Thickness dependence of structural and transport properties of Co-doped BaFe2As2 on Fe buffered MgO substrates
Kazumasa Iida, Jens Haenisch, Sascha Trommler, Silvia Haindl, Fritz Kurth, Ruben Huehne, Ludwig Schultz, Bernhard Holzapfel
TThickness dependence of structural and transportproperties of Co-doped BaFe As on Fe bufferedMgO substrates Kazumasa Iida, Jens H¨anisch, Sascha Trommler, Silvia HaindlFritz Kurth, Ruben H¨uhne, Ludwig Schultzand Bernhard Holzapfel
Leibniz-Institut f¨ur Festk¨orper-und Werkstoffforschung (IFW) DresdenP. O. Box 270116, 01171 Dresden, GermanyE-mail: [email protected]
Abstract.
We have investigated the influence of the superconducting layer thickness, d , on the structural and transport properties of Co-doped BaFe As films depositedon Fe-buffered MgO substrates by pulsed laser deposition. The superconductingtransition temperature and the texture quality of Co-doped BaFe As films improvewith increasing d due to a gradual relief of the tensile strain. For d ≥
90 nm anadditional 110 textured component of Co-doped BaFe As was observed, which leadsto an upward shift in the angular dependent critical current density at H (cid:107) c . Theseresults indicate that the grain boundaries created by the 110 textured component maycontribute to the c -axis pinning.PACS numbers: 74.70.Xa, 81.15.Fg, 74.78.-w, 74.25.Sv, 74.25.F- Submitted to:
Supercond. Sci. Technol. a r X i v : . [ c ond - m a t . s up r- c on ] N ov hickness dependence of structural and transport properties of Co-doped BaFe As
1. Introduction
Crystalline quality, structural parameters and surface morphology of thin films aregenerally influenced by their thickness due to several factors such as strain, defectformation and change in growth mode. Such changes affect the physical properties ofthin films. Hence, whenever new functional thin films are prepared, it always becomesimmediately interesting to explore the effect of layer thickness on the structural andphysical properties.Among the family of newly discovered Fe-based superconducting compounds,increasing the layer thickness of FeSe and FeSe . Te . thin films improves thesuperconducting transition temperature, T c , presumably due to lattice distortion bystrain [1, 2].For Co-doped BaFe As (Ba-122) films, only thickness studies regarding the bufferlayers have been reported to date. Tarantini et al and Lee et al have found that singlecrystalline (La,Sr)(Al,Ta)O substrates with 100 unit cells of epitaxial SrTiO resultedin the highest T c and the largest critical current density, J c [3, 4]. Even higher T c andsharper out-of-plane and in-plane textures of the Fe/Ba-122 bilayers can be realizedfor 20 nm thick epitaxial Fe buffer layers [5]. However, no investigation of the effectof layer thickness of Co-doped Ba-122 on the structural and transport properties havebeen published to date. In this article, we report on the influence of layer thickness onthe structural and transport properties of the Fe/Ba-122 bilayer system with a fixed Felayer thickness.
2. Experiment
Epitaxial, smooth Fe buffer layers (20 nm) were prepared by a two-step process, whichinvolves a room temperature deposition of Fe on MgO (001) single crystalline substratesby pulsed laser deposition, PLD, followed by a high-temperature annealing at 750 ◦ C,both in a UHV chamber (base pressure of 10 − mbar). Prior to the Fe deposition, thesubstrate was heated to 1000 ◦ C, held at this temperature for 30 min, subsequently cooledto room temperature for cleaning. A KrF excimer laser (248 nm) has been employed ata frequency of 5 Hz for the deposition with an energy density of 3–5 Jcm − on the target.After the Fe buffer preparation, Co-doped Ba-122 layers were deposited at 750 ◦ C witha laser repetition rate of 10 Hz. Each deposition step was monitored by reflection high-energy electron diffraction, RHEED. The layer thickness, d , was varied in the range of30 nm to 225 nm by controlling the number of laser pulses. Each layer thickness wasconfirmed by cross-sectional focused ion beam, FIB, cuts on multiple sample areas. Thenominal composition of the PLD target was Ba:Fe:Co:As = 1:1.84:0.16:2. The detailedtarget preparation can be found in reference [6]. The phase purity of the target wasdetermined by x-ray diffraction using a standard Bragg-Brentano geometry with Co-K α radiation. All the observed peaks were identified as Co-doped Ba-122. The latticeparameters refined via Rietveld analyses were a = 0 . c = 1 . hickness dependence of structural and transport properties of Co-doped BaFe As (d)(c)(b)(a) Figure 1.
Representative RHEED images of Fe/Ba-122 bilayer ( d =150 nm). (a)MgO single crystalline substrate at room temperature after the heat treatment.Yellow arrows indicate the diffraction spots lie on the Laue circles. (b) Fe at roomtemperature at the end of deposition, (c) Fe at 750 ◦ C, and (d) Co-doped Ba-122 atroom temperature. The incident electron beam is along the MgO [110] azimuth. respectively.Surface morphology of the films was observed by atomic force microscopy, AFM.Out-of-plane texture and phase purity were investigated by x-ray diffraction in Bragg-Brentano geometry with Co-K α radiation. In-plane orientation of both Fe and Co-doped Ba-122 were investigated by using the 110 and 103 poles respectively in a texturegoniometer operating with Cu-K α radiation. In order to evaluate the in-plane andout-of-plane lattice parameters of Co-doped Ba-122 precisely, high resolution reciprocalspace maps, RSM, around the 109, 1011 and 1110 reflections were performed with Cu-K α radiation. Here, the 204 reflection of MgO was used as a reference to eliminate anyerrors by a misalignment of the substrate.After the structural characterization, Au layers were deposited on the films byPLD at room temperature followed by ion beam etching to form bridges of 0.5 mmwidth and 1 mm length for transport measurements. Superconducting properties weremeasured in a Physical Property Measurement System (PPMS, Quantum Design) bya standard four-probe method with a criterion of 1 µ Vcm − for evaluating J c . In theangular-dependent J c measurements, J c (Θ), the magnetic field, H , was applied in themaximum Lorentz force configuration ( H perpendicular to J ) at an angle Θ measuredfrom the c -axis. T c is defined as 50% of the normal state resistance at 30 K.
3. Results and discussion
The diffraction pattern in the RHEED images of MgO substrate shows a series of spotslying on the Laue circles, indicative of a perfectly flat surface (figure 1(a)). For the Fe hickness dependence of structural and transport properties of Co-doped BaFe As (c) (a)(b) (d) Figure 2. (a) 2-Dimensional and (b) 3-dimensional AFM images (1 µ m × µ m) ofFe/Ba-122 bilayer ( d =150 nm) exhibits a large number of terraced islands. (c), (d)The corresponding images of the 30 nm thick film show that the grains are smaller andnot well connected. buffer preparation, the RHEED images of Fe in figure 1(b) confirm the epitaxial growtheven at room temperature for d =150 nm. The diffraction spots turn into streaks withincreasing temperature (figure 1(c)), indicative of smoothing of the surface [7]. For Co-doped Ba-122, the diffraction patterns with long streaks centered at positions on theLaue circles is typical for a multilevel surface (figure 1(d)); i.e. for a high number ofsmooth terraces separated by steps [7]. Additionally, the spacing of the observed streaksindicates a surface reconstruction, which is consistent with the observation on singlecrystals reported in reference [8].The AFM image of this film in figures 2 (a) and (b) further confirmed that thesurface was flat with a root mean-square roughness, R rms , of 0.83 nm. The AFM imagealso shows that Co-doped Ba-122 grows in the terraced-island mode with an averagestep height of 0.65 nm, which is almost identical to half the lattice parameter c . All thefilms in this study have the same surface morphology, and their surface roughness aresummarized in Table 1. However, the grains of the 30 nm thick film are smaller and notwell connected compared with the other films (figures. 2(c) and (d)).The θ/ θ - scans for the Fe/Ba-122 bilayers with different layer thickness do notshow any secondary phases (figure 3). The pronounced 00 l reflections of Co-doped Ba-122 together with the 002 reflection of MgO and Fe are observed for all films, indicating a c -axis orientation. For thickness d ≥
90 nm, a 110 component is observed whose intensitybecomes gradually stronger with increasing d . In addition, the ratio of the diffractionintensity for the 110 and 004 is increased with d (Table 1). This 110 component is alsovisible in the 103 pole figure measurement, and its epitaxial relation to the substrateis (110)[001]Ba-122 (cid:107) (001)[110]MgO and (110)[001]Ba-122 (cid:107) (001)[110]MgO. It should be hickness dependence of structural and transport properties of Co-doped BaFe As I n t e n s it y ( a r b . un it s ) Figure 3.
The θ/ θ - scans of Fe/Ba-122 bilayers with various layer thicknesses on(001) MgO substrates. Intensity of the 110 reflection is observed to increase with d . noted, however, that the amount of the 110 component is small since the ratio ofthe diffraction intensity for the 110 and 004 is less than 0.01 for all films. Here, thecorresponding value for a randomly oriented grain is 3.28 (ICDD card number 01-077-6875). Indeed, this small amount of 110 component does not compromise the crystallinequality as shown in Table 1. The full width at half maximum (FWHM), ∆ ω , of the 004rocking curve and the average ∆ φ of the 103 reflection of Co-doped Ba-122 are gettingsmaller with increasing d .Another prominent feature is a shift of the 00 l reflections to lower angles withincreasing d , indicating an increase in the lattice parameter c . Calculated latticeparameters c of the films using the Nelson-Riley function are increasing from 1.274 nm( d = 30 nm) to 1.289 nm ( d = 225 nm) [9], as shown in Table 1. 1. In this calculation,the 002 reflection is omitted in order to avoid excessive extrapolation. The correlationbetween the layer thickness and both the in-plane and the out-of-plane lattice parametersevaluated by RSM are exhibited in figures 4 (a) and (b). The lattice parameter a is observed to decrease with d , while the out-of-plane lattice parameter behaves theopposite way. The evaluated lattice parameter c by both methods (i.e. RSM and the θ/ θ - scans) are almost identical for all the films within the experimental uncertainty,albeit high angle data of 2 θ ≥ ◦ are not measured in the θ/ θ - scans. Since the FeAstetrahedron in the Ba-122 bonds coherently to bcc Fe [10], the lattice parameter a of athin Co-doped Ba-122 layer is close to that of Fe multiplied by √ a of bulk Co-doped Ba-122 (i.e. PLD target), suggesting tensilestrain in the film. The lattice parameter of Fe is almost constant at around 0.287 nmconfirmed by RSM. Here the respective lattice misfit of Fe/MgO and Co-doped Ba-122/Fe are -3.9% and -2.4%.The unit cell volume, V = a c , of Co-doped Ba-122 films is almost constant with d in the range of 30 nm ≤ d ≤
90 nm, while the thicker films deviate from this trendpresumably due to a relatively large contents of grain boundaries, GBs (figure 4(c)).Nevertheless, the volume for all the films is larger than that of the bulk Co-doped Ba- hickness dependence of structural and transport properties of Co-doped BaFe As Table 1.
Surface roughness, average FWHM values of the φ -scans and the ω -scans,lattice parameter c and diffraction intensity ratio, I / I , for Co-doped Ba-122 thinfilms with different layer thickness, d . d (nm) R rms (nm) ∆ ω ( ◦ ) ∆ φ ( ◦ ) c (nm) I / I
30 1.14 0.82 0.99 1.274 n. d.70 0.85 0.96 1.26 1.285 n. d.90 1.47 0.65 0.88 1.284 ≤ . c ( n m ) (b) PLD target0.4040.4000.396 a ( n m ) PLD target (a)ˆ2a Fe V ( n m ) Figure 4. (a) The lattice parameter a is observed to decrease with d . (b)Correspondingly, the lattice parameter c are increased with d . a Fe is the latticeparameter of Fe. (c) The unit cell volume is almost constant with d in the rangeof 30 nm ≤ d ≤
90 nm, while the thicker films deviate from this trend. Lines are guideto the eye. N o r m a li ze d R T c ( K ) Figure 5. (a) The normalized resistive traces of the Fe/Ba-122 bilayers show a gradualincrease in the T c with increasing d . (b) The T c of the Co-doped Ba-122 film is sensitiveto the lattice distortion. Errors of the T c is defined as a delta T c . hickness dependence of structural and transport properties of Co-doped BaFe As d . The film with d = 70 nm shows the lowest T c of around 18 K. The30 nm thick film shows a broad transition width, which is a direct consequence of thesmall grain size together with poor connectivity. Another prominent feature is a clearjump of T c between 70 nm and 90 nm thickness. From the θ/ θ - scans in figure 3, the110 component is absent in the 70 nm thick film whereas the 90 nm thick film containeda small amount of the 110 grains. For d ≥
90 nm, the T c s are gradually improved up to23 K together with sharpening of the transition width by increasing d . It is clear fromfigure 5 (b) that T c of the films improves with c/a , which is consistent with our previousresults for films on different substrates [6].The E − J curves for the Co-doped Ba-122 film with d = 225 nm measured in variousmagnetic fields at 12.65 K show a power-law relation, indicative of current limitation bydepinning of flux lines rather than GB effects (figure 6 (a)). As stated earlier, thickerfilms contained a small amount of the 110 component. However, the film does not showany sign of weak-link behavior.The J c (Θ) measured at a reduced temperature of t = 0 .
538 ( t = T /T c , , where the T c , is onset temperature of zero resistance) are exhibited in figure 6 (b). All the filmsexcept the 30 nm thick film can carry a high J c of over 0 .
18 MA / cm in the whole angularrange. The 30 nm thick film shows one order of magnitude lower J c (Θ) than that ofthe other films, which is due to the small grain size together with poor connectivity.Indeed, E − J curves of this film show a non-ohmic linear differential, NOLD, signature,indicative of J c limitation by GBs [12].Another prominent feature is a shift upward of J c at Θ = 180 ◦ ( H (cid:107) c ) as thethickness of Co-doped Ba-122 films increases. In particular a small c -axis angular peakis observed for d ≥
150 nm (fig. 6 (c)). These films show very similar forms of J c (Θ) witha low J c anisotropy, γ J = J c (90 ◦ ) /J c (180 ◦ ). For example, γ J of the film with d ≥
150 nmis only around 1.2, whereas the corresponding values of the 90 nm and 30 nm thick filmsare 1.5 and 2, respectively. These results suggest that the GBs may contribute to thepinning along the c -axis [13], which opens the opportunity to tune γ J by controlling theamount of the 110 textured component.The implementation of very thin Fe buffer layers also yields the 110 texturedcomponent [5]. However, the crystalline quality and the superconducting properties ofCo-doped Ba-122 are compromised due to the presence of too many GBs. Hence, theremay exist a threshold of the amount of GBs at which deterioration of the structural andsuperconducting properties sets in. hickness dependence of structural and transport properties of Co-doped BaFe As J c ( M A / c m ) (b)H ^ c H ^ cH || ct = 0.538m H = 1 T 30 nm70 nm90 nm150 nm225 nm110100 E ( m V / c m ) ) 0 T0.20.61 T2 T3 T5 T7 T9 T12.65 KH || c(a)1.00.80.6 J c / J c ¡ Figure 6. (a) The E − J curves for the Co-doped Ba-122 film with d = 225 nmmeasured in various magnetic fields at 12.65 K. Applied magnetic fields are parallel tothe c -axis of the film. (b) The J c (Θ) of the films with various d measured in an appliedfield of 1 T at a reduced temperature of 0.538. The data of the 225 nm thick film arenot visible, since it shows the almost identical form of J c (Θ) as the 150 nm thick film.(c) J c (Θ) presented fig. 6 (b) are normalized to the value at Θ = 90 ◦ .
4. Conclusion
The effect of the superconducting layer thickness on the structural and superconductingproperties of Co-doped Ba-122 films has been investigated. The texture quality and thesuperconducting transition temperature are improved by increasing the layer thicknessdue to stress relief. Increasing the layer thickness yields an additional 110 texturedcomponent, creating GBs. However, these GBs may constitute c -axis pinning centerswithin a certain amount, which leads to an increase in J c at H (cid:107) c without compromisingthe structural and superconducting properties. Acknowledgments
The authors thank J. Scheiter for help with FIB cut samples and E. Barbara for helpwith the AFM observation. We are also grateful to M. K¨uhnel and U. Besold for theirtechnical support and S. F¨ahler for his RHEED software. hickness dependence of structural and transport properties of Co-doped BaFe As References [1] Wang M J, Luo J Y, Huang T W, Chang H H, Chen T K, Hsu F C, Wu C T, Wu P M, Chang AM and Wu M K 2009
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