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Direct study of structural phase transformation in single crystalline bulk and thin film BaFe2As2
Micron 119 (2019) 1–7
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Direct study of structural phase transformation in single crystalline bulk and thin film BaFe2As2
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A. Pukenasa, , P. Chekhonina, M. Meißnerb, E. Hieckmannb, S. Aswarthamc, J. Freudenbergerc, J. Engelmannc, R. Hühnec, S. Wurmehlc, B. Büchnerc, W. Skrotzkia a
Institut für Festkörper- und Materialphysik, Technische Universität Dresden, 01062 Dresden, Germany Institut für Angewandte Physik, Technische Universität Dresden, 01062 Dresden, Germany c IFW Dresden, 01069 Dresden, Germany b
A R T I C LE I N FO
A B S T R A C T
Keywords: BaFe2As2 low temperature Superconductivity Phase Transformation Cryogenic EBSD
The ternary iron arsenide compound BaFe2As2 exhibits a structural phase transition from tetragonal to orthorhombic at a temperature of about 140 K. The twin lamellae arising below this transition temperature were studied in undoped single crystalline bulk and epitaxial thin film samples using electron backscatter diffraction in a scanning electron microscope equipped with a helium cryostat. Applying this technique on bulk single crystals a characteristic twin lamella size in the range of 0.1 μm up to a few μm was observed. In contrast, in epitaxially strained thin films the phase transition is not observed at temperatures above 19 K.
1. Introduction The ternary iron arsenide BaFe2As2 is a parent compound, which belongs to the family of superconducting iron pnictides (Paglione and Greene, 2010). It has been shown that BaFe2As2 becomes superconducting under pressure (Alireza et al., 2008), by chemical substitution (Rotter et al., 2008a) or by the application of in-plain strain through epitaxy (Engelmann et al., 2013). Upon cooling the undoped tetragonal parent compound (space group I4/mmm, a = b = 0.3963 nm and c = 1.3017 nm) exhibits simultaneously a magnetic and diffusionless structural phase transition. Below the structural transition temperature close to 140 K the orthorhombic structure (space group Fmmm, a = 0.5615 nm, b = 0.5574 nm and c = 1.2945 nm) is formed (Rotter et al., 2008b; Huang et al., 2008). The resulting unit cell almost doubles in size and rotates about 45° about the c-axis (Goldman et al., 2008). The formation of the orthorhombic lattice (black lattice in Fig. 1) can be illustrated as a stretching of the tetragonal lattice (red lattice in Fig. 1) along its [110] and [1–10] diagonals, respectively. Microstructural investigations reveal that the low temperature orthorhombic BaFe2As2 consists of two different types of domain patterns which can span the entire crystal surface (Tanatar et al., 2009). Fig. 1 schematically shows the formation of one type of domain, where the common twin plane corresponds to a (100)tetr plane (marked by pink dotted double line) of the former tetragonal lattice. According to polarized light microscopy (PLM) and transmission
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electron microscopy (TEM) studies on BaFe2As2 single crystals, the twin spacing in the orthorhombic structure is in the range of 10–50 μm (Tanatar et al., 2009) and 100–400 nm (Ma et al., 2009), respectively. This large difference may be explained by the used BaFe2As2 crystal growth methods including differences in crystal quality and by the TEM sample preparation technique that may introduce crystal defects. On the other hand, PLM cannot resolve very fine structures (Tanatar, et al., 2009; Ma, et al., 2009). To avoid these problems, scanning electron microscopy (SEM) was applied in the present work to study twin lamellae in single crystalline BaFe2As2. Using electron backscatter diffraction (EBSD) (Dingley, 2004; Randle, 2000) in a SEM the lateral resolution along the EBSD tilt axis is about few 10 nm (Harland et al., 1981; Humphreys, 2004; Zaefferer, 2007). On the one hand, this resolution is higher in comparison to light microscopy based analyses. On the other hand, sample preparation artefacts are avoided which may affect the results of TEM analyses. Therefore, the EBSD technique has promising advantages over the other techniques applied so far. In comparison to conventional EBSD, there are of course experimental difficulties for similar measurements at cryogenic temperatures, which will be discussed here. The second part of the paper focuses on undoped single crystalline BaFe2As2 thin films with different thicknesses and strain states. In contrast to chemical and pressure tuning of the superconducting phase transition, there are neither disorder effects nor sensitivity to the degree of non-hydrostaticity and therefore the crystal structure is well-defined
Correspondence author. E-mail address: [email protected] (A. Pukenas).
https://doi.org/10.1016/j.micron.2018.12.009 Received 9 June 2018; Received in revised form 24 December 2018; Accepted 24 December 2018 Available online 27 December 2018 0968-4328/ © 2018 Elsevier Ltd. All rights reserved.
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attached sample was fixed on top of the sample holder using silver conducting paste. In the final position of the sample the tetragonal BaFe2As2 [100]-direction was aligned parallel to the SEM X- direction (tilt axis for EBSD). To keep sample contamination at a minimum, best possible vacuum conditions were targeted by mounting the stage including the sample into the SEM chamber several days before experiment. The structural characterization of several BaFe2As2 single crystals was performed after cooling them to 100 K. Due to the condensation of residual gas molecules a contamination layer is formed on the sample surface which results in blurred SEM images in comparison to RT imaging (cf. Figs. 3a, b). Since BaFe2As2 is sensitive to humidity and may react with water this may be an explanation for a persistent surface contamination. In the present experimental setup there are parts in the SEM chamber which are definitely colder than the specimen surface, e.g. parts of the cryostat and the helium supply pipes, which are always significantly below 100 K. However, obviously their combined surface area is not large enough and/or too far away from the sample surface in order to be an effective cold trap. After irradiating a small specimen surface area with the electron beam for several minutes, the contamination layer disappears with the exception of a few spots (Fig. 3b). These contamination spots typically have a diameter smaller than 0.5 μm and some of them are thick enough to lower the EBSD pattern quality locally. Otherwise, the EBSD pattern quality was very high in the area irradiated by the electron beam and not prone to carbon contamination like at RT. For indexing the recorded EBSD patterns with the Channel 5 software, a tetragonal unit cell was used with lattice parameters a= b = 0.3963 nm and c = 1.3017 nm. This setting allows to resolve the crystallographic orientation differences between twin lamellae in form of pseudo-rotations in the ab - plane. An arbitrary single point inside one of the twin lamellae was taken as an arbitrary reference for the absolute orientation and each point of the mapping was plotted as relative misorientation to this reference point on a color scale. Of course, the difference in the EBSD patterns is not a rotation, as the Kikuchi band that corresponds to the twin plane remains at the exact same position. Noteworthy, the remaining contamination spots as well as the cleavage steps on the specimen surface did not substantially affect the subsequent evaluation. While the spatial and angular resolution of the EBSD mapping technique is sufficient for imaging of twin lamellae in bulk single crystals, the low small-scale orientation changes in thin films aggravate identification of twin lamellae. In addition to the automated misorientation analysis, several line scans were evaluated by manual straight line fitting to (100)tetr and (010)tetr Kikuchi bands in the EBSD patterns and estimating the angles φ1 and φ2 between (100)/(-100)tetr and (010)/(0–10)tetr Kikuchi lines respectively (Fig. 4). The mean value of the φ1 and φ2 is denoted by φ = (φ1+φ2)/2 and for better comparison between different line scans given as relative degree.
Fig. 1. Formation of an orthorhombic lattice (black lattice points) from a tetragonal one (red lattice points) below the transition temperature. In the orthorhombic phase the unit cell vectors aO and bO are rotated about 45° about the c-axis in comparison to the unit cell vectors aT and bT in the tetragonal phase and the unit cell doubles in size (blue and green rectangles). The pink dotted double line marks a (110)orth twin boundary (TB) (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
(Wu et al., 2013). However, substrate constraints may be responsible for the suppression of structural transformation of thin films. 2. Experimental Plate-shaped single crystals of BaFe2As2 with millimeter size were grown by the self-flux technique as described in (Aswartham et al., 2011). The normal direction of the plates is aligned parallel to the caxis. Specimens for SEM investigations and EBSD studies were prepared at room temperature (RT) by gluing them between two 1 mm thick copper plates using a silver-filled epoxy adhesive. Cleaving the crystals by separating the copper plates results in a clean and undistorted (001) surface parallel to the copper plates. In addition to bulk single crystals, two undoped BaFe2As2 thin films were epitaxially grown by pulsed laser deposition on (001)-oriented single crystalline MgAl2O4 (spinel) substrates with an iron buffer layer in between. The first sample with 20 nm thickness is fully strained along the two in-plane directions resulting in bulk superconductivity, whereas the second one is a 60 nm thin film featuring partial relaxation due to cracks and only signs of a superconducting transition. More details on sample preparation, structural and functional properties are published elsewhere (Engelmann et al., 2013). The characterization of the crystal surfaces as well as orientation imaging microscopy of the domain patterns was done in a Zeiss Ultra 55 scanning electron microscope with a field emission gun operated at 20 kV using a 120 μm aperture. The microscope is equipped with a Nordlys HKL EBSD system. EBSD patterns were recorded and indexed using the Channel 5 acquisition software (HKL Technology). The frontpiece of the EBSD detector additionally contains a forward scatter detector (FSD) enabling backscatter electron imaging of the crystal in the tilted state. Low temperature investigations were carried out on a helium-cooled cryogenic SEM stage (Kammrath and Weiss), which facilitates specimen temperatures down to 5 K by screening the sample with a radiation shield. Originally this stage was not designed for EBSD studies. Therefore, due to geometrical issues a special sample holder of annealed pure copper having a fixed tilt angle of 70° was manufactured (Fig. 2). A custom semiconductor sensor built-in in the cryostat sample holder measures the temperature which can be controlled and kept stable within ± 0.02 K. A further temperature sensor mounted in a small hole drilled into the EBSD sample holder measures the temperature directly below the surface at the position, where the copper plate with the sample is glued. The SEM stage used does not allow tilting and rotation of the sample. Therefore, to mount the sample in a well-defined orientation, the sample (cleaved single crystal or thin film) was analyzed prior to cryogenic EBSD using the conventional SEM stage. In accordance to the sample orientation, then the copper plate with the
3. Results and discussion 3.1. Bulk single crystals Room temperature EBSD misorientation mappings on BaFe2As2 (Fig. 5) bulk single crystals confirm high crystal quality, since the degree of misorientation is between 0° and about 0.25°. At temperatures below the structural transition temperature domains with vertical and horizontal oriented twin lamellae are formed. A section from a domain with vertical twin lamellae is shown in Figs. 6a, b. Fig. 6c schematically demonstrates the corresponding EBSD patterns with twisted (110)orth Kikuchi lines. The width of the twin lamellae ranges from 0.1 μm up to few μm. A few EBSD measurements under comparable conditions done on other BaFe2As2 samples from the same single crystal batch gave comparable widths. Hence, the observed lamella sizes are somewhat larger than those reported in the TEM study of 2
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Fig. 2. Experimental setup of cryogenic EBSD: (a) Cleaved BaFe2As2 single crystal mounted on the sample holder. (b) Side view of the sample holder inside the SEM vacuum chamber.
Fig. 3. Secondary electron images showing (a) bulk single crystal surface at RT and (b) surface artefacts caused by contamination after cooling to 100 K. The bright rectangular area is cleaned by irradiation with the electron beam. The black arrows mark cleavage steps.
Fig. 5. EBSD mapping at RT of BaFe2As2 bulk single crystal. The step size used was 100 nm.
Fig. 4. EBSD pattern of BaFe2As2 and the definition of the angle φ = (φ1+φ2) / 2.
lamella spacing is smaller than 0.5 μm. The existence of vertical TBs on the right side of the domain wall, which corresponds to the (100)tetr plane of the tetragonal BaFe2As2, was proven by EBSD. However, only the type of horizontal TBs is revealed in the FSD image in Fig. 6d. This observation can be explained by the direction of the electron beam vector. On the right side of Fig. 6d the electron beam vector is inside the twin plane. As the twin plane is identical for both orientations their backscatter coefficient is the same, as a consequence these twins are out of contrast. On the left side of this figure the electron beam vector is inside a plane that is oriented slightly different for both orientations,
Ma et al. (2009), however they are much smaller than those reported in the PLM study of Tanatar et al. (2009). Under certain conditions the direct imaging of twins is also possible with the FSD. The contrast is based on electron channeling (Simkin and Crimp, 1999; Zaefferer and Elhami, 2014). Fig. 6d shows a sample area on the BaFe2As2 single crystal, where both types of domains border each other (the domain boundary is marked by the red dotted line). On the left side of Fig. 6d, a pattern of horizontally aligned twin boundaries is revealed. These boundaries correspond to (010)tetr planes of the tetragonal BaFe2As2 lattice (Fig. 1). In this specific specimen area the 3
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Fig. 6. EBSD mapping illustrating a section of a single domain of orthorhombic BaFe2As2 at 100 K with vertical TBs. The color scale denotes the misorientation between 0° and 0.5°, where one of the two orientations is set as reference. The mapping dimensions are 300 × 50 points using 50 nm step size. (b) Enlarged section of (a) with structural model of twins. (c) Sketch of EBSD pattern with (110)orth Kikuchi lines twisted in relation to the (010)tetr one. The twists are strongly overestimated for better visibility. (d) Section of FSD image presenting horizontal twin boundaries (cf. Figs. S1-2). The non-horizontal dark and bright lines are cleavage steps. The blue and green stripes visualize horizontal twins. The red dotted line marks the boundary of the two domain types bordering each other (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
color scale shows 0° to 1° misorientation with respect to the orientation of an arbitrary chosen point of the mapping. The EBSD mappings indicate that irregular shaped areas may have misorientations less than 0.5° on a length scale of 100 nm or less, as some of the pixels show abrupt colour change. Qualitatively this is in agreement with previously published results obtained by high resolution EBSD on similar thin films (Chekhonin et al., 2015). Fig. 10 shows an example distribution of φ along a line. The maximum value of Δφ does not exceed 0.6° for measurements performed at RT (Fig. 10a) and 19 K (Fig. 10b) as well. Example EBSD patterns for this diagram are given in supplementary material (Figs. S5-8). In the bulk single crystals intermediate φ values between -0.45° and 0.45° appear in the vicinity of TBs (cf. Fig. 7) where an overlapping of the EBSD pattern from two neighbouring twins occurs. The minimum distance from the TB that allows EBSD patterns to be obtained with no overlap from the adjacent twin lamella is below about 50 nm for vertically and below about 150 nm for horizontally aligned TBs. Therefore, smaller twin sizes cannot be resolved by conventional misorientation analysis of EBSD patterns. An alternative way to prove a phase transition resulting in structures smaller than the lateral resolution of EBSD is a comparison between EBSD patterns recorded at RT and respectively 19 K averaged over a small sample area. 50 averaged patterns at magnification of 30,000 over the field of view (2.7 × 3.7) μm2 did not exhibit broadening of (100)tetr and (010)tetr Kikuchi bands from RT to 19 K (see Supplementary material, Figs. S9-10). Therefore, it is concluded that a phase transformation in this thin film sample did not occur in that temperature range. The results obtained on the 60 nm thin film are shown in Fig. 11. EBSD mappings were acquired with a step size of 30 nm. The overall distribution in the misorientation mapped on the sample at RT (Fig. 11a) is comparable to that measured at 19 K (Fig. 11b). Several other EBSD mappings and line scans obtained from different locations on the same sample gave similar results. The mappings again contain irregularly shaped areas of the same orientation with misorientations up to about 1° between them. The distribution of φ scatters stronger than that of the 20 nm thin film. Δφ values larger than 0.9° were additionally checked by comparison of corresponding EBSD patterns (see Supplementary material, Figs. S11-14). Only a small area of about (120 × 60) nm2 was found indicating a structural phase transformation. It can be confirmed by switching between two EBSD patterns from this area, making it clearly visible as a twist of the (-110)orth Kikuchi band (see Supplementary material, Figs. S15-16). Additionally, in the
which leads to different backscatter coefficients (see also supplementary material, Figs. S3-4). Low temperature measurements on bulk single crystalline BaFe2As2 have also shown that by crossing twin boundaries φ varies between approximately -0.45° and 0.45° (Fig. 7). This corresponds to a difference Δφ between two adjacent twin lamellae of about 0.9°. In comparison, within a twin lamella Δφ is always below 0.32°. Therefore, the observation of such values in thin films would be a signature of a structural phase transition. 3.2. Single crystalline thin films The following results were obtained on epitaxially grown single crystalline thin film samples. Surface sensitive secondary electron images of the thin films reveal a plane surface (Figs. 8a, b) with atomic steps indicating pure island growth mode. The typical size of growth islands is about 115 and 160 nm in the 20 nm and 60 nm thin film, respectively. In contrary to the 20 nm thin film, the 60 nm thin film (Fig. 8b) is cracked and therefore (partially) strain relaxed (Engelmann et al., 2013). EBSD mappings on the thin films were carried out at RT and at about 19 K. Representative results for the 20 nm thin film are shown in Figs. 9a, b. EBSD mappings were acquired with a step size of 50 nm. The
Fig. 7. Distribution of φ along a line in bulk single crystalline BaFe2As2 below transition temperature. 4
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Fig. 8. Secondary electron images of the surfaces of a (a) fully strained 20 nm and (b) partially relaxed 60 nm thin film.
same way as for 20 nm thin film, the averaged EBSD patterns does not exhibit broadening of (100)tetr and (010)tetr Kikuchi bands from RT to 19 K (see Supplementary material, Figs. S17-18). Therefore, it is concluded that in general the phase transformation is suppressed in this thin film, too. The few areas which appear to undergo a phase transition are likely strain relaxed and probably detached from the substrate/ buffer layer and thus are behaving like bulk BaFe2As2. However, the investigation of phase transition is aggravated by the fact that mosaicity is already pronounced at RT (cf. Fig. 11a).
EBSD experiments on metallic samples where long irradiation times and small step sizes are useful and where strong carbon contamination is a limitation, cooling the sample to low temperatures can provide better conditions. b) True sample temperature: Another important issue is the question about the true temperature on the sample surface after cooling during the measurement. To answer this, the cooling experiment was repeated with a second Si diode functioning as a temperature sensor but with the electron beam off. The Si diode was attached to the copper plate in exactly the same way as mentioned for the bulk single crystalline BaFe2As2 samples. At temperatures above 19 K the temperatures measured by the diode are about 1 K–2 K higher than the regulating temperature measured at the cryostat below the sample holder. However, at temperatures below 19 K this difference is increasing. This experiment showed that the true temperature that can be achieved on the top of the EBSD sample holder with the current setup is about 12 K. To find out the true temperature in the experiments done on the thin films, the Si diode was glued instead of the thin film onto a pure spinel plate of the same thickness like the ones used as thin film substrates and attached in a similar way to the sample holder. The lowest temperature on the spinel plate that was measured in this way is about 19 K. c) Constraints for cryogenic EBSD: The area presented in Fig. 6a contains TBs that are perpendicular to the SEM tilt axis. As the lateral resolution of EBSD is highly anisotropic (Zaefferer, 2007), in the
3.3. Peculiarities of cryogenic EBSD Several experimental issues and results are important and will be discussed in this paragraph. a) Contamination: The effect of disappearing surface contamination and even its reversal (etching) at low temperatures was observed and analyzed in older publications, a review is provided in (Hren, 1979). In a very truncated way, the contamination at RT can be described as the ionization and subsequent polymerization of hydrocarbon molecules by the electron beam leading to an immobile carbon deposit. If the temperature of the specimen surface is low enough, water vapor and presumably other gaseous molecules are adsorbed and may react under the influence of the electron beam with the carbon layer (etching) to volatile molecules, thus effectively cleaning the sample. Therefore, for
Fig. 9. EBSD mappings (50 nm step size) done on the 20 nm thin film: (a) RT, (b) 19 K. 5
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Fig. 10. Distribution of angle φ (20 nm thin film) along a horizontal line scan: (a) RT, (b) 19 K.
d) Verification of phase transition in BaFe2As2: By analyzing the misorientation mappings and distribution of the angle φ between (100)tetr and (010)tetr Kikuchi bands obtained from different areas in the thin film samples at RT, it was found that the values may show noticeable variations. These discrepancies most likely arise as a consequence of the mosaicity of the thin film samples. Because of that the angle φ is a less reliable indicator for the presence of a phase transformation. The latter effect is even more pronounced in the 60 nm thin film where the degree of mosaicity is noteworthy higher than in the 20 nm one. The absence of the structural phase transformation in the 20 nm thin film may be due to epitaxy-induced strains leading to suppression of antiferromagnetic order and induction of superconductivity. However, the structural phase transformation appears to be also suppressed in most areas of the partially relaxed 60 nm thin film.
present case the resolution across the TBs is about a few 10 nm. Therefore, twin lamellae aligned in this way can be resolved if they are not smaller than about 50 nm. Since there are no contamination issues, an even lower acceleration voltage in combination with smaller step size could be applied, thus reducing this value even further. However, there are sample positions were the TBs are aligned parallel to the tilt axis (like in the left side of Fig. 6d). In this case, using 70° as tilt angle the lateral resolution is worse by about a factor of 3. In comparison to conventional EBSD at RT there are several noteworthy drawbacks regarding the setup presented. The most important ones are the missing rotation and tilt axes in the SEM stage used, but also a limited measurement time (defined by the liquid helium storage and stability issues after cooling) and a significantly higher measurement effort (issues regarding preparation, set-up time and dismounting) making cryogenic EBSD experiments much more time and material consuming.
Fig. 11. EBSD mappings (30 nm step size) done on the 60 nm thin film and distribution of angle φ along a horizontal line scan: (a, c) RT, (b, d) 19 K. 6
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4. Conclusions
K., Hühne, R., Hänisch, J., Hoffmann, M., Kurth, F., Schultz, L., Holzapfel, B., 2013. Strain induced superconductivity in the parent compound BaFe2As2. Nat. Commun. 4, 2877. Goldman, A.I., Argyriou, D.N., Ouladdiaf, B., Chatterji, T., Kreyssig, A., Nandi, S., Ni, N., Bud’ko, S.L., Canfield, P.C., McQueeney, R.J., 2008. Lattice and magnetic instabilities in CaFe2As2: a single-crystal neutron diffraction study. Phys. Rev. B 78 100506. Harland, C.J., Akhter, P., Venables, J.A., 1981. Accurate microcrystallography at high spatial resolution using electron back-scattering patterns in a field emission gun scanning electron microscope. Sci. Instrum. 14, 175–182. Hren, J.J., 1979. Barriers to AEM: contamination and etching. In: Hren, J.J., Goldstein, J.I., Joy, D.C. (Eds.), Introduction to Analytical Electron Microscopy, Plenum Pr. pp. pp. 481–505 New York. Huang, Q., Qiu, Y., Bao, W., Green, M.A., Lynn, J.W., Gasparovic, Y.C., Wu, T., Wu, G., Chen, X.H., 2008. Neutron-diffraction measurements of magnetic order and a structural transition in the parent BaFe2As2 compound of FeAs-based high-temperature superconductors. Phys. Rev. Lett. 101 257003. Humphreys, F.J., 2004. Characterisation of fine-scale microstructures by electron backscatter diffraction (EBSD). Scr. Mater. 51, 771–776. Ma, C., Yang, H.X., Tian, H.F., Shi, H.L., Lu, J.B., Wang, Z.W., Zeng, L.J., Chen, G.F., Wang, N.L., Li, J.Q., 2009. Microstructure and tetragonal-to-orthorhombic phase transition of AFe2As2 (A = Sr, Ca) as seen via transmission electron microscopy. Phys. Rev. B 79 060506. Paglione, J., Greene, R., 2010. High-temperature superconductivity in iron-based materials. Nat. Phys. 6, 645–658. Randle, V., 2000. Theoretical framework for electron backscatter diffraction. In: Schwartz, A.J., Kumar, M., Adams, B.L. (Eds.), Electron Backscatter Diffraction in Materials Science. Kluwer Academic/Plenum Publishers, New York, pp. pp. 19–30. Rotter, M., Tegel, M., Johrendt, D., 2008a. Superconductivity at 38 K in the iron arsenide (Ba1−xKx)Fe2As2. Phys. Rev. Lett. 101 107006. Rotter, M., Tegel, M., Schellenberg, I., Hermes, W., Pöttgen, R., Johrendt, D., 2008b. Spindensity-wave anomaly at 140 K in the ternary iron arsenide BaFe2As2. Phys. Rev. B 78 020503. Simkin, B.A., Crimp, M.A., 1999. An experimentally convenient configuration for electron channeling contrast imaging. Ultramicroscopy 77, 65–75. Tanatar, M.A., Kreyssig, A., Nandi, S., Ni, N., Bud’ko, S.L., Canfield, P.C., Goldman, A.I., Prozorov, R., 2009. Direct imaging of the structural domains in iron pnictides AFe2As2 (A = Ca, Sr, Ba). Phys. Rev. B 79, 180508. Wu, J.J., Lin, J.F., Wang, X.C., Liu, Q.Q., Zhu, J.L., Xiao, Z.M., Chow, P., Jin, C., 2013. Pressure-decoupled magnetic and structural transitions of the parent compound of iron-based 122 superconductors BaFe2As2. Proc. Nat. Acad. Sci. 110, 17263–17266. Zaefferer, S., 2007. On the formation mechanisms, spatial resolution and intensity of backscatter Kikuchi patterns. Ultramicroscopy 107, 254–266. Zaefferer, S., Elhami, N.N., 2014. Theory and application of electron channelling contrast imaging under controlled diffraction conditions. Acta Mater. 75, 20–50.
The structural phase transformation in undoped BaFe2As2 samples was investigated in a SEM at low temperatures applying EBSD. Bulk single crystals undergo a tetragonal to orthorhombic phase transition at low temperature resulting in regular twin lamellae. Spatially highly resolved EBSD mappings and FSD imaging provide additional information on the structure compared to previous TEM and light microscopy based analyses. Measurements done on BaFe2As2 thin films generally indicate the suppression of a structural phase transition. Acknowledgments The authors thank M. Siegel, M. Petrovsky and D. Andris for technical assistance and Dr. V. Grinenko for many fruitful discussions. This work was supported by the DFG via Graduate Training Group GRK 1621. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.micron.2018.12.009. References Alireza, P.L., Ko, Y.T.C., Gilett, J., Petrone, C.M., Cole, J.M., Lonzarich, G.G., Sebastian, S.E., 2008. Superconductivity up to 29 K in SrFe2As2 and BaFe2As2 at high pressures. J. Phys. Condens. Matter 21 012208. Aswartham, S., Nacke, C., Friemel, G., Leps, N., Wurmehl, S., Wizent, N., Hess, C., Klingeler, R., Behr, G., Singh, S., Büchner, B., 2011. Single crystal growth and physical properties of superconducting ferro-pnictides Ba(Fe, Co)2As2 grown using selfflux and Bridgman techniques. J. Cryst. Growth 314, 341–348. Chekhonin, P., Engelmann, J., Langer, M., Oertel, C.G., Holzapfel, B., Skrotzki, W., 2015. Strain inhomogeneities in epitaxial BaFe2As2 thin films. Cryst. Res. Technol. 50, 891–902. Dingley, D., 2004. Progressive steps in the development of electron backscatter diffraction and orientation imaging microscopy. J. Microsc. 213, 214–224. Engelmann, J., Grinenko, V., Chekhonin, P., Skrotzki, W., Efremov, D., Oswald, S., Iida,
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Micron journal homepage: www.elsevier.com/locate/micron
Direct study of structural phase transformation in single crystalline bulk and thin film BaFe2As2
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A. Pukenasa, , P. Chekhonina, M. Meißnerb, E. Hieckmannb, S. Aswarthamc, J. Freudenbergerc, J. Engelmannc, R. Hühnec, S. Wurmehlc, B. Büchnerc, W. Skrotzkia a
Institut für Festkörper- und Materialphysik, Technische Universität Dresden, 01062 Dresden, Germany Institut für Angewandte Physik, Technische Universität Dresden, 01062 Dresden, Germany c IFW Dresden, 01069 Dresden, Germany b
A R T I C LE I N FO
A B S T R A C T
Keywords: BaFe2As2 low temperature Superconductivity Phase Transformation Cryogenic EBSD
The ternary iron arsenide compound BaFe2As2 exhibits a structural phase transition from tetragonal to orthorhombic at a temperature of about 140 K. The twin lamellae arising below this transition temperature were studied in undoped single crystalline bulk and epitaxial thin film samples using electron backscatter diffraction in a scanning electron microscope equipped with a helium cryostat. Applying this technique on bulk single crystals a characteristic twin lamella size in the range of 0.1 μm up to a few μm was observed. In contrast, in epitaxially strained thin films the phase transition is not observed at temperatures above 19 K.
1. Introduction The ternary iron arsenide BaFe2As2 is a parent compound, which belongs to the family of superconducting iron pnictides (Paglione and Greene, 2010). It has been shown that BaFe2As2 becomes superconducting under pressure (Alireza et al., 2008), by chemical substitution (Rotter et al., 2008a) or by the application of in-plain strain through epitaxy (Engelmann et al., 2013). Upon cooling the undoped tetragonal parent compound (space group I4/mmm, a = b = 0.3963 nm and c = 1.3017 nm) exhibits simultaneously a magnetic and diffusionless structural phase transition. Below the structural transition temperature close to 140 K the orthorhombic structure (space group Fmmm, a = 0.5615 nm, b = 0.5574 nm and c = 1.2945 nm) is formed (Rotter et al., 2008b; Huang et al., 2008). The resulting unit cell almost doubles in size and rotates about 45° about the c-axis (Goldman et al., 2008). The formation of the orthorhombic lattice (black lattice in Fig. 1) can be illustrated as a stretching of the tetragonal lattice (red lattice in Fig. 1) along its [110] and [1–10] diagonals, respectively. Microstructural investigations reveal that the low temperature orthorhombic BaFe2As2 consists of two different types of domain patterns which can span the entire crystal surface (Tanatar et al., 2009). Fig. 1 schematically shows the formation of one type of domain, where the common twin plane corresponds to a (100)tetr plane (marked by pink dotted double line) of the former tetragonal lattice. According to polarized light microscopy (PLM) and transmission
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electron microscopy (TEM) studies on BaFe2As2 single crystals, the twin spacing in the orthorhombic structure is in the range of 10–50 μm (Tanatar et al., 2009) and 100–400 nm (Ma et al., 2009), respectively. This large difference may be explained by the used BaFe2As2 crystal growth methods including differences in crystal quality and by the TEM sample preparation technique that may introduce crystal defects. On the other hand, PLM cannot resolve very fine structures (Tanatar, et al., 2009; Ma, et al., 2009). To avoid these problems, scanning electron microscopy (SEM) was applied in the present work to study twin lamellae in single crystalline BaFe2As2. Using electron backscatter diffraction (EBSD) (Dingley, 2004; Randle, 2000) in a SEM the lateral resolution along the EBSD tilt axis is about few 10 nm (Harland et al., 1981; Humphreys, 2004; Zaefferer, 2007). On the one hand, this resolution is higher in comparison to light microscopy based analyses. On the other hand, sample preparation artefacts are avoided which may affect the results of TEM analyses. Therefore, the EBSD technique has promising advantages over the other techniques applied so far. In comparison to conventional EBSD, there are of course experimental difficulties for similar measurements at cryogenic temperatures, which will be discussed here. The second part of the paper focuses on undoped single crystalline BaFe2As2 thin films with different thicknesses and strain states. In contrast to chemical and pressure tuning of the superconducting phase transition, there are neither disorder effects nor sensitivity to the degree of non-hydrostaticity and therefore the crystal structure is well-defined
Correspondence author. E-mail address: [email protected] (A. Pukenas).
https://doi.org/10.1016/j.micron.2018.12.009 Received 9 June 2018; Received in revised form 24 December 2018; Accepted 24 December 2018 Available online 27 December 2018 0968-4328/ © 2018 Elsevier Ltd. All rights reserved.
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attached sample was fixed on top of the sample holder using silver conducting paste. In the final position of the sample the tetragonal BaFe2As2 [100]-direction was aligned parallel to the SEM X- direction (tilt axis for EBSD). To keep sample contamination at a minimum, best possible vacuum conditions were targeted by mounting the stage including the sample into the SEM chamber several days before experiment. The structural characterization of several BaFe2As2 single crystals was performed after cooling them to 100 K. Due to the condensation of residual gas molecules a contamination layer is formed on the sample surface which results in blurred SEM images in comparison to RT imaging (cf. Figs. 3a, b). Since BaFe2As2 is sensitive to humidity and may react with water this may be an explanation for a persistent surface contamination. In the present experimental setup there are parts in the SEM chamber which are definitely colder than the specimen surface, e.g. parts of the cryostat and the helium supply pipes, which are always significantly below 100 K. However, obviously their combined surface area is not large enough and/or too far away from the sample surface in order to be an effective cold trap. After irradiating a small specimen surface area with the electron beam for several minutes, the contamination layer disappears with the exception of a few spots (Fig. 3b). These contamination spots typically have a diameter smaller than 0.5 μm and some of them are thick enough to lower the EBSD pattern quality locally. Otherwise, the EBSD pattern quality was very high in the area irradiated by the electron beam and not prone to carbon contamination like at RT. For indexing the recorded EBSD patterns with the Channel 5 software, a tetragonal unit cell was used with lattice parameters a= b = 0.3963 nm and c = 1.3017 nm. This setting allows to resolve the crystallographic orientation differences between twin lamellae in form of pseudo-rotations in the ab - plane. An arbitrary single point inside one of the twin lamellae was taken as an arbitrary reference for the absolute orientation and each point of the mapping was plotted as relative misorientation to this reference point on a color scale. Of course, the difference in the EBSD patterns is not a rotation, as the Kikuchi band that corresponds to the twin plane remains at the exact same position. Noteworthy, the remaining contamination spots as well as the cleavage steps on the specimen surface did not substantially affect the subsequent evaluation. While the spatial and angular resolution of the EBSD mapping technique is sufficient for imaging of twin lamellae in bulk single crystals, the low small-scale orientation changes in thin films aggravate identification of twin lamellae. In addition to the automated misorientation analysis, several line scans were evaluated by manual straight line fitting to (100)tetr and (010)tetr Kikuchi bands in the EBSD patterns and estimating the angles φ1 and φ2 between (100)/(-100)tetr and (010)/(0–10)tetr Kikuchi lines respectively (Fig. 4). The mean value of the φ1 and φ2 is denoted by φ = (φ1+φ2)/2 and for better comparison between different line scans given as relative degree.
Fig. 1. Formation of an orthorhombic lattice (black lattice points) from a tetragonal one (red lattice points) below the transition temperature. In the orthorhombic phase the unit cell vectors aO and bO are rotated about 45° about the c-axis in comparison to the unit cell vectors aT and bT in the tetragonal phase and the unit cell doubles in size (blue and green rectangles). The pink dotted double line marks a (110)orth twin boundary (TB) (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
(Wu et al., 2013). However, substrate constraints may be responsible for the suppression of structural transformation of thin films. 2. Experimental Plate-shaped single crystals of BaFe2As2 with millimeter size were grown by the self-flux technique as described in (Aswartham et al., 2011). The normal direction of the plates is aligned parallel to the caxis. Specimens for SEM investigations and EBSD studies were prepared at room temperature (RT) by gluing them between two 1 mm thick copper plates using a silver-filled epoxy adhesive. Cleaving the crystals by separating the copper plates results in a clean and undistorted (001) surface parallel to the copper plates. In addition to bulk single crystals, two undoped BaFe2As2 thin films were epitaxially grown by pulsed laser deposition on (001)-oriented single crystalline MgAl2O4 (spinel) substrates with an iron buffer layer in between. The first sample with 20 nm thickness is fully strained along the two in-plane directions resulting in bulk superconductivity, whereas the second one is a 60 nm thin film featuring partial relaxation due to cracks and only signs of a superconducting transition. More details on sample preparation, structural and functional properties are published elsewhere (Engelmann et al., 2013). The characterization of the crystal surfaces as well as orientation imaging microscopy of the domain patterns was done in a Zeiss Ultra 55 scanning electron microscope with a field emission gun operated at 20 kV using a 120 μm aperture. The microscope is equipped with a Nordlys HKL EBSD system. EBSD patterns were recorded and indexed using the Channel 5 acquisition software (HKL Technology). The frontpiece of the EBSD detector additionally contains a forward scatter detector (FSD) enabling backscatter electron imaging of the crystal in the tilted state. Low temperature investigations were carried out on a helium-cooled cryogenic SEM stage (Kammrath and Weiss), which facilitates specimen temperatures down to 5 K by screening the sample with a radiation shield. Originally this stage was not designed for EBSD studies. Therefore, due to geometrical issues a special sample holder of annealed pure copper having a fixed tilt angle of 70° was manufactured (Fig. 2). A custom semiconductor sensor built-in in the cryostat sample holder measures the temperature which can be controlled and kept stable within ± 0.02 K. A further temperature sensor mounted in a small hole drilled into the EBSD sample holder measures the temperature directly below the surface at the position, where the copper plate with the sample is glued. The SEM stage used does not allow tilting and rotation of the sample. Therefore, to mount the sample in a well-defined orientation, the sample (cleaved single crystal or thin film) was analyzed prior to cryogenic EBSD using the conventional SEM stage. In accordance to the sample orientation, then the copper plate with the
3. Results and discussion 3.1. Bulk single crystals Room temperature EBSD misorientation mappings on BaFe2As2 (Fig. 5) bulk single crystals confirm high crystal quality, since the degree of misorientation is between 0° and about 0.25°. At temperatures below the structural transition temperature domains with vertical and horizontal oriented twin lamellae are formed. A section from a domain with vertical twin lamellae is shown in Figs. 6a, b. Fig. 6c schematically demonstrates the corresponding EBSD patterns with twisted (110)orth Kikuchi lines. The width of the twin lamellae ranges from 0.1 μm up to few μm. A few EBSD measurements under comparable conditions done on other BaFe2As2 samples from the same single crystal batch gave comparable widths. Hence, the observed lamella sizes are somewhat larger than those reported in the TEM study of 2
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Fig. 2. Experimental setup of cryogenic EBSD: (a) Cleaved BaFe2As2 single crystal mounted on the sample holder. (b) Side view of the sample holder inside the SEM vacuum chamber.
Fig. 3. Secondary electron images showing (a) bulk single crystal surface at RT and (b) surface artefacts caused by contamination after cooling to 100 K. The bright rectangular area is cleaned by irradiation with the electron beam. The black arrows mark cleavage steps.
Fig. 5. EBSD mapping at RT of BaFe2As2 bulk single crystal. The step size used was 100 nm.
Fig. 4. EBSD pattern of BaFe2As2 and the definition of the angle φ = (φ1+φ2) / 2.
lamella spacing is smaller than 0.5 μm. The existence of vertical TBs on the right side of the domain wall, which corresponds to the (100)tetr plane of the tetragonal BaFe2As2, was proven by EBSD. However, only the type of horizontal TBs is revealed in the FSD image in Fig. 6d. This observation can be explained by the direction of the electron beam vector. On the right side of Fig. 6d the electron beam vector is inside the twin plane. As the twin plane is identical for both orientations their backscatter coefficient is the same, as a consequence these twins are out of contrast. On the left side of this figure the electron beam vector is inside a plane that is oriented slightly different for both orientations,
Ma et al. (2009), however they are much smaller than those reported in the PLM study of Tanatar et al. (2009). Under certain conditions the direct imaging of twins is also possible with the FSD. The contrast is based on electron channeling (Simkin and Crimp, 1999; Zaefferer and Elhami, 2014). Fig. 6d shows a sample area on the BaFe2As2 single crystal, where both types of domains border each other (the domain boundary is marked by the red dotted line). On the left side of Fig. 6d, a pattern of horizontally aligned twin boundaries is revealed. These boundaries correspond to (010)tetr planes of the tetragonal BaFe2As2 lattice (Fig. 1). In this specific specimen area the 3
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Fig. 6. EBSD mapping illustrating a section of a single domain of orthorhombic BaFe2As2 at 100 K with vertical TBs. The color scale denotes the misorientation between 0° and 0.5°, where one of the two orientations is set as reference. The mapping dimensions are 300 × 50 points using 50 nm step size. (b) Enlarged section of (a) with structural model of twins. (c) Sketch of EBSD pattern with (110)orth Kikuchi lines twisted in relation to the (010)tetr one. The twists are strongly overestimated for better visibility. (d) Section of FSD image presenting horizontal twin boundaries (cf. Figs. S1-2). The non-horizontal dark and bright lines are cleavage steps. The blue and green stripes visualize horizontal twins. The red dotted line marks the boundary of the two domain types bordering each other (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
color scale shows 0° to 1° misorientation with respect to the orientation of an arbitrary chosen point of the mapping. The EBSD mappings indicate that irregular shaped areas may have misorientations less than 0.5° on a length scale of 100 nm or less, as some of the pixels show abrupt colour change. Qualitatively this is in agreement with previously published results obtained by high resolution EBSD on similar thin films (Chekhonin et al., 2015). Fig. 10 shows an example distribution of φ along a line. The maximum value of Δφ does not exceed 0.6° for measurements performed at RT (Fig. 10a) and 19 K (Fig. 10b) as well. Example EBSD patterns for this diagram are given in supplementary material (Figs. S5-8). In the bulk single crystals intermediate φ values between -0.45° and 0.45° appear in the vicinity of TBs (cf. Fig. 7) where an overlapping of the EBSD pattern from two neighbouring twins occurs. The minimum distance from the TB that allows EBSD patterns to be obtained with no overlap from the adjacent twin lamella is below about 50 nm for vertically and below about 150 nm for horizontally aligned TBs. Therefore, smaller twin sizes cannot be resolved by conventional misorientation analysis of EBSD patterns. An alternative way to prove a phase transition resulting in structures smaller than the lateral resolution of EBSD is a comparison between EBSD patterns recorded at RT and respectively 19 K averaged over a small sample area. 50 averaged patterns at magnification of 30,000 over the field of view (2.7 × 3.7) μm2 did not exhibit broadening of (100)tetr and (010)tetr Kikuchi bands from RT to 19 K (see Supplementary material, Figs. S9-10). Therefore, it is concluded that a phase transformation in this thin film sample did not occur in that temperature range. The results obtained on the 60 nm thin film are shown in Fig. 11. EBSD mappings were acquired with a step size of 30 nm. The overall distribution in the misorientation mapped on the sample at RT (Fig. 11a) is comparable to that measured at 19 K (Fig. 11b). Several other EBSD mappings and line scans obtained from different locations on the same sample gave similar results. The mappings again contain irregularly shaped areas of the same orientation with misorientations up to about 1° between them. The distribution of φ scatters stronger than that of the 20 nm thin film. Δφ values larger than 0.9° were additionally checked by comparison of corresponding EBSD patterns (see Supplementary material, Figs. S11-14). Only a small area of about (120 × 60) nm2 was found indicating a structural phase transformation. It can be confirmed by switching between two EBSD patterns from this area, making it clearly visible as a twist of the (-110)orth Kikuchi band (see Supplementary material, Figs. S15-16). Additionally, in the
which leads to different backscatter coefficients (see also supplementary material, Figs. S3-4). Low temperature measurements on bulk single crystalline BaFe2As2 have also shown that by crossing twin boundaries φ varies between approximately -0.45° and 0.45° (Fig. 7). This corresponds to a difference Δφ between two adjacent twin lamellae of about 0.9°. In comparison, within a twin lamella Δφ is always below 0.32°. Therefore, the observation of such values in thin films would be a signature of a structural phase transition. 3.2. Single crystalline thin films The following results were obtained on epitaxially grown single crystalline thin film samples. Surface sensitive secondary electron images of the thin films reveal a plane surface (Figs. 8a, b) with atomic steps indicating pure island growth mode. The typical size of growth islands is about 115 and 160 nm in the 20 nm and 60 nm thin film, respectively. In contrary to the 20 nm thin film, the 60 nm thin film (Fig. 8b) is cracked and therefore (partially) strain relaxed (Engelmann et al., 2013). EBSD mappings on the thin films were carried out at RT and at about 19 K. Representative results for the 20 nm thin film are shown in Figs. 9a, b. EBSD mappings were acquired with a step size of 50 nm. The
Fig. 7. Distribution of φ along a line in bulk single crystalline BaFe2As2 below transition temperature. 4
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Fig. 8. Secondary electron images of the surfaces of a (a) fully strained 20 nm and (b) partially relaxed 60 nm thin film.
same way as for 20 nm thin film, the averaged EBSD patterns does not exhibit broadening of (100)tetr and (010)tetr Kikuchi bands from RT to 19 K (see Supplementary material, Figs. S17-18). Therefore, it is concluded that in general the phase transformation is suppressed in this thin film, too. The few areas which appear to undergo a phase transition are likely strain relaxed and probably detached from the substrate/ buffer layer and thus are behaving like bulk BaFe2As2. However, the investigation of phase transition is aggravated by the fact that mosaicity is already pronounced at RT (cf. Fig. 11a).
EBSD experiments on metallic samples where long irradiation times and small step sizes are useful and where strong carbon contamination is a limitation, cooling the sample to low temperatures can provide better conditions. b) True sample temperature: Another important issue is the question about the true temperature on the sample surface after cooling during the measurement. To answer this, the cooling experiment was repeated with a second Si diode functioning as a temperature sensor but with the electron beam off. The Si diode was attached to the copper plate in exactly the same way as mentioned for the bulk single crystalline BaFe2As2 samples. At temperatures above 19 K the temperatures measured by the diode are about 1 K–2 K higher than the regulating temperature measured at the cryostat below the sample holder. However, at temperatures below 19 K this difference is increasing. This experiment showed that the true temperature that can be achieved on the top of the EBSD sample holder with the current setup is about 12 K. To find out the true temperature in the experiments done on the thin films, the Si diode was glued instead of the thin film onto a pure spinel plate of the same thickness like the ones used as thin film substrates and attached in a similar way to the sample holder. The lowest temperature on the spinel plate that was measured in this way is about 19 K. c) Constraints for cryogenic EBSD: The area presented in Fig. 6a contains TBs that are perpendicular to the SEM tilt axis. As the lateral resolution of EBSD is highly anisotropic (Zaefferer, 2007), in the
3.3. Peculiarities of cryogenic EBSD Several experimental issues and results are important and will be discussed in this paragraph. a) Contamination: The effect of disappearing surface contamination and even its reversal (etching) at low temperatures was observed and analyzed in older publications, a review is provided in (Hren, 1979). In a very truncated way, the contamination at RT can be described as the ionization and subsequent polymerization of hydrocarbon molecules by the electron beam leading to an immobile carbon deposit. If the temperature of the specimen surface is low enough, water vapor and presumably other gaseous molecules are adsorbed and may react under the influence of the electron beam with the carbon layer (etching) to volatile molecules, thus effectively cleaning the sample. Therefore, for
Fig. 9. EBSD mappings (50 nm step size) done on the 20 nm thin film: (a) RT, (b) 19 K. 5
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Fig. 10. Distribution of angle φ (20 nm thin film) along a horizontal line scan: (a) RT, (b) 19 K.
d) Verification of phase transition in BaFe2As2: By analyzing the misorientation mappings and distribution of the angle φ between (100)tetr and (010)tetr Kikuchi bands obtained from different areas in the thin film samples at RT, it was found that the values may show noticeable variations. These discrepancies most likely arise as a consequence of the mosaicity of the thin film samples. Because of that the angle φ is a less reliable indicator for the presence of a phase transformation. The latter effect is even more pronounced in the 60 nm thin film where the degree of mosaicity is noteworthy higher than in the 20 nm one. The absence of the structural phase transformation in the 20 nm thin film may be due to epitaxy-induced strains leading to suppression of antiferromagnetic order and induction of superconductivity. However, the structural phase transformation appears to be also suppressed in most areas of the partially relaxed 60 nm thin film.
present case the resolution across the TBs is about a few 10 nm. Therefore, twin lamellae aligned in this way can be resolved if they are not smaller than about 50 nm. Since there are no contamination issues, an even lower acceleration voltage in combination with smaller step size could be applied, thus reducing this value even further. However, there are sample positions were the TBs are aligned parallel to the tilt axis (like in the left side of Fig. 6d). In this case, using 70° as tilt angle the lateral resolution is worse by about a factor of 3. In comparison to conventional EBSD at RT there are several noteworthy drawbacks regarding the setup presented. The most important ones are the missing rotation and tilt axes in the SEM stage used, but also a limited measurement time (defined by the liquid helium storage and stability issues after cooling) and a significantly higher measurement effort (issues regarding preparation, set-up time and dismounting) making cryogenic EBSD experiments much more time and material consuming.
Fig. 11. EBSD mappings (30 nm step size) done on the 60 nm thin film and distribution of angle φ along a horizontal line scan: (a, c) RT, (b, d) 19 K. 6
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4. Conclusions
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The structural phase transformation in undoped BaFe2As2 samples was investigated in a SEM at low temperatures applying EBSD. Bulk single crystals undergo a tetragonal to orthorhombic phase transition at low temperature resulting in regular twin lamellae. Spatially highly resolved EBSD mappings and FSD imaging provide additional information on the structure compared to previous TEM and light microscopy based analyses. Measurements done on BaFe2As2 thin films generally indicate the suppression of a structural phase transition. Acknowledgments The authors thank M. Siegel, M. Petrovsky and D. Andris for technical assistance and Dr. V. Grinenko for many fruitful discussions. This work was supported by the DFG via Graduate Training Group GRK 1621. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.micron.2018.12.009. References Alireza, P.L., Ko, Y.T.C., Gilett, J., Petrone, C.M., Cole, J.M., Lonzarich, G.G., Sebastian, S.E., 2008. Superconductivity up to 29 K in SrFe2As2 and BaFe2As2 at high pressures. J. Phys. Condens. Matter 21 012208. Aswartham, S., Nacke, C., Friemel, G., Leps, N., Wurmehl, S., Wizent, N., Hess, C., Klingeler, R., Behr, G., Singh, S., Büchner, B., 2011. Single crystal growth and physical properties of superconducting ferro-pnictides Ba(Fe, Co)2As2 grown using selfflux and Bridgman techniques. J. Cryst. Growth 314, 341–348. Chekhonin, P., Engelmann, J., Langer, M., Oertel, C.G., Holzapfel, B., Skrotzki, W., 2015. Strain inhomogeneities in epitaxial BaFe2As2 thin films. Cryst. Res. Technol. 50, 891–902. Dingley, D., 2004. Progressive steps in the development of electron backscatter diffraction and orientation imaging microscopy. J. Microsc. 213, 214–224. Engelmann, J., Grinenko, V., Chekhonin, P., Skrotzki, W., Efremov, D., Oswald, S., Iida,
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