Transformational dynamics of BZO and BHO nanorods imposed by Y 2 O 3 nanoparticles for improved isotropic pinning in YBa 2 Cu 3 thin films

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the layered structure of the YBCO.2][23][24] However, the perfectly aligned or small-angle splayed (∼10 o for BZO and ∼13 o for BHO at their optimal growth temperature of 810 o C -830 o C) 1D-APCs may not benefit vortex pinning when H is oriented considerably away from the c-axis. 16,23,25This results in prominent J c peaks at H//c-axis (θ=0) and H//ab-plane (θ=90 o due to the intrinsic pinning) separated by a low J c valley at the θ angles in between.This J c anisotropy with respect to the H orientations may be quantified using the normalized J c variation between the highest J c (J c,max ) at the larger of the J c peaks and the lowest at the J c valley by (J c,max -J c,min )/J c,min .For example: at 77 K, the J c anisotropy is around 80.9% and 95.6%, respectively, at 1 T and 3 T in 2% BZO APC/YBCO nanocomposite films. 16In order to achieve a more isotropic J c , several APC structural control approaches have been explored including double-doping with a combination of 1D-APCs (BZO or BSO or BHO, Ba 2 (Y/Gd)(Nb/Ta)O 6 ) and 0D-APCs (Y 2 O 3 , or (Y/Gd) 2 O 3 ), 15,[25][26][27][28][29][30] and length and splay control of 1D-APCs through alternate/segmented layer deposition. 31These approaches indeed have reduced the J c anisotropy compared to single doped YBCO thin films.Jha et al shows that the J c anisotropy changed from 181% to 82.3% at 77 K and 1 T for 3.0 vol.% BSO +3.0 area % Y 2 O 3 /YBCO film as compared to 3% BSO/YBCO film. 32The reduced J c anisotropy could be attributed to the shorter length of 1D-APCs along the c-axis and suggests that the presence of the secondary APCs may provide a unique approach to control the morphology of the primary APCs for strong and isotropic pinning. 16,29n this paper, we illustrate the alignment of the BZO and BHO 1D-APCs, although having comparable diameters of ∼5-7 nm, 33 can be tuned differently by the addition of secondary APCs of Y 2 O 3 0D-APCs, leading to a different pinning landscape and hence J c (H, θ) behaviors.It should be noted that BZO and BHO have subtle differences both in their lattice mismatches with the YBCO matrix and in their elastic properties.Based on an elastic strain model we developed recently in assessing the materials selection for 1D-APCs, 34 the lattice mismatch between APC and YBCO along with the elastic properties of both were found to affect the morphology of the APCs formed in the APC/YBCO nanocomposite films.By minimizing the elastic energy of the APC/YBCO nanocomposite film, a lattice mismatch vs. elastic constant phase boundary has been obtained between materials that can form c-axis aligned 1D-APCs and those that cannot. 34uantitatively, the rigidity of the c-axis aligned 1D-APCs should be proportional to its distance to the phase boundary.This means that the shorter the distance, the less rigidity is, and the more tunable the 1D-APCs would be by modification of the strain field.To confirm this hypothesis, this work has calculated the rigidity of the BZO and BHO 1D-APCs, and measured the transport J c (H) of YBCO films double-doped with BZO+Y 2 O 3 (BZO DD) or BHO+Y 2 O 3 (BHO DD), respectively.Remarkably, we have found that the BHO 1D-APCs have lower rigidity and confirmed experimentally that a stronger and more isotropic pinning can be obtained in the BHO DD case due to higher tunability of the BHO 1D-APC microstructures.
BZO DD (or BHO DD) samples with 2 vol.% BZO (or BHO) mixed with 3 vol.%Y 2 O 3 were fabricated at 825 o C for BZO and 810 o C for BHO respectively, with oxygen partial pressure of 300 mTorr using pulsed laser deposition on (100) SrTiO 3 (STO) single crystal substrates.A Lambda Physik LPX 300 KrF excimer laser (248 nm) was utilized for the depositions at fluence of approximately 1.6 J/cm 2 on target and repetition frequency of 8.0 Hz.All samples were further annealed for 30 minutes at 500 °C within an oxygen atmosphere.The film thicknesses were determined using a Tencor profilometer, with an average thickness of 160 nm for BHO DD and 140 nm for BZO DD thin films.
For electrical transport measurements, samples with two parallel microbridges of 500 µm (length) x20 µm or 40 µm (width), respectively, were patterned using photolithography.Quantum Design Evercool II Physical Properties Measurement System (PPMS) with a vibrating sample magnetometer was employed for measuring T c and the transport J c (H, θ) as function of temperature T, magnetic field H (up to 9.0 T).Current-voltage characteristics were measured using Keithley 2430 1 KW pulsed current source meter and HP 34420.J c was determined by applying the standard 1 µV/ cm criterion.
Figure 1 shows the lattice mismatch |f 1 /f 3 | vs. elastic constant phase diagram for several APC materials in YBCO matrix calculated using the elastic strain model. 34The f 1 and f 3 represent High crystallinity was confirmed in the X-ray diffraction (XRD) θ-2θ spectra for the BZO DD (black) and BHO DD (red) samples.Both of them are c-axis-oriented with c-axis lattice constant of 1.172 nm BZO DD and 1.177 nm for BHO DD.These slightly larger c-axis lattice constants than that of the undoped YBCO film are expected from the coherent interfaces between the BZO (or BHO) with the YBCO matrix. 24,35,36The insets of Figure 2a  the BZO DD and BHO DD samples, respectively.A considerably larger full-width-at a half maximum (FWHM) of the former (0.839 deg), than that of the latter (0.353 deg) indicates the YBCO lattice is disturbed (such as ab-plane buckling), considerably more in the BZO DD sample.This may explain its more severe T c degradation to 85.71 K as compared to 87.08 K of the BHO DD sample (R-T curve Figure S2 of the supplementary material).Figure S1 of the supplementary material compares transmission electron microscopy (TEM) images taken on both BZO DD and BHO DD samples.In both cases, massive large nanoparticles of a several nanometers in diameter can be clearly seen in addition to the BZO and BHO 1D-APCs aligned in the c-axis of YBCO films.This confirms the mixed APC morphologies in the BZO DD samples reported earlier by Maiorov et al and a similar APC landscape in the BHO DD samples.
Figure 3 compares the J c (H) curves of the BZO DD and BHO DD samples at 77 K and 65 K, respectively, at θ=0 o (H//c-axis), θ=90 o (H//ab-plane), and θ=45 o (H at the mid-point between the c-axis and ab-plane).At H//c-axis (Figure 3a), the c-axis aligned BZO 1D-APCs (or BHO 1D-APCs) are anticipated to provide strong correlated pinning at up to the so-called matching fields determined by the areal density of the 1D-APCs.This enhanced correlated pinning is typically illustrated in the smaller H-field susceptibility, or the low α values of J c (H) ∼ H -α , 25,37 as compared to α ∼ 0.5 for the undoped YBCO. 38At 77 K, the α value is ∼0.49for BZO DD and ∼0.27 for the BHO DD sample, indicating more enhanced pinning in the latter.The α value of the BHO DD sample is ∼0.27, which is considerably lower than that (∼0.32) of the BHO SD sample, 39 indicative of the improved pining in the former.One possible explanation of the negligible lower pinning efficiency of the BZO 1D-APCs may be the lower T c too close to 77 K at which J c (H) was measured.This same T c effect may explain the lower J c (H) observed on the BZO DD sample at θ=45 o (Figure 3b) and θ=90 o (Figure 3c) at 77 K.At 77K and 1.0 T, J c (H//c-axis) for the BHO DD (∼0.24 MA/cm 2 ) is lower than ∼ 7.0 mol % BZO + 7.0 mol%Y 2 O 3 DD sample (0.45 MA/cm 2 ) 40 and 3 wt.%BZO+2 wt.% Y 2 O 3 DD sample (0.4 MA/cm 2 ). 41Although the J c value in BHO DD sample is not superior to BZO DD, 41 the lower α value for BHO DD (0.27) compared BZO DD (0.31) 41 indicates the slightly stronger vortex pinning in the former.In addition, the higher H max of ∼3.5 T for BHO DD film as compared to BZO DD film's 3.0 T 41 indicates a higher concentration of strong APCs in the former.
This T c effect becomes insignificant at 65 K, which is consistent with the comparable J c (H) curves observed on the two samples.At H//c-axis, the J c (H) for the BZO DD sample is even slightly higher than that of BHO DD sample in the field range of 0-9.0 T (Figure 3a).Moreover, comparable α values of 0.17 for BZO DD and 0.16 for BHO DD samples were obtained from the J c (H) ∼ H -α fitting at H//c-axis.This suggests the most probable explanation for the slightly lower J c (H) in the BHO DD sample is the more disturbance of the BHO 1D-APC alignment in the c-axis.The effectively longer length of the former provides more effective pinning at H//c-axis and result in an overall higher J c (H) at 65 K.
The less c-axis alignment in the BHO DD case implies a possible enhanced pinning as H is not oriented at H//c-axis.Indeed, a higher J c (H) at H ≥ 2.0 T for BHO DD sample was observed at 65 K at both θ=45 o (Figure 3b) and θ=90 o (Figure 3c) due to a crossover of the J c (H) curves for these samples at ∼ 2.0 T. The trends in the θ=45 o and θ=90 o cases indicate the BHO APCs enhance the pinning at H//ab-plane and also at angles in between the c-axis and ab-plane.This enhancement in such a wide angle range is unlikely associated to the small angle splay of the BHO 1D-APCs around the c-axis.Rather, we argue that the presence of the secondary Y 2 O 3 0D-APCs affects the kinetic diffusion process during the BHO 1D-APCs formation along the c-axis, resulting in many shorter, misaligned segments of the BHO 1D-APCs.In a comparative study we did earlier on BZO SD and BZO DD cases, it was found that the BZO 1D-APC length can be reduced by a factor 2-3 from the former to the latter case at the same growth conditions. 16In addition, the shorter segments of the 1D-APCs tend to misalign away from the c-axis.While a similar effect is expected to happen in the BHO DD case, the level of the misalignment could be more significant considering the lower rigidity of the BHO 1D-APCs than that of the BZO 1D-APCs as shown in Figure 1.
There are two benefits from less c-axis alignment of the 1D-APCs.First, it may result in reduced strain on the YBCO lattice, which seems consistent with the 1.3 K higher T c observed on BHO DD sample as compared with its BZO DD.This leads to significantly higher J c (H) of BHO DD samples at higher temperatures, such as 77 K as shown in Figure 3. On the other hand, it provides isotropic pinning at other H orientations away from H//c-axis.Indeed, at 65 K, significantly lower α values of 0.25 (θ = 45 o ) and 0.19 (θ = 90 o ) were observed for the BHO DD sample compared to that of 0.36 and 0.39 for the BZO DD counterpart.In particular, this results in smaller anisotropy of the α value at different H orientations.
Figure 4 compares the pinning force density (F p ) calculated from F p =J c x H at 77 K and 65 K as function of H at different H-field orientations of θ=0 o , 45 o and 90 o , respectively.All F p (H) curves exhibit a typical inverted bell shape.The H max at the peak of the F p (F p,max ) corresponds to the matching field associated to the areal density of the strong pins, such as BZO and BHO APCs, in this work.At θ=0 o (Figure 4a), the H max values for the BZO DD and BHO DD samples are comparable at both 77 K and 65 K.This suggests the concentrations BZO-1D APCs and BHO-1D APCs aligned in the c-axis are reasonably close to each other.However, the F p,max value of 52.48 GN/m 3 for BZO DD film is about 1.5 times higher than that of the BHO DD counterpart at 65 K, at which point the T c effect is negligible.The comparable H max and higher F p,max values in the former confirm our earlier argument on the effectively longer length of the c-axis aligned BZO 1D-APCs.
This trend is certainly altered at θ=45 o and 90 o as shown in Figures 4b and 4c.In both cases, higher H max and F p,max values were observed on BHO DD films at both 77 K and 65 K.At 65 K and at θ=45 o , the H max ∼ 8.5 T in the BHO DD film is more than twice of the ∼4.0 T for the BZO DD film (Figure 4b).In addition, the F p , max ∼26.71GN/m 3 for the BHO DD sample is 1.14 times higher than that of 22.43 GN/m 3 for the BZO DD counterpart.Since a similar behavior occurs also at H//ab-plane (Figure 4c), the stronger pinning in the BHO DD case may be attributed most probably to the shorter, misaligned BHO APCs.Inset of Figure 4b shows that the F p,max and H max both increase monotonically with the decreasing temperatures for BHO DD sample at θ=0 This argument is supported by the more isotropic J c (θ) curves observed on BHO DD samples as compared to the BZO DD ones at both 77 K (Figure 5a) and 65 K (Figure 5b).A major difference is in the absence of the J c valley as θ is in between θ=0 • and θ=90 • for the BHO DD samples.The appearance of the J c valley is the consequence of weaker pinning in this angle range as compared to the stronger pinning at θ=0 due to the aligned 1D-APCs and at θ=90 • due to the ab-plane intrinsic pinning.This leads to the J c anisotropy with respect to H orientations and the larger the difference between the J c peaks and valley as in most 1D-APC/YBCO nanocomposites, the larger the J c anisotropy. 16,39or example, the J c anisotropy is about 100% for 2.0 vol.%BZO SD films at 77 K and 1.0 T, 16 which is similar to the 3.0 vol.%BSO SD case. 32However, higher J c anisotropy of 159.0% for BZO DD with J c,min = 0.59*10 5 A/cm 2 in this study suggests most BZO 1D APCs remained aligned in the c-axis in the 2.0 vol.%BZO DD samples.An opposite trend was observed in comparison of BHO SD and DD samples.At 77 K and H=1.0 T, the overall J c (θ) values for the entire angular range from θ=0 o (H//c-axis) to θ=90 o (H//ab-plane) are higher in the BHO DD film than in the BHO SD film. 39n addition, the J c anisotropy is about 41% for the BHO DD film, which is about a half of ∼80% for the BHO SD film. 42This result seems consistent with the higher orientation tunability of the BHO 1D-APCs in the DD case predicted in Fig. 1.The J c anisotropy is about 40% for the BHO DD samples measured at 77 K at 1 T. The anisotropy reduces further to about 18% for the BHO DD films at 65 K and 9.0 T (red diamonds), in contrast to about 100% for the BZO DD counterpart (black diamonds) as shown in Figure 5b.This result illustrates the importance of engineering the APC morphology and orientation.In summary, a comparative study has been carried out to probe the tunability of the BZO and BHO 1D-APCs alignment along the c-axis by a secondary Y 2 O 3 0D-APCs in BZO DD/YBCO and BHO DD/YBCO nanocomposite films.Since this tunability correlates to the microstructure of the APCs, which in turn affects the J c anisotropy with respect to the H orientations, transport J c was measured in the temperature range of 50-77 K at H up to 9.0 T, and different H orientations.The experimentally observed higher tunability of the BHO APCs by the Y 2 O 3 0D-APCs seems to agree well with the theoretically predicted lower rigidity of the BHO 1D-APCs as compared to the BZO 1D-APC counterpart's, and lead to achievement of much smaller J c anisotropy due to formation of short, unaligned BHO APCs that enhance pinning at most H orientations.

FIG. 1 .
FIG. 1. Calculated (f 1 /f 3 ) vs (C 11 -C 12 2 /C 11 ) dopant phase boundary (solid line) above which the dopant materials energetically prefer to form c-axis aligned 1D-APCs in YBCO thin films.f 1 and f 3 are the lattice mismatch of film and dopants along a-and c-axes respectively.C 11 and C 12 are elastic constants of the dopant.The points for Y 2 O 3 and CeO 2 are below the limit of the y-axis.