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Pd epitaxial multilayer films
Journal of Magnetism and Magnetic Materials 324 (2012) 1059–1062
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Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Influence of layer thickness on the structure and the magnetic properties of Co/Pd epitaxial multilayer films Kousuke Tobari n, Mitsuru Ohtake, Katsumasa Nagano, Masaaki Futamoto Faculty of Science and Engineering, Chuo University, Bunkyo-ku, Tokyo 112–8551, Japan
a r t i c l e i n f o
abstract
Article history: Received 31 October 2010 Received in revised form 6 August 2011 Available online 31 October 2011
Co/Pd epitaxial multilayer films were prepared on Pd(111)fcc underlayers hetero-epitaxially grown on MgO(111)B1 single-crystal substrates at room temperature by ultra-high vacuum RF magnetron sputtering. In-situ reflection high energy electron diffraction shows that the in-plane lattice spacing of Co on Pd layer gradually decreases with increasing the Co layer thickness, whereas that of Pd on Co layer remains unchanged during the Pd layer formation. The CoPd alloy phase formation is observed around the Co/Pd interface. The atomic mixing is enhanced for thinner Co and Pd layers in multilayer structure. With decreasing the Co and the Pd layer thicknesses and increasing the repetition number of Co/Pd multilayer film, stronger perpendicular magnetic anisotropy is observed. The relationships between the film structure and the magnetic properties are discussed. & 2011 Elsevier B.V. All rights reserved.
Keywords: Co/Pd multilayer MgO single-crystal substrate Epitaxial growth Perpendicular magnetic anisotropy Layer thickness
1. Introduction Co/Pd multilayer films with perpendicular magnetic anisotropy have been investigated for applications like magnetic recording media [1–3], magnetic random access memory devices [4], etc. The effects of layer thickness [5], underlayer material [6], and Ar gas pressure [7] on the magnetic properties have been investigated by employing Co/Pd multilayer polycrystalline films prepared on glass substrates. The films generally consist of fcc(111) preferred-texture crystals including small volume of crystals with other orientations. The reduction in the Co layer thickness is reported to promote perpendicular magnetic anisotropy [5]. The microstructure and the magnetic properties are influenced by the crystallographic orientation and the layer thickness of Co and Pd layers. Furthermore, the interface structures as well as the strain in the Co and the Pd layers are expected to influence the magnetic properties, where the lattice mismatch between fcc-Co and fcc-Pd is fairly large at about 9%. There is a possibility that the crystal structure of Co may change between fcc and hcp depending on the strain in the multilayer structure. In order to investigate the detailed structural and the magnetic properties, well-defined epitaxial films are useful, since the film uniformity and the magnetic anisotropy are well controlled. In our previous study [8], Co/Pd epitaxial multilayer films were prepared on Pd underlayers with different orientations by RF magnetron sputtering. The Co/Pd multilayer film formed on Pd(111)fcc underlayer
n
Corresponding author. Tel.: þ81 3 3817 1862; fax: þ 81 3 3817 1847. E-mail address: [email protected] (K. Tobari).
0304-8853/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2011.10.026
showed a stronger perpendicular anisotropy than that formed on Pd(011)fcc underlayer. On the contrary, the multilayer formed on Pd(001)fcc underlayer was easily magnetized when the magnetic field was applied along the in-plane direction. In the present study, Co/Pd multilayer films were prepared on Pd(111)fcc underlayers hetero-epitaxially grown on MgO(111)B1 single-crystal substrates by varying the Co and the Pd layer thicknesses from 8.6 and 11.4 nm to 0.09 and 0.11 nm, respectively. Co50Pd50 (at%) alloy films were also prepared on Pd(111)fcc underlayers by employing a CoPd alloy target in order to compare the structural and the magnetic properties with those of Co/Pd multilayer film. The detailed Co and Pd layer structures were studied by in situ RHEED during film preparation process and the resulting multilayer film structures were examined by in-plane X-ray diffraction (XRD).
2. Experimental procedure Thin films were prepared on polished MgO(111)B1 substrates by using an RF magnetron sputtering system, where the base pressures were lower than 4 10 7 Pa. Substrates were heated at 600 1C for 1 h in the ultra-high vacuum chamber. The distance between the target and the sample was fixed at 150 mm. The Ar gas pressure was 0.67 Pa and the rf powers for Pd, Co, and Co50Pd50 (at%) targets with 3 in. diameter were, respectively, kept constant at 30, 49, and 38 W, where the deposition rate was 0.02 nm/s for all materials. The film layer structures are [Pd(tPd nm)/Co(tCo nm)]n/Pd(10 nm)/ MgO(111)B1 and CoPd(20 nm)/Pd(10 nm)/MgO(111)B1. A 10 nmthick Pd(111)fcc underlayer was prepared by hetero-epitaxial growth
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Fig. 1. (a)–(d) RHEED patterns observed for a Pd(11.4 nm)/Co(8.6 nm)/Pd(10 nm)/MgO(111)B1 specimen and the RHEED intensity profiles along the fcc[112] direction. (a) A 10 nm-thick Pd underlayer grown on MgO(111)B1 substrate, a Co layer of (b) 1.0 nm and (c) 2.5 nm formed on the Pd underlayer, and an upper Pd layer of (d) 0.2 nm deposited on the Co layer. The incident electron beam is parallel to the MgO[11¯0]B1 direction. The in-plane lattice spacings, d(112), are determined from the distances between RHEED spots shown in the arrows in the intensity profiles of (a)–(d). The spot maps of (e) and (f) correspond to fcc(111) and hcp(0001) textures, respectively. The symbols, A and B, correspond to the orientation relationships of Types A and B explained in the text, respectively.
on an MgO(111)B1 substrate at 300 1C. The epitaxial orientation relationship of Pdð111Þ½110fcc ,ð111Þ½110fcc :MgOð111Þ½110B1 was determined by comparing the reflection high energy electron diffraction (RHEED) patterns observed for the Pd underlayer [Fig. 1(a)] and the substrate. And then, a [Pd(tPd nm)/Co(tCo nm)]n multilayer or a 20 nm-thick Co50Pd50 alloy layer was deposited on the underlayer at room temperature. The total thickness and the composition of Co/Pd multilayer were fixed at 20 nm and Pd-50 at% Co, respectively. Here, the thicknesses of Pd and Co layers were, respectively, defined as t Pd ¼ 20=fn½1 þ ðaCo Þ3 =ðaPd Þ3 g,
t Co ¼ ½ðaCo Þ3 =ðaPd Þ3 t Pd ,
where aPd and aCo are lattice constants of bulk fcc-Pd crystal (aPd ¼ 0.390 nm [9]) and bulk fcc-Co crystal (aCo ¼0.354 nm [10]). The repetition number, n, was varied from 1 to 100. The compositions of Co/Pd multilayer and Co50Pd50 alloy layer were confirmed by energy dispersive X-ray spectroscopy and the errors were less than 5 at%. The details of film preparation are similar to our previous study [8]. The surface structure during RF-sputter deposition process was studied by RHEED. The film structure was investigated by inplane X-ray diffraction with Cu-Ka radiation (l ¼ 0.15418 nm). The magnetization curves were measured using a vibrating sample magnetometer.
3. Results and discussion Co/Pd multilayer films epitaxially grew on Pd(111)fcc underlayers for all the samples. Co50Pd50 (at%) alloy films were also obtained on Pd(111)fcc underlayers. Fig. 1 shows the RHEED patterns observed for a Pd(11.4 nm)/Co(8.6 nm) bi-layer film (n¼1) grown on Pd(111)fcc underlayer. The RHEED patterns observed for the Co layer and the upper Pd layer consist of spots corresponding to fcc(111) texture and streaks along the fcc[111] direction, as shown in Fig. 1(b)–(d). The spots consist of two fcc(111) reflections, as shown by the symbols, A and B, in the RHEED spot map of Fig. 1(e). The streaks observed for the upper Pd layer show that the layer has atomically flat terraces. On the other hand, the streaks observed for the Co layer indicate that the layer has atomically flat terraces and/or involves stacking faults
along the fcc[111] direction. The crystal structure of Co varies between hcp and fcc by introduction of stacking faults parallel to the close-packed plane. The existence of Co(0001)hcp crystal is confirmed by XRD analysis, which is described later. The epitaxial orientation relationships of Pdð111Þ½110fccðTypeAÞ ,ð111Þ½110fccðTypeBÞ :Coð111Þ½110fccðTypeAÞ , ð111Þ½110fccðTypeBÞ ,ð0001Þ½1120hcp :MgOð111Þ½110B1 are determined by RHEED. The Co layer consists of two-type fcc and one-type hcp variants, whereas the upper and the lower Pd layers consist of two-type fcc variants. The orientations of twotype fcc variants are rotated around the film normal by 1801 with respect to each other and the variants with Type A and B relationships have atomic stacking sequences of ABCABCy and ACBACBy, respectively. The in-plane lattice spacings of Pd/Co bilayer film were measured from the distances between RHEED spots [8]. The in-plane lattice spacing of Co layer gradually decreases and reaches a constant value of 0.22 nm beyond 2.5 nm-thick Co deposition, as shown in Fig. 1(b) and (c). This gradual change in lattice spacing is considered to be partially due to lattice strain caused by the lattice mismatch between Co and Pd layers ( 9.0%) and partially due to atomic mixing around the Co/Pd interface. Atomic sites of Co crystal are partly replaced by Pd atoms with larger atomic radius and there is a possibility that a CoPd alloy phase is formed. The formation of CoPd alloy phase is also recognized by XRD. The CoPd alloy phase is formed in the crystallographic orientation relationships of CoPdð111Þ½110fccðType AÞ ,ð111Þ½110fccðType BÞ , ð0001Þ½1120hcp :MgOð111Þ½110B1 The in-plane lattice spacing of upper Pd layer is kept constant at 0.24 nm from the beginning of Pd deposition till the end of 11.4 nm-thick Pd layer formation. At the Pd/Co interface, a CoPd alloy phase is also considered to be formed, since the lattice constant of CoPd alloy close to that of Pd crystal [11]. Fig. 2(a) and (b) shows the RHEED patterns observed for a [Pd(0.11 nm)/ Co(0.09 nm)]100 multilayer film (n ¼100). The variation of inplane lattice spacing is not recognized for all the Co and the Pd layers, and the lattice spacings of Co and Pd layers are kept constant at 0.24 nm. The atomic mixing is enhanced for thinner Co and Pd layers in Co/Pd multilayer structure. Fig. 2(c) shows the
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Fig. 2. RHEED patterns observed for [(a),(b)] [Pd(0.11 nm)/Co(0.09 nm)]100/Pd(10 nm)/MgO(111)B1 and (c) Co50Pd50(20 nm)/Pd(10 nm)/MgO(1 1 1)B1 specimens and the RHEED intensity profiles along the fcc[112] direction. The uppermost (a) Co and (b) Pd layers of Co/Pd multilayer film and (c) the 20 nm-thick CoPd alloy films grown on Pd(111) underlayers. The incident electron beam is parallel to the MgO[110]B1 direction. The in-plane lattice spacings, d(112), are measured from the distances between RHEED spots shown in the arrows in the intensity profiles of (a)–(c).
Fig. 4. Magnetization curves of (a) [Pd(5.68nm)/Co(4.32nm)]2, (b) [Pd(2.27nm)/ Co(1.73nm)]5, (c) [Pd(1.14nm)/Co(0.86nm)]10, (d) [Pd(0.19nm)/Co(0.14nm)]60, (e) [Pd(0.11nm)/Co(0.09nm)]100 multilayer films and (f) a Co50Pd50(20nm) alloy film grown on Pd underlayers. The values in parenthesis are layer thicknesses in nanometers. Fig. 3. In-plane XRD spectra of Co/Pd multilayer films and a Co50Pd50 alloy film grown on Pd(111) underlayers. The scattering vector of in-plane XRD is along (a) the MgO[11¯0]B1 or (b) the MgO[112]B1 direction. The intensity is shown in a logarithmic scale.
RHEED pattern observed for a Co50Pd50 alloy film. The in-plane lattice spacing is 0.24 nm. Fig. 3 shows the in-plane XRD spectra of Co/Pd multilayer films with different n values and Co50Pd50 film. In the in-plane XRD spectra measured by making the scattering vector parallel to the MgO[11¯0] direction [Fig. 3(a)], fcc(22¯0) and/or hcp(112¯0) reflections from Co and CoPd crystals are observed in addition to MgO(22¯0) and Pd(22¯0) reflections. In the in-plane XRD spectra measured by making the scattering vector parallel to the MgO[112¯] direction [Fig. 3(b)], Co(11¯00) and/or CoPd(11¯00) reflections are recognized. The epitaxial orientation relationships determined by RHEED are also confirmed by the in-plane XRD. The XRD data apparently indicate that the films include hcp phases in addition to fcc phase. With increasing the n value, the fcc(22¯0) and/or hcp(112¯0) and the (11¯00) reflection intensities from CoPd alloy become stronger, whereas those from Co crystal weaken, which indicates that the CoPd alloy phase formation is promoted by increasing the n value. Fig. 4(a)–(f) shows the magnetization curves of Co/Pd multilayer films with different n values and Co50Pd50 alloy film. The magnetization curves are almost isotropic in in-plane measurements for all the samples. The multilayer films with n ¼1–2 show in-plane magnetic anisotropies, as shown, for example, in Fig. 4(a). The multilayer film with n¼5 shows almost magnetic properties in in-plane and out-of-plane measurements, as shown
Fig. 5. n value (Co layer thickness) dependences on the ratios of (a) Mr///Ms//, (b) Mr?/Ms?, and (c) Mr?/Mr// of Co/Pd multilayer films. (d) Mr///Ms//, (e) Mr?/Ms?, and (f) Mr?/Mr// ratios of a Co50Pd50 alloy film.
in Fig. 4(b). The multilayer films with n ¼10–100 show perpendicular anisotropies, as shown in Fig. 4(c)–(e). The interface anisotropy is enhanced with increasing the n value (decreasing the Co
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layer thickness). This result is similar to the case of Co/Pd multilayer polycrystalline film [5]. The CoPd alloy film shows almost isotropic anisotropies in in-plane and out-of-plane measurements, as shown in Fig. 4(f). Fig. 5(a), (b), and (c), respectively, show the n value (the Co layer thickness) dependences on the ratios of Mr///Ms//, Mr?/Ms?, and Mr?/Mr// of Co/Pd multilayer films, where Mr is the remanent magnetization, Ms is the saturation magnetization, and the subscripts, // and ?, refer to in-plane and out-of-plane magnetizations, respectively. With increasing the n value beyond 100, the perpendicular anisotropy tends to weaken, possibly due to reduction in the interface anisotropy caused by atomic mixing around the Co/Pd interface. The magnetic properties are apparently influenced by the film structure depending on the Co and the Pd layer thicknesses.
4. Conclusions Co/Pd epitaxial multilayer films were obtained on Pd(111)fcc underlayers. The film structure is investigated by in-situ RHEED and ex-situ XRD. CoPd alloy phase is formed around the Co/Pd interface. The atomic mixing is enhanced for thinner Co and Pd layers in multilayer structure. With increasing the n value (with decreasing the Co and the Pd layer thicknesses and increasing the repetition number of Co/Pd multilayer film), stronger perpendicular anisotropy is observed for the multilayer film. Further increase in n value beyond 100 (Co layer thickness is smaller than
1.73 nm) causes reduction in perpendicular anisotropy. The magnetic properties are delicately influenced by the film structure.
Acknowledgments Authors thank Prof. Fumiyoshi Kirino of Tokyo National University of Fine Arts and Music for energy dispersive X-ray spectroscopy analysis. A part of this work was supported by METI–Japan, MEXT–Japan, and Chuo university Grant for Special Research. References [1] P.F. Carcia, A.D. Meinhaldt, A. Suna, Applied Physics Letters 47 (1985) 178. [2] B.M. Lairson, J. Perez, C. Baldwin, IEEE Transactions on Magnetics 30 (1994) 4014. [3] A. Ajian, K. Sato, N. Aoyama, T. Tanaka, Y. Miyaguchi., K. Tsumagari, T. Morita, T. Nishihashi, A. Tanaka, T. Uzumaki, IEEE Transactions on Magnetics 46 (2010) 2020. [4] D. Lim, S. Kim, S. Lee, Journal of Applied Physics 97 (2005) 10C902. [5] H.J.G. Draaisma, W.J.M. de Jonge, Journal of Applied Physics 62 (1987) 3318. [6] T. Asahi, K. Kuramochi, J. Kawaji, T. Onoue, T. Osaka, M. Saigo, Journal of Magnetism and Magnetic Materials 235 (2001) 87. [7] S. Hashimoto, Y. Ochiai, K. Aso, Journal of Applied Physics 66 (1989) 4909. [8] K. Tobari, M. Ohtake, K. Nagano, M. Futamoto, Japanese Journal of Applied Physics 50 (2011) 073001. [9] E. Owen, E. Yates, Philosophical Magazine Series 7 (15) (1933) 472. [10] A. Hull, Physical Review 17 (1921) 571. [11] R.M. Bozorth, P.A. Wolff, D.D. Davis, V.B. Compton, J.H. Wernick, Physical Review 122 (1961) 1157.
Contents lists available at SciVerse ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Influence of layer thickness on the structure and the magnetic properties of Co/Pd epitaxial multilayer films Kousuke Tobari n, Mitsuru Ohtake, Katsumasa Nagano, Masaaki Futamoto Faculty of Science and Engineering, Chuo University, Bunkyo-ku, Tokyo 112–8551, Japan
a r t i c l e i n f o
abstract
Article history: Received 31 October 2010 Received in revised form 6 August 2011 Available online 31 October 2011
Co/Pd epitaxial multilayer films were prepared on Pd(111)fcc underlayers hetero-epitaxially grown on MgO(111)B1 single-crystal substrates at room temperature by ultra-high vacuum RF magnetron sputtering. In-situ reflection high energy electron diffraction shows that the in-plane lattice spacing of Co on Pd layer gradually decreases with increasing the Co layer thickness, whereas that of Pd on Co layer remains unchanged during the Pd layer formation. The CoPd alloy phase formation is observed around the Co/Pd interface. The atomic mixing is enhanced for thinner Co and Pd layers in multilayer structure. With decreasing the Co and the Pd layer thicknesses and increasing the repetition number of Co/Pd multilayer film, stronger perpendicular magnetic anisotropy is observed. The relationships between the film structure and the magnetic properties are discussed. & 2011 Elsevier B.V. All rights reserved.
Keywords: Co/Pd multilayer MgO single-crystal substrate Epitaxial growth Perpendicular magnetic anisotropy Layer thickness
1. Introduction Co/Pd multilayer films with perpendicular magnetic anisotropy have been investigated for applications like magnetic recording media [1–3], magnetic random access memory devices [4], etc. The effects of layer thickness [5], underlayer material [6], and Ar gas pressure [7] on the magnetic properties have been investigated by employing Co/Pd multilayer polycrystalline films prepared on glass substrates. The films generally consist of fcc(111) preferred-texture crystals including small volume of crystals with other orientations. The reduction in the Co layer thickness is reported to promote perpendicular magnetic anisotropy [5]. The microstructure and the magnetic properties are influenced by the crystallographic orientation and the layer thickness of Co and Pd layers. Furthermore, the interface structures as well as the strain in the Co and the Pd layers are expected to influence the magnetic properties, where the lattice mismatch between fcc-Co and fcc-Pd is fairly large at about 9%. There is a possibility that the crystal structure of Co may change between fcc and hcp depending on the strain in the multilayer structure. In order to investigate the detailed structural and the magnetic properties, well-defined epitaxial films are useful, since the film uniformity and the magnetic anisotropy are well controlled. In our previous study [8], Co/Pd epitaxial multilayer films were prepared on Pd underlayers with different orientations by RF magnetron sputtering. The Co/Pd multilayer film formed on Pd(111)fcc underlayer
n
Corresponding author. Tel.: þ81 3 3817 1862; fax: þ 81 3 3817 1847. E-mail address: [email protected] (K. Tobari).
0304-8853/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2011.10.026
showed a stronger perpendicular anisotropy than that formed on Pd(011)fcc underlayer. On the contrary, the multilayer formed on Pd(001)fcc underlayer was easily magnetized when the magnetic field was applied along the in-plane direction. In the present study, Co/Pd multilayer films were prepared on Pd(111)fcc underlayers hetero-epitaxially grown on MgO(111)B1 single-crystal substrates by varying the Co and the Pd layer thicknesses from 8.6 and 11.4 nm to 0.09 and 0.11 nm, respectively. Co50Pd50 (at%) alloy films were also prepared on Pd(111)fcc underlayers by employing a CoPd alloy target in order to compare the structural and the magnetic properties with those of Co/Pd multilayer film. The detailed Co and Pd layer structures were studied by in situ RHEED during film preparation process and the resulting multilayer film structures were examined by in-plane X-ray diffraction (XRD).
2. Experimental procedure Thin films were prepared on polished MgO(111)B1 substrates by using an RF magnetron sputtering system, where the base pressures were lower than 4 10 7 Pa. Substrates were heated at 600 1C for 1 h in the ultra-high vacuum chamber. The distance between the target and the sample was fixed at 150 mm. The Ar gas pressure was 0.67 Pa and the rf powers for Pd, Co, and Co50Pd50 (at%) targets with 3 in. diameter were, respectively, kept constant at 30, 49, and 38 W, where the deposition rate was 0.02 nm/s for all materials. The film layer structures are [Pd(tPd nm)/Co(tCo nm)]n/Pd(10 nm)/ MgO(111)B1 and CoPd(20 nm)/Pd(10 nm)/MgO(111)B1. A 10 nmthick Pd(111)fcc underlayer was prepared by hetero-epitaxial growth
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Fig. 1. (a)–(d) RHEED patterns observed for a Pd(11.4 nm)/Co(8.6 nm)/Pd(10 nm)/MgO(111)B1 specimen and the RHEED intensity profiles along the fcc[112] direction. (a) A 10 nm-thick Pd underlayer grown on MgO(111)B1 substrate, a Co layer of (b) 1.0 nm and (c) 2.5 nm formed on the Pd underlayer, and an upper Pd layer of (d) 0.2 nm deposited on the Co layer. The incident electron beam is parallel to the MgO[11¯0]B1 direction. The in-plane lattice spacings, d(112), are determined from the distances between RHEED spots shown in the arrows in the intensity profiles of (a)–(d). The spot maps of (e) and (f) correspond to fcc(111) and hcp(0001) textures, respectively. The symbols, A and B, correspond to the orientation relationships of Types A and B explained in the text, respectively.
on an MgO(111)B1 substrate at 300 1C. The epitaxial orientation relationship of Pdð111Þ½110fcc ,ð111Þ½110fcc :MgOð111Þ½110B1 was determined by comparing the reflection high energy electron diffraction (RHEED) patterns observed for the Pd underlayer [Fig. 1(a)] and the substrate. And then, a [Pd(tPd nm)/Co(tCo nm)]n multilayer or a 20 nm-thick Co50Pd50 alloy layer was deposited on the underlayer at room temperature. The total thickness and the composition of Co/Pd multilayer were fixed at 20 nm and Pd-50 at% Co, respectively. Here, the thicknesses of Pd and Co layers were, respectively, defined as t Pd ¼ 20=fn½1 þ ðaCo Þ3 =ðaPd Þ3 g,
t Co ¼ ½ðaCo Þ3 =ðaPd Þ3 t Pd ,
where aPd and aCo are lattice constants of bulk fcc-Pd crystal (aPd ¼ 0.390 nm [9]) and bulk fcc-Co crystal (aCo ¼0.354 nm [10]). The repetition number, n, was varied from 1 to 100. The compositions of Co/Pd multilayer and Co50Pd50 alloy layer were confirmed by energy dispersive X-ray spectroscopy and the errors were less than 5 at%. The details of film preparation are similar to our previous study [8]. The surface structure during RF-sputter deposition process was studied by RHEED. The film structure was investigated by inplane X-ray diffraction with Cu-Ka radiation (l ¼ 0.15418 nm). The magnetization curves were measured using a vibrating sample magnetometer.
3. Results and discussion Co/Pd multilayer films epitaxially grew on Pd(111)fcc underlayers for all the samples. Co50Pd50 (at%) alloy films were also obtained on Pd(111)fcc underlayers. Fig. 1 shows the RHEED patterns observed for a Pd(11.4 nm)/Co(8.6 nm) bi-layer film (n¼1) grown on Pd(111)fcc underlayer. The RHEED patterns observed for the Co layer and the upper Pd layer consist of spots corresponding to fcc(111) texture and streaks along the fcc[111] direction, as shown in Fig. 1(b)–(d). The spots consist of two fcc(111) reflections, as shown by the symbols, A and B, in the RHEED spot map of Fig. 1(e). The streaks observed for the upper Pd layer show that the layer has atomically flat terraces. On the other hand, the streaks observed for the Co layer indicate that the layer has atomically flat terraces and/or involves stacking faults
along the fcc[111] direction. The crystal structure of Co varies between hcp and fcc by introduction of stacking faults parallel to the close-packed plane. The existence of Co(0001)hcp crystal is confirmed by XRD analysis, which is described later. The epitaxial orientation relationships of Pdð111Þ½110fccðTypeAÞ ,ð111Þ½110fccðTypeBÞ :Coð111Þ½110fccðTypeAÞ , ð111Þ½110fccðTypeBÞ ,ð0001Þ½1120hcp :MgOð111Þ½110B1 are determined by RHEED. The Co layer consists of two-type fcc and one-type hcp variants, whereas the upper and the lower Pd layers consist of two-type fcc variants. The orientations of twotype fcc variants are rotated around the film normal by 1801 with respect to each other and the variants with Type A and B relationships have atomic stacking sequences of ABCABCy and ACBACBy, respectively. The in-plane lattice spacings of Pd/Co bilayer film were measured from the distances between RHEED spots [8]. The in-plane lattice spacing of Co layer gradually decreases and reaches a constant value of 0.22 nm beyond 2.5 nm-thick Co deposition, as shown in Fig. 1(b) and (c). This gradual change in lattice spacing is considered to be partially due to lattice strain caused by the lattice mismatch between Co and Pd layers ( 9.0%) and partially due to atomic mixing around the Co/Pd interface. Atomic sites of Co crystal are partly replaced by Pd atoms with larger atomic radius and there is a possibility that a CoPd alloy phase is formed. The formation of CoPd alloy phase is also recognized by XRD. The CoPd alloy phase is formed in the crystallographic orientation relationships of CoPdð111Þ½110fccðType AÞ ,ð111Þ½110fccðType BÞ , ð0001Þ½1120hcp :MgOð111Þ½110B1 The in-plane lattice spacing of upper Pd layer is kept constant at 0.24 nm from the beginning of Pd deposition till the end of 11.4 nm-thick Pd layer formation. At the Pd/Co interface, a CoPd alloy phase is also considered to be formed, since the lattice constant of CoPd alloy close to that of Pd crystal [11]. Fig. 2(a) and (b) shows the RHEED patterns observed for a [Pd(0.11 nm)/ Co(0.09 nm)]100 multilayer film (n ¼100). The variation of inplane lattice spacing is not recognized for all the Co and the Pd layers, and the lattice spacings of Co and Pd layers are kept constant at 0.24 nm. The atomic mixing is enhanced for thinner Co and Pd layers in Co/Pd multilayer structure. Fig. 2(c) shows the
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Fig. 2. RHEED patterns observed for [(a),(b)] [Pd(0.11 nm)/Co(0.09 nm)]100/Pd(10 nm)/MgO(111)B1 and (c) Co50Pd50(20 nm)/Pd(10 nm)/MgO(1 1 1)B1 specimens and the RHEED intensity profiles along the fcc[112] direction. The uppermost (a) Co and (b) Pd layers of Co/Pd multilayer film and (c) the 20 nm-thick CoPd alloy films grown on Pd(111) underlayers. The incident electron beam is parallel to the MgO[110]B1 direction. The in-plane lattice spacings, d(112), are measured from the distances between RHEED spots shown in the arrows in the intensity profiles of (a)–(c).
Fig. 4. Magnetization curves of (a) [Pd(5.68nm)/Co(4.32nm)]2, (b) [Pd(2.27nm)/ Co(1.73nm)]5, (c) [Pd(1.14nm)/Co(0.86nm)]10, (d) [Pd(0.19nm)/Co(0.14nm)]60, (e) [Pd(0.11nm)/Co(0.09nm)]100 multilayer films and (f) a Co50Pd50(20nm) alloy film grown on Pd underlayers. The values in parenthesis are layer thicknesses in nanometers. Fig. 3. In-plane XRD spectra of Co/Pd multilayer films and a Co50Pd50 alloy film grown on Pd(111) underlayers. The scattering vector of in-plane XRD is along (a) the MgO[11¯0]B1 or (b) the MgO[112]B1 direction. The intensity is shown in a logarithmic scale.
RHEED pattern observed for a Co50Pd50 alloy film. The in-plane lattice spacing is 0.24 nm. Fig. 3 shows the in-plane XRD spectra of Co/Pd multilayer films with different n values and Co50Pd50 film. In the in-plane XRD spectra measured by making the scattering vector parallel to the MgO[11¯0] direction [Fig. 3(a)], fcc(22¯0) and/or hcp(112¯0) reflections from Co and CoPd crystals are observed in addition to MgO(22¯0) and Pd(22¯0) reflections. In the in-plane XRD spectra measured by making the scattering vector parallel to the MgO[112¯] direction [Fig. 3(b)], Co(11¯00) and/or CoPd(11¯00) reflections are recognized. The epitaxial orientation relationships determined by RHEED are also confirmed by the in-plane XRD. The XRD data apparently indicate that the films include hcp phases in addition to fcc phase. With increasing the n value, the fcc(22¯0) and/or hcp(112¯0) and the (11¯00) reflection intensities from CoPd alloy become stronger, whereas those from Co crystal weaken, which indicates that the CoPd alloy phase formation is promoted by increasing the n value. Fig. 4(a)–(f) shows the magnetization curves of Co/Pd multilayer films with different n values and Co50Pd50 alloy film. The magnetization curves are almost isotropic in in-plane measurements for all the samples. The multilayer films with n ¼1–2 show in-plane magnetic anisotropies, as shown, for example, in Fig. 4(a). The multilayer film with n¼5 shows almost magnetic properties in in-plane and out-of-plane measurements, as shown
Fig. 5. n value (Co layer thickness) dependences on the ratios of (a) Mr///Ms//, (b) Mr?/Ms?, and (c) Mr?/Mr// of Co/Pd multilayer films. (d) Mr///Ms//, (e) Mr?/Ms?, and (f) Mr?/Mr// ratios of a Co50Pd50 alloy film.
in Fig. 4(b). The multilayer films with n ¼10–100 show perpendicular anisotropies, as shown in Fig. 4(c)–(e). The interface anisotropy is enhanced with increasing the n value (decreasing the Co
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layer thickness). This result is similar to the case of Co/Pd multilayer polycrystalline film [5]. The CoPd alloy film shows almost isotropic anisotropies in in-plane and out-of-plane measurements, as shown in Fig. 4(f). Fig. 5(a), (b), and (c), respectively, show the n value (the Co layer thickness) dependences on the ratios of Mr///Ms//, Mr?/Ms?, and Mr?/Mr// of Co/Pd multilayer films, where Mr is the remanent magnetization, Ms is the saturation magnetization, and the subscripts, // and ?, refer to in-plane and out-of-plane magnetizations, respectively. With increasing the n value beyond 100, the perpendicular anisotropy tends to weaken, possibly due to reduction in the interface anisotropy caused by atomic mixing around the Co/Pd interface. The magnetic properties are apparently influenced by the film structure depending on the Co and the Pd layer thicknesses.
4. Conclusions Co/Pd epitaxial multilayer films were obtained on Pd(111)fcc underlayers. The film structure is investigated by in-situ RHEED and ex-situ XRD. CoPd alloy phase is formed around the Co/Pd interface. The atomic mixing is enhanced for thinner Co and Pd layers in multilayer structure. With increasing the n value (with decreasing the Co and the Pd layer thicknesses and increasing the repetition number of Co/Pd multilayer film), stronger perpendicular anisotropy is observed for the multilayer film. Further increase in n value beyond 100 (Co layer thickness is smaller than
1.73 nm) causes reduction in perpendicular anisotropy. The magnetic properties are delicately influenced by the film structure.
Acknowledgments Authors thank Prof. Fumiyoshi Kirino of Tokyo National University of Fine Arts and Music for energy dispersive X-ray spectroscopy analysis. A part of this work was supported by METI–Japan, MEXT–Japan, and Chuo university Grant for Special Research. References [1] P.F. Carcia, A.D. Meinhaldt, A. Suna, Applied Physics Letters 47 (1985) 178. [2] B.M. Lairson, J. Perez, C. Baldwin, IEEE Transactions on Magnetics 30 (1994) 4014. [3] A. Ajian, K. Sato, N. Aoyama, T. Tanaka, Y. Miyaguchi., K. Tsumagari, T. Morita, T. Nishihashi, A. Tanaka, T. Uzumaki, IEEE Transactions on Magnetics 46 (2010) 2020. [4] D. Lim, S. Kim, S. Lee, Journal of Applied Physics 97 (2005) 10C902. [5] H.J.G. Draaisma, W.J.M. de Jonge, Journal of Applied Physics 62 (1987) 3318. [6] T. Asahi, K. Kuramochi, J. Kawaji, T. Onoue, T. Osaka, M. Saigo, Journal of Magnetism and Magnetic Materials 235 (2001) 87. [7] S. Hashimoto, Y. Ochiai, K. Aso, Journal of Applied Physics 66 (1989) 4909. [8] K. Tobari, M. Ohtake, K. Nagano, M. Futamoto, Japanese Journal of Applied Physics 50 (2011) 073001. [9] E. Owen, E. Yates, Philosophical Magazine Series 7 (15) (1933) 472. [10] A. Hull, Physical Review 17 (1921) 571. [11] R.M. Bozorth, P.A. Wolff, D.D. Davis, V.B. Compton, J.H. Wernick, Physical Review 122 (1961) 1157.