Cu seed-layer on perpendicular magnetic anisotropy of Co80Pt20 films

Journal of Magnetism and Magnetic Materials xxx (2017) xxx–xxx

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Effect of Cu, Cu/Ru, or Ru/Cu seed-layer on perpendicular magnetic anisotropy of Co80Pt20 films S.C. Chen a,b, S.U. Jen c,⇑, R.Z. Chen d, C.F. Lu a, C.M. Wang e, P.C. Kuo e a

Department of Materials Engineering and Center for Thin Film Technologies and Applications, Ming Chi University of Technology, Taipei 243, Taiwan Department of Electric Engineering, Chang Gung University, Taoyuan 333, Taiwan c Institute of Physics, Academia Sinica, Taipei 11529, Taiwan d New Materials R&D Dept., China Steel Corporation, Kaohsiung 812, Taiwan e Institute of Materials Science and Engineering, National Taiwan University, Taipei 106, Taiwan b

a r t i c l e

i n f o

Article history: Received 7 June 2017 Received in revised form 29 October 2017 Accepted 28 November 2017 Available online xxxx Keywords: Co80Pt20 thin films Ru/Cu bilayers Perpendicular magnetic anisotropy Columnar grains

a b s t r a c t The magnetic anisotropy of Co80Pt20 films deposited directly on glass substrates (without any seed layer) at room temperature tends to be inclined. The degree of perpendicular magnetic anisotropy can be slightly increased by introducing a Cu layer or a Cu/Ru (Cu on Ru) bilayer under the Co80Pt20 film. However, when Co80Pt20 films are deposited onto a Ru/Cu (Ru on Cu) bilayer, the magnetic anisotropy of the films becomes perpendicular. Moreover, from this study, we found that an fcc Cu (1 1 1) underlayer can promote the hcp Ru (0 0 0 2) seed layer, which can, in turn, induce a good (0 0 0 2) texture in the hcp Co80Pt20 magnetic layer. As a result, Co80Pt20/Ru/Cu/glass films display excellent (or improved) perpendicular magnetic anisotropy. In conclusion, our results showed that columnar Co-rich Co-Pt nano-grains with perpendicular coercivity of 4580 Oe and perpendicular squareness of 0.77 can be achieved by depositing Co80Pt20 (50 nm) film onto a Ru (30 nm)/Cu (100 nm) bilayer, with a glass substrate, at room temperature. Ó 2017 Elsevier B.V. All rights reserved.

1. Introduction To achieve ultra-high magnetic recording density, magnetic materials with high magneto-crystalline anisotropy constant (KU) are required to maintain the thermal stability of magnetic grains. Hexagonal Co3Pt alloy is one of the potential candidates for magnetic recording material because of its large KU value of 2.0  107 erg/cm3 [1–3] and high corrosion resistance. For it to be used as a perpendicular magnetic recording medium, the Co3Pt film must not only form magnetically isolated grains to obtain a high signal-to-noise ratio (SNR) but also possess perpendicular magnetic anisotropy (PMA). The magnetic easy axis of the Co3Pt film is along the [0 0 0 2] axis; however, it is usually difficult to achieve (0 0 0 2) texture in a single-layered Co3Pt film. Ru film can be used as an under-layer to improve the magneto-crystalline PMA of the film due to the small lattice mismatch (about 4%) between Ru (0 0 0 2) and Co3Pt (0 0 0 2) films. Moreover, Refs. [4–7] have discovered that inserting a (1 1 1) textured Pt under-layer beneath the Ru layer, it is beneficial in enhancing the (0 0 0 2) texture of Ru films. This enhancement is due to the lattice-plane match ⇑ Corresponding author. E-mail address: [email protected] (S.U. Jen).

between the Pt (1 1 1) under-layer and Ru (0 0 0 2) layer (i.e., the lattice misfit is small, around 2.6%). Since the misfit between Cu (1 1 1) and Ru (0 0 0 2) is also small (around 5.0%), we propose in this paper the replacement of the Pt (1 1 1) with the Cu (1 1 1) under-layer, while keeping the other structure layers unchanged. We hope that the goal of a large PMA can also be achieved after our modifications. In particular, the effects of the Ru/Cu (Ru on Cu) and/or Cu/Ru (Cu on Ru) structures and thickness of the Ru layer on the texture, microstructure and magnetic properties of the Co80Pt20 magnetic layer are thoroughly investigated in this study. 2. Experimental details We used the DC magnetron sputtering technique to deposit all the layers, including the Co80Pt20, Ru, and Cu layers, at room temperature (RT). First, 100 nm thick Cu under-layers were deposited on Corning 1737F glass substrates. Then, Ru seed layers with thicknesses of 0–100 nm were deposited onto the Cu under-layer, respectively. Finally, Co80Pt20 magnetic layers of 50 nm thickness were deposited sequentially onto the Ru/Cu bilayer films by co-sputtering of Co and Pt targets. The base pressure of the chamber was lower than 5.0  10 7 Torr. The Ar working pressures

https://doi.org/10.1016/j.jmmm.2017.11.119 0304-8853/Ó 2017 Elsevier B.V. All rights reserved.

Please cite this article in press as: S.C. Chen et al., Effect of Cu, Cu/Ru, or Ru/Cu seed-layer on perpendicular magnetic anisotropy of Co80Pt20 films, Journal of Magnetism and Magnetic Materials (2017), https://doi.org/10.1016/j.jmmm.2017.11.119

S.C. Chen et al. / Journal of Magnetism and Magnetic Materials xxx (2017) xxx–xxx

(PAr) were 3 mTorr during the deposition of the Cu under-layer and Ru seed layer, and 10 mTorr for deposition of the Co80Pt20 magnetic layer. The sputtering power densities of the Cu, Ru, Co and Pt targets were fixed at 9.87  10 3, 9.87  10 3, 1.48  10 2, and 2.47  10 3 watt/mm2, respectively. In order to obtain uniform film composition, the substrate was rotated at 10 rpm. The magnetization curves of the samples were measured using a vibrating sample magnetometer (VSM) with a maximum applied field of 20 kOe at RT. The structure of the films was examined by X-ray diffraction (XRD) using Cu-Ka radiation. The chemical composition of the Co80Pt20 films was analyzed with an electron probe micro-analyzer (EPMA). The microstructures of the films were investigated by field emission gun transmission electron microscopy (TEM). 3. Results and discussion

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Fig. 1 shows the easy-axis (or out-of-plane) and hard-axis (or in-plane) M-H hysteresis loops of Co80Pt20 films with various under-layer structures. As shown in Fig. 1, the saturation magnetization (MS) of our Co80Pt20 films is only about 600 emu/cm3, much smaller than that (1100 emu/cm3) of the bulk Co80Pt20 [3]. This does not indicate there are serious problems, such as off-stoichiometric, filmoxidation, or otherwise, in our Co80Pt20 films. First, the EPMA result indicated that the compositions of our Co80Pt20 films were all stoichiometric. Second, according to Refs. [4,5], even for the Pt-capped Co72Pt28 films, made by the same DC magnetron sputtering at RT,

their MS is also low or roughly in agreement with ours. This may certainly rule out the possibility of film-oxidation in our case. In particular, Ref. [4] also found that MS depends on PAr: i.e., MS decreases from 670 to 490 emu/cm3, as PAr increases from 0.5 to 20 mTorr. Furthermore, from the hard-axis (or in-plane) magnetic hysteresis loop in Fig. 1, we can determine the uniaxial KU of each Co80Pt20 film. All the KU data from this study are summarized in Table 1. Also, in this table, the quality factor Q is defined as Q = KU/2p(MS)2. According to Refs. [8,9], if Q > 1, the magnetic film exhibits PMA, and if 1 > Q > 0.4, the magnetic film exhibits tilted (or oblique) magnetic anisotropy (TMA). TMA means that the easy-axis is slightly tilted from the film normal. More details are discussed below. In this study, when a single layer of 50 nm thick Co80Pt20 film is deposited directly on glass substrates at RT, the magnetic anisotropy of the Co-Pt film tends towards a slightly tilted orientation, as its Q is only 0.91. When a Cu (100 nm) under-layer is introduced, the degree of PMA of Co80Pt20 film is enhanced slightly, i.e., Q increases from 0.91 to 0.95. But, when a Cu (100 nm)/Ru (30 nm) bilayer is used, the degree of PMA becomes worse, i.e., Q = 0.87. Finally, if the 50 nm thick Co80Pt20 film is deposited onto a Ru (30 nm)/Cu (100 nm) bi-layered films, the magnetic anisotropy of the film clearly changes to perpendicular (or truly PMA); its perpendicular coercivity, Q value, and perpendicular squareness are 4580 Oe, 1.38, and 0.77, respectively. Fig. 2 shows the XRD patterns of Co80Pt20 films with various under-layer structures. First, a weak Co3Pt (0 0 0 2) peak appears for the single-layered Co80Pt20

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Fig. 1. In-plane and perpendicular magnetization curves of Co80Pt20 (50 nm) films deposited on various under-layers: (a) no under-layer, (b) Cu (100 nm) under-layer, (c) Cu (100 nm)/Ru (30 nm) bi-layer, and (d) Ru (30 nm)/Cu (100 nm) bi-layer.

Please cite this article in press as: S.C. Chen et al., Effect of Cu, Cu/Ru, or Ru/Cu seed-layer on perpendicular magnetic anisotropy of Co80Pt20 films, Journal of Magnetism and Magnetic Materials (2017), https://doi.org/10.1016/j.jmmm.2017.11.119

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S.C. Chen et al. / Journal of Magnetism and Magnetic Materials xxx (2017) xxx–xxx

Table 1 MS is the saturation magnetization. Ku is the perpendicular magnetic anisotropy energy. Q is the quality factor, defined as Q = Ku/2p(MS)2. F = Co80Pt20. g = glass substrate. PMA = perpendicular magnetic anisotropy. TMA = tilted magnetic anisotropy. Films

F(50 nm)/g [Fig. 1(a)] F(50 nm)/Cu(100 nm)/g [Fig. 1(b)] F(50 nm)/Cu(100 nm)/Ru(30 nm)/g [Fig. 1(c)] F(50 nm)/Ru(30 nm)/Cu(100 nm)/g [Fig. 1(d)] F(50 nm)/Ru(10 nm)/Cu(100 nm)/g [Fig. 4(a)] F(50 nm)/Ru(20 nm)/Cu(100 nm)/g [Fig. 4(b)] F(50 nm)/Ru(50 nm)/Cu(100 nm)/g [Fig. 4(c)] F(50 nm)/Ru(100 nm)/Cu(100 nm)/g [Fig. 4(d)]

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thin film, with no under-layer (Fig. 2(a)). Second, when a 100 nm thick Cu under-layer is introduced, not only a Cu (1 1 1) but also a weaker Cu (2 0 0) peak appears (Fig. 2(b)). As a result, the peak intensity of Co3Pt (0 0 0 2) is still weak, reflecting the fact that the texture of Co3Pt (0 0 0 2) is unable to form on the Cu (2 0 0) surface. Third, a stronger Co3Pt (0 0 0 2) peak is achieved, when the Co80Pt20 film is deposited onto Ru(30 nm)/Cu(100 nm) bilayered films (Fig. 2(d)). As shown in Fig. 3, the interatomic distance of fcc Cu (1 1 1), hcp Ru (0 0 0 2) and hexagonal Co3Pt (0 0 0 2) are 2.56 Å, 2.70 Å, and 2.56 Å, respectively, and the fractions between parentheses for Co and Pt denote the heights above the base plane, respectively [10]. As a result, a Cu under-layer with (1 1 1) preferred orientation could enhance the preferred texture growth of the Ru (0 0 0 2) seed layer. In turn, this mechanism results in a highly textured Co80Pt20 (0 0 0 2) film with a good PMA, as expected, as checking Figs. 1(d), 2(d) and Table 1. Fourth, when the Co80Pt20 film is deposited onto Cu(100 nm)/Ru(30 nm) bilayer films, we could observe two results: [A] the Cu (2 0 0) peak is stronger in Fig. 2(c) than in Fig. 2(d), and [B] an obvious Ru (1 0 1 1) peak appears in Fig. 2(c). The appearance of the Ru (1 0 1 1) peak structure helps the growth of the Cu (2 0 0) texture (Fig. 2(c)). But, in Fig. 3b, since there is no Ru layer or Ru (1 0 1 1) peak, the Cu (2 0 0) peak is much weaker. As a result, this effect of Fig. 2(c) (i.e., with the Cu/Ru structure) is not beneficial to the growth of a highly (1 1 1) texture in the Cu seed layer, which is required to promote the (0 0 0 2) texture of the Co80Pt20

Fig. 3. Schematic of epitaxial growth of hexagonal Co3Pt (0 0 0 2) film on the hcpRu (0 0 0 2)/fcc-Cu (1 1 1) under-layer.

layer. Notice that the Cu (1 1 1) peak in Fig. 2(c) is lower than that in Fig. 2(b). Therefore, the magnetic anisotropy of Co80Pt20 film is degraded from PMA to TMA [see Fig. 2(b), (c), and Table 1]. Fig. 4 shows the M-H hysteresis loops of Co80Pt20 (50 nm) films deposited onto Ru/Cu bi-layered films; with the Cu seed layer fixed at 100 nm, and Ru seed layers at various thicknesses from 10 to 100 nm. Comparing Fig. 4 with the M-H loop of the Co80Pt20/Cu bi-layered film with no Ru seed layer [Fig. 1(b)], the degree of PMA of the Co80Pt20 magnetic layer can be improved significantly when a Ru seed layer is introduced between the Co80Pt20 and Cu films (see Figs. 4, 1(b), and Table 1). The variations of perpendicular coercivity (Hc\) and squareness (S\) of Co80Pt20 films deposited onto Ru/Cu bi-layered films, with Ru under-layer of different thicknesses, are shown in Fig. 5. The Hc\ of Co80Pt20/Cu bilayer films with no Ru seed-layer is 3910 Oe. The highest Hc\ value, e.g., 4925 Oe, can be obtained by the introduction of a 10 nm thick Ru seed-layer between the Co80Pt20 and Cu layers. However, when the thickness of the Ru layer is further increased above 10 nm, the Hc\ value of the films drops gradually. Meanwhile, as shown in Fig. 5(b), the value of S\ of the

Please cite this article in press as: S.C. Chen et al., Effect of Cu, Cu/Ru, or Ru/Cu seed-layer on perpendicular magnetic anisotropy of Co80Pt20 films, Journal of Magnetism and Magnetic Materials (2017), https://doi.org/10.1016/j.jmmm.2017.11.119

S.C. Chen et al. / Journal of Magnetism and Magnetic Materials xxx (2017) xxx–xxx

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Co80Pt20/Cu bi-layered films is 0.47. The maximum S\ value, e.g., 0.77, is achieved by introducing a 30 nm Ru seed-layer between the Co80Pt20 and Cu layers. Further raising the Ru thickness exceeds 30 nm, the S\ value decreases. These findings indicate that (A) the PMA of the films can be enhanced markedly as the thickness of the Ru layer is increased from 10 to 30 nm; (B) further increasing the Ru thickness beyond 30 nm is non-beneficial to increase the PMA of the magnetic films. Pandey et al. [4] reported that when a thicker Ru layer is introduced under Co-rich Co-Pt films, reduced grain isolation could alter the inter-grain dipole or exchange interaction, and thus cause the neighboring grains to have weaker PMA and early switching, which should result in reduced values of Hc\ and S\. The XRD patterns of Co80Pt20/Ru/Cu multilayer films with Ru seed-layers of various thicknesses are shown in Fig. 6. A very faint Co3Pt (0 0 0 2) peak appears in the Co80Pt20/Cu bi-layered films; without any Ru seed layer. When the Ru seed layers are introduced between the Co80Pt20 magnetic layer and Cu underlayer, clear Co3Pt (0 0 0 2) peaks can be seen, as shown in Fig. 6 (b)–(f), indicating that the PMA of Co80Pt20 films can be improved greatly by the introduction of a Ru seed-layer, which agrees with Table 1. Once again, this result also confirms that the (0 0 0 2) texture of the Co80Pt20 magnetic layer can be easily grown on Ru (0 0 0 2) seed layers. Nevertheless, Kitakami et al. [11] reported that a Ta under-layer can promote higher (0 0 0 2) texture of Co films than a Ag or Cu under-layer. This difference results from the surface free energy of Ta being larger than that of Ag and Cu, which prefers the layered-growth mode, and, therefore, the enhancement

of the (0 0 0 2) preferred texturing in the magnetic Co or Co3Pt film. In our work, the surface free energy of Ru (3.409 J/m2) is also greater than that of Cu (1.934 J/m2) [12]. We believe this surface free energy difference is another main reason why the degree of PMA of Co80Pt20 films deposited on Ru/Cu bilayers is better than those deposited on a Cu under-layer (Table 1). In addition, as shown in Fig. 6(f), a weak Ru (1 0 1 1) peak starts to show up, when tRu = 100 nm. As discussed previously, the appearance of the Ru (1 0 1 1) peak does not favor the required texture growth. Thus, it explains why when tRu  30 nm, the degree of PMA decreases, as shown in Fig. 4 and Table 1. Fig. 7 shows TEM cross-section images of Co80Pt20/Cu bi-layered and Co80Pt20/Ru/Cu multi-layered films. It is found that as shown in Fig. 7(a), only a small amount of columnar grains are grown on the Cu under-layer in the Co80Pt20/Cu bi-layered case. This finding reflects the difficulty of obtaining a highly (1 1 1) texture in the Cu under-layer deposited on glass substrate. However, in Fig. 7(b), significantly more amounts of columnar grains in the Co80Pt20 magnetic layer are grown on the Ru seed-layer, as in the Co80Pt20/ Ru/Cu multi-layered case. Therefore, the situation of Ru/Cu bilayers in achieving (0 0 0 2) texture and crystallinity in the Co80Pt20 layer is much better than that of single-layered Cu films. That is why we can have an achievement of S\ = 0.77 for the film with a Ru layer, which is much larger S\ = 0.47 of the film without a Ru layer. The same reason also applies to Hc\: i.e., Hc\ = 4925 Oe with a Ru layer, which is also much larger than Hc\= 3910 Oe without a Ru layer.

Please cite this article in press as: S.C. Chen et al., Effect of Cu, Cu/Ru, or Ru/Cu seed-layer on perpendicular magnetic anisotropy of Co80Pt20 films, Journal of Magnetism and Magnetic Materials (2017), https://doi.org/10.1016/j.jmmm.2017.11.119

S.C. Chen et al. / Journal of Magnetism and Magnetic Materials xxx (2017) xxx–xxx

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Fig. 7. Cross-sectional TEM images of (a) Co80Pt20 (50 nm)/Cu (100 nm) bi-layered and (b) Co80Pt20 (50 nm)/Ru (30 nm)/Cu (100 nm) multi-layered films.

4. Conclusions It is well known that in the conventional (or commercial) method of making a Co3Pt film with good PMA and high MS, one needs to employ the high-temperature deposition, e.g., TS  400 °C, or high-temperature annealing technique. One merit of this study is that by inserting a proper Ru/Cu bi-layer, we can already obtain a Co3Pt film with PMA by room-temperature deposition only. Columnar Co80Pt20 films with nano-grains and PMA are achieved by depositing the films onto a Ru(30 nm)/Cu(100 nm) bilayer at room temperature. Its Hc\ = 4580 Oe, S\ = 0.77, KU = 2.95  106 erg/cc, and Q = 1.38, revealing its significant potential as a perpendicular magnetic recording medium. Acknowledgements

Fig. 6. XRD patterns of Co80Pt20 (50 nm) deposited onto Ru/Cu (100 nm) bi-layer using Ru seed layers of various thicknesses: (a) 0 nm, (b) 10 nm, (c) 20 nm, (d) 30 nm, (e) 50 nm and (f) 100 nm.

This work was supported by the Ministry of Science and Technology and Ministry of Economic Affairs of Taiwan through Grant Nos. 105–2221-E-131-010 and 99-EC-17-A-08-S1-006, respectively. SUJ would also like to thank IOP-AS for the financial support to attend the MISM-2017 conference.

Please cite this article in press as: S.C. Chen et al., Effect of Cu, Cu/Ru, or Ru/Cu seed-layer on perpendicular magnetic anisotropy of Co80Pt20 films, Journal of Magnetism and Magnetic Materials (2017), https://doi.org/10.1016/j.jmmm.2017.11.119

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Please cite this article in press as: S.C. Chen et al., Effect of Cu, Cu/Ru, or Ru/Cu seed-layer on perpendicular magnetic anisotropy of Co80Pt20 films, Journal of Magnetism and Magnetic Materials (2017), https://doi.org/10.1016/j.jmmm.2017.11.119