INFLUENCE OF Pr2Co7 MAGNETIC NANOMETER PARTICLES ADDITION ON YBa2Cu3Oy SUPERCONDUCTOR MAGNETO-CONDUCTIVITY’S IN FLUCTUATION REGIME

* D. Mani Kongnine 1 , M. Baneto 1 , K. Tepe 1 , A. Hamrita 2 and K. Napo 1 . 1. Département de Physique, Faculté des Sciences, Université de Lomé, BP 1515, Lomé, Togo. 2. Département de Physique, Faculté des Sciences de Bizerte, Zarzouna 7021, Université de Carthage, Bizerte, Tunisie. ...................................................................................................................... Manuscript Info Abstract ......................... ........................................................................ Manuscript History

We perform magneto-conductivity fluctuation study on polycrystalline samples of YBa 2 Cu 3 O y (YBCO) superconductor added with x% (x=0.0, 0.1 and 0.3) in weigh of magnetic nanometer particles (20 nm) of Pr 2 Co 7 . Standard solid state reaction method is used to elaborate samples. X-ray diffraction patterns refinement by Rietveld method show the orthorhombic to tetragonal phase transition from 0.3% wt of Pr 2 Co 7 addition. Magneto-resistance measurements are performed in magnetic field ranging from 0 to 40 mT, orthogonal to current. The magneto-conductivity fluctuation is analyzed assuming excess conductivity in magnetic field follows power law. The results show two steps of transition. The pairing transition above (maximum of temperature derivative of resistivity) and the coherence transition below . Transition width goes large for each samples when magnetic field increases and is more higher in samples containing Pr 2 Co 7 . Above , the fluctuation regime in presence of magnetic field is not changed in comparison to the results obtained out of magnetic field. Magnetoconductivity fluctuation analysis near zero resistance temperature, , shows two critical regimes with samples containing 0.0% and 0.1% in weight of Pr 2 Co 7 but only one critical regime in sample containing 0.3% wt of Pr 2 Co 7 .

Introduction:-
Due to their small coherence length and anisotropy, high temperature superconductors induce large fluctuation phenomena around the transition region in their physical properties such as electrical conductivity, magnetic susceptibility, specific heat …(Y. A. Opata et al., 2011;N. M. Vladimirova et al., 1998). Their granular character leads to high density of grains boundaries which behave as Josephson junctions that are sensitive to external magnetic fields makes this phenomenon pronounced (R. M. Costa et al., 2007).
Particularly, their resistive transition shows a characteristic two-stage behavior. When decreasing temperature, one first observes a pairing transition, at the bulk critical transition temperature , arising in homogenous regions called grains which are not necessarily coincident to crystallographic grains; and the coherence transition at where the In high magnetic fields, the superconducting transition should be described by lowest-Landau-level~LLL! approximation of the GL theory (R. M. Costa et al., 2001).
In this paper, we report the effect of Pr 2 Co 7 addition on magneto-conductivity fluctuation of YBa 2 Cu 3 O y superconductor in low magnetic field, by using the excess conductivity formula obtained from the approximation of  The resistivity temperature dependence were carried out in a CCS 450 cryostat system, using standard four-probe technique, on samples cut as rectangular with nearly similar dimensions and using silver paint for electrical contact with resistivity less than 0.5 Ω. Magnetic fields were applied perpendicular to current ranging from 0 to 40 mT. In order to extract the derivative of resistivity and other parameters, the resistivity measurements are spaced closely.

X-ray diffraction characterization:-
The crystalline structures of these samples were characterized by powder XRD using a Philips 1710 diffractometer with CuKα radiation ( ) at room temperature. Detailed structural parameters were obtained by Rietveld refinements using FULLPROF program. We discuss about the x-ray powder diffraction patterns and the crystallographic parameters characteristics of all the sintered samples elsewhere (D. Mani Kongnine et al., 2015). For example, Fig. 1 shows a Rietveld structural refinement of 0.3% in weight of Pr 2 Co 7 addition on YBCO sample. In general manner, crystallographic parameters vary with increasing Pr 2 Co 7 level, decreasing the orthorhombicity of sample which becomes tetragonal from 0.3% wt of Pr 2 Co 7 addition.

Method of analysis:-
We assume that magneto-conductivity diverges as a power law given by : where , is the critical amplitude, the critical exponent. The conductivity ( is obtained by resistivity measurements under magnetic field and the regular conductivity is obtained by extrapolation from the high temperature behavior (R. M. Costa et al., 2001). To determine simultaneously and , the numerical logarithmic derivative of is done from experimental data : And with the combination of Eqs. (1) and (2), we obtain : After the determination of and , the amplitude can be deduced from Eq. (1) by substituting in it the values of and within the region of temperature the scaling is observed.

Results and discussion:-
In Fig. 2  The magneto-resistance curves are not almost affected by applied field in the region above the maximum of the temperature derivative , whose is denoted by in Fig. 3. This temperature is approximately the same with pairing transition temperature . This result is consistent with those of J. Roja-Roja (J. Roa-Rojas et al., 2000). Below and near zero resistance temperature, the magneto-resistance curves are strongly dependent of samples granularity and applied magnetic fields. When applied magnetic field increases, the zero resistance temperature decreases, enlarging the transition region width due to vortex motion thermally activated. In our case, this phenomenon is not minimized because applied magnetic field is perpendicular to current (R. M. Costa et al., 2007). We called the transition temperature under magnetic field irreversible temperature and the corresponding magnetic field the irreversible field. In Fig. 2. (d), the irreversible lines obtained with this method show the pinning properties degradation induced by Pr 2 Co 7 addition.
The fluctuation regime above under magnetic field does not change significantly and is not presented here. Below , we assume the magneto-conductivity follows power law. In Fig. 3, we represent the temperature derivative of resistivity and the curves of versus temperature according to Eq. We note for each sample that ( does not change with applied magnetic field and the critical exponent is sample dependent only. Indeed, we observe, for each percentage of Pr 2 Co 7 content : x=0.0%, two critical exponents and ; for x=0.1%, and and for x=0.3% only one critical exponent suggesting that the two critical exponents observed in other samples collapse to one critical exponent. Aside from, 0.1% wt Pr 2 Co 7 whose critical exponent is consistent with 2D Aslamazov and Larkin theory (P. Marra  , probably due to experiment conditions and samples characteristics'. We apply magnetic field perpendicular to current and the dissipation phenomena, associated to disorder introduced by Pr 2 Co 7 in microscopic/mesoscopic level may play a predominant role disturbing magneto-conductivity fluctuation phenomena.

Conclusion:-
We have studied the effect of Pr 2 Co 7 addition on the magneto-conductivity in fluctuation regime of YBa 2 Cu 3 O y superconductor. The obtained critical exponents do not permit to identify clearly any specific regime of fluctuation except for sample containing 0.1% wt Pr 2 Co 7 . However, we identify two critical regimes in samples containing 0.0% and 0.1% in weight of Pr 2 Co 7 and only one critical regime in sample containing 0.3% of Pr 2 Co 7 . Dissipation phenomena, related to the thermal activated flux flow may be responsible of these results.