C films

Thin Solid Films 445 (2003) 127–130

Structural and magnetic properties of nanogranular Co–PtyC films Ai-ling Wang, Tong Li, Yun-song Zhou, Hong-wei Jiang, Wu Zheng* Department of Physics, Capital Normal University, Beijing 100037, PR China Received 8 November 2002; received in revised form 17 August 2003; accepted 17 August 2003

Abstract The structural and magnetic properties of Co–PtyC nanocomposite films were investigated as functions of Pt and C concentration, respectively. Under the same conditions two series multilayer samples with different Pt and C concentrations were prepared. The as-deposited films are amorphous Co–Pt–C alloys and magnetically soft. All samples were annealed in vacuum. The lattice structure is observed to transform and grain size is reduced to 7 nm. These structural transitions are accompanied by large changes in magnetic anisotropy. A maximum coercivity of approximately 5.4 kOe is obtained and a shoulder in hysteresis loops develops. These results show that Co–Pt particles are transformed by annealing from a disordered phase to ordered facecentered-cubic CoPt3 and face-centered-tetragonal CoPt, commixed in a C matrix, and the various Co–Pt crystalline particles interact. 䊚 2003 Elsevier Science B.V. All rights reserved. PACS: 75.30.GW; 75.50.Vv; 78.20.Ls Keywords: Carbon; Platinum; Crystallization; Magnetic properties and measurements

1. Introduction Cobalt–platinum alloy films have long been attractive as materials for permanent magnets due to their high magnetic anisotropy and high coercivity. Co–Pt alloys with compositions close to equi-atom have been studied extensively w1x. These alloys undergo a phase transformation from a disordered face-centered-cubic (fcc) phase at higher temperatures to an ordered face-centeredtetragonal (fct) phase at lower temperatures. The fct phase has a large value of magnetocrystalline anisotropy. At the same time, Co–Pt alloys with different compositions have been receiving recently considerable attention for magnetic and magneto-optical recording applications w2,3x. Requirements for higher magnetic recording density with low noise would need to have a coercivity (Hc) of approximately 4 kOe w4x and weakly exchange-coupled grains of less than 10 nm in size. Co–Pt and C are immiscible and metastable Co–Pt carbides decompose easily into Co–Pt and C w2,3,5x. In fact the macroscopic *Corresponding author. Tel.: q86-10-6890-1594; fax: q86-106890-2178. E-mail address: [email protected] (W. Zheng).

interfusion coefficient between Co–Pt and C is negative. Therefore, Co–PtyC composite films can be formed by a good system to investigate the magnetic properties of nanostructured Co–Pt particles, since C can provide enough space between neighboring Co–Pt particles to reduce the intergrain exchange interaction. Current studies have been focused on nanocrystalline Co–PtyC films because of their high anisotropy. Most reports have dealt with the films consisting of Co-rich Co–Pt particles. We prefer to investigate the similar films but consisting of Pt-rich Co–Pt particles. In this article we report the structural and magnetic properties of Co–PtyC nanocomposite films prepared by sputtering and postannealing. The effects of Pt and C content on coercivity are discussed. 2. Experimental details The granular structure was obtained by depositing CoyPtyC onto water-cooled Si(1 0 0) substrates in a multilayer form (consisting of 80 repetitions) and subsequently annealing to form the nanoparticles. The films were prepared by magnetron sputtering deposition from pure targets of Co, Pt and C. The base pressure of the

0040-6090/03/$ - see front matter 䊚 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/j.tsf.2003.08.061

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˚ ˚ ˚ Fig. 1. Hc of films structured as C 200 AywCo2 AyPt x AyC12 ˚ 80yC150 A ˚ (xs0–20) annealed in vacuum at 700 8C for 1 h as a Ax function of Pt content.

chamber was 3=10y5 Pa and high-purity Ar (99.999%) was used for deposition at ambient temperature with a pressure of 0.5 Pa. The substrates used were Si(1 0 0) with a naturally grown oxide on the surface. The C layers were sputtered using a d.c. power at a rate of 0.4 ˚ ˚ Ays. The sputtering rates of both Co and Pt were 1 Ay s. The chemical compositions of Co–PtyC films were checked by energy-dispersive X-ray (in a SEM). The magnetic measurements at room temperature were performed on an alternating gradient force magnetometer with the applied field parallel to the film plane. The structure of the alloy film was obtained from X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM) analysis. A 20 nm buffer and 15 nm overcoat of C layer was used for all samples to ensure similar growth conditions. Two series samples were prepared. The first was struc˚ ˚ ˚ x80 where x varied from tured as wCo2 AyPt x AyC4 A 0 to 20 for a Pt element percent range from 0 to 90 at.% in Co–Pt, and then annealed in vacuum at 300– 700 8C for 1–5 h. It was used to determine the optimum Pt concentration and suitable annealing conditions. The ˚ second series samples, structured as wCo31Pt698 AyC x ˚ x80 (xs0–12) and annealed for 1 h at 700 8C in A vacuum, were used to find the proportional C content.

phase at 700 8C. The amorphous phase of Co–Pt particles are magnetically soft. Another is that the magnetic enhancement is not remarkable within much C surrounded. In this case, C concentration is over 60 at.% in the film except C buffer and cover layers. It is easy to understand that more nonmagnetic element in the alloy decreases the magnetic moment of the film w4 x . Fig. 2 shows the plan view HRTEM images of the ˚ ˚ x80, films which were structured as wCo31Pt698 AyC xA where x was 0 or 4, and annealed at 700 8C. When xs 0, it means no inserting C layer, C element from buffer and capping layers seeps into the film in the annealing process. In Fig. 2a, two Co–Pt grains, as single approximately 9 nm in size, are connecting together, and surrounded by C. While xs4, in Fig. 2b, since there are C layers between Co and Pt layers, the Co–Pt particle size is reduced to 7 nm, and Co–Pt particles are separated by C. These images also show that the C matrix is amorphous and the Co–Pt grain is a single crystal. Two conclusions may be drawn. First, 700 8C, as the annealing temperature, is high enough to blend C layers, including the buffer and capping layers, with CoyPt layers together. Second, an appropriate amount of C could not only reduce the size of Co–Pt particles, but also offer the enough space to take connecting Co– Pt particles apart. ˚ Fig. 3 shows XRD patterns of wCo31Pt698 AyC4 ˚ x80 film annealed at 700 8C for 1 h. All visible peaks A

3. Results and discussion All the as-deposited films are magnetically soft and some of them become hard after annealing. It is found that Hc of films changes remarkably from a few tens Oe to 5.4 kOe with the changes of Pt content, as shown in Fig. 1. The maximum of Hc is obtained at 70 at.% Pt. If the Pt ratio in Co–Pt is less than 50%, the effect of annealing is negligible. This result was not expected and contradicts some reports w6–8x. Co and Pt might form some stable phase by annealing, and it could introduce coercivity enhancement. One possible explanation for our experimental result is that the Co–Pt particles with a low Pt concentration may not be completely transformed to stable phase from disordered

Fig. 2. Plan view HRTEM images of films structured as C 200 ˚ ˚ ˚ ˚ AywCo 31Pt698 AyC x Ax80 yC 150 A, annealed at 700 8C for 1 h, where (a) xs0 and (b) xs4.

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Table 1 ˚ ˚ ˚ Comparison of XRD values of C 200 AywCo 31 Pt69 8 AyC4 Ax80 C 150 ˚ film annealed at 700 8C for 1 h with normal values of fcc CoPt3 A and fct CoPt

˚ ˚ ˚ ˚ Fig. 3. XRD pattern of C 200 AywCo 31 Pt69 8 AyC4 Ax80 yC 150 A film annealed at 700 8C for 1 h.

can be attributed to the fcc CoPt3 or fct CoPt phase. This indicates that the most C remains as a pure element matrix rather than forming any carbide phase. The Co– Pt grain size is estimated approximately 8 nm by Scherrer’s formula from the (1 1 1) peak width. These conclusions are in good agreement with the results obtained from HRTEM pictures. But only by XRD measurement, it is difficult to distinguish between fcc CoPt3 and fct CoPt phases, because their XRD patterns are very similar, refer Table 1 w9,10x, and all peaks are considerably broadened due to the very small grain sizes. On the other hand, the magnetic measurement can also provide the phase transforming evidence. All the as-deposited films, with Hc approximately 100 Oe, most likely consist of disordered fcc and amorphous Co–Pt w11x. But the coercivity of all films, structured as ˚ x80, increases dramatically over 2 kOe wCo31Pt698 AyC after annealed at 700 8C. It means that a part of the Co–Pt particles transform into ordered phase, e.g. the fcc CoPt3 andyor fct CoPt. ˚ Fig. 4 shows the hysteresis loops of wCo31Pt698 AyC ˚ ˚ x Ax80 films, where x from 0 to 12 A, annealed at 700 8C. It is very interesting that shoulders develop in the demagnetization curves. It is clear that Hc and the demagnetization curve shape of the film can be controlled by C. If there is no inserting C layer, Hc is low and no shoulder appears. With C layer thickened sequentially, the Hc increases, reaches its maximum of approximately 5.4 kOe at xs4, then decreases, and a shoulder develops in the demagnetization curve simultaneously. ˚ the shoulder tends to When the C layer equals to 12 A, disappear. One can note that shoulder is the most visible while Hc reaches the maximum approximately. The same remanence (Mr yMs) of approximately 0.9 is detained in all films. There are several mechanisms to cause the shoulder in the demagnetization curves. But the most important one is that there are two kinds of Co–Pt crystals, e.g. fcc CoPt3 and fct CoPt, in the film. They have the

CoPt3(fcc) d (nm) h k l

CoPt(fct) d (nm) h k l

Co–Pt–C d (nm) h k l

0.2224 (1 1 1) 0.1927 (2 0 0) 0.1723 (2 1 0)

0.2176 (1 1 1) 0.1908 (2 0 0) 0.1692 (2 0 1)

0.2183 (1 1 1) 0.1906 0.1700

different magnetic properties. The fcc CoPt3 has a smaller Hc and higher Ms while fct CoPt has a larger Hc and lower Ms w7x. If they were connected in the films, as shown in Fig. 2a, consequently coupled in magnetism, the unique magnetic character of them would not be displayed in an external field, respectively. It means that no shoulder could appear in a hysteresis loop. If two kinds of Co–Pt crystals are separated by C, as shown in Fig. 2b, their different magnetization processes would become visible in the loop. And the shoulder would be expected to appear in the loops as in the case of a soft-phase exchange-coupled to the magnetically hard fct CoPt phase w12x. Here C content plays the key role. When C content is low, two phases, fcc CoPt3 and fct CoPt, interact strongly and appear just like a single phase. If C content is high, e.g. C layer ˚ which means C concentration thickness reaching 12 A, is over 68 at.% in the Co–PtyC film except buffer and cover C layers, the distance between Co–Pt crystallites increases, it reduces the intensity of exchange-couple and enhances the demagnetization process. That causes

Fig. 4. Hysteresis loop (MryMs vs. H) development dependent on C ˚ ˚ 80 films, where x is 0, 1, 2, 4 and 12, content in wCo31Pt698 AyC x Ax annealed at 700 8C for 1 h.

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the loop shoulder to disappear and the coercivity to descend w13x. Of course, the real situation is more complex. For coherent rotation in a random distribution of noninteracting fct CoPt nanoparticles with uniaxial anisotropy the coercivity would be several tens of kilooersted on theory. The values of measured coercivity are smaller by one order of magnitude. The difference between theory and our experiments is mainly due to the low degree of ordering of the Co–Pt phase as Christodoulides et al. reported w1x. The relatively low annealing temperature and short time used to optimize the microstructure and coercivity may not be sufficient for atomic ordering to be complete. 4. Conclusions In summary, we were successful in fabricating high coercivity Co–PtyC granular films. The films consist of ordered fcc CoPt3 and fct CoPt nanoparticles embedded in a C matrix. The annealing condition, at 700 8C for 1 h, is suitable to commix the C layers with CoyPt layers, but insufficient to complete atomic ordering. The different magnetic anisotropies, characterized by two ordered phases, fcc CoPt3 and fct CoPt, may develop a shoulder in the hysteresis loops. The coercivity and hysteresis loop of the films can vary with the different Pt and C content. The results of this research are very promising and present these materials attractive as candidates for magnetic recording.

Acknowledgments The authors gratefully acknowledge the support of the National Natural Science Foundation of China (Grant No. 50071038) and the Development Foundation of Beijing Education Commission. References w1x J.A. Christodoulides, Y. Huang, Y. Zhang, G.C. Hadjipanayis, I. Panagiotopoulos, D. Niarchos, J. Appl. Phys. 87 (2000) 6938. w2x J.J. Delaunay, T. Hayashi, M. Tomita, S. Hirono, IEEE. Trans. Magn. 34 (1998) 1627. w3x J.P. Hu, P. Lin, IEEE. Trans. Magn. 32 (1996) 4096. w4x M. Yu, Y. Liu, A. Moser, D. Weller, D.J. Sellmyer, Appl. Phys. Lett. 75 (1999) 3992. w5x T.J. Konno, R. Sinclair, Acta Metall. Mater. 42 (1994) 1231. w6x J.J. Delaunay, T. Hayashi, M. Tomita, S. Hirono, S. Umemura, Appl. Phys. Lett. 71 (1997) 3427. w7x S.H. Liou, S. Huang, E. Klimek, R.D. Kirby, J. Appl. Phys. 85 (1999) 4334. w8x R.S. Bandhu, R. Sooryakumar, R.F.C. Farrow, D. Weller, M.F. Toney, T.A. Rabedeau, J. Appl. Phys. 91 (2002) 2737. w9x M. Li, Z.H. Jiang, Z.Q. Zou, D.F. Shen, J. Magn. Magn. Mater. 176 (1997) 331. w10x J. Zhou, K. Xun, D.F. Chen, G.Q. Xia, Y.X. Zheng, L.Y. Chen, Acta Physica Sinica 48 (1999) s218, in Chinese. w11x S.H. Liou, Y. Liu, S.S. Malhotra, M. Yu, D.J. Sellmyer, J. Appl. Phys. 79 (1996) 5060. w12x I. Panagiotopoulos, L. Withanawasam, G. Hadjipanayis, J. Magn. Magn. Mater. 152 (1996) 353. w13x C.W. Spratt, P.R. Bissell, R.W. Chantrell, E.P. Wohlfarth, J. Magn. Magn. Mater. 75 (1988) 309.