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ZrO2 multilayer
Applied Surface Science 252 (2006) 2910–2914 www.elsevier.com/locate/apsusc
A study of the effect of ZrO2 on the magnetic properties of FePt/ZrO2 multilayer Xiao-Hong Xu *, Xiao-Li Li, Fang Wang, Feng-Xian Jiang, Hai-Shun Wu School of Chemistry and Materials Science, Shanxi Normal University, Linfen Shanxi 041004, China Received 17 February 2005; received in revised form 28 April 2005; accepted 29 April 2005 Available online 1 June 2005
Abstract FexPt100 x(30 nm) and [FexPt100 x(3 nm)/ZrO2]10 (x = 37, 48, 57, 63, 69) films with different ZrO2 content were prepared by RF magnetron sputtering technique, then were annealed at 550 8C for 30 min. This work investigates the effect of ZrO2 doping on the microstructural evolution, magnetic properties, grain size, as well as the ordering kinetics of FePt alloy films. The asdeposited films behaved a disordered state, and the ordered L10 structure was obtained by post-annealing. The magnetic properties of the films are changed from soft magnetism to hard magnetism after annealing. The variation of the largest coercivities of [FexPt100 x/ZrO2]10 films with the Fe atomic percentage, x and differing amounts of ZrO2 content reveals that as we increase the ZrO2 content we must correspondingly increase the amount of Fe. This phenomenon suggests that the Zr or O atoms of ZrO2 preferentially react with the Fe atoms of FePt alloy to form compounds. In addition, introducing the nonmagnetic ZrO2 can reduce the intergrain exchange interactions of the FePt/ZrO2 films, and the interactions are decreased as the ZrO2 content increases, the dipole interactions are observed in FePt/ZrO2 films as the ZrO2 content is more than 15%. # 2005 Elsevier B.V. All rights reserved. pacs: 81.15.C; 75.60.G; 75.70.K Keywords: FePt/ZrO2 multilayers; Coercivity; Annealing; Exchange interaction
1. Introduction It has been pointed out that for ultrahigh-density magnetic recording, high magnetic anisotropy materials such as L10 phase of FePt is essential as the recording medium material, if one concerns thermal * Corresponding author. Tel.: +86 357 2052468; fax: +86 357 2051375. E-mail address: [email protected] (X.-H. Xu).
stability of written bits [1]. L10 phase of FePt alloy film exhibits magnetocrystalline anisotropy (Ku) of the order of 107 erg/cm3, which is an order of magnitude higher than that of the currently used Co-based alloy materials in commercialized hard disks [2]. Generally, the sputtered FePt thin films have the disordered FCC structure. This disordered phase is magnetically soft, and it transforms to the L10 structure through substrate heating during deposition and/or post-annealing [3]. Recent studies on micro-
0169-4332/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2005.04.042
X.-H. Xu et al. / Applied Surface Science 252 (2006) 2910–2914
structural properties of FePt thin films [4] and on FePt nanoparticle assemblies [5] reveal that the magnetic grains in the FePt system are interconnected in a certain degree due to the in situ or ex situ heating [6]. If we wish to get ultrahigh-density magnetic recording with low media noise, it is better to prepare the specific thin films in which the magnetic grains are isolated each other and the grain size is below 10 nm [7]. So, in order to control magnetic grain size as well as to achieve magnetic isolation among grains, various substitutions of nonmagnetic elements (C [6,8], Si3N4 [9], BN [10], SiO2 [11], Al2O3 [12], B2O3 [13], AlN [14], etc.) have been used in the FePt system. All these above substitutions do play an important role both in the hindering grain size and in the isolating of the magnetic grains. However, up to now, by our knowledge, there is no report about ZrO2-doped FePt films. In current work, FexPt100 x(30 nm) (x = 37, 48, 57, 63, 69) and [FexPt100 x(3 nm)/ZrO2]10 films with different ZrO2 content were prepared by RF magnetron sputtering technique. The goal of this study is to investigate how the ZrO2 doping affects the microstructural evolution, magnetic properties, grain size, as well as the ordering kinetics of FePt alloy films. Furthermore we aim to show the differences between ZrO2 doping as compared with other ceramic nonmagnetic ones.
2. Experimental details FexPt100 x(30 nm) (x = 37, 48, 57, 63, 69) singlelayer film and [FexPt100 x(3 nm)/ZrO2(a nm)]10 (a = 0.16, 0.53, 1.28, 2.45) multilayer films were deposited on glass substrates by RF magnetron sputtering. Through controlling the thickness of the ZrO2 layer, the ZrO2 content can be controlled. For example, the ZrO2 contents of [FexPt100 x(3 nm)/ ZrO2(a nm)]10 (a = 0.16, 0.53, 1.28, 2.45) multilayer films are 5, 15, 30, 45 vol.%, respectively. The base pressure was reduced to 5 10 5 Pa before deposition. A Fe–Pt composite target is made of Fe target with various Pt chips, by which Fe or Pt contents can be changed. And a separated ZrO2 target is used. Argon was introduced in the sputtering chamber during the deposition process. A magnetron cathode was a water-cooled target with a diameter of 2 in. During the deposition, there are not any controls of
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substrate temperature. After deposition, the samples were annealed in a vacuum condition with the pressure of 2 10 4 Pa at 550 8C for 30 min. The thickness of films was measured by Surface Profile. The structures and compositions were determined by X-ray diffraction (XRD) and X-ray energy dispersive analysis (EDX). The magnetic properties of the thin films were measured using a vibrating sample magnetometer (VSM).
3. Results and discussion Fig. 1 shows the coercivities (Hc) of FexPt100 x (30 nm) and [FexPt100 x(3 nm)/ZrO2(a nm)]10 thin films as a function of the atomic percent of Fe, x. FexPt100 x thin films deposited at room temperature are FCC disordered phase and show soft magnetic behavior. After annealed at 550 8C for 30 min, the coercivities are rapidly raised. It can be seen from Fig. 1 that the coercivities of FexPt100 x single-layer without ZrO2 are increased initially, and then are decreased with an increase of x. And its coercivity presents the maximum values (Hc,max) at the Fe atomic percent of 48, where the ratio of Fe/Pt is closely 1:1. It is in agreement with the fact that the FePt thin films exhibit the maximum coercivity at the around equalatomic ratio, below or above which the Hc,max would decrease [1–3,14,15]. The hysteresis loops of FexPt100 x films, which are shown in Fig. 2, also reflect the trend of the variations of Hc with x.
Fig. 1. The variation of coercivities of FexPt100 ZrO2]10 films with x.
x
and [FexPt100 x/
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X.-H. Xu et al. / Applied Surface Science 252 (2006) 2910–2914
Fig. 2. The hysteresis loops of various FexPt100
x
films.
However, once adding the ZrO2, the Hc,max dependence of x would be different. The coercivities of [FexPt100 x(3 nm)/ZrO2(a nm)]10 thin films are also increased initially, then are decreased with an increase of x from 37 to 69. The very interesting result we discovered is that the Hc,max does not occur for x = 48, but is closely related to the ZrO2 content in the films. The corresponding x of Hc,max is higher as ZrO2 content is higher. Hc,max are obtained at x = 57, 63, 69, when ZrO2 contents are 5, 30, 45 vol.%, respectively. If ZrO2 content is 15 vol.%, its Hc,max could correspond to the range of x between 57 and 63. It is seen that when the ZrO2 content is increased to 45 vol.%, the coercivities are all very small in various Fe atomic percent films. Why Hc,max of the ZrO2-added FexPt100 x films are not obtained at x = 48, but are regularly dependent on the ZrO2 content? This means ZrO2 plays an important and special role in the films. Supposing Zr or O atoms of ZrO2 react with Fe or Pt atoms of FePt alloy and form compounds, the content of Fe or Pt in the FePt alloy would be changed. And the ratio of Fe/Pt would be deviated from the equal-atomic composition, which leads to a decrease of coercivity. A question arises as to which atom in FePt alloy more possible react with ZrO2, Fe or Pt? It is assumed that Fe of FePt phase reacted with Zr or O of ZrO2. There might be a few loss of Fe in FePt alloy. If we wish to obtain the equal-atomic ratio L10-FePt alloy that possesses a maximum of Hc, we need to increase the value of x first, which means x should be more than 48. Base on this hypothesis, the more ZrO2 contents in
Fig. 3. The variation of coercivities of [FexPt100 x/ZrO2]10 films with ZrO2 content.
FePt/ZrO2 films were, the more Fe atoms would be needed. Accordingly, Hc,max of FexPt100 x/ZrO2 films with more ZrO2 content will be obtained at the higher atomic percent of Fe, x. The experimental results in Fig. 1 do support our hypothesis. Therefore, the hypothesis that Fe atom of FePt phase react with Zr or O of ZrO2 is reasonable. For further investigation, the experimental data can be rearranged as in Fig. 3. The coercivities of Fe34Pt66 with different ZrO2 contents are all small, which is because the value of x is below the particular range of compositions where the phase transformation from FCC to L10 ordered could take place. The coercivities of [Fe48Pt52/ZrO2]10 films are decreased monotonously with increasing ZrO2 content. Whereas, the ones of [FexPt100 x/ZrO2]10 (x = 57, 63, 69) films increase initially, then decrease with an increase of ZrO2 content from 0 to 45%. It is quite different with our previous works [8,14] in which the coercivities of the AlN and C doped FePt films are all decreased monotonously with increasing AlN and C contents. This comparative observation provides additional evidence that ZrO2 not only acts the barrier to hinder the phase transform but also acts the reactant to react with Fe atoms of FePt alloy. It is a common knowledge that the reaction of Fe and O is much easier than that of Pt and O. And then, how to prove FeZr alloy is easier to form than PtZr alloy? Here we present a theoretical investigation in the bond length and energy of FeZr and ZrPt diatomic molecules. The ab initio calculations are performed by using DMol3 package based on density functional
X.-H. Xu et al. / Applied Surface Science 252 (2006) 2910–2914
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theory (DFT) [16]. And the calculations were done at the all-electron level and double numerical basis sets with polarization functions (DNP) were used. The density function is treated within the generalized gradient approximation (GGA) with exchange-correlation potential parameterized by Wang and Perdew (GGA-PW91) [17]. Through the calculation, we find that the bond length of Zr–Fe and Zr–Pt is 1.902 and ˚ , and their energy is 2.449 A 5.47994 and 5.33341 eV, respectively. The lower its energy is, the more stable it is. This also may be visualized as the evidence that Zr atom in ZrO2 reacts preferentially with Fe atom in FePt alloy. It is well known that the properties of a material should be concordant with its structures, so we determine the structures of films using X-ray diffraction (XRD). Fig. 4 shows the XRD patterns of [Fe57Pt43/ZrO2]10 films. Superlattice (0 0 1) peaks can be found in [Fe57Pt43/ZrO2]10 films as the ZrO2 is less than 45 vol.% after annealing at 550 8C for 30 min, which indicate that the FCC phase has been partially transformed into L10 phase. Here when ZrO2 reach to 45 vol.%, L10 phase cannot be obtained because it needs more energy to break the thicker ZrO2 layer, this is in agreement with the results of coercivities shown in Fig. 1. FePt (1 1 1) and (0 0 1) peaks get weaker and weaker with the increase of ZrO2 content. This means that increasing ZrO2 content leads to the decrease of FePt grain size. The grain size estimated by the Scherrer formula from the (1 1 1) peak width of XRD patterns are 14.23, 13.33,
12.40, 7.29, 6.83 nm, respectively, and the corresponding ZrO2 content are 0, 5, 15, 30 and 45%. In [FePt(3 nm)/ZrO2(a nm)]10, where the number of bilayer repetition is 10, and FePt layer thickness in each bilayer repetition is 3 nm. Apparently FePt grain size of these films is larger than 3 nm. This indicates that the multilayer structure of as-deposited FePt/ZrO2 films have been broken down during annealing. Actually, in order to reduce intergrain interaction, the high-density recording requires magnetic grains to be isolated, which leads to lower media noise. dM curve has been proposed as means of estimating the amount of intergranular exchange present in thin film recording media. Fig. 5 shows dM curves of Fe57Pt43 and [Fe57Pt43/ZrO2]10 films with different ZrO2 content. The positive dM of FePt and FePt/ZrO2 films with 5% ZrO2 shows that there are intergrain exchange interactions in them. Whereas, dM becomes negative as ZrO2 content are 15 and 30%, which indicates that the interactions are dipole interactions. Without the addition of ZrO2, the intergrain interaction of single FePt films is very strong, and with the increase of ZrO2 content, the intergrain interactions are gradually reduced, and dipole interactions are observed as ZrO2 content is more than 15%. It is indicated that introducing the nonmagnetic ZrO2 is a very effective method for reducing the intergrain interactions. Comparing with FePt/C and FePt/AlN multilayer system reported by us previously [8,14], the ZrO2 matrix has a remarkable advantage in reducing the intergrain interaction.
Fig. 4. XRD patterns of [Fe57Pt43/ZrO2]10 films.
Fig. 5. dM curves for Fe57Pt43 and [Fe57Pt43/ZrO2]10 films annealed at 550 8C.
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It might be because that Zr, ZrO2, the iron oxide or Zr– Fe compounds prefer to segregated at the grain boundaries, and effectively reduce the intergrain interaction. In the FePt/C and FePt/AlN films, even if the C or AlN content exceeds 30 vol.%, they still have a strong intergrain interaction. However adding more nonmagnetic material will affect the quality of magnetic thin films. So ZrO2 is a potential and excellent nonmagnetic matrix for FePt magnetic recording media.
4. Conclusion In this work, the magnetic properties and structures of FexPt100 x and FexPt100 x/ZrO2 thin films have been studied. The results are summarized as follows. As-deposited [FexPt100 x/ZrO2] with the ZrO2 content of less than 45 vol.% films has been partially transformed into L10 phase after annealing at 550 8C for 30 min. Hc,max of FexPt100 x single-layer is obtained at x = 48, and the ones of FexPt100 x/ZrO2 films are all obtained at x > 48. The corresponding x of Hc,max is higher when ZrO2 content is higher. Both from theoretical and experimental results, they all provides the evidence that ZrO2 not only acts the barrier to hinder the phase transform but also acts the reactant to react with Fe atoms of FePt alloy. In addition, introducing the nonmagnetic ZrO2 not only hinders the growth of FePt particles, but also reduces the intergrain exchange interactions. And the interactions are decreased with increasing ZrO2 content, dipole interactions are observed when the ZrO2 content is more than 15 vol.%. So ZrO2 is expected to be a potential and excellent nonmagnetic matrix for FePt magnetic recording media.
Acknowledgements The authors would like to express their thanks to Natural Science Foundation of China and Natural Science Foundation of Shanxi Province (20041032) for their support to this research.
References [1] N. Honda, K. Ouchi, S. Iwasaki, IEEE Trans. Magn. 38 (2002) 1615. [2] B. Bian, D.E. Laughlin, K. Sato, Y. Hirotsu, J. Appl. Phys. 87 (9) (2000) 6962. [3] Y. Zhang, J. Wan, M.J. Bonder, G.C. Hadjipanayis, D. Weller, J. Appl. Phys. 93 (10) (2003) 7175. [4] M.G. Kim, S.C. Shin, K. Kang, Appl. Phys. Lett. 80 (2002) 3802. [5] H. Zeng, S. Sun, T.S. Vedantam, J.P. Liu, Z.R. Dai, Z.L. Wang, Appl. Phys. Lett. 80 (2002) 2583. [6] H.S. Ko, A. Perumal, S.C. Shin, Appl. Phys. Lett. 82 (14) (2003) 2311. [7] C. Tsang, M. Chen, T. Yogi, Proc. IEEE 81 (1993) 1346. [8] Xiao-Hong Xu, Hai-Shun Wu, Xiao-Li Li, Fang Wang, JingFang Duan, Physica B 348 (2004) 436. [9] C.M. Kuo, P.C. Kuo, J. Appl. Phys. 87 (1) (2000) 419. [10] M. Daniil, P.A. Farber, H. Okumura, G.C. Hadjipanayis, D. Weller, J. Magn. Magn. Mater. 246 (2002) 297. [11] C.P. Luo, S.H. Liou, D.J. Sellmyer, J. Appl. Phys. 87 (9) (2000) 6941. [12] J. Bai, Z. Yang, F. Wei, M. Matsumoto, A. Morisako, J. Magn. Magn. Mater. 257 (2003) 132. [13] M.L. Yan, H. Zeng, N. Powers, D.J. Sellmyer, J. Appl. Phys. 91 (10) (2002) 8471. [14] Xiao-Hong Xu, Xiao-Li Li, Hai-Shun Wu, Physica B 352 (2004) 48. [15] Xiao-Hong Xu, Hai-Shun Wu, Fang Wang, Xiao-Li Li, Thin Solid Films 472 (2005) 222. [16] B. Delley, J. Chem. Phys. 92 (1990) 508. [17] Y. Wang, J.P. Perdew, Phys. Rev. B 43 (1991) 8911.
A study of the effect of ZrO2 on the magnetic properties of FePt/ZrO2 multilayer Xiao-Hong Xu *, Xiao-Li Li, Fang Wang, Feng-Xian Jiang, Hai-Shun Wu School of Chemistry and Materials Science, Shanxi Normal University, Linfen Shanxi 041004, China Received 17 February 2005; received in revised form 28 April 2005; accepted 29 April 2005 Available online 1 June 2005
Abstract FexPt100 x(30 nm) and [FexPt100 x(3 nm)/ZrO2]10 (x = 37, 48, 57, 63, 69) films with different ZrO2 content were prepared by RF magnetron sputtering technique, then were annealed at 550 8C for 30 min. This work investigates the effect of ZrO2 doping on the microstructural evolution, magnetic properties, grain size, as well as the ordering kinetics of FePt alloy films. The asdeposited films behaved a disordered state, and the ordered L10 structure was obtained by post-annealing. The magnetic properties of the films are changed from soft magnetism to hard magnetism after annealing. The variation of the largest coercivities of [FexPt100 x/ZrO2]10 films with the Fe atomic percentage, x and differing amounts of ZrO2 content reveals that as we increase the ZrO2 content we must correspondingly increase the amount of Fe. This phenomenon suggests that the Zr or O atoms of ZrO2 preferentially react with the Fe atoms of FePt alloy to form compounds. In addition, introducing the nonmagnetic ZrO2 can reduce the intergrain exchange interactions of the FePt/ZrO2 films, and the interactions are decreased as the ZrO2 content increases, the dipole interactions are observed in FePt/ZrO2 films as the ZrO2 content is more than 15%. # 2005 Elsevier B.V. All rights reserved. pacs: 81.15.C; 75.60.G; 75.70.K Keywords: FePt/ZrO2 multilayers; Coercivity; Annealing; Exchange interaction
1. Introduction It has been pointed out that for ultrahigh-density magnetic recording, high magnetic anisotropy materials such as L10 phase of FePt is essential as the recording medium material, if one concerns thermal * Corresponding author. Tel.: +86 357 2052468; fax: +86 357 2051375. E-mail address: [email protected] (X.-H. Xu).
stability of written bits [1]. L10 phase of FePt alloy film exhibits magnetocrystalline anisotropy (Ku) of the order of 107 erg/cm3, which is an order of magnitude higher than that of the currently used Co-based alloy materials in commercialized hard disks [2]. Generally, the sputtered FePt thin films have the disordered FCC structure. This disordered phase is magnetically soft, and it transforms to the L10 structure through substrate heating during deposition and/or post-annealing [3]. Recent studies on micro-
0169-4332/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2005.04.042
X.-H. Xu et al. / Applied Surface Science 252 (2006) 2910–2914
structural properties of FePt thin films [4] and on FePt nanoparticle assemblies [5] reveal that the magnetic grains in the FePt system are interconnected in a certain degree due to the in situ or ex situ heating [6]. If we wish to get ultrahigh-density magnetic recording with low media noise, it is better to prepare the specific thin films in which the magnetic grains are isolated each other and the grain size is below 10 nm [7]. So, in order to control magnetic grain size as well as to achieve magnetic isolation among grains, various substitutions of nonmagnetic elements (C [6,8], Si3N4 [9], BN [10], SiO2 [11], Al2O3 [12], B2O3 [13], AlN [14], etc.) have been used in the FePt system. All these above substitutions do play an important role both in the hindering grain size and in the isolating of the magnetic grains. However, up to now, by our knowledge, there is no report about ZrO2-doped FePt films. In current work, FexPt100 x(30 nm) (x = 37, 48, 57, 63, 69) and [FexPt100 x(3 nm)/ZrO2]10 films with different ZrO2 content were prepared by RF magnetron sputtering technique. The goal of this study is to investigate how the ZrO2 doping affects the microstructural evolution, magnetic properties, grain size, as well as the ordering kinetics of FePt alloy films. Furthermore we aim to show the differences between ZrO2 doping as compared with other ceramic nonmagnetic ones.
2. Experimental details FexPt100 x(30 nm) (x = 37, 48, 57, 63, 69) singlelayer film and [FexPt100 x(3 nm)/ZrO2(a nm)]10 (a = 0.16, 0.53, 1.28, 2.45) multilayer films were deposited on glass substrates by RF magnetron sputtering. Through controlling the thickness of the ZrO2 layer, the ZrO2 content can be controlled. For example, the ZrO2 contents of [FexPt100 x(3 nm)/ ZrO2(a nm)]10 (a = 0.16, 0.53, 1.28, 2.45) multilayer films are 5, 15, 30, 45 vol.%, respectively. The base pressure was reduced to 5 10 5 Pa before deposition. A Fe–Pt composite target is made of Fe target with various Pt chips, by which Fe or Pt contents can be changed. And a separated ZrO2 target is used. Argon was introduced in the sputtering chamber during the deposition process. A magnetron cathode was a water-cooled target with a diameter of 2 in. During the deposition, there are not any controls of
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substrate temperature. After deposition, the samples were annealed in a vacuum condition with the pressure of 2 10 4 Pa at 550 8C for 30 min. The thickness of films was measured by Surface Profile. The structures and compositions were determined by X-ray diffraction (XRD) and X-ray energy dispersive analysis (EDX). The magnetic properties of the thin films were measured using a vibrating sample magnetometer (VSM).
3. Results and discussion Fig. 1 shows the coercivities (Hc) of FexPt100 x (30 nm) and [FexPt100 x(3 nm)/ZrO2(a nm)]10 thin films as a function of the atomic percent of Fe, x. FexPt100 x thin films deposited at room temperature are FCC disordered phase and show soft magnetic behavior. After annealed at 550 8C for 30 min, the coercivities are rapidly raised. It can be seen from Fig. 1 that the coercivities of FexPt100 x single-layer without ZrO2 are increased initially, and then are decreased with an increase of x. And its coercivity presents the maximum values (Hc,max) at the Fe atomic percent of 48, where the ratio of Fe/Pt is closely 1:1. It is in agreement with the fact that the FePt thin films exhibit the maximum coercivity at the around equalatomic ratio, below or above which the Hc,max would decrease [1–3,14,15]. The hysteresis loops of FexPt100 x films, which are shown in Fig. 2, also reflect the trend of the variations of Hc with x.
Fig. 1. The variation of coercivities of FexPt100 ZrO2]10 films with x.
x
and [FexPt100 x/
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X.-H. Xu et al. / Applied Surface Science 252 (2006) 2910–2914
Fig. 2. The hysteresis loops of various FexPt100
x
films.
However, once adding the ZrO2, the Hc,max dependence of x would be different. The coercivities of [FexPt100 x(3 nm)/ZrO2(a nm)]10 thin films are also increased initially, then are decreased with an increase of x from 37 to 69. The very interesting result we discovered is that the Hc,max does not occur for x = 48, but is closely related to the ZrO2 content in the films. The corresponding x of Hc,max is higher as ZrO2 content is higher. Hc,max are obtained at x = 57, 63, 69, when ZrO2 contents are 5, 30, 45 vol.%, respectively. If ZrO2 content is 15 vol.%, its Hc,max could correspond to the range of x between 57 and 63. It is seen that when the ZrO2 content is increased to 45 vol.%, the coercivities are all very small in various Fe atomic percent films. Why Hc,max of the ZrO2-added FexPt100 x films are not obtained at x = 48, but are regularly dependent on the ZrO2 content? This means ZrO2 plays an important and special role in the films. Supposing Zr or O atoms of ZrO2 react with Fe or Pt atoms of FePt alloy and form compounds, the content of Fe or Pt in the FePt alloy would be changed. And the ratio of Fe/Pt would be deviated from the equal-atomic composition, which leads to a decrease of coercivity. A question arises as to which atom in FePt alloy more possible react with ZrO2, Fe or Pt? It is assumed that Fe of FePt phase reacted with Zr or O of ZrO2. There might be a few loss of Fe in FePt alloy. If we wish to obtain the equal-atomic ratio L10-FePt alloy that possesses a maximum of Hc, we need to increase the value of x first, which means x should be more than 48. Base on this hypothesis, the more ZrO2 contents in
Fig. 3. The variation of coercivities of [FexPt100 x/ZrO2]10 films with ZrO2 content.
FePt/ZrO2 films were, the more Fe atoms would be needed. Accordingly, Hc,max of FexPt100 x/ZrO2 films with more ZrO2 content will be obtained at the higher atomic percent of Fe, x. The experimental results in Fig. 1 do support our hypothesis. Therefore, the hypothesis that Fe atom of FePt phase react with Zr or O of ZrO2 is reasonable. For further investigation, the experimental data can be rearranged as in Fig. 3. The coercivities of Fe34Pt66 with different ZrO2 contents are all small, which is because the value of x is below the particular range of compositions where the phase transformation from FCC to L10 ordered could take place. The coercivities of [Fe48Pt52/ZrO2]10 films are decreased monotonously with increasing ZrO2 content. Whereas, the ones of [FexPt100 x/ZrO2]10 (x = 57, 63, 69) films increase initially, then decrease with an increase of ZrO2 content from 0 to 45%. It is quite different with our previous works [8,14] in which the coercivities of the AlN and C doped FePt films are all decreased monotonously with increasing AlN and C contents. This comparative observation provides additional evidence that ZrO2 not only acts the barrier to hinder the phase transform but also acts the reactant to react with Fe atoms of FePt alloy. It is a common knowledge that the reaction of Fe and O is much easier than that of Pt and O. And then, how to prove FeZr alloy is easier to form than PtZr alloy? Here we present a theoretical investigation in the bond length and energy of FeZr and ZrPt diatomic molecules. The ab initio calculations are performed by using DMol3 package based on density functional
X.-H. Xu et al. / Applied Surface Science 252 (2006) 2910–2914
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theory (DFT) [16]. And the calculations were done at the all-electron level and double numerical basis sets with polarization functions (DNP) were used. The density function is treated within the generalized gradient approximation (GGA) with exchange-correlation potential parameterized by Wang and Perdew (GGA-PW91) [17]. Through the calculation, we find that the bond length of Zr–Fe and Zr–Pt is 1.902 and ˚ , and their energy is 2.449 A 5.47994 and 5.33341 eV, respectively. The lower its energy is, the more stable it is. This also may be visualized as the evidence that Zr atom in ZrO2 reacts preferentially with Fe atom in FePt alloy. It is well known that the properties of a material should be concordant with its structures, so we determine the structures of films using X-ray diffraction (XRD). Fig. 4 shows the XRD patterns of [Fe57Pt43/ZrO2]10 films. Superlattice (0 0 1) peaks can be found in [Fe57Pt43/ZrO2]10 films as the ZrO2 is less than 45 vol.% after annealing at 550 8C for 30 min, which indicate that the FCC phase has been partially transformed into L10 phase. Here when ZrO2 reach to 45 vol.%, L10 phase cannot be obtained because it needs more energy to break the thicker ZrO2 layer, this is in agreement with the results of coercivities shown in Fig. 1. FePt (1 1 1) and (0 0 1) peaks get weaker and weaker with the increase of ZrO2 content. This means that increasing ZrO2 content leads to the decrease of FePt grain size. The grain size estimated by the Scherrer formula from the (1 1 1) peak width of XRD patterns are 14.23, 13.33,
12.40, 7.29, 6.83 nm, respectively, and the corresponding ZrO2 content are 0, 5, 15, 30 and 45%. In [FePt(3 nm)/ZrO2(a nm)]10, where the number of bilayer repetition is 10, and FePt layer thickness in each bilayer repetition is 3 nm. Apparently FePt grain size of these films is larger than 3 nm. This indicates that the multilayer structure of as-deposited FePt/ZrO2 films have been broken down during annealing. Actually, in order to reduce intergrain interaction, the high-density recording requires magnetic grains to be isolated, which leads to lower media noise. dM curve has been proposed as means of estimating the amount of intergranular exchange present in thin film recording media. Fig. 5 shows dM curves of Fe57Pt43 and [Fe57Pt43/ZrO2]10 films with different ZrO2 content. The positive dM of FePt and FePt/ZrO2 films with 5% ZrO2 shows that there are intergrain exchange interactions in them. Whereas, dM becomes negative as ZrO2 content are 15 and 30%, which indicates that the interactions are dipole interactions. Without the addition of ZrO2, the intergrain interaction of single FePt films is very strong, and with the increase of ZrO2 content, the intergrain interactions are gradually reduced, and dipole interactions are observed as ZrO2 content is more than 15%. It is indicated that introducing the nonmagnetic ZrO2 is a very effective method for reducing the intergrain interactions. Comparing with FePt/C and FePt/AlN multilayer system reported by us previously [8,14], the ZrO2 matrix has a remarkable advantage in reducing the intergrain interaction.
Fig. 4. XRD patterns of [Fe57Pt43/ZrO2]10 films.
Fig. 5. dM curves for Fe57Pt43 and [Fe57Pt43/ZrO2]10 films annealed at 550 8C.
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It might be because that Zr, ZrO2, the iron oxide or Zr– Fe compounds prefer to segregated at the grain boundaries, and effectively reduce the intergrain interaction. In the FePt/C and FePt/AlN films, even if the C or AlN content exceeds 30 vol.%, they still have a strong intergrain interaction. However adding more nonmagnetic material will affect the quality of magnetic thin films. So ZrO2 is a potential and excellent nonmagnetic matrix for FePt magnetic recording media.
4. Conclusion In this work, the magnetic properties and structures of FexPt100 x and FexPt100 x/ZrO2 thin films have been studied. The results are summarized as follows. As-deposited [FexPt100 x/ZrO2] with the ZrO2 content of less than 45 vol.% films has been partially transformed into L10 phase after annealing at 550 8C for 30 min. Hc,max of FexPt100 x single-layer is obtained at x = 48, and the ones of FexPt100 x/ZrO2 films are all obtained at x > 48. The corresponding x of Hc,max is higher when ZrO2 content is higher. Both from theoretical and experimental results, they all provides the evidence that ZrO2 not only acts the barrier to hinder the phase transform but also acts the reactant to react with Fe atoms of FePt alloy. In addition, introducing the nonmagnetic ZrO2 not only hinders the growth of FePt particles, but also reduces the intergrain exchange interactions. And the interactions are decreased with increasing ZrO2 content, dipole interactions are observed when the ZrO2 content is more than 15 vol.%. So ZrO2 is expected to be a potential and excellent nonmagnetic matrix for FePt magnetic recording media.
Acknowledgements The authors would like to express their thanks to Natural Science Foundation of China and Natural Science Foundation of Shanxi Province (20041032) for their support to this research.
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