The influence of magnetic nano metal oxides doping on structure and electrical properties of YBCO superconductor

Nano metal oxides have been prepared by the co-precipitation method [15–17]. The chemicals used were of analytical grade and used without further purification. Manganese chloride (MnCl₂ · 4H₂O), cobalt chloride (CoCl₂ · 6H₂O), chromium chloride (CrCl₃ · 6H₂O) and oxalic acid (C₂H₂O₄ · 2H₂O) with purity 99.5% are used. Oxalic acid and each metal chloride were dissolved in water separately to a concentration of 0.1 M. The aqueous metal chloride solution was put in a 100 ml burette and is allowed to drip in the flask containing the oxalic acid solution. The dripping process was completed in 30 min. After which, 5 ml ammonia solution (NH₄OH) was added drop by drop in the solution to increase the pH value of the prepared solution and then the two solutions interact and the corresponding metal oxalate precipitate in water. During precipitation the solution is subjected to magnetic stirring at the rate of 550 rpm. The resultant precipitated oxalate solution has been kept in an oven for drying over night at 90 °C then the dried product is grinded in powder form and put in furnace at 700 °C for 5 h to obtain nano metal oxide powder except of manganese oxalate which calcinated at 1100 °C for 2 h to obtain Mn₃O₄ phase.

YBCO powder is prepared by the solid state reaction route by mixing stochiometric amount of Y₂O₃, BaCO₃, CuO followed by grinding, calcinations and sintering as previously described in details in our previous work [18].

A series of polycrystalline composite samples of YBCO + x where x = 0.1, 0.2, 0.3, 0.4 and 0.5 wt% of each metal oxide namely Cr₂O₃, Mn₃O₄ or Co₃O₄ were mixed separately and pressed into pellets. The pellets were sintered according to the same curve of heat treatment as in reference [18]. XRD was done for phase conformation and structural analysis using x-ray diffractograms provided with computer controlled formally made by the PHILIPS®MPDX´PERT x-ray diffratometer equipped with Cu radiation CuKα (λ = 1.540 56 Å). The x´pert diffractometer has the Bragg-Brentano geometry. The x-ray tube used was a copper tube operating at 40 KV and 30 mA. The scanning range (2θ) was 20–80° with step size of 0.02° and counting time of 3 s/step. Quartz was used as the standard material to correct for the instrumental broadening. The transmission electron microscope (TEM) images were taken for the prepared metal oxides using a JEDL model 1230. Scanning electron microscope (SEM) was used for microstructural analysis using Philips XL 30 equipped with EDX unit, accelerating voltage of 30 kV, magnification up to 400 000×, and resolution for W (3.5 nm). Temperature dependent resistivity ρ(T) was measured using the standard four-probe techniques with a 182‐Keithley nanovoltmeter and constant current source (224‐Keithley), with the voltage resolution of 10–8 V of the nanovoltmeter, a constant current source of 1 mA flowing through the samples. A closed cycle helium refrigerator consists of cryo‐compressor (model 531‐120‐IBARA), cooled head (model CCS‐100EB‐JAUS RESEARCH Co.), supplied with a suitable holder, vacuum pump (model RVB‐EDWARDS) and a temperature controller (321‐autotuning temperature controller‐LAKESKORE) having a temperature resolution of ±0.1 K was used for temperature variation.

The metal oxides of Mn₃O_4, Cr₂O₃ and Co₃O₄ are investigated by transmission electron microscope and its results are shown in figure 1. In our previous study [18], x-ray powder diffraction was done to obtain the particle size of the prepared metal oxides. The particle size is in the nano scale range (75–82 nm).

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From TEM measurement, all the prepared samples are in the nano scale and show spherical shape with 75–82 nm size range in agreement with the particle size calculated from the XRD patterns of the all prepared metal oxide samples.

Figure 2 shows the powder x-ray diffraction patterns of the YBCO samples doped with different nano metal oxides. The analysis of the data indicates a predominantly single phase perovskite structure YBCO with orthorhombic Pmmm symmetry and small quantities of secondary phases.

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The lattice parameters of YBCO with orthorhombic structure are a₀ = 3.823 Å, b₀ = 3.885 Å, c₀ = 11.7 Å. It should be mentioned that no peaks corresponding to any other compounds were detected by x-ray diffraction. Unit cell volumes and the lattice parameters of these samples are presented in table 1. It may be noticed that the lattice constants a, b and c changes slightly with doping with different nano metal oxides as shown from table 1. Generally, the addition of nano metal oxides slightly decrease the difference between a and b parameters and thus reduces the orthorhombicity (δ). The high value of orthorhombicity of undoped YBCO sample is the result of a high oxygen content of the undoped YBCO sample with fully occupied O(1) sites in the CuO chains along the b-axis as shown from the O_7−δ value which increased with nano metal oxide content indicate that the orthorhombicity of the system decreases. The low variation in the a-axis and changes in the unit cell volume with increasing nano metal oxide (Mn⁺², Mn⁺³, Co⁺², Co⁺³ and Cr⁺³) doping level most probably indicate that metal ions are incorporated into crystal structure.

The scanning electron microscopic (SEM) images of YBCO doped 0.2, 0.4 and 0.5 wt% of Co₃O₄, Mn₃O₄ and Cr₂O₃ samples are presented in figures 3, 4 and 5, respectively. These pictures depict that the average grain size in all samples mostly does not change in most samples and the YBCO samples exhibits large grains randomly oriented in all directions with the presence of pores between the grains. For each doping of nano oxides, it is shown that nanodots are distributed randomly over the YBCO grains mostly. From graphs it is obvious that the grains of all samples are in the same range 1–4 μm.

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The EDX analysis of 0.4 wt% Co₃O₄ and 0.5 wt% of Mn₃O₄, Cr₂O₃ samples are represented in table 2, one can note that each of Mn₃O₄ and Co₃O₄ is distributed over the entire surface of the granules as points in the nanosize range, conversely Cr₂O₃ tends to merges through the structure between or covering the YBCO grains (as shown in figures 3, 4 and 5).

Table 3.  Variation of normal state and superconducting parameters in different samples.

Samples	${{\rm{T}}}_{{{\rm{c}}}_{0}}$ (K)	Tc (K)	ΔT (K)	ρ0 (mΩ.cm)
YBCO	87	92	5	2.18
YBCO + 0.1 wt% Co3O4	83	92	9	2.1
YBCO + 0.2 wt% Co3O4	71	81	10	3.13
YBCO + 0.3 wt%Co3O4	60	74	14	30
YBCO + 0.4 wt%Co3O4	43	60	17	27
YBCO + 0.5 wt% Co3O4	62	83	21	15
YBCO + 0.1 wt% Mn3O4	110	119	9	5.3
YBCO + 0.2 wt%Mn3O4	106	112	6	5.8
YBCO + 0.3 wt% Mn3O4	78	89	11	6.4
YBCO + 0.4 wt% Mn3O4	62	71	9	7.6
YBCO + 0.5 wt%Mn3O4	90	100	10	10
YBCO + 0.1 wt%Cr2O3	70	78	8	6.1
YBCO + 0.2 wt%Cr2O3	68	80	12	4.7
YBCO + 0.3 wt%Cr2O3	70	79	9	5.2
YBCO + 0.4 wt%Cr2O3	54	62	8	4.5
YBCO + 0.5 wt%Cr2O3	78	86	8	5.65

The temperature dependence of resistivity ρ(T) for doped YBCO samples with different concentrations of Co, Mn and Cr nano oxides are shown in figure 6. Generally, all samples show metallic behavior in the normal state (dρ/dT > 0) and superconducting transition to zero resistance. At higher temperature, all samples exhibited linear temperature dependence. The resistive transition exhibits two different regimes. The first is characterized by the normal state that shows a metallic behavior (above 2T_c). The normal state resistivity is found to be linear from room temperature to a certain temperature and follows Andersom and Zou relation ρ(T) = A + BT, where ρ(T) is calculated by using the values of A and B parameters which are obtained from the linear fitting of resistivity in the temperature range 2T_c to 300 K and extrapolation to 0 K gives resistivity slope (dρ/dT) and residual resistivity ρ₀ as seen from figure 6. The second region is characterized by the contribution of Cooper pairs fluctuation to the conductivity below T_c, where ρ(T) is deviated from linearity. This is mainly due to the increasing rate of Cooper pair formation on decreasing the temperature. Note that doping with Co₃O₄ shows degradation of T_c from 92 to 83 with increasing the doping concentration, whereas the residual resistivity ρ₀ increased with the increasing the doping concentration as seen in figure 6(a). Figure 6(b) shows that doping YBCO with different concentrations of nano Mn₃O₄ improves T_c to 119 K and 112 K at low doping ratios and then decreases T_c with increasing concentrations (0.3 and 0.4 wt%) and, finally slightly improves again at 0.5 wt% to become 100. The residual resistivity ρ₀ of YBCO samples doped with Mn₃O₄ increased with the increasing the doping ratio. Figure 6(c) shows that YBCO samples doped with different concentrations of Cr₂O₃ exhibit the decay of the resistivity at T_c for all doping ratios, and the value of the residual resistivity ρ₀ oscillates, decreasing and increasing again with the increase of doping concentration. The transition width (ΔT) increases with increasing doping concentration of all the three doping nano metal oxides (Co₃O₄, Mn₃O₄ and Cr₂O₃) and this may be due to the gradual occurrence of non-superconducting additional phases and the effect of microscopic in-homogeneity.

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Figure 7 represents the variation of T_c of all the prepared samples as a function of concentration for each doping. One can note that T_c decreases with increasing magnetic moment for all nano metal oxide doping. The decrease of T_c with increasing concentration of Cr₂O₃, Co₃O₄ doping evidences substitutions in the YBCO. It was reported earlier that, the CuO₂ sheet in the YBCO structure is a superconducting layer (S) whereas the CuO chain layer is a non-superconducting (N) [19]. The hopping interaction between these S-N layers leads to the suppression of T_c [20, 21]. Additionally, the decrease of T_c with doping nominal concentration confirms that Cu atoms in YBCO crystals lattice were partially substituted. Some differences in T_c decrease in comparison with undoped YBCO may be caused by solidification process as well as by the formation of secondary phases rich in doping. For Mn₃O₄ doping one can note that, there is the increase in transition temperature up to 0.2 wt% which may be due to the Mn inclusion which reduces the formation of other YBCO phases and enhances the formation of YBCO. However, the decrease in T_c with increasing Mn content above 0.2 wt% indicates that Mn ions have been incorporated into the YBCO structure, which results in some changes of microstructure and chemical properties of CuO₂ planes. Moreover, the possible Cooper-pair breaking effect of magnetic ions is known to depress the transition temperature through the short range exchange scattering.

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Table 3 represents the variation of normal state and superconducting parameters of all samples. It indicates that the values of ρ₀ depend on the type of doping. In general ρ₀ increases due to the doping. When the doping ratio increases, the value of ρ₀ for Co₃O₄-doped YBCO samples at first increases, then at 0.3 wt% it turns into a decay. Conversely, Mn₃O₄- and Cr₂O₃-doped samples show the monotone behavious: ρ₀ of Mn₃O₄-doped samples increases and that of Cr₂O₃-doped ones decreases with increasing doping ratio. The increasing value of ρ₀ and the decreasing trend in the value of zero-resistance critical temperature (${{\rm{T}}}_{{{\rm{c}}}_{0}})$ indicate that the connectivity between grains decreases gradually with the increasing the doping concentration of all oxides. All these effects are due to increased inhomogeneities in the intergranular regions. Point defects and chemical dopants may occupy various positions in a real crystal forming substituent or interstitial impurities.