Impact of Co2C Nanoparticles on Enhancing the Critical Current Density of Bi-2223 Superconductor

We have investigated the superconducting properties of nanocomposite pellets made from Bi-2223 and Co2C powders. There is loss of superconducting fraction in the nanocomposites, but the retained superconducting fraction exhibits robust bulk superconducting properties, having Tc ~ 109 K which was found to be comparable to that of the pure Bi-2223 pellet. We found that the composites net magnetization response is a superposition of ferromagnetic and superconducting fractions contributions. We also found the surviving superconducting fraction exhibits a robust Meissner response. In the nanocomposite the irreversibility field of the superconducting fraction at 77 K is found to increase by almost three times compared to the pristine material, thereby showing strong vortex pinning features. We also find a broadened magnetic field regime over which we observe a single vortex pinning regime sustained in the nanocomposite. The critical current density, Jc, of the nanocomposite was found to be approximately five times higher than that of the pristine Bi-2223 pellet at low T. In fact, the enhancement in Jc is most significant in the high T regime, where at temperatures close to Tc in the nanocomposite we see almost two orders of magnitude increase of Jc compared to the pristine Bi-2223 pellet. The larger sized agglomeration of magnetic nanoparticles of Co2C leads to loss of superconductivity in the nanocomposite. However, there are also unagglomerated Co2C nanoparticles distributed uniformly throughout the nanocomposite which acts as efficient pinning centres allowing for collective vortex pinning centres to be retained, even upto temperatures near Tc, and these nanoparticles also do not compromise the bulk Tc of the superconducting fraction. Our study shows that these nanocomposites exhibit enhanced Jc especially in the high T regime are potentially useful for high current applications.


Introduction:
The critical current density (  ) of type II superconductor, is the maximum dissipationless current density a superconductor can sustain.It is a crucial quantity for applications since it clearly indicates how strongly vortices are pinned in a superconductor.Pinning localizes the vortices and prevents dissipation from appearing in a superconductor until  ≤   1,2 .In realistic superconductors, the vortices are trapped or pinned on these sites which can be regions with local crystalline imperfections like regions with dislocations, vacancy sites or sites with impurity atoms 3,4 .In a superconductor, impurities and/or defects are either present naturally or are artificially introduced in the material 1,4,5,6 .It is well known that strong pinning centers are produced via heavy ion irradiation of the superconducting samples and also by artificial nano-structured patterning of superconductors 7 .Often nano -pinning centers whose size is comparable to ξ (the superconducting coherence length) or λ (the superconducting pellets were prepared by subjecting the powders to mechanical pressure without any additional heat or chemical treatment.A comparison of the magnetization (M) versus temperature (T) and M versus field (H) measurements of pristine and composite pellets, shows all the pellets possess similar Tc ~ 109 K.
The magnetization response of these composite samples is complex, where the FM contribution to magnetization i.e., MFM and the superconducting contribution, i.e.,   are mixed.We analyse M(T) and M(H) data to separate out the two contributions.Our study shows about 97% ,97.3% and 98.34% loss by weight of superconducting fraction in the nanocomposite pellets with increasing admixture of Co2C for 0.05%, 2% and 10% respectively.Our analysis as a function of H and T shows that although there has been a significant loss of the superconducting fraction, the shape of the extracted behavior of the magnetization of the superconducting (  ) phase is found to be identical to that of the original superconducting Bi-2223 pellet.This indicates that the FM-SC composite retains the bulk superconducting pinning properties.We see that the superconducting phase in the composite exhibits an enhanced irreversibility, especially in the high  regime near   .At 77 K compared to the pristine Bi-2223 pellet, the irreversibility field increases by almost three times in the composite pellet.From our analysis of the   (, ) extracted from the   (, ), the superconducting phase shows the presence of a strong pinning single vortex pinning (SVP) regime which crosses over at a field  * into a field dependent collective vortex pinning (CVP) regime.Notably, we see a significant enhancement in  * value for the composite pellet in the high T regime of 77 K, compared to the pristine Bi-2223 pellet where the  * value is negligibly small at a similar T range.We find in the composite pellet at 77 K the appearance of a strong single vortex pinning regime extending from 0 <  ≤  * , while this regime is nearly absent in the pristine Bi-2223 pellet.A comparison of the composite with the pristine Bi-2223 pellet in the low  regime, shows the   of the composite increases by a factor of almost 10 times at low T and we observe an increase by almost 100 times at high T (77 K).Our studies show while the superconducting phase in the SC -FM composites made with Bi-2223 NP's although have a reduced superconducting volume fraction, their pinning properties get significantly enhanced, especially in the high T regime near Tc.Such composites are potentially useful for developing superconductors for high current applications.

Co2C and Bi-2223 nanoparticles and their nanocomposite preparation:
Cobalt acetate tetrahydrate [Co(CH3CO2)2⋅4 H2O], tetra ethylene glycol (TEG), sodium hydroxide (NaOH), polyvinylpyrrolidone (PVP), and absolute ethanol from Sigma-Aldrich Co. are used as primary chemicals to synthesize the cobalt carbide nanoparticle clusters.We synthesize the samples via the one-pot polyol reduction process 36,37 .At first, 0.8 g of sodium hydroxide (NaOH) is dissolved into 20 ml TEG in a glass beaker (Borosil) by heating it to 100 °C.Another 30 ml of TEG is poured into a 100 ml European flask containing 2.5 mmol of Co(CH3CO2)2⋅4 H2O and 0.75 g of PVP.The whole mixture is then stirred for 20 minutes at room temperature by using a magnetic stirrer.After mixing it for 20 minutes, it becomes a homogeneous mixture, and then NaOH is poured into the European flask, and we heat the mixture to 373 K for 30 min to remove water from the solution.The solution is heated further till it reaches its boiling point of TEG (583 K) for 1 hour.Then we allow the solution to cool down to room temperature naturally.After that, the solution is poured into methanol, in a test tube and the nanoparticles are precipitated.The precipitated nanoparticles are separated from the solution using a rare earth magnet.The residual solution is drained, and ethanol is added to the precipitate, and this process is repeated for several times.Later, using a vacuum oven, the extracted nanoparticles are dried, and the nanoparticle powders are compacted into pellets weighing 4 mg 36,37 .
Polycrystalline pieces of Bi-2223 (CAN superconductors, Czech Republic) are ground into a fine powder using alumina pestle mortar continuously for 5 hours and repeated for 3 cycles (microstructure details of the polycrystal from which the powders are ground are presented later in fig.2).We prepared two batches using the powders of Bi-2223 and Co2C NP's: Batch-1 has Bi-2223 powder mixed with 0.05% by weight and batch-2 has 2% by weight of Co2C powder, respectively.The powders were mixed continuously for 4 to 5 hours in a pestle mortar and the process was repeated 3 times.A portion of the mixture from each batch was pressed with a hydraulic press to make pellets with dimensions 3 mm × 2 mm × 0.5 mm (batch-1, pellet with 0.05% by wt. of Co2C, 14.4 mg) which we will refer to as 0.05%-Bi-2223 and 4.5 mm × 3 mm × 0.5 mm (batch 2, pellet with 2% by wt.Co2C, 31.7 mg) which we will refer to as 2%-Bi-2223, also give information about the 10% pellet which we refer to as 10%-Bi-2223.Co2C dissociation temperature is between 500 to 600C, so the composite pellets weren't subjected to high-temperature (beyond 450 C) sintering.We also used for our measurements pristine Co2C pellet with dimensions of 2.13 mm × 1.23 mm × 0.36 mm (4 mg) and pristine Bi-2223 (0% Co2C) pellet with dimensions of 2.8 mm × 2.2 mm × 1 mm (25.5 mg), which we will refer to as 0%-Bi-2223.Note that the 0%-Bi-2223 pellet was also prepared from the same batch of pristine Bi-2223 ground powder.

Experimental Details and Microstructure investigation of the pellets:
For preliminary characterization of the samples, Powder X-ray diffraction (XRD) was performed on the as-synthesized powder of Co2C nanoparticles using a Panalytical X-ray diffractometer with Cu Kα radiation  , where D is the average crystallite size,  is the dimensionless shape factor ~ 0.9,  is the full width at half maximum of XRD peak, and  is the Bragg angle.Using the value of 2 42.5 The clustering leads to the collective magnetic behaviour and strong magnetic moment associated with each cluster of Co2C.The magnetic features of these nanoparticles have been explored in detail earlier 37 .The indexing of peaks seen in the powder XRD pattern fits well to the Bi-2223 phase (see fig. 2(a)) 40 and (see supplementary section I) chemically confirms a Bi2Sr2Ca2Cu3O8 (Bi-2223) stoichiometry of the powder, with a small concentration of Pb.From the width of the XRD peak at 59.63° in Fig. 2

Magnetization measurements of the nanocomposite pellets:
The magnetization (M) versus applied field (H) (M-H) hysteresis loops measured at different temperatures (T) for the Co2C NP pellet is shown in Figure 1(d).The loops show FM in the NPs surviving up to room temperature (300 K).The FM hysteresis loop widths remain nearly constant from 5 K up to 300 K. We measure the magnetization of the pellets containing 0%, 0.05%, and 2% Bi-2223.
For low-field M-T measurements, the pellets were first zero field cooled down to 2.3 K and then a low field of 100 G was applied.From the M(T) behaviour in fig. 3  To separate out the contributions to the net  from the FM component (i.e.,   ) and the superconducting contribution, i.e.,   , to a first approximation, we consider  =   +   .Since we already know from fig. 1(d) that   doesn't change significantly with T. At  >   = 109 K, we consider the  =   as   = 0 here.Figure 1(d) shows that   of Co2C NP's is only weakly T dependent (the FM loops change very slightly between low  and 100 K), therefore in fig.2(c Based on similar analysis as above, we find that mixing 10% by wt. of Co2C NP's powder in Bi-2223 powder, the superconducting volume fraction gets suppressed by 98.34% i.e., only 1.66% superconducting volume fraction survives in the sample.It must be mentioned that although there is a loss of superconducting volume fraction, the fraction which retains superconductivity exhibits robust Meissner effect and diamagnetism signatures characteristic of bulk superconductivity.Subsequently we will investigate the pinning in the surviving superconducting phase.Between 0.05% and 2% pellet we influence the pinning properties without destroying superconductivity, a feature we explore in greater detail subsequently.We find that r() decreases with increasing T. Note that the behaviour of superconducting r() with increasing T mimics the () behaviour of the pristine Bi-2223 superconductor.The rate at which r() drops is determined by ratio of the rates at which MSC(T) for the pristine and composite pellets decreases towards zero with increasing T near Tc.The r() is identical for both 0.05% -Bi-2223 and 2%-Bi-2223 pellet, and both curves extrapolate down to zero at a T ~ 110 K  Tc.It is clear that by mixing the Co2C NP's in Bi-2223, although there is some loss of superconducting phase due to clustering of Co2C, the Tc of the retained superconducting fraction is however unaffected compared to the pristine 0%-Bi-2223 pellet.We now investigate the effect on pinning in the superconducting fraction due to the presence of Co2C NP's distributed across the pellet.

Exploring the pinning properties of the nanocomposite and pristine pellets:
In order to explore the pinning properties of the nanocomposite and pristine Bi-2223 pellets , we analyse the magnetization hysteresis loops of these systems.The Figs. 4(a Note that above a field marked the penetration field, Hp, the M(H) curve begins to deviate from linearity and the diamagnetism decreases due to the penetration of vortices in the pellet.The deviation from linear M(H) behaviour represents the presence of a penetration field   = 0.067 T (at 5 K), which is typical feature of bulk superconductors.Usually   is higher than  1 due to demagnetization, surface barrier, uniformity of sample edges, effects 41 .The presence of a penetration field feature along with a linear M(H) curve at  <   , suggests the presence of robust Meissner diamagnetic response in the composite (similar feature is also seen in fig. 5 for the 2%-Bi-2223 pellet).The presence of a robust Meissner effect feature in M(H), along with signatures of a bulk penetration field suggests that although in the SC-FM composite pellet, the superconducting volume fraction has shrunk, the retained superconducting phase in the pellet still possesses features of robust bulk superconductivity with a welldefined bulk   .The nature of the superconducting response we observe is one of macroscopic phase coherent superconductivity and is not associated with features related to weak superconducting fluctuations present locally in the pellet and where macroscopic phase coherent superconductivity is absent.The M(T) of the 0.05%-Bi-2223 pellet shows that instead of M(H) remaining in the fourth quadrant of the plot (viz., in the diamagnetic sector with a -|M| value), it moves into the first quadrant (+|M| value) for H > 0.5 T (see inset and main panel of Fig. 4(a)).While the curve displays a hysteresis between the forward (f) and reverse (r) legs, the overall shape of the () loop is distorted compared to the M-H curve of a pristine HTSC superconductor.Here too the observed distortion of the M(H) curve is related to the FM Co2C NP's contributing to M(H).In order to extract out the magnetization versus H response of the superconducting fraction, i.e.,   (), we use a method which has been employed in the past for other superconducting-magnetic systems 42  ).This feature again confirms the macroscopic bulk superconductivity present in the superconducting fraction present in the composite pellet (and its not superconducting fluctuations).Due to the relatively weak hysteresis width of Co2C NP's M(H) about its   (), it doesn't affect the MSC(H) behaviour we have obtained.It is important to note that Figures 4 and 5 represents the un-normalized M values in e.m.u units for the pristine 0%-Bi-2223 pellet as well as the 0.05 % and 2 % -Bi-2223 pellets (Fig. 5).Therefore, one should not attempt to compare the magnitude of the M values for the 0.05 % and 0 % -Bi-2223 pellets in these figures (and 0%-Bi-2223 pellet and 2%-Bi-2223 pellet in Fig. 5).Note that as the superconducting fraction is very small for the 10%-Bi-2223 pellet therefore the hysteresis in Msc(H) is small (especially at high T in the range of 77 K).Hence we have restricted our analysis to the 0.05% and 2%-Bi-2223 pellet here, as well as for our subsequent analysis.T the pristine pellet exhibits a diamagnetic, reversible M(H) response between 0.3T upto 1T.At 77 K below Hirr, the pristine pellet has a weak irreversibility (fig.4(e) inset).By comparing the shape of the MSC(H) hysteresis width for 0.05%-Bi-2223 pellet (Fig. 4(f)) with that of the 0%-Bi-2223 pellet (Fig. 4(e) inset), we see that at 77 K the vortex pinning strength of the superconducting state in the composite pellet at 77 K has significantly enhanced compared to the pristine pellet.Note that in 0.05%-Bi-2223 pellet the Hirr(77 K) has increased by three times to 0.9 T compared to that in the pristine 0%-Bi-2223 pellet.There is also a significant enhancement in the M(H) hysteresis loop width at 77 K in the 0.05%-Bi-2223 pellet.Hence the effect of mixing Co2C in Bi-2223 on pinning in the high T regime of the HTSC seems to be very significant, especially in the high T regime.Note that similar analysis as above was for the 2%-Bi-2223 pellet (cf.Fig. 5).By determining Mavg(H) from the M(H) (following the same procedure outlined for fig.4), we obtain the behaviour of MSC(H) at different T for the 2%-Bi-2223 pellet.The behaviour of MSC(H) at different T seen in Fig. 5 for the 2%-Bi-2223 pellet is like that in Fig. 4 for the 0.05%-Bi-2223 pellet.

Understanding the enhancement of Jc different pinning regimes of Co2C-Bi-2223 nanocomposites:
Using the MSC(H) determined for the composite as well as pristine pellets and superconducting volume fraction determined using the r() of Fig. 3(d), we estimate the width of the hysteresis, M(H), in A.m -1 units, at different T. From ∆M we estimate the bulk critical current density (Jc) as a function of H at different T, using the expression   = 20 ∆/[(1 − /3)] where  and  are the crystal dimensions perpendicular the applied  43,44 .Using this for the 0%-Bi-2223 pellet the estimated   = 10 6 A.m -2 (Fig. 6(a)) in nominally zero magnetic field at 77 K, compares well with the value of   = 5.7 x 10 6 A.m -2 at 77 K determined using I-V measurements 45 .This shows that the process of mechanically grinding and pelletizing doesn't significantly modify the Jc of the 0%-Bi-2223 pellet compared to the unground Bi-2223 polycrystal.It is immediately clear from fig. 6 there is a significant enhancement in Jc(H) over all H and also at different T (of 5 K and 77 K) for composite pellet.While we will return to this feature later; however below we analyse the Jc(H) behaviour in greater details.For the 0%-Bi-2223 pellet at 5 K, the Jc(H) shows a field-independent nature in the low H regime (shown with the horizontal dashed line).This is the strong single vortex pinning (SVP) regime, where due to weak intervortex interactions at low H (spacing between vortices >> ) the weakly interacting vortices get individually very strongly pinned 1 .In this regime, due to low inter-vortex interactions, the Jc(H) exhibit weak H dependence. Increasing H leads to stronger intervortex interaction between closely spaced vortices thereby enhancing the collective rigidity of the vortex state.This rigid vortex state exhibits weaker effective pinning.The CVP regime is characterized by a field dependence of the form, Jc(H)  H - , where  is a positive constant 46,47,48,49,50,51,52 .We see in fig.6(a1) that beyond a field  * , there is a transformation from SVP regime to the collective vortex pining (CVP) regime.In the 0%-Bi-2223 pellet the value of  ~0.45  0.02 at 5 K is within the range of value  found in the CVP regime 46,47,48,49,50,51,52 .
At 77 K we find an enhanced  ~ 1.49  0.08 (~1.5) at 77 K.Note that at higher T (as seen at 77 K) there is the added effect of thermally induced suppression of vortex pinning which produces a faster rate of suppression of   with increasing H, i.e., | (  ) (log ) | = , compared to that at lower T. The effects of thermal smearing of pinning at high T is seen clearly in fig.6.For the pristine 0%-Bi-2223 pellet at 5 K as the effective pinning is much stronger, hence the SVP regime is clearly identifiable over a wide H regime in fig.6(a1) (solid green triangles in fig.6(a1)) whereas at 77 K the SVP regime has shrunk and cannot be identified (Fig. 6(a2) the solid pink coloured triangles).We see that in the Co2C NP's and Bi-2223 powder composite pellets, there is a significant enhancement in Jc at both low (5 K) and high T  From fig. 6(a2) we see that the high T regime of 77 K there is a substantial increase in Jc of the 0.05%-Bi-2223 pellet compared to the pristine 0%-Bi-2223 pellet.At 77 K the CVP regime in the 0.05%-Bi-2223 pellet extends upto a field of 1 T where the Jc ~ 0.01 A.m -2 while at K in the pristine 0%-Bi-2223 pellet, by 0.4 T the CVP regime hits the reversible Jc ~ 0. Thus, at high T of 77 K in the 0.05%-Bi-2223 pellet the pinning strength is not smeared out by thermal fluctuations and has significantly strengthened.Furthermore, the field range over which pinning remains effective is significantly enhanced in the composite.() plot in Fig. 6(b) shows an enhancement by a factor of 100 times at low H, which can reach upto almost 1000 times at higher H (due to the expansion of the CVP regime in the composite samples).From fig. 6(b) we see that in the SC-FM composite pellet (0.05%-Bi-2223) at 5K the enhancement in   is by a factor of ten and is almost uniform at all H.However, the same composite pellet shows an order of magnitude greater enhancement in   at higher T where the enhancement increases in high H regime.In figs.6(c1), 6(c2) and 6(d) we compare the   behaviour for the 2%-Bi-2223 pellet and 0%-Bi-2223 pellet and here too we see similar features as that found in figs.
6(a1), 6(a2) and 6(b).In a table below, we have summarized all the important findings of both the batches of nanoparticle-superconductor composites.Recall here that the clustering of Co2C NP's (pink blobs seen in figs.2(e) and 2(f)) in the nanocomposite is responsible for the loss of superconducting fraction (discussed earlier).The increase in Jc of the retained superconducting fraction in the nanocomposite is a result of the Co2C nanoparticles uniformly distributed in the matrix of Bi-2223 nanoparticles (the pink speckles in fig.2(e) and (f)).The ferromagnetism of these NP's is weak enough that they do not destroy superconductivity of the nanocomposite.However, the exchange interaction of these magnetic Co2C NP's and their local fields are strong enough that they locally act as Cooper pair breakers for the superconducting condensate around their vicinity.Thereby, they locally suppress the superconducting order parameter of Bi-2223, without affecting the bulk Tc of the superconducting fraction.It is this local order parameter suppression by the FM -NP's in the nanocomposite pellet which generates the strong pinning effects.The pins we believe are local point like features rather than extended pins, because in fig.6 we see the clear evidence of collective vortex pinning regime, which is known to arise from collective interactions of vortices with point like pinning centre's 1,46 .We see that these Co2C NP pinning effects are not smeared out due to thermal fluctuation effects which is a very significant effect in HTSC materials like Bi-2223.The effectiveness of the vortex pinning depends on, the ratio of the pinning potential well depth U to thermal energy, and also the Ginzburg number (Gi) 1 for a superconductor (Gi is the ratio of Tc to the zero temperature superconducting condensation energy).The Gi governs the width of the critical thermal fluctuation regime or the reversible regime in the field -temperature vortex phase diagram of the superconductors.Infact as the thermal smearing out of the pinning potential strength is significantly high in HTSC due to their large Tc and the Gi of HTSC is also high 1 (~ 10 -1 to 10 -2 for HTSC), all of these factors combine to weaken the pinning effects in most HTSC materials, especially in the high T regime (especially 77 K and above).However we that the ferromagnetic Co2C -NP's act as efficient pinning centres, allowing for collective vortex pinning to survive even in the high temperatures regimes (both Hirr and Jc at 77 K in the nanocomposite is enhanced compared to pristine superconducting sample).One may recall that, raising the temperature causes thermal fluctuations to become more pronounced, thereby softening the vortex state; conversely, raising H increases inter vortex interactions, which strengthen the vortex state's rigidity 1 .When T increases and the effects of enhanced thermal fluctuations also haven't completely smeared out the pinning strength in the sample (i.e., pinning strength ≠ 0 at finite T), then thermally softened vortex lines find it easier to adapt to pinning and enter a strongly pinned vortex state regime.In contrast a more rigid state is comparatively in a weaker pinned state 1  () at 44 K is intriguing.We observe that in the weak interaction regime of low H (< 0.1 T) the thermally softened vortex state at 44 K accommodates the pinning centers of Co2C NP's well.As a result there is an increase in pinning as T increases (from 5K → 44 K →77 K) (see increase in .This feature at 44 K arises, because the net vortex state becomes effectively more rigid with increase in H (> 0.1 T), as the enhancement in vortex state rigidity at higher H is a much larger than the thermally induced softening of the vortex state at 44 K. Thus, a net higher vortex rigidity found at 44 K in the H > 0.1 T regime, results in the progressive weakening of vortex pinning with increase in H. Consequently the,  ,0.05% ,0% () or  ,2%  ,0% (), at 44 K shows a decreasing trend with increase in H > 0.1 T. At 77 K, however, the significantly enhanced thermal softening effect completely dominates over the increase in vortex state rigidity caused by the increase in H. Hence, even at high H (> 0.1 T) a net softened vortex state at 77 K accommodates the pinning centers of Co2C NP's well.In contrast, at very low T of 5 K due to very weak thermal smearing out of pinning effects, the pinning strength remains at a very high value at 5 K.

(
Fig:2(a).Electron Dispersive X-ray (EDX) measurements, using Instrument Model: JSM-6010LA; JEOL, were performed to get the distribution of basic elements (Bi, Sr, Pb, Ca, Cu, O and Co, C) of the admixture samples.For magnetic measurements of the samples, we used the SQUID magnetometer facility of IIT-Kanpur from Cryogenics, UK.Transmission Electron Microscopy (TEM) of the Co2C nanoparticles was performed using Model: FEI-Titan G2 60-300 KV TEM, from Advanced Imaging Centre, IIT-Kanpur.EBSD (Electron Backscattered Diffraction) measurements were performed to image the grain size of the

Fig. 1 :
Fig. 1: (a) Peak position at XRD intensity data shows the signature of Co2C nanoparticles.(b) TEM images of Co2C nanoparticles.(c) Histogram of size distribution of Co2C nanoparticles.(c, inset) SEM images of clusters of Co2C nanoparticles.(d) Magnetic Hysteresis loop of Co2C nanoparticles for 5 K, 50K, and 300 K.It shows Ferromagnetic nature upto 5K.

,
we estimate D = 30.7 nm ± 7 nm.The TEM image (Fig.1(b)) of the synthesized nanoparticle and a histogram analysis (Fig.1(c)) of the particle sizes, suggest an average size in the range of 40 ±15 nm.The TEM image in Fig.1(b) and the SEM image in Fig.1(c, inset) show the tendency of the magnetic Co2C nanoparticles to agglomerate into clusters.
(a), the average size of crystallites in the Bi-2223 powder is ~ 15.3  0.1 nm.Inset of Fig. 2(a) shows an SEM image of a region on the surface of 0%-Bi-2223 pellet.The original polycrystalline pellet was also characterized prior to grinding it to form the powder.EBSD image (fig.2(b)) of this polycrystalline pellet of the 0%-Bi-2223 shows grain of size in the range of 2-10  (Fig. 2(b)) and they are randomly oriented having different crystal orientations (Fig. 2(c)).Analysis of the grain size distribution in fig.2(d) showed log-normal distribution with a mean value~(5.18±0.01)m.EDX mapping of the spatial distribution of the Co and C (see Figs. 2(e), (f) and (g), (h)) in the composite pellets showed a nearly uniform distribution, with low density of clustering of Co (larger sized pink spots) due to the agglomeration of Co2C NP's.It may be noted from figs.1(e) and 1(f), that while we see blobs of Co suggesting presence of aggregates of Co2C NP's, there is also a uniform distribution of pink speckles of Co distributed across the scan, suggesting also a uniform dispersion of Co2C NP's across the pellet.The blobs we believe arises from the Co2C strong tendency to cluster.Between 0.05%-Bi-2223 and 2%-Bi-2223 pellet there is a 40 times increase in the admixed amount of Co2C (by weight).Analysis of a number of EDX elemental scan analysis similar to those in figs.2(e) and (f) reveals that the average number density of the Co2C cluster's changes from ~ 0.022 clusters/ 2 in 0.05% pellet to ~ 0.025 clusters/ 2 in the 2% pellet, namely, an increase in the number density of large sized clusters (blobs) by only ~ 10%, in going from 0.05%-Bi-2223 to 2%-Bi-2223 pellet.The remaining Co2C-NP's are uniformly dispersed in the Bi-2223 NP composite.

Fig. 2 :
Fig. 2: (a), Peak Positions of XRD intensity plots of Bi-2223 pellet after grinding the powders for 15 hrs (Inset shows SEM image of Bi-2223) .(b)EBSD image of the original polycrystalline Bi-2223 pellet (which was ground to obtain the Bi-2223 nano-powder).It shows the grains of the polycrystal (c) EBSD image shows the crystal grain orientation image.It shows each grains are oriented randomly, confirming its polycrystallinity of the original samples, (d) Distribution of the grain size within the original pristine Bi-2223 polycrystalline sample, and we see its grain size distribution follows a log-normal distribution, with a mean grain size of 5.18 micron ,(e) EDS image of Cobalt (Co) in Bi-2223 admixed with 0.05% Co2C and (f) ) EDS image of Cobalt (Co) in Bi-2223 admixed with 2% Co2C respectively, showing homogeneous distribution of Co-across the samples (pink speckles in the images), with some larger sized pink blobs (larger sized clustering of Co2C NP's).(g)EDS image of Carbon (C) in Bi-2223 admixed with 0.05% Co2C nanoparticles (h) EDS image of Carbon (C) in Bi-2223 admixed with 2% Co2C nanoparticles .
(a) (and inset of fig.3(b)) for 0%-Bi-2223, 0.05%-Bi-2223 and 2%-Bi-2223 pellets, note that with the addition of Co2C the superconducting transition temperature Tc of the pellets remains unchanged.From the onset of diamagnetism in M(T) we estimate Tc ~ 109  0.5 K. Figure 3(b) inset shows the M(T) behaviour near Tc for the 0.05% and 2% pellets normalized by their respective mass, i.e., e.m.u/gm units, and in the main panel of fig.3(b) we show the M(T) data in un-normalized (e.m.u units).
Figure 3(b) shows the expected saturated Meissner diamagnetic  signal at low .The M sharply increases from this saturated value with increasing  near   .A new feature in fig.3(b) main panel which is not seen in the typical behaviour of () of pristine superconducting systems, is the ( * )~ 0 at  =  * which is less than Tc, and at  >  * the M(T) increases and saturates to a finite positive M above   = 109 K.This feature seen between  * and   , is due to the FM contribution to M from the Co2C fraction competing with the M contribution from the superconducting fraction present in the composite pellet.In the normal state above Tc = 109 K the FM, positive  response from Co2C dominates, so that the net M(T) response is positive (see Fig.3(b) main panel).In fig.3(b) the un-normalized e.m.u scale for the main panel helps to show that the positive magnetization signal changes in proportion to the amount of Co2C (0.05% or 2%) admixed in the composite pellet.As T decreases below Tc = 109 K, the diamagnetic M contribution to the net measured M signal from the superconducting fraction increases until it completely compensates the positive M contribution at  * .Figure 3(b) main panel shows that at T* of 99.5  0.2 K (for 0.05%-Bi-2223 pellet) and 93.3  0.2 K (for 2%-Bi-2223 pellet) the two responses completely balance resulting in ( * ) →0.
) we determine that   ( <   ) ~ 3 x 10 -4 emu (for 0.05%-Bi-2223) and   ( <   ) ~ 9.5 x 10 -4 emu (for 2%-Bi-2223).Using these values in fig.2(c) we plot   () = (() −   ) for both composite (Co2C -Bi2223) pellets and compare it with that for 0%-Bi-2223 pellet.In fig.3(c), we see that the saturated Meissner M(T) signal at T = 2.3 K for the 0.05%-Bi-2223 and 2%-Bi-2223 pellet is significantly suppressed compared to the pristine 0%-Bi-2223 pellet.The suppression is directly related to a loss of superconducting fraction due to the presence of FM pair breakers in the SC-FM composite pellets.To estimate the change in the superconducting volume fraction in the SC-FM pellets, we use the   () values to calculate the ratio r() =   (,0%−−2223) [  (,0.05%−−2223)   (,2%−−2223)] .Parameter r() measures the ratio of the superconducting volume present in the pristine pellet to that in the composite pellet.From fig. 3(d) we see that at 2.3 K the Meissner superconducting volume of pristine 0%-Bi-2223 pellet is larger than that in of 0.05%-Bi-2223 pellet by almost 33 times and 2%-Bi-2223 pellet by 38 times, respectively.Thus, the ferromagnetism of Co2C causes a loss of superconducting volume fraction of 97% in the 0.05%-Bi-2223 pellet and a loss of about 97.3 % in the 2 %-Bi-2223 pellet when compared to the original pristine 0%-Bi-2223 pellet.We have also studied the Msc vs T of a 10%-Bi-2223 pellet, viz., with 10% Co2C NP's powder admixed by weight in the pristine Bi -2223 powder (see the comparison of Msc vs T for 0%, 0.05%, 2% and 10%-Bi-2223 pellets in the supplementary section 1).

observe a 10 %
additional loss of superconducting volume fraction although there is a 40 times increase in the weight percentage of Co2C in the nanocomposite pellet.Clearly the additional loss of superconducting volume fraction has only a weak dependence on the weight percentage increase of Co2C in the admixed pellet.This is because as already discussed earlier, the weight percent change of Co2C in Bi -2223 translates to a slow increase in the density Co2C larger sized clusters (blobs).Our earlier analysis of the EDX data revealed a change in number density of clusters of about ~ 10% which agrees well with the 10% drop in the superconducting fraction in going from the 0.05%-Bi-2223 to 2%-Bi-2223 pellet.Thus the agglomerated clusters (blobs) of Co2C in the nanocomposite are responsible for destroying bulk superconductivity.These large blobs of Co2C are strong ferromagnetic macroscopic regions in the superconducting medium which destroy superconductivity around them.However, the finely dispersed nanoparticles of Co2C amongst the Bi-2223 (fine pink speckles seen in figs.2(e) and 2(f)) are likely to
) and (d) shows the five quadrant M(H) hysteresis loop for the 0.05%-Bi-2223 pellet at 5 K and 77 K, respectively.The low field portion of the virgin forward M(H) leg of the curve (shown with a blue arrow marked in Fig. 4(a) shows a linear M(H) behaviour.The zoomed-in portion of the virgin linear M(H) is shown in the inset of Fig. 4(a).

: 2 for 5 K
Fig. 4(f) for the composite pellet, 0.05%-Bi-2223.Note the shape of MSC(H) superconducting fraction in the composite pellet in figs.4(c) and 4(f), is identical to the shape of the irreversible M(H) hysteresis loops seen in our pristine 0%-Bi-2223 pellet measured at 5 K (see inset of fig.4(b)).This feature again

Fig. 4 :
Fig. 4: All the Figures are for 0.05% of Co2C added with Bi-2223 (0.05%-Bi-2223).(a) Five quadrants M(H) loop at 5 K. Inset shows first quadrant of M(H) loop and the deviation from linear M(H) behaviour represents the penetration field Hp of the superconductor (b) The black filled circles, show the behaviour of the calculated average magnetization as function of field at 5 K (see text for details) for the 0.05%-Bi-2223 composite.The magnetization data is normalized with the average M value determined at 6T.In the same plot.we have also shown with red symbols the average M value of pure Co2C NPs powder determine from the () in fig.1(d).The Inset shows five quadrants magnetization loop of 0%-Bi-2223 at 5 K. (c) Magnetization as function of field after subtraction of average magnetization from forward run and reverse run of 5 K 0.05%-Bi-2223 magnetization loop.(d) Five quadrants M(H) loop at 77 K. (e) Average magnetization of forward run (6 T to -6 T) and reverse run (-6 T to 6 T) for 77 K.

Fig. 5 :
Fig. 5: All the Figures are for 2% of Co2C added with Bi-2223 (2%-Bi-2223).(a) Five quadrants M(H) loop at 5 K, (b) Average magnetization (Mavg) of forward run (6 T to -6 T) and reverse run (-6 T to 6 T) as function of field at 5 K, (c) Magnetization (MSC) as function of field after subtraction of average magnetization (Mavg) from forward run and reverse run of 2%-Bi-2223 loop at 5 K.(d) Five quadrants M(H) loop at 77 K, (e) Average magnetization of forward run (6 T to -6 T) and reverse run (-6 T to 6 T) as function of field at 77 K, (f) Magnetization (MSC) as function of field after subtraction average magnetization (Mavg) from forward run and reverse run of 2%-Bi-2223 at 77 K.

(
77 K) (see figs.6(a1) and 6(a2)).In fig.6(a1) comparing the data for 0%-Bi-2223 in 0.05%-Bi-2223 pellet, shows only a slight increase in  * at 5 K, however the same comparison in fig.6(a2) at 77 K shows a significant increase  * for the composite pellet.In fig.6(b) we compare the composite and pristine pellets at different T the values of   () by determining the ratio of Jc for 0.05%-Bi-2223 ( ,0.05% ) to that for 0%-Bi-2223 ( ,0% ) pellet, i.e.,  ,0.05% ,0% ().From fig.6(b) we see an increase in Jc of the 0.05%-Bi-2223 pellet by almost 10 compared to the pristine pellet and the increase in Jc is uniformly retained at all H.At 5 K between the composite and pristine pellet, nature of the Jc(H) curves are nearly similar with the   () plot shifted for the composite.Hence the ratio  ,0.05% ,0% () is feature less for 5 K.
. From figs.6(b) and 6(d) the behaviour of