Epitaxial growth and magnetic properties of electrochemically multilayered [CoPtP/Cu]n films

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m lay g H ology form : 14 N Electrochemically fabricated [CoPtPð100 nmÞ/Cuðx nmÞ] multilayers and their resulting magnetic properties were investigated. It was creasingly…


m lay g H ology form : 14 N Electrochemically fabricated [CoPtPð100 nmÞ/Cuðx nmÞ] multilayers and their resulting magnetic properties were investigated. It was creasingly used in those fields requiring high perfor- as CVD and PVD [3–5]. Thin film CoPtP alloys are known as one of the fer- romagnetic alloys with the best PMA (perpendicular magnetic anisotropy) among those magnetic alloys which can be prepared from electrodeposition. There- and their relatively simple fabrication process [6–8]. hypothesis provided the motivation for this study. Moreover, we recently observed that the magnetic properties of electrodeposited [CoPtP/Cu]n films were also severely conditional on the thickness of the Cu in- terlayers. Therefore, in order to produce thick magnetic films having high magnetization and coercivity, we attempted the electrodeposition of [CoPtP/Cu] multi- ation * mance magnetic materials, such as MRAM (magnetic random access memory), ultrahigh density perpendicu- lar recording media, and NEMS/MEMS (nano/micro electro mechanical systems), which are used for fabri- cating devices with the dimensions in nanometers/mi- crons [1–5]. In these areas, the electrodeposition process has regained its popularity, mainly due to its inherent ability to fill up high aspect ratio patterns and to tailor the magnetic properties to specific needs, as compared with conventional vacuum evaporation techniques such However, it is also known that when the thickness of the CoPtP film exceeds about 1 lm, its inherently high PMA rapidly deteriorates with increasing thickness, due to the formation of a columnar structure with larger grains [7]. Based on this result, it might be expected that the su- perior PMA of the CoPtP film would be maintained or even improved, if it were applied in the form of ½CoPtPð 2. Experimental 3. Results and discussion films prepared by the DBT method are shown in Fig. 1. The films consist of 100 nm thick CoPtP layers grown alternatively with either 20, 50 or 100 nm thick Cu layers. It was observed that while other types of multi- layers [9,10] had diffuse or wavy interfaces, the multi- layers shown in Fig. 1 have well-defined interfaces with a uniform layer thickness successfully controlled by our self-made DBT apparatus. y Communications 6 (2004) 115–119 There are two electrochemical methods which can be used to fabricate multilayered structures: SBT (single bath technique) and DBT (dual bath technique). SBT allows the creation of a multilayer through the appli- cation of a pulsed current in a single bath containing both of the precursors for each layer, while DBT uses two separate baths each containing one of the two in- dividual precursors for each layer. DBT was employed in this study, mainly due to its ability to provide a sharp interface as well as more homogeneous layers. Cross- [CoPtP/Cu]n multilayers were electrochemically pre- pared by means of the DBT (dual bath technique). Each layer was galvanostatically electrodeposited at 0.5 A/ dm2 and 40 �C. The working electrode (1 cm� 1 cm) consisted of a 200 nm thick Au layer on a (1 0 0) Si wafer. Electrolytic cobalt plates (99.9% purity) and a SCE (Saturated Calomel Electrode) were used as the counter electrode and the reference electrode, respec- tively. The bath for the CoPtP layer was composed of 0.12 M CoSO4, 0.45 M Na4P2O7, 0.01 M H2PtCl6 and 0.05 M NaH2PO2, and that for the Cu interlayer was made up of 0.3 M CuSO4 and 0.45 M Na4P2O7. All solutions were prepared using ultra pure deionized water (over 18 MX at 40� 1 �C). The thickness of each layer was adjusted by controlling the electrodeposition time, taking into consideration the current efficiency, which was ascertained in our preliminary experiments through SEM (Scanning Electron Microscopy) observation and weight gain measurements. The magnetic properties of the prepared specimens were measured by means of a VSM (Vibrating Sample Magnetometer, 7400, LakeShore, USA). The micro- structural and crystallographic features of the [CoPtP/ Cu] multilayer films were investigated by means of a transmission electron microscope (TEM, CM30, Philips, The Netherlands) operated at 200 kV, through imaging and electron diffraction, respectively. thickness limitation by means of microstructural modi- fication. The microstructural features of the multilayer structures, including their peculiar epitaxial growth, are also examined, in order to elucidate the role of the Cu interlayer thickness and the relationship between the microstructure and the magnetic properties of the elec- trodeposited CoPtP multilayers, which will lead to a plausible explanation for the role of the Cu layers in controlling the magnetic properties of the electrochem- ically multilayered [CoPtP/Cu] films. 116 K.H. Lee et al. / Electrochemistr sectional TEM images of the [CoPtPð100 nmÞ/Cuðx nmÞ] Fig. 1. Cross-sectional TEM images of [CoPtP/Cu] multilayers with different Cu thicknesses; (a) 20 nm, (b) 50 nm, and (c) 100 nm, re- spectively. Fig. 2 shows the variation of the magnetic hysteresis loops of the [CoPtPð100 nmÞ/Cuðx nmÞ] films with respect to the thickness of the Cu interlayer. It is apparent in Fig. 2 that the magnetic properties of the [CoPtPð100 nmÞ/ Cuðx nmÞ] films exhibit a strong dependency on the Cu layer thickness, i.e., the PMA characteristics of the film increase in proportion to the thickness of the Cu layer. More specifically, the perpendicular coercivities of the multilayer films were enhanced from 2770 Oe at tCu ¼ 20 nm to 4150 Oe at tCu ¼ 100 nm, while the in-plane coercivities exhibited the opposite variation from 1930 Oe at tCu ¼ 20 nm to 1010 Oe at tCu¼ 100 nm. The squareness (M r/M s) showed the same trend with respect to the thickness of the Cu interlayers as did the PMA. The changes in magnetic properties of a given mate- rial are often closely related to its microstructural vari- ations. Our previous study [1] demonstrated that the magnetic properties of an electrodeposited Co(P) alloy could be altered by varying the concentration of am- monium chloride, whose presence causes microstruc- tural and crystallographic differences and eventually magnetic property variations even with the same amount of P in the Co(P) alloys. It is also speculated that the variation in the thickness of the Cu layers may bring about unique microstructural variations in the multilayered specimens. Microstructural investigations were carried out by TEM to confirm this hypothesis. The DF (dark field) images obtained using cross-sec- tional TEM, which are shown in Fig. 3, reveal the ex- istence of microstructural differences between the two [CoPtPð100 nmÞ/Cuðx nmÞ] samples having the Cu layer thicknesses of 50 and 100 nm. One striking feature of these multilayered samples is that strong crystallo- graphic alignment exists throughout the thickness of the film, even though each layer is grown successively and has a different crystal structures. Evidence for this strong crystallographic alignment is found in the central region of Fig. 3(b), where a bright band with a width of about 200 nm extends from the substrate to the top of the film. This implies that there is an epitaxial or highly preferred orientation relationship between the Cu and CoPtP layers. It is also worth mentioning that the width of the ‘‘aligned region’’ increases with increasing Cu interlayer thickness, as can be seen by comparing Fig. 3(a) and (b). More detailed crystallographic characterization was performed using electron diffraction analysis of the multilayered samples. The electron diffraction patterns obtained from the [CoPtPð100 nmÞ/Cuðx nmÞ] (x¼ 20, 50, (a) Easy Magnetization High Squareness High Coercivity K.H. Lee et al. / Electrochemistry Communications 6 (2004) 115–119 117 (b) tCu = 20 nm tCu = 50 nm tCu = 100 nm Thickness of Cu interlayer M ag n et iza tio n c-axisc-axisApplied Field (b)(a) Fig. 2. The variation of the magnetic hysteresis loops measured in the direction perpendicular (a) and parallel (b) to the film plane according to the thickness of the Cu layers; 20 nm (dotted line), 50 nm (dashed line), and 100 nm (solid line), respectively. Fig. 3. Dark field images of [CoPtP/Cu] multilayers with Cu layer thicknesses of (a) 50 nm and (b) 100 nm. 100) films are shown in Fig. 4. They provide clear evi- dence for the effect of the Cu layer thickness on the mi- crostructural modification of the multiplayer films. For example, the [CoPtPð100 nmÞ/Cuð20 nmÞ] films (Fig. 4(a)) exhibited a completely random crystallographic orien- tation of both the Cu and CoPtP layers. On the other hand, in the case of the multilayered films having a Cu layer thickness of more than 50 nm (Fig. 4(c) and (d)), there existed a specific orientation relationship between the Cu and CoPtP layers, as well as a strong texture, as shown by the streaked diffraction spots. An enlarged diffraction pattern from the [CoPtPð100 nmÞ/Cuð100 nmÞ] film (Fig. 4(d)) was indexed, in order to extract quanti- tative information on the orientation relationship. Those diffraction spots, which are numbered and marked with an arrow, were indexed as listed in Table 1, and it was interesting to note that the spot marked could be in- dexed as both (0 0 2)hex and (1 1 1)fcc, which suggested that the (0 0 2) planes of the CoPtP layer and the (1 1 1) planes of the Cu layers could grow epitaxially. The analysis of the electron diffraction patterns thus implied that the predominant growth of the (1 1 1) Cu grains exerted a direct influence on the preferential growth of the CoPtP layers, with their (0 0 2) planes parallel to the direction of growth. Since the easy magnetization di- properties shown in Fig. 2(a) were measured in the per- pendicular direction of the film, the c-axis of the hexag- onal CoPtP layer had to be aligned with the direction of the applied magnetic field for the film with increasing thickness of Cu interlayers, as shown in Fig. 4. The fact that the degree of PO (preferred orientation) was stron- ger in the [CoPtPð100 nmÞ/Cuð100 nmÞ] films than in the [CoPtPð100 nmÞ/Cuð50 nmÞ] films, as shown in Fig. 4, also provided a plausible explanation for the experimental observation that the multilayer films with tCu¼ 100 nm exhibited a superior PMA than those with tCu¼ 20 and 50 nm in Fig. 2. Therefore, the microstructural features ayers with different Cu thicknesses: (a) 20 nm, (b) 50 nm, (c) 100 nm and (d) Table 1 Indexing of the electron diffraction pattern shown in Fig. 4(d) Ring # Inter-planar spacing (nm) Index Cobalt (hex) Copper (fcc) 0.2055 0 0 2 1 1 1 0.1931 1 0 1 0.1775 2 0 0 0.1255 1 1 0 0.1246 2 2 0 0.1152 1 0 3 0.1065 3 1 1 118 K.H. Lee et al. / Electrochemistry Communications 6 (2004) 115–119 rection is along the c-axis of the hexagonal structure, the alignment of the c-axis with the direction of an applied magnetic field gives rise to easier magnetization, i.e., in- duces high squareness and coercivity. Since the magnetic Fig. 4. Cross-sectional electron diffraction patterns of [CoPtP/Cu] multil enlarged pattern of (c). shown in Fig. 4 provide an accurate explanation for the magnetic properties observed in Fig. 2. In summary, varying the thickness and crystallinity of the Cu layers has an effect on the electrodeposited CoPtP layers, causing microstructural differences to appear in the form of different grain sizes and textures, which in turn, manifest themselves in the variation of various magnetic properties, such as the coercivity and square- ness. In particular, the epitaxial growth of the Cu and CoPtP layers induces the strong PO of the [0 0 2]hex of CoPtP layers, which is a main cause of the superior PMA characteristics of the films. To the best of our knowledge, such epitaxial growth of the multilayer electrochemically fabricated by DBT has never before been reported. Acknowledgements The financial support from the ‘‘R&D Program for NT-IT Fusion Strategy of Advanced Technologies’’ is gratefully acknowledged. References [1] K.H. Lee, G.H. Kim, W.Y. Jeung, Electrochem. Commun. 4 (2002) 605. [2] K.H. Lee, H.Y. Lee, W.Y. Jeung, W.Y. Lee, J. Appl. Phys. 91 (2002) 8513. [3] W.Y. Jeung, D.H. Choi, K.H. Lee, Mater. Sci. 21 (2003) 147. [4] T.M. Liaksopoulos, W. Zhang, C.H. Ahn, IEEE Trans. Magn. 32 (1996) 5154. [5] H.J. Cho, C.H. Ahn, IEEE Trans. Magn. 36 (2000) 686. [6] L. Callegaro, E. Puppin, P.L. Cavallotti, G. Zangari, J. Magn. Magn. Mater. 155 (1996) 190. [7] P.L. Cavallotti, N. Lecis, H. Fauser, A. Zielonka, J.P. Celis, G. Wouters, J. Machado da Silva, J.M. Brochado Oliveira, M.A. S�a, Surf. Coat. Tech. 105 (1998) 232. [8] G. Zangari, P. Bucher, N. Lecis, P.L. Cavallotti, L. Callegaro, E. Puppin, J. Magn. Magn. Mater. 157–158 (1996) 256. [9] C. Yang, H.Y. Cheh, J. Electrochem. Soc. 142 (1995) 3040. [10] C.A. Ross, L.M. Goldman, F. Spaepen, J. Electrochem. Soc. 140 (1993) 91. K.H. Lee et al. / Electrochemistry Communications 6 (2004) 115–119 119 Epitaxial growth and magnetic properties of electrochemically multilayered [CoPtP/Cu]n films Introduction Experimental Results and discussion Acknowledgements References


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