Fabrication of grain boundary junctions using NdFeAs(O,F) superconducting thin films
Taito Omura, Takuya Matsumoto, Takafumi Hatano, Kazumasa Iida, Hiroshi Ikuta
FFABRICATION OF GRAIN BOUNDARY JUNCTIONS USINGNdFeAs(O,F) SUPERCONDUCTING THIN FILMS
Taito Omura, ∗ Takuya Matsumoto, TakafumiHatano,
1, 2
Kazumasa Iida,
1, 2 and Hiroshi Ikuta
1, 2 Department of Crystalline Materials Science, Nagoya University,Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan Department of Materials Physics, Nagoya University,Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan (Dated: November 10, 2018)We report on the growth of NdFeAs(O,F) thin films on [001]–tilt MgO bicrystal substrates withmisorientation angle θ GB = 6 ◦ , ◦ , ◦ and 45 ◦ , and their inter- and intra-grain transport properties.X-ray diffraction study confirmed that all our NdFeAs(O,F) films are epitaxially grown on the MgObicrystals. The θ GB dependence of the inter-grain critical current density J c shows that, unlikeCo-doped BaFe As and Fe(Se,Te), its decay with θ GB is rather significant. As a possible reason ofthis result, fluorine may have diffused preferentially to the grain boundary region and eroded thecrystal structure. I. INTRODUCTION
It is very important to examine the current-limiting effect of grain boundaries (GBs) forfabricating superconducting wires and tapesbased on high critical temperature ( T c ) su-perconductors. For cuprate superconductorYBa Cu O − δ (YBCO), the J c characteristicacross the grain boundary ( J interc ) has beenintensively investigated using a bicrystal sub-strate in which two single crystals with dif-ferent orientations are thermally jointed. Al-though J interc of YBCO does not decrease withincreasing the misorientation angle θ GB untila critical angle θ c (3 ◦ ∼ ◦ ), J interc attenuatesexponentially with θ GB above θ c . Therefore,for practical use as superconducting tapes, bi-axially textured YBCO conductors whose in-plane orientation distribution is smaller than θ c are required. However, this demands acostly fabrication process and delayed signif-icantly conductor applications.Iron-based superconductors (FBS) form amaterial class that has a high T c next tothe cuprates and a high upper critical field( H c2 ) exceeding 100 T with a moderate H c2 anisotropy. Regarding the grain boundarycharacteristics, Katase et al . reported that θ c is as large as 9 ◦ for Co-doped BaFe As , whichis about twice larger than that of YBCO. Inaddition, the attenuation of J interc above θ c wasmore moderate than YBCO. As a result, J interc of Co-doped BaFe As was higher than that ofYBCO at 4 K and θ GB ≥ ◦ . Our group re-ported that P-doped BaFe As showed also aslow decay of J interc with θ GB and a high J interc of 10 A/cm for θ GB = 24 ◦ at 4 K, whichis the highest value ever reported for YBCOand FBS for this misorientation angle. Sim-ilar to the BaFe As system, the transitionfrom strong-link to weak-link occurs at θ c ∼ ◦ for Fe(Se,Te) grown on SrTiO bicrystals. Allthese results are very promising for high fieldconductor applications.Among the various FBSs discovered to date, Ln Fe P n (O,F) ( Ln : lanthanoid, P n : pnictogen,and 1111) has the highest T c up to 58 K, indicative of potentially high J c . Howeverthe characteristics of grain boundary has notyet been examined in detail since the crystalgrowth of this system is very difficult. Wehave succeeded in growing high quality epitax-ial NdFeAs(O,F) thin films by molecular beamepitaxy (MBE). This opened the opportu-nity to fabricate 1111 films on bicrystal sub-strates. Here, we report on the transport prop-erties of NdFeAs(O,F) films grown on bicrystalsubstrates with various misorientation angles.
II. EXPERIMENTAL DETAILS
NdFeAs(O,F) thin films were grown byMBE. Solid sources of Fe, Fe O , As, NdF , a r X i v : . [ c ond - m a t . s up r- c on ] J u l IG. 1. A schematic illustration of the micro-bridge for transport measurements. V GB and I GB are the voltage and current contacts for the GBjunction, while V NGB and I NGB are those for theintra-grain bridge. and Ga were charged in Knudsen cells. Here,Ga was used as a F-getter to control theamount of fluorine. The films were grown on6 ◦ , 12 ◦ , 24 ◦ and 45 ◦ [001]–tilt MgO bicrys-tal substrates and the substrate temperaturewas fixed at 800 ◦ C. We have first grown a Nd-FeAsO parent phase layer, and then NdOFsubsequently. This resulted in a superconduct-ing 1111 film with a high T c as fluorine dif-fused into the 1111 phase from the top flu-oride layer . The film thickness of Nd-FeAsO was about 160 nm and that of NdOFabout 50 nm. Reflection high energy elec-tron diffraction (RHEED) was used for in – situ monitoring the surface structure duringthe thin film growth. Phase purity and crys-talline quality of the obtained thin films wereexamined by X-ray diffraction (XRD) usingCu–K α radiation. Microstructural characteri- zation was performed by transmission electronmicroscope (TEM).After the NdOF over-layer was removedby Ar-ion milling, the films were photolitho-graphically patterned and etched by Ar-ionmilling to form micro-bridges for transportmeasurements. As shown in Figure 1, the filmswere patterned so as to measure the trans-port properties of both grain boundary andintra-grain on the same sample. The bridgeswere 20–40 µ m-wide and 1 mm–long across thegrain boundary and 0.25 mm–long within thegrain. The current–voltage characteristics andthe temperature dependence of resistivity weremeasured by a four-probe method. A crite-rion of 1 µ V/cm was employed for determin-ing intra-grain J c . On the other hand, inter-grain J c was defined as the intersection be-tween E = 0 and a linear fit to the non-ohmiclinear differential region (i.e., the region wherethe electric field E depended linearly on thecurrent density J ). III. RESULTS AND DISCUSSION
Figure 2 shows the result of structural char-acterization by XRD of the NdFeAs(O,F) thinfilm on [001]-tilt MgO bicrystal with θ GB =45 ◦ . The NdFeAs(O,F) film has grown c –axis oriented as revealed by the θ –2 θ scan,Figure 2(a). Impurity phase was observed asthe by-product of chemical reaction betweenNdOF and NdFeAsO. The φ –scan of the 102NdFeAs(O,F) peak showed an eightfold pat-tern with a 45 ◦ separation as shown in Fig-ure 2(b). Hence, the NdFeAs(O,F) film hasgrown epitaxially on the [001]-tilt MgO bicrys-tal. We confirmed that all films on the bicrys-tal substrates with various θ GB were biaxiallytextured.Figure 3 compares the resistivity curves forthe inter-grain bridges with various θ GB . Here θ GB = 0 ◦ is represented by the intra-grainbridge that was fabricated from the Nd-FeAs(O,F) film on the θ GB = 6 ◦ bicrys-tal substrate (see Figure 1). The intra-grainbridge had an onset T c ( T onsetc ) of 47 K andzero T c ( T ) of 46 K. On the other hand, forinter-grain bridges, both T onsetc and T de-creased with increasing the misorientation an-2 IG. 2. (a) θ –2 θ scan of the NdFeAs(O,F) film on [001]–tilt MgO bicrystal with θ GB = 45 ◦ measuredwith the Bragg-Brentano geometry using Cu–K α radiation. (b) φ –scan of the 102 NdFeAs(O,F) peak. gle. Additionally, a finite resistivity even be-low T onsetc was observed for the inter-grainbridges with θ GB ≥ ◦ the as shown in Fig-ure 3(b), whereas the resistivity of the otherbridges dropped well below the instrumentallimitation.To clarify the reason why the bridges withhigher misorientation angles had a finite re-sistivity, we performed microstructural char-acterization by TEM. Shown in Figure 4(a) isthe cross-sectional view of the NdFeAs(O,F)thin film with θ GB ≥ ◦ in the vicinity ofGB region. Clearly, both the NdFeAs(O,F)layer and the MgO substrate around GB aredamaged. Elemental mappings acquired fromthe same area showed the absence of Fe andAs, and the segregation of Mg (Figure 4(b)).Other than the GB region, we did not see suchdamage to the NdFeAs(O,F) film and the sub-strate. These results show that F preferen-tially diffused through GB and thereby erodedNdFeAs(O,F) and MgO. It was reported for a YBCO/Ca-doped YBCO bilayer film thatCa diffused from the over-layer into the grainboundary region of the bottom YBCO layer,causing hole overdoping and consequently in-creased the inter-grain J c . Similarly, fluo-rine had probably diffused more easily alongthe grain boundary in our films, but unlikeYBCO, the excess fluorine gave a negative im-pact on the structural and transport proper-ties because of its strong reactivity.Shown in Figure 5(a) is the E – J character-istics of the inter-grain bridges for various θ GB measured at 4.2 K. For comparison, the data ofthe intra-grain bridge is also plotted. As canbe seen, the intra-grain curve showed a typi-cal power-law behavior (i.e., E ∼ J n ), suggest-ing that J c is limited by the intra-grain itself.On the other hand, the inter-grain bridges for θ GB = 6 ◦ and 12 ◦ showed a linear behavior atlow E due to flux flow along the GB, indica-tive of J c limitation by GB (Figure 5(b)). With further increasing θ GB , a finite resistivity3 IG. 3. (a) ρ – T curves of the inter-grain bridges with various misorientation angles and the intra-grainbridge. (b) Semi-logarithmic plots of the same ρ – T curves. was observed as shown in Figure 3(b), and ac-cordingly, the inter-grain J c could not be eval-uated.Figure 6 shows the inter-grain J c for θ GB =6 ◦ measured at various temperatures withoutapplying a magnetic field. The intra-grain J c of the same film is also plotted for compar-ison. At low temperatures, the self-field J c of the intra-grain exceeded 1 MA/cm . How-ever, J interc reduced by almost 30% comparedto J intrac . Hence, the critical angle is less than9 ◦ , which is different from Co-doped BaFe As and Fe(Se,Te).Figure 7 summarizes the ratio of inter- andintra-grain J c as a function of misorientationangle. For comparison, the data for Co-dopedBaFe As and Fe(Se,Te) are also plotted.As can be seen, the ratio is nearly unity un-til θ GB ∼ ◦ for both Co-doped BaFe As andFe(Se,Te), indicative of strong-coupling be-tween the grains. On the other hand, our Nd-FeAs(O,F) film showed a weak link behavioreven at θ GB = 6 ◦ . However, the present resultmay not reflect the intrinsic GB properties ofNdFeAs(O,F). As stated above, the inter-grain bridges for θ GB ≥ ◦ had a finite resistivitydue to the erosion of GB region by F. Althoughwe observed a non-zero J c for θ GB = 6 ◦ and12 ◦ , it is well likely that fluorine had diffusedin excess along the GB region in these filmsas well and caused some damages. Hence thepresent result of inter-grain J c is probably anunderestimation.To characterize the intrinsic nature of grainboundary for NdFeAs(O,F), the excess F-diffusion to GB should be prevented. Inour earlier studies, the growth temperaturewas 670 ◦ C for both NdFeAsO and NdOF.
Later we found that increasing the growthtemperature improved the crystallinity of the1111 phase, and we have grown both the1111 and NdOF phases at 800 ◦ C in the presentstudy. However, the growth temperature ofNdOF over-layer would not influence the crys-tallinity of the 1111 layer. Hence, suppressingthe excess diffusion of fluorine without com-promising the crystallinity of the 1111 phasemight be achieved by lowering the growth tem-perature of NdOF whilst keeping that of Nd-FeAsO at 800 ◦ C, which would be the direction4
IG. 4. (a) Cross-sectional view of the NdFeAs(O,F) thin film with θ GB = 24 ◦ in the vicinity of thegrain boundary region. (b) Elemental mappings acquired from the same area.FIG. 5. (a) E – J characteristics of the inter-grain bridges for various misorientation angles and theintra-grain bridge. (b) Enlarged view of the E – J curves at low E . IG. 6. Self-field intra- and inter-grain ( θ GB = 6 ◦ ) J c as a function of temperature.FIG. 7. The ratio of inter- and intra-grain J c ( J interc /J intrac ) as a function of θ GB . For com-parison, the data of Co-doped BaFe As andFe(Se,Te) are also shown. of a future study. That method would alsobe useful to improve the performance of 1111-based coated conductors. Previously, we re-ported the field ( H ) dependence of J c for Nd-FeAs(O,F) coated conductors. Although T c was around 43 K, almost twice as high as thatof Co-doped BaFe As , the J c – H performance was inferior to that of Co-doped BaFe As .However, this NdFeAs(O,F) coated conductorwas fabricated at 800 ◦ C. Based on the resultsof the present study, we think that the grainboundaries of the coated conductor had suf-fered damages from excess fluorine, and thesuppression of excess F-diffusion would im-prove the J c – H properties. IV. SUMMARY
NdFeAs(O,F) thin films were grown on 6 ◦ ,12 ◦ , 24 ◦ and 45 ◦ [001]–tilt MgO bicrystal sub-strates and their electrical transport proper-ties were investigated. Structural character-ization by X-ray diffraction revealed that allfilms have grown epitaxially. Finite resistivitywas observed for the inter-grain bridges withhigher misorientation angles ( θ GB ≥ ◦ ).Additionally, by comparing with Co-dopedBaFe As and Fe(Se,Te) grain boundary junc-tions, the inter-grain J c showed a larger decaywith θ GB . As a possible reason of this result,fluorine may have diffused preferentially to thegrain boundary region and degraded the struc-tural and transport properties. The suppres-sion of excess F-diffusion to GB would there-fore be the key for studying the intrinsic grainboundary properties. ACKNOWLEDGMENTS
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