The influence of residual stress and crystallite size on the magnetic properties of electrodeposited nanocrystalline Pd–Co alloys

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Surface & Coatings Technology 202 (2007) 107 – 113 www.elsevier.com/locate/surfcoat

The influence of residual stress and crystallite size on the magnetic properties of electrodeposited nanocrystalline Pd–Co alloys R.D. Noce a , N. Barelli a , R.F.C. Marques a , P.T.A. Sumodjo b,⁎, A.V. Benedetti a a

Instituto de Química, Universidade Estadual Paulista, UNESP, 14801-970 Araraquara, SP, Brazil b Instituto de Química, Universidade de São Paulo, USP 05508-900 São Paulo, SP, Brazil Received 13 November 2006; accepted in revised form 22 April 2007 Available online 1 May 2007

Abstract Nanocrystalline Pd–Co alloys were obtained by electrodeposition from an ammoniacal chloride bath. The influence of the crystallite size and the residual stress on the magnetic properties of the alloys was investigated. The residual stress increased as the applied current density was increased. It was associated to the high nucleation rate during electrodeposition and correlated to the lattice strain, estimated from the XRD patterns. Also from the XRD patterns the average crystallite size and the lattice constant were determined by Scherrer's and Rietveld's methods, respectively. Both parameters were directly influenced by the applied current density. Magnetic properties such as coercivity, remanence, saturation magnetization and squareness showed strong dependence on the residual stress and crystallite size. Coercivity higher than 1 kOe was achieved when a high current density was applied. High coercivity was attributed to the presence of residual stress and to the small crystallite size of deposits. © 2007 Elsevier B.V. All rights reserved. Keywords: Electroplating; Pd–Co alloys; X-ray diffraction; Magnetic properties

1. Introduction Magnetic alloys have been the subject of fundamental and applied studies, because of their applications such as in high density magneto-optical recording devices and Micro-ElectroMechanical systems (MEMS) [1–3]. The characteristics of the alloys such as composition, microstructure, temperature, residual stress and crystallite size, among others, can influence significantly their magnetic properties. In fact, it was already observed that small variations in these parameters, with emphasis on the residual stress and crystallite size, can change drastically the coercivity and remanence [4–10]. In general, the presence of stress appreciably affects the magnetic properties of the deposits [11–13]. It is frequently desired to minimize the residual stress because of its negative effect on some fundamental properties of electrodeposits. However, the complete ⁎ Corresponding author. Av. Prof. Lineu Prestes, 748, Instituto de Química, Universidade de São Paulo, USP, 05508-900 São Paulo, SP, Brazil. Tel./fax: +55 11 30912154. E-mail address: [email protected] (P.T.A. Sumodjo). 0257-8972/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2007.04.082

reduction of the residual stress in some magnetic alloys can cause the worsening of their magnetic properties [12]. It was theoretically shown that magnetic materials with very fine crystallites have their magnetic properties improved because of the tendency to exhibit the behavior of a single magnetic domain [14]. In fact, it was reported that nanocomposite (Pr0.17Co0.83)69C31 films with considerable small crystallite size (7.8 nm) showed relatively large coercivity (5.2 kOe) [15]. Some deposition techniques such as vacuum techniques [16– 18] (sputtering and vapor physical deposition), laser techniques [19–21] and electrodeposition [22–24] can provide the appropriate conditions to produce deposits with very fine crystallites. Among these methods electrodeposition stands out due to its advantages when compared to the other ones because is well known that it is a simple technique, of great versatility, and possesses a relatively low cost of operational conditions, energy and equipments. Pd–Co alloys have excellent mechanical properties such as high hardness and also high corrosion resistance [25]. Besides, these alloys also exhibit interesting magnetic properties such as the perpendicular magnetic anisotropy and high Kerr rotation

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angles [19,20,26], two important properties required in the magneto-optical recording field. A very peculiar property in relation to these alloys, is the fact that small amounts of Co, as little as 0.1 at.%, when added to Pd, produces a ferromagnetic material, although Pd is paramagnetic. These properties make Pd–Co alloys promising candidates in magnetic recording and magneto-optical recording fields [27]. In this study, Pd–Co alloys were electrodeposited from an ammoniacal chloride bath with different applied current densities. In our previous paper [28], we studied the effect of the bath pH on the electrodeposition of nanocrystalline Pd–Co alloys obtained using an applied current density of 250 mA cm− 2 for different values of bath pH. In the present paper we have focused the investigation on the influence of residual stress and crystallite size on the magnetic properties of electrodeposited nanocrystalline Pd–Co alloys. Their morphological and structural characteristics were analyzed by scanning electron microscopy (SEM) and X-rays diffraction (XRD), and the magnetic properties were evaluated using a vibrating sample magnetometer (VSM). Therefore, the aim of this work is to correlate residual stress, crystallite size and magnetic properties shown by the deposits with the applied current density. 2. Experimental details Pd–Co alloys were electrodeposited at constant current (galvanostatic mode) on commercial brass plates with a geometric area of 4 cm2. The plating bath composition was: 0.005 M CoCl2, 0.01 M Pd(NH3)4Cl2 and 1.68 M NH4Cl. The pH was kept at 9.5 adjusted with NH3(aq). The applied current density was varied from 10 to 250 mA cm− 2 and the electric charge passed through the cell was kept constant and equal to 100 C in all depositions. The anode was a platinum net and the plating solution was unstirred during deposition. All depositions were made at room temperature. A Kraft Dynatronix model DPR 20-5-10 (Dynatronix, Inc., Amery, WI 54001 USA) power source was used in all electroplatings. The morphology and crystal structure of the deposits were determined by SEM and XRD. XRD experiments were performed using a Siemens D 5000 X-ray generator. CuKα (1.54 ) radiation was used at 40 kV and 30 mA with monochromator at the diffracted beam. For a general pattern the range recorded was 20–75° with step size of 0.05° and step time of 15 s. To determine the average crystallite size and obtain the lattice constant, a slower scanning rate was used to avoid peak widening. In this case, the range recorded was 30–75° with step size of 0.01° and step time of 5 s. The average crystallite size and the lattice constant were estimated using the Rietveld's [29] method. The Rietveld analysis was performed with the Rietveld refinement software GSAS (General Structure Analysis System) which allowed obtaining the average crystallite size by using the Scherrer's equation [30]. To determine these values with good accuracy the XRD patterns were corrected for the instrumental broadening by using an internal standard provided in the respective software. XRD patterns also allowed estimating the lattice strain correlated with the residual stress [31,32] originated from the film electrodeposition process on the substrate. The compo-

sition of the alloys was estimated by Energy Dispersive X-ray Spectrometry (EDS) and confirmed by the Vegard's law [33] plotting the lattice constant as a function of the alloy composition. The hysteresis loops and their respective parameters were obtained using a VSM (model 1660 ADE Technologies Inc.). Small discs of 0.31 cm2 were used for the magnetic measurements. The magnetic field applied in all experiments varied from −10 kOe to 10 kOe; at this upper limit saturation magnetization (Ms) was practically reached. 3. Results and discussion Table 1 shows the Pd–Co deposit thickness and composition as function of the applied current density. Thicknesses were estimated from the mass of the deposit. All depositions were run by the passage of a constant electric charge and the film thickness decreased with increasing current density. This occurred possibly because of the increase of hydrogen evolution reaction rate as the current density increases and therefore, a decrease in the current efficiency is expected. As Pd is the major component of the bath and the alloy electrodeposition is well behaved, Pd is always the major component in the film. Although the metal concentration ratio in solution was [Pd2+ ]/[Co2+ ] = 2, the corresponding ratio in the deposit was around 4. Fig. 1 shows the SEM micrographs of deposits obtained by the application of different current densities. The micrograph of the deposit obtained with a current density of 10 mA cm− 2 (Fig. 1a) shows a completely dendritic morphology and parallel arranged dendrites in relation to the substrate surface. Fig. 1b and c shows that with increasing current density the dendrites decreased in size and almost disappeared. In Fig. 1d (100 mA cm− 2) some dendrites are still observed; however, cauliflower-like morphology is already predominant. Apparently the deposited particles tended to be arranged perpendicularly to the electrode surface. A completely cauliflower-like morphology with particles arranged making an angle of around 90° in relation to the substrate surface was observed for a film deposited applying 250 mA cm− 2 (Fig. 1e). Probably the kinetics of the alloy deposition imposed by the current density and the hydrogen gas evolution among others are the main factors responsible for the morphological changes of the deposit. Morphological changes in electrodeposits are governed by many and complex aspects (applied current density, bath pH, additives, concentration of reagents, nucleation and growth rate, electrode reaction mechanism and crystallographic structure of the substrate) and this subject has been often explored in the literature [34–45]. Another factor that provokes changes in Table 1 Thickness and composition of Pd–Co alloys determined by EDS. Alloys electrodeposited at different current densities by passing a total electric charge of 100 C i/mA cm− 2

Thickness/μm

Pd wt.%

Co wt.%

10 20 50 100 250

3.35 3.01 2.47 2.26 1.83

86.7 88.3 89.4 88.7 88.0

13.3 11.7 10.6 11.3 12.0

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Fig. 1. SEM micrographs of electrodeposited Pd–Co alloys obtained applying different current densities: (a) 10, (b) 20, (c) 50, (d) 100 and (e) 250 mA cm− 2.

the electrodeposits morphology is the use of pulsed electrodeposition. Landolt and Marlot [34] have described some interesting aspects of the pulsed electrodeposition when compared to the corresponding dc mode. They reported that using pulse plating in the electrodeposition of some metals and alloys, different morphologies can be obtained depending on the current and frequency modulations. A morphology usually observed in electrodeposits is the dendritic one [37,39]. As reported by Lopez and Choi [39] who studied the electrochemical synthesis of dendritic zinc films, the dendritic growth can be promoted when the growth rate of crystals exceeds the mass transport rate of ions or molecules that feed the growing crystals. For deposits of zinc from alkaline solutions different morphologies like heavy spongy, dendritic, boulder, laylerlike and mossy were obtained depending on the direct current density, alkaline concentration, temperature and solution

stirring [40]. For example dendritic initiation is favored at high current density, low zincate concentration, high KOH concentration, unstirred solution or highly viscous gelled electrolyte, indicating a diffusion control process. On the other hand compact and mossy morphologies cannot be obtained under diffusion control according to Wang et al. [40]. Also, electrodeposit growth and morphology were found substrate dependent whilst the crystal structure was independent [41]. Another morphology hereby described is the cauliflower-like one. Jovic et al. [42] showed that this morphology appears when high current densities (≈0.5 A cm− 2) are applied in the electrodeposition of nickel powders. In addition, some papers [43–45] have mentioned the existence of the cauliflower-like morphology and it is commonly observed on most of the electrodeposited binary selenide films [46]. One important factor contributing to the morphology of the deposit is the residual

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stress. It is well known that residual stress is very frequent in electrodeposits [47–50] and many factors contribute to its occurrence. One possible cause for the appearance of stress is the use of high current densities because in these cases larger amount of small nuclei are formed. Moreover, an increase of the hydrogen evolution reaction rate increases the possibility of hydrogen permeation in the substrate, which is also a factor of residual stress development. All XRD patterns obtained for the different deposited Pd–Co alloys are similar. Fig. 2 shows the Rietveld plot at the end of the refinement for deposits obtained using current densities of 10, 50 and 250 mA cm− 2. As indicated by the vertical bars three peaks related to Pd and three attributed to brass were observed. S denotes the brass substrate peaks. No peaks that could be attributed to Co were noted, suggesting that almost all the cobalt is dissolved into the Pd matrix as a substitutional solid solution. This result agrees with the phases diagram for Pd–Co system [51], which shows total miscibility of these elements in the whole composition range. However, to fit the experimental data with the calculated one using the Rietveld's method, another phase had to be considered to adjust the peak in about 43°. In this case, a small percentage of a phase (6.8% for i = 10 mA cm− 2 and 3.5% for i = 20 mA cm− 2) where Pd is dissolved into Co was taken into account. Furthermore, a comparison of the XRD patterns (Fig. 2) with the corresponding pattern for brass (Fig. 2d) reveals a displace-

Table 2 Lattice strain values in percentage, crystallite size and magnetic properties of the deposits obtained by applying different current densities i (mA cm− 2)

Lattice Longitudinal Perpendicular Hc Ms strain crystallite crystallite (kOe) (T) (%) size (nm) size (nm)

MR (T)

S

10 20 50 100 250

0.062 0.148 0.177 0.257 0.321

0.0424 0.0534 0.100 0.124 0.144

0.350 0.390 0.430 0.470 0.480

25.6 22.3 16.7 11.3 11.2

10.1 14.7 15.4 12.1 11.1

0.609 0.695 0.767 0.942 1.08

0.121 0.136 0.233 0.264 0.300

ment of the peaks attributed to the substrate. This is an indication of lattice strain development in the substrate caused by the presence of a deposit, and therefore, by the residual stress. From the XRD patterns, lattice strain percentage was estimated [31,32] and crystallite sizes determined by using the Scherrer's equation [30]. To estimate the lattice strain we first recorded an XRD pattern of only brass substrate (Fig. 2d) and considered it as an internal standard where no strain is observed. According to Klug and Alexander [31] strain can be measured as a change in the dspacing line in a strained sample compared to an unstrained one and is usually expressed as ε = b|▵d/d|Nwhere ε represents the lattice strain in % relative. Therefore, if d-spacing changes in the XRD pattern of our samples are observed compared to the XRD

Fig. 2. Rietveld plot at the end of the refinement performed for samples obtained using a current density of: (a) 10, (b) 50 and (c) 250 mA cm− 2. S denotes the brass substrate peaks. XRD pattern for the brass substrate (d).

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Table 3 Results obtained by EDS and analysis of the lattice constant parameter from the Vegard's law (VL) for different electrodeposition rates i (mA cm− 2)

Pd at.% (EDS)

Co at.% (EDS)

Lattice constant (Å)

Pd at.% (VL)

Co at.% (VL)

10 20 50 100 250

78.3 80.7 82.4 81.3 80.2

21.7 19.3 17.6 18.7 19.8

3.845 3.855 3.870 3.865 3.860

77.4 79.6 82.1 81.0 80.1

22.6 20.4 17.9 19.0 19.9

pattern of only brass, these changes are attributed to the lattice strain appearance in the substrate caused by the film electrodeposition process on it. According to the SEM images (Fig. 1) the shape of the crystallites is anisotropic, principally at low current densities (needles). Thus, to calculate the average crystallite size, two crystallographic directions were considered: longitudinal and perpendicular. As the crystalline system for these alloys is cubic (fcc), the axis chosen was the c one. Therefore, it was calculated two different crystallite sizes, parallel and perpendicular to c axis [29]. Table 2 shows the lattice strain values in percentage, crystallite size and magnetic properties of the deposits obtained by applying different current densities. Analysis of the data reveals that the lattice strain percentage increased with the applied current density. Also, the lattice constant values of the alloys were obtained from XRD patterns using the Rietveld's method [29]. By the lattice constant and considering the Vegard's law, it was possible to confirm the alloys composition obtained by EDS. In this case, we assumed that our data followed the plot described by Zangari et al. [27], which shows the variation of lattice constant with the composition of Co–Pd alloys thin films. In that paper, the authors demonstrated that the lattice constant of Co–Pd alloys for a composition range between 25 and 70 at.% Co follows the plot predicted by the Vegard's law. For compositions lower than 25 at.%Co, there was a deviation of this law and a non-linear region was observed. Thus, to obtain the Pd–Co alloys composition, we simply correlated our data of the lattice constant of the alloys to the plot in Ref. [27]. The

comparison of the results obtained by EDS and the analysis of the lattice constant parameter from the Vegard's law is shown in Table 3. A careful look of the data in Table 2 allows us to infer that a good coherence between the data obtained by EDS and the analysis of the lattice constant from the Vegard's law (VL) for Pd–Co alloys exists. These results support the slight variation in the composition of the alloys when the applied current density is changed as shown before (Table 1). Fig. 3 shows the dependence of the deposited crystallite size with the used current density. As the current density increased, longitudinal crystallite size decreased tending to a constant value for current densities above 250 mA cm− 2. In relation to the crystallite size, a model about the shape can be inferred as it follows. At low current densities the crystallite presents a needle shape. As the current density increases, the longitudinal crystallite size decreases whilst the perpendicular one increases. This dependence is stronger observed up to a current density of 20 mA cm − 2 . At 50 mA cm − 2 both longitudinal and perpendicular growth velocities contribute in similar intensity. At 100 and 250 mA cm− 2, the values of the two directions are almost the same, indicating that the needle shape of the crystallites disappeared and now the spherical shape takes place,

Fig. 3. Variation of Pd–Co deposits crystallite size with the applied current density.

Fig. 5. Dependence of coercivity of Pd–Co alloys with deposit parallel crystallite size.

Fig. 4. Longitudinal M–H hysteresis loop for Pd–Co alloys electrodeposited at 20 mA cm− 2 and 250 mA cm− 2.

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which can explain the deposits with a cauliflower-like morphology obtained at higher current densities. These results are in agreement with the SEM images shown in Fig. 1. Fig. 4 shows hysteresis M–H loops for Pd–Co alloys deposited using two different current densities, 20 and 250 mA cm− 2. In the literature Pd–Co alloys are classified as magnetic materials with relatively low coercivity [19,20,27]. However, from Fig. 4 it can be noted that the coercivity is relatively high, reaching a maximum of 1.08 kOe for the alloys deposited at 250 mA cm− 2. Besides, we obtained alloys with even higher values of coercivity, around 1.69 kOe, by simply changing the pH of the plating bath [28]. Table 3 summarizes the magnetic properties of the different deposits. The values of saturation magnetization (Ms) and remanence (MR) are reported in units of Tesla (T) and were corrected considering the mass of deposits. The analysis of the data reveals that deposits with smaller crystallites, and therefore, higher residual stress, have higher coercivity. Though different authors have also observed that a decrease in the crystallite size results in a deposit with higher coercivity [52,53], many other reported studies show that coercivity decreases with crystallite size in the range from 10 to 100 nm [4,6,8,54]. The increase of the saturation magnetization (Ms) with applied current density is explained in terms of the crystallite shapes as shown in the SEM images (Fig. 1) considering that an out-of-plane saturation magnetization is more difficult to reach than an in-plane one. Squareness showed little influence of the applied current density, ranging from 0.35 to 0.48 at i = 250 mA cm− 2. Remanence increases as the crystallite size decreases and this observation is in agreement with other studies reported in the literature [5,10], which show that some materials can present this behavior when their crystallites are diminished to nanometric scale. Moreover, an interesting feature is observed from coercivity vs. crystallite size plot. Fig. 5 shows the dependence of the coercivity on the parallel crystallite size of deposits. It shows that although the current density has little influence on the crystallite size, 11 to 26 nm, a sharp increase in the Hc values was observed for crystallite size smaller than 17 nm. Therefore, if an optimization is desired more studies have to be done for investigating and understanding the influence of the different electrodeposition parameters on the final magnetic properties. 4. Conclusions The electrodeposition technique was shown to be of great utility in the fabrication of nanocrystalline Pd–Co alloys. Using an ammoniacal chloride bath, deposits with crystallite size in the range from 10 to 26 nm were obtained. The average composition of the alloys was 88 wt.%Pd–12 wt.%Co (80 at.% Pd–20 at.%Co) and the applied current density influenced significantly the morphology of the deposits and, consequently, the magnetic properties. From XRD patterns the lattice constant and strain were determined. The lattice constant was useful to support the results of alloys composition obtained by EDS. Also, the XRD patterns confirmed the presence of a fcc Pd–Co substitutional solid solution, where almost all Co was dissolved

in the Pd matrix. At lower current densities, i ≤ 20 mA cm− 2, a little amount of a Co–Pd solid solution (b 6.7%) was detected. The applied current density modified the morphology and crystallites size, and produced an increase of residual stress. The magnetic properties of the alloys were strongly dependent on the crystallite size and the residual stress. Coercivities as high as 1.08 kOe were observed for alloys deposited with a current density of 250 mA cm− 2. The increase of the coercivity was attributed to the presence of residual stress and the small size of the crystallite. Acknowledgements The authors would like to thank B.Y. Yoo for helpful discussions. The authors also thank CNPq (Conselho Nacional de Desenvolvimento Científico e Tecnológico), CAPES (Coordenação de Aperfeiçoamento de Pessoal de Nível Superior) for Ph.D. scholarships, and FAPESP (Fundação de Amparo à Pesquisa do Estado de São Paulo) for financial support. References [1] N.V. Myung, D.-Y. Park, B.-Y. Yoo, P.T.A. Sumodjo, J. Magn. Magn. Mater. 265 (2003) 189. [2] Y. Zhang, G. Ding, H. Wang, S. Fu, B. Cai, IEEE Trans. Magn. 42 (2006) 51. [3] S. Guan, B.J. Nelson, J. Magn. Magn. Mater. 292 (2005) 49. [4] G.B. Han, R.W. Gao, S.S. Yan, H.Q. Liu, S. Fu, W.C. Feng, W. Li, X.M. Li, J. Magn. Magn. Mater. 281 (2004) 6. [5] G.P. Zhao, C.K. Ong, Y.P. Feng, J. Magn. Magn. Mater. 192 (1999) 543. [6] M.K. Griffiths, J.E.L. Bishop, J.M. Tucker, H.A. Davies, J. Magn. Magn. Mater. 183 (1999) 49. [7] A. Manaf, R.A. Buckley, H.A. Davies, M. Leonowicz, J. Magn. Magn. Mater. 101 (1991) 360. [8] G. Herzer, IEEE Trans. Magn. 26 (1990) 1397. [9] J. Löffler, H. VanSwygenhoven, W. Wagner, J. Meier, B. Doudin, J.-Ph. Ansermet, Nanostruct. Mater. 9 (1997) 523. [10] J. Bauer, M. Seeger, A. Zern, H. Kronmüller, J. Appl. Phys. 80 (1996) 1667. [11] D. Sander, R. Skomski, A. Enders, C. Schmidthals, D. Reuter, J. Kirschner, J. Phys. D: Appl. Phys. 31 (1998) 663. [12] P. Zou, W. Yu, J.A. Bain, IEEE Trans. Magn. 38 (2002) 3501. [13] V.A. Vas'ko, J.O. Rantschler, M.T. Kief, IEEE Trans. Magn. 40 (2004) 2335. [14] W. Rave, K. Ramstöck, J. Magn. Magn. Mater. 171 (1997) 69. [15] P. Chen, S.P. Wong, M.F. Chiah, H. Wang, W.Y. Cheung, N. Ke, Z.S. Xiao, Appl. Phys. Lett. 81 (2002) 4799. [16] P. Poulopoulos, M. Angelakeris, D. Niarchos, R. Krishnan, M. Porte, C. Batas, N.K. Flevaris, J. Magn. Magn. Mater. 148 (1995) 78. [17] J.-R. Jeong, S.-C. Shin, IEEE Trans. Magn. 39 (2003) 2705. [18] B. Lesiak, J. Zemek, P. de Haan, A. Jozwik, Surf. Sci. 346 (1996) 79. [19] E. Gan'shina, V. Guschin, I. Romanov, A. Tselev, J. Magn. Magn. Mater. 185 (1998) 258. [20] E. Gan'shina, V. Guschin, I. Romanov, A. Skobelev, A. Tselev, J. Magn. Magn. Mater. 193 (1999) 174. [21] E. Gan'shina, V. Guschin, S. Kirov, O. Shabanova, A. Tselev, J. Magn. Magn. Mater. 203 (1999) 244. [22] A. Robertson, U. Erb, G. Palumbo, Nanostruct. Mater. 12 (1999) 1035. [23] M. Takai, K. Hayashi, M. Aoyaki, T. Osaka, J. Electrochem. Soc. 144 (1997) L203. [24] T. Nakanishi, M. Ozaki, H.-S. Nam, T. Yokoshima, T. Osaka, J. Electrochem. Soc. 148 (2001) C627. [25] J.A. Abys, G.F. Breck, H.K. Straschil, I. Bogustavsky, G. Holmbom, Plating Surf. Finish. 86 (1999) 108. [26] R. Gontarz, L. Smardz, B. Szymanski, P. Juzikis, J. Magn. Magn. Mater. 120 (1993) 278.

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