The method of atomic replacement in YBa₂Cu₃0₇ _ (YBCO) has proven to give valuable insight into the superconducting properties. YBCO coated conductors are candidates for future practical applications [1,2]. Doping of Ca⁺² with Y⁺³ in YBCO films increases holes concentration 3, improves superconductor coupling between grains [4,5] and thus turns an oxygen deficient YBCO sample from insulator to superconductor. Ca preferentially occupies the Y site of YBCO especially when the doping concentration is lower than 11 wt.% [6]. This doping also increases the intergrain critical current J_c in spite of minimal lowering of T. in the higher concentrations of Ca-doped YBCO films 4, However, J_c rapidly decreases as the temperature increases under magnetic ûelds. The main reasons for this J_c depression are the intrinsic crystalline anisotropy of the system and the thermal fluctuations. Nevertheless, the lack of effective pinning centers should be noted as another important reason. A Variety of oxides such as BaM0₃ (M= Zr, Ir, Hf, Sn) have been found to be effective pinning centers [79]. Investigations reveals that one of most promising approaches to enhance critical current density of HTSCs in presence of magnetic field is to introduce three dimensional defects of artificial pinning centers (APCs) in the YBCO matrix [10] one such example is Y₂0₃ nanoparticles [11]. Another significant approach of pinning is irradiation. Swift Heavy Ion (SHI) irradiation is very useful tool for modification of the properties of thick and thin films. It penetrates deep into the surface producing long and narrow disordered zone along its trajectory. This disordered columnar zone provides flux pinning along its entire length rather than occasional pinning at randomly distributed points or impurities. Columnar defects due to SHI are more effective than point defects at pinning flux lines at high temperature and fields [12]. In this present work we have studied the synergetic effect of non superconducting inclusion of Y₂0₃ (10 wt. %) and SHI irradiation of 200 MeV of silver ions on Y_1-xCa_xBa₂Cu₃O_7-a (YCaBCO) thick films. Magnetic studies are further carried to investigate critical current density at 40 K

Magnetization measurements have been performed to estimate the current density. It is effective for estimation of critical current density (J_c) as well as for investigation of the magnetic pinning force.Fig.(a) 2 shows magnetization as a function of applied field for YCaBCO thick film irradiated with 200 MeV silver ions at 40 K. Each loop has a peak near the lower critical point H_c1, beyond this point flux penetrates the material and the magnetization decreases gradually. The width of loops increases with irradiation fluences. This behavior of the irradiated sample is attributed to increase in flux pinning which reflects higher capacity to pin the vortices hence enhancement in J_c. Magnetization width of Y_1-xCa_xBa₂Cu Aa+ Y₂O₃ decreases with irradiation doses. This can be accounted for large number of defects created by inclusion of Y₂O₃and columnar defects. Large numbers of defects are responsible for inhibiting the pinning energy to be used effectively. We observed that unirradiated YCaBCO doped with Y₂0₃ has the largest magnetization width and with the highest fluence of 5x10¹¹ions/cm²the width is the smallest. Critical current was estimated from the magnetization width employing Beans critical state model [15] using equation (1)

Here ∆M = M₊-M which is extracted from M (H) loop and a < b where a and b is the thickness and width of the bar shaped sample respectively. Fig. 2(b) shows J_cvs H graph. The highest J_c is recorded for YCaBCO doped Y₂0₃unirradiated thick film. We observe enhancement of J_c in all the cases expect for YCaBCO composite irradiated with fluence of 5×10¹¹ ions/cm². The maximum values of J_c are tabulated in Table 1.

The synergetic effect of Y₂O₃ creating defects and irradiation with SHI producing columnar defect have resulted in lower J_c. This may-be due to the fact that vortex-defects interaction energy dominant over pinning energy. Hence J_c decreases with more defects at higher fluences. Fig. 3. shows the temperature dependence of magnetization in a low applied field of H=0.05T for YCaBCO irradiated with silver ions with fluence of 5×10¹¹ ions/cm².

Both field-cooled (FC) and zero-field-cooled (ZFC) magnetization was studied. In ZFC measurements the sample is cooled in zero fields where a small magnetic field (0.05T) is applied. In the FC procedure, where the Meissner expulsion is measured, the sample is cooled in a small applied field. Below about 92 K the ZFC and FC curves start to open, indicating an irreversibility effect induced by the flux pinning. This measurement confirms that the transition temperature of the irradiated YCaBCO does not drastically reduce T_c. The onset of irreversibility where the two curves merge is connected with the onset of the upper transition at 83 K. Higher magnetization is observed in the FC case as compared to the ZFC case because of columnar defects introduced by irradiation acts as pinning centers in the matrix of YCaBCO.

Thick films of YCaBCO composite with Y₂O₃were irradiated with 200 MeV of silver ions. The greater magnetization width show higher J_c due to pinning effect of nanosized columnar defects. Unirradiated YCaBCO+10 wt. % Y₂O₃ have highest J_c with value 8.3 × 10⁴A/cm² as pinning force is optimum. As the interaction energy between vortex and defects dominates over the pinning energy the J_c decreases at higher fluence of the composite. Both FC and ZFC magnetization studies reveal that transition temperature is not drasticallty affected by irradiation in the range studied.

The authors are thankful to the Pelletron group of IUAC, New Delhi for providing ion irradiation. We also acknowledge the Centre for UGC-DAE-CSR, Indore for providing the facility of high field magnetic measurement.

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