupling but the deposition process usually needs a high substrate temperature, which limits the minimum grain size and c-axis control. Oxides, especially SiO2 , are effective isolation agents to decouple the magnetic nano-grains in media but they are also incorporated into magnetic grains, which seriously de-
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teriorates the magnetic anisotropy energy density of media[20] . Previous researches show that the addition of phosphor into Co-Pt films can obtain well isolated grains and fined grains[16,17,19,21] at room temperature. These Co-Pt-P films are fabricated by electroless or electric deposition, which increases the surface roughness and hardly controls film thickness at ∼10 nm compared to magnetron sputtering. Therefore, it is indispensable to look for a way to prepare CoPt-P films with lower surface roughness and tunable inter-granular exchange decoupling. Moreover, there are few researches on structural and magnetic properties of the Co-Pt-P thin films with different phosphor contents. In this paper, a series of ternary Co-PtP thin films with different phosphorus contents were successfully fabricated by using DC magnetron sputtering. The crystalline structure and magnetic properties of single layer Co-Pt-P thin films were investigated to confirm the exchange decoupling by the addition of phosphor. The results indicate that the inter-granular exchange coupling among HCP structured Co-Pt magnetic grains were decreased by the addition of phosphor. 2. Experimental In order to obtain Co-Pt-P films by magnetron sputtering, platinum and home-made Co-P chips were pasted on cathode target. The phosphorus and platinum concentration were controlled through changing areas of Co-P and Pt chips. For the detailed preparation procedure of Co-P chips, please refer to our recent work[22] . A series of Co-Pt-P thin films with various phosphor contents were deposited on Si (100) substrates at room temperature by DC magnetron sputtering. All the thin films were deposited at a working Ar pressure of 0.6 Pa and the base pressure before depositing was better than 9.0×10−5 Pa. The sputtering rate was about 0.16 nm/s. The nominal thickness of Co-Pt-P films was fixed at 25 nm and confirmed by stylus profiler (DEKTAK 150). The composition of Co-Pt-P films was estimated by energy dispersive analysis of X-ray spectroscopy (SEM-EDAX, JSM-7001F). Vibrating sample magnetometer (VSM, Lakeshore 7407) was used to measure the magnetic prosperities of Co-Pt-P thin films. Magnetic ΔM curves were obtained based on the isotherm remanent magnetization (MIRM ) and direct current demagnetization remanence (MDCD ). The crystalline structure was investigated by X-ray diffraction (XRD, PW3040/60) with CuKα radiation. The structure of the thin films were also characterized by high resolution transmission electron microscopy (HR-TEM) performed on JEOL JEM-2100F with an operation voltage of 200 kV, and the composition analysis of a series of points across one grain boundary was measured with a beam size of ∼1 nm. Magnetic force microscopy (MFM, SPI3800N) was used to char-
Fig. 1 Effects of phosphorous content on coercivities of Co-12 at.% Pt-xP (x=0–3.7 at.%) thin films sputtered at room temperature (a) and the corresponding in-plane M -H loops (b)
acterize the magnetic domains of Co-Pt-P thin films. The samples were observed without external magnetic field and in a vacuum about 5×10−3 Pa. 3. Results and Discussion Figure 1 shows the dependence of in-plane coercivity on phosphor contents (Fig. 1(a)) and the in-plane hysteresis loops of Co-12 at.% Pt-xP(x=0–3.7at.%) films (Fig. 1(b)). The coercivities of Co-12 at.% PtxP(x=0–3.7 at.%) films considerably increase with the phosphor content. Only a small amount addition of phosphor (1.7 at.%P) results in a great increase in coercivity from 1034 to 1420 Oe. A further increase in coercivity (1525 Oe) is achieved as the phosphor content is 3.7 at.%. According to the micromagnetic simulations, it is well-established that the coercivity increase with the decrease of exchange coupling[23–25] . The effect of phosphor content on exchange decoupling will be analyzed and discussed in detail in the following part. The drastic increase in coercivity of Co-12at.%PtP thin films indicates that the inter-granular exchange coupling strength has been considerably reduced with the addition of phosphor, as shown in Fig. 1. In order to further confirm it, the ΔM (H) technique is used to determine the magnetic interaction among the adja-
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phor precipitating at Co-Pt grain boundaries[27,28] . It can be clearly seen that the magnetic domain size greatly reduces with increasing phosphor content. For the Co-12 at.% Pt thin film, the magnetic domains were complex and the domain walls are not clear, i.e. the magnetic domains are hardly distinguished. In contrast, the domain walls can be clearly observed only by the addition of 2.3 at.% phosphor, as shown in Fig. 3(b), in spite of the fact that there are still some areas not clear (marked by arrows). For Co12 at.% Pt-3.7 at.% P, the situation is better, multidomain structure and domain walls are clear as seen in Fig. 3(c). Fig. 2 ΔM plot of Co-12 at.% Pt (a) and Co-12 at.% Pt-3.7 at.% P (b) thin films
cent magnetic grains. In ΔM (H) technique, ΔM (H) is defined as[26] ΔM (H) = 2MIRM (H) + MDCD (H) − 1 where MIRM is the isotherm remanent magnetization and MDCD is the direct current demagnetization remanence. As shown in Fig. 2. The ΔM plot of the Co-12 at.% Pt film shows a positive peak and a negative peak, and the former implies a strong ferromagnetic exchange coupling among the Co-Pt magnetic grains while the later means a long period magnetostatic coupling. With the addition of 3.7 at.% P, however, no positive peak was found, only a negative peak illustrates some dipolar or magnetostatic coupling. These results of ΔM measurement show that the inter-granular exchange coupling has been significantly decreased by the addition of phosphor, which is well consistent with coercivity dependence on the phosphor content above. The second direct evidence of exchange decoupling by the addition of phosphor is the change of magnetic domain size, as shown in Fig. 3. It is attributed to the reduction in magnetic domains size, exchange decoupling and the pinning effect on domain walls by phos-
As depicted above, the inter-granular exchange decoupling of the Co-12 at.% Pt thin film by the addition of phosphor is confirmed by ΔM curves and MFM images. All these results indicate that the addition of phosphor is an effective way to decouple the magnetic grains in the recording media, which is instructive for both longitudinal and perpendicular recording media. Since, it needs a full exchange decoupled magnetic grains for longitudinal recording media while a moderate inter-granular exchange decoupling for perpendicular recording media, which is helpful to the thermal stability and SNR. In order to clarify the mechanism of exchange decoupling, TEM and composition analysis with nanobeam were employed to examine the sample. Figure 4(a) and (b) show the TEM image of Co-12 at.% Pt and Co-12 at.% Pt-3.7 at.% P. As shown in Fig. 4(a), the Co-12 at.% Pt thin film is of HCP structure and randomly oriented. Compared to Co-12 at.% Pt, the Co-12 at.% Pt-3.7 at.% P thin films show a smaller grain size. The composition distributions of Co, Pt and P elements in grains and at grain boundaries were analyzed by nano-beam composition analysis. For Co12 at.% Pt thin films, the content of Co and Pt is equably distributed in grains and at grain boundaries, as shown in Fig. 4(c). However, for Co-12 at.% Pt3.7 at.% P thin films, the phosphor content is almost negligible in grains, but enriched at grain boundaries,
Fig. 3 MFM images of Co-12 at.% Pt-xP films without external magnetic field: (a) x=0, (b) x=2.3 at.%, (c) x=3.7 at.%
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Fig. 4 TEM images of Co-12 at.% Pt (a) and Co-12 at.% Pt-3.7 at.% P (b), and the corresponding composition distribution in grains and at grain boundaries for the Co-12 at.% Pt (c) and Co-12 at.% Pt-3.7 at.% P (d) films. GB means Co-Pt grain boundary and the dash line means the nominal phosphor content (3.7 at.%)
as shown in Fig. 4(d). Meanwhile, both the Co and Pt contents at grain boundaries are smaller than that in grains. It indicated that the phosphor element segregates at grain boundaries. As is well known, Pt has a large solid solubility in cobalt. The addition of 12 at.% Pt will not change the HCP structure. Since there is no solid solubility of phosphor in HCP structured cobalt, it is inevitable that the phosphor will precipitate in cobalt matrix, i.e. there will be two-phase microstructure in Co-PtP system with a small amount of the phosphor addition. From the viewpoint of diffusion kinetics, there is a large atomic size difference between cobalt (135 pm in radius) and phosphor (100 pm in radius). The phosphor can diffuse rapidly in cobalt and precipitate at grain boundary, especially at the sputtering condition. Thus, the inter-granular exchange coupling can be effectively decreased. X-ray diffraction technique (XRD) was also used to examine the crystalline structure of Co-12 at.% PtP thin films. Figure 5 shows the XRD patterns of Co12 at.% Pt-xP(x=0, 2.3, 3.7 at.%) films. The HCP structured (00.2) peak considerably increases with increasing phosphor content. For Co-12 at.% Pt, both HCP (00.2) and (10.0) peaks can be observed, while
Fig. 5 X-ray diffraction pattern of Co-12 at.% Pt-xP (x=0, 2.3, 3.7 at.%) thin films sputtered at room temperature
the weak (10.0) peaks and strong (00.2) peaks appear with the addition of phosphor. On the other hand, with increasing the phosphor content, the peak intensity ratio of (00.2)/(10.0) of Co-12 at.% Pt-P thin films increases drastically, suggesting that the addition of phosphor can be helpful to obtain c-axis tex-
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ture Co-Pt thin films. From the XRD and TEM results, it can be known that the platinum content in grains is larger than that at grain boundaries, and thus leads to the (00.2) peak in XRD pattern shifting from 44.13 to 43.82 deg. Generally the Pt enriched in the grains by the addition of phosphor promotes the increase of magnetic anisotropy energy and thus contributes to the thermal stability of recorded bits. But the detailed magnetic anisotropy energy of the Co-Pt-P should be determined experimentally in the future to confirm the effect of phosphor. Even so, the evidence that phosphor is enriched at grain boundaries is clear in this study, contributing to exchange decoupling and increase of coercivity as well as c-axis control of the Co-Pt-P thin films, and all of these suggests the CoPt-P system be a potential candidate of perpendicular recording media by appropriate alloy composition design and introduction of underlayer in the future. 4. Conclusion In this paper, a series of ternary Co-Pt-P thin films with different phosphorus contents were successfully fabricated by using DC magnetron sputtering. The crystalline structure and magnetic properties of single layer Co-Pt-P thin films with different phosphor contents were investigated. The coercivities of Co-12 at.% Pt-xP(x=0–3.7 at.%) thin films increase with increasing phosphor contents, mainly due to the intergranular exchange decoupling. ΔM measurement and MFM images directly show that the inter-granular exchange coupling was significantly decreased by the addition of phosphor. TEM results show that phosphor precipitated at Co-Pt grain boundaries is contributed to the inter-granular exchange decoupling.
Acknowledgements This work was supported by the Program for New Century Excellent Talents in University (No. NCET-09-0272) and the Liaoning Program for Excellent Talents in University (No. 2009R23). One of authors, B. Yang appreciates Northeastern University Research Foundation for Excellent Doctor Candidates (No. 200904). REFERENCES [1 ] N. Honda, K. Ouchi and S.I. Iwasaki: IEEE. Trans. Magn., 2002, 38(4), 1615. [2 ] J.J. Miles: IEEE. Trans. Magn., 2007, 43(3), 955. [3 ] G.W. Qin, Y.P. Ren, N. Xiao, B. Yang, L. Zuo and K. Oikawa: Int. Mater. Rev., 2009, 54(3), 157.
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