Irreversibility and critical current density of FeSr2ErCu2O6+y

FeSr2ErCu2O6+y (ErFe1212) and non-superconducting FeSr2ErCu1.9Zn0.1O6+y were synthesized to study the property of the superconductivity and the irreversibility of ErFe1212. A large irreversibility in the temperature dependence of magnetization and a hysteresis in the magnetization curve were observed in ErFe1212. By comparison with non-superconducting FeSr2ErCu1.9Zn0.1O6+y, it was found that the most part of the hysteresis at high magnetic eld originates from the magnetism of Fe ion and some part of the hysteresis at low magnetic eld originates from the superconductivity. Using the magnetization curve of ErFe1212 and FeSr2ErCu1.9Zn0.1O6+y, the Jc of ErFe1212 in individual grains at 10 K under 0.1 T was estimated by the Bean model and Jcintra was 2.6 × 109 A/m2. The critical current density across inter-grain boundaries at 10 K estimated by V − I measurement was Jcinter = 5.7 × 104 A/m2. A large difference between Jcinter and Jcintra was observed in ErFe1212. Jcinter and Jcintra of ErFe1212 are 2.2 and 5.2 times larger than these of YFe1212, respectively.


Introduction
The crystal structure of FeSr 2 YCu 2 O 6+y (YFe1212) is related to that of YBa 2 Cu 3 O 7−δ , in which Cu ion is partially replaced by Fe ion and Ba ion is fully replaced by Sr ion. In YFe1212, superconductivity has been found to be associated with CuO 2 plane and magnetism also has been associated with FeO δ layers. YFe1212 shows superconductivity only after three separate annealing process [1]. The first annealing is performed in N 2 flow, and this relates to suppression of Fe substitution at Cu(2) sites. The second and third annealing are in O 2 flow and in a highpressure O 2 atmosphere, respectively, to supply carriers to the CuO 2 planes [2,3]. The optimized annealing condition is annealing in N 2 flow at 790 • C, annealing in O 2 flow and under high O 2 pressure at 270 • C [4]. Superconductivity of YFe1212 was much affected by the annealing temperature in N 2 flow rather than the annealing temperature in O 2 flow. After this annealing process, YFe1212 shows zero resistivity below 40 K.
Two stage transition was observed in resistivity measurement under the magnetic field. The critical current densities of YFe1212 in individual grains, J intra c , and across inter-grain boundaries, J inter c , were studied [5]. J inter c was about six orders of magnitude lower than J intra c at 2 K under 0.1 T and this seems to be responsible for the two stage transition.
The superconducting properties of YBa 2 Cu 3 O 7−δ are affected by the substitution of rare earth (RE) ions for Y ion, which are sandwiched between CuO 2 plane. Most type of RE ions substituted for Y ion and the effects of the substituting ions on the superconducting 2 1234567890 ''"" transition temperature [6] and magnetic irreversibility field [7] have been reported. The Y sites in YFe1212 is also occupied by RE ions. The crystal structure of FeSr 2 ErCu 2 O 6+y (ErFe1212) and FeSr 2 NdCu 2 O 6+y was studied [8] and the superconducting properties and Fe ion distribution of ErFe1212, FeSr 2 EuCu 2 O 6+y , FeSr 2 GdCu 2 O 6+y and FeSr 2 NdCu 2 O 6+y were studied [9]. A large irreversibility was observed in the temperature dependence of magnetization for ErFe1212 and the origin of this irreversibility was unclear. On the assumption that this large irreversibility originates from the superconductivity, it implies that ErFe1212 has large critical current density. In this study, the proper annealing condition was determined and the magnetic and electric property of ErFe1212 were studied to reveal the origin of this irreversibility and the superconducting property.

Experimental
Polycrystalline samples of ErFe1212 were prepared by a solid state reaction using a stoichiometric mixture of Er 2 O 3 , SrCO 3 , CuO and Fe 2 O 3 powders. The mixture was calcined at 850 • C for 24 h in air, ground and then pressed into pellets. The pellets were sintered at 950 • C for 24 h in air. Then, they were subsequently annealed at T N 2 = 730 ∼ 850 • C for 24 h in N 2 flow, at 270 • C for 24 h in O 2 flow, and finally at 270 • C for 24 h in O 2 atmosphere under a high pressure of 17.6 MPa. The non-superconducting FeSr 2 YCu 1.9 Zn 0.1 O 6+y were synthesized and annealed by the same process.
The crystal structure and phase purity of the samples were characterized using powder xray diffraction. The magnetization curves and the temperature dependence of magnetization were measured using a SQUID magnetometer (MPMS-XL5, Quantum Design). The resistivity was measured by a four-probe dc technique. The critical current density in individual grains, J intra c was estimated by the Bean model [10] and that across inter-grain boundaries, J inter c was estimated from voltage-current measurement.

Result and discussion
For as-synthesized sample, all of the peaks in x-ray diffraction pattern can be assigned to singlephase ErFe1212 with a tetragonal crystal structure. After the annealing in N 2 flow, the crystal structure changes to an orthorhombic structure and after the annealing in O 2 flow, it reverts to tetragonal structure.
The temperature dependence of the resistance for FeSr 2 ErCu 2 O 6+y after various annealing temperature in N 2 flow is shown in Fig. 1 Figure 3 shows the temperature dependence of the magnetization under an applied magnetic field of 1.0 mT after zero field cooling and field cooling. The magnetization of ErFe1212 begins to decrease at T c = 56.6 K and diamagnetism was observed. The temperature dependence of magnetization consists of diamagnetism of superconductivity and the magnetism of Fe ion and paramagnetism of Er ion. A large irreversibility was observed between zero field cooled and field cooled curves below 26.6 K. On the other hand, no diamagnetism was observed and similar irreversibility was observed below 21.6 K in non-superconducting FeSr 2 YCu 1.9 Zn 0.1 O 6+y .     The temperature dependence of resistivity was shown in Fig. 4. In zero magnetic field, the resistivity was decreased due to the onset of the superconductivity at 60.5 K. Zero resistivity was observed below T zero  Magnetic field dependence of ∆M for ErFe1212 and FeSr 2 YCu 1.9 Zn 0.1 O 6+y at 2 K.
∆M is the difference of the magnetization per unit volume between ascending and descending field process and r is the radius of the grain. The magnetic field dependence of ∆M is shown in Fig. 6. A peak was observed at around the magnetic field of 1.3 T in ErFe1212 and FeSr 2 YCu 1.9 Zn 0.1 O 6+y . Since the magnetism of Er ion is paramagnetism down to 2 K [9], this peak may originate from the magnetism of Fe ion. The irreversibility of ErFe1212 under low magnetic field may mainly originate from the superconductivity and to avoid the effect of the irreversibility of magnetism, J intra c of ErFe1212 was deduced from ,where ∆M ′ is ∆M of FeSr 2 YCu 1.9 Zn 0.1 O 6+y . J intra c of ErFe1212 at 10 K under 0.1 T is estimated as 2.6 × 10 9 A/m 2 and this value is 2.2 times larger than that of YFe1212 [5].
The voltage-current curve for ErFe1212 is shown in Fig. 7. The values of the voltage and current are normalized by the sample dimension. The critical current density across the intergrain boundaries, J inter   A large irreversibility was observed in the temperature dependence of magnetization below 26.6 K. The temperature dependence of magnetization consists of diamagnetism of superconductivity and the magnetism of Fe ion and Er ion. The magnetism of Er ion kept Curie-Weiss type paramagnetism down to 2 K.
The magnetization curve consists of the type-II superconductivity and the magnetism of Fe ion and paramagnetism of Er ion. A peak of ∆M , that is the difference of the magnetization per unit volume between ascending and descending field process, was observed at around 1.3 T and large ∆M was observed at zero magnetic field. Since a similar irreversibility was observed for non-superconducting FeSr 2 ErCu 1.9 Zn 0.1 O 6+y , the irreversibility at around 1.3 T may originate from the magnetism of the Fe ion.
Some part of the irreversibility under low magnetic field may originate from the superconductivity. Using the magnetization curve of ErFe1212 and FeSr 2 ErCu 1.9 Zn 0.1 O 6+y , J intra