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Magnetic-Superconducting phase-diagram of Eu2-xCexRuSr2Cu2O10

Eu2-xCexRuSr2Cu2O10-d (Ru-2122) is the first Cu-O based system in which superconductivity (SC) in the CuO2 planes and weak-ferromagnetism (W-FM) in the Ru sub-lattice coexists. The hole doping in the CuO2 planes, is controlled by appropriate variation of the Ce concentration. SC occurs for Ce contents of 0.4-0.8, with the highest TC=35 K for Ce=0.6. The as-prepared non-SC EuCeRuSr2Cu2O10 (x=1) sample exhibits magnetic irreversibility below Tirr=125 K and orders anti-ferromagnetically (AFM) at TM =165 K. The saturation moment at 5 K is Msat=0.89 mB /Ru close to the expected 1 mB for the low-spin state of Ru5+. Annealing under oxygen pressures, does not affect these parameters, whereas depletion of oxygen shifts both Tirr and TM up to 169 and 215 K respectively. Systematic magnetic studies on Eu2-xCexRuSr2Cu2O10-d show that TM, Tirr and Msat decrease with x, and the Ce dependent magnetic-SC phase diagram is presented. A simple model for the SC state is proposed. We interpret the magnetic behavior in the framework of our ac and dc magnetic studies, and argue that: (i) the system becomes AFM ordered at TM; (b) at Tirr<TM, W-FM is induced by the canting of the Ru moments, and (c), at lower temperatures the appropriate samples become SC at TC. The magnetic features are not affected by the SC state, and the two states coexist.


Introduction
Much attention has been focused on a phase resembling the superconducting RBa 2 Cu 3 O 7 (R=rare-earth) materials, having the composition R 2-x Ce x MSr 2 Cu 2 O 10 (M-2122, M= Nb, Ru or Ta) 1  with fivefold pyramidal coordination, exists. The hole doping of the Cu-O planes, which results in metallic behavior and SC, can be optimized with appropriate variation of the R/Ce ratio 2 . SC occurs for Ce contents of 0.4-0.8, and the highest T C was obtained for Ce=0. 6. The Nb-2122 and Ta-2122 materials are SC with T C 28-30 K.
In both systems, the magnetic order does not vanish when SC sets in at T C , and remains unchanged and coexists with the SC state. The Ru-2122 materials (for R=Eu) display a magnetic transition at T M = 125-180 K and bulk SC below T C = 32-50 K (T M >T c ) depending on oxygen concentration and sample preparation 6 . SC survives because the Ru moments probably align in the basal planes, which are practically decoupled from the CuO 2 planes, so that there is no pair breaking. Specific heat studies show a sizeable typical jump at T C and the magnitude of the ∆C/T (0.08 mJ/gK 2 ) indicates clearly the presence of bulk SC 8 . The specific heat anomaly is independent of the applied magnetic field. Scanning tunneling spectroscopy 3 , muon-spin rotation 9 and magneto-optic experiments 10 have demonstrated that all materials are microscopically uniform with no evidence for spatial phase separation of superconducting and magnetic regions. That is, both states coexist intrinsically on the microscopic scale.
In the Ru-2122 system, the W-FM state, as well as irreversibility phenomena, arise as a result of an anti-symmetric exchange coupling of the Dzyaloshinsky-Moriya (DM) type 3 between neighboring Ru moments, induced by a local distortion that breaks the tetragonal symmetry of the RuO 6 octahedra. Due to this DM interaction, the field causes the adjacent spins to cant slightly out of their original direction and to align a component of the moments with the direction of the applied field. Below the irreversible temperature (T irr ,), which is defined as the merging temperature of the zero-field (ZFC) and field-cooled (FC) curves, the Ru-Ru interactions begin to dominate, leading to reorientation of the Ru moments, which leads to a peak in the magnetization curves. The Ce concentration affects the hole carrier concentration as a result the SC properties and the T C values of R 2-x Ce x RuSr 2 Cu 2 O 10-δ system. The remaining unresolved question concerns the effect of Ce concentration on magnetic properties of this system. It was shown that the peak position of the ZFC magnetization data increases with increasing Ce 11 , however, the detailed magnetic features of this system are lacking.
In attempting to understand the mechanism of SC and W-FM in the Ru-2122 system, an approach involving a systematic magnetic properties of Eu 2-x Ce x RuSr 2 Cu 2 O 10-δ for 0.4<x<1 was employed. Eu 3+ is used in order to diminish the paramagnetic contribution of Gd 3+ . This paper is organized as follows:

Experimental details
Ceramic samples with nominal composition Eu 2-x Ce x RuSr 2 Cu 2 O 10-δ (x = 0.4-1) were prepared by a solid-state reaction technique. Prescribed amounts of Eu 2 O 3, CeO 2, SrCO 3, Ru and CuO were mixed and pressed into pellets and preheated at 1000° C for about 1 day at atmospheric pressure. The product was cooled, reground and sintered at 1050°C for 50 h in a slightly pressurized oxygen atmosphere (∼1.1 atm) and then furnace cooled to ambient temperature (asp samples). All the asp materials reported were prepared at the same time and under the same conditions. Part of the asp EuCeRuSr 2 Cu 2 O 10 sample was re-heated for 24 h at 800° C under high oxygen pressure (60 atm.) and another part was quenched from 1050° C to room temperature, denoted as hop and quenched materials, respectively. Determination of the absolute oxygen content in the asp Ru-2122 material, as well as in the hop and quenched samples, is difficult because CeO 2 is not completely reducible to a stoichiometric oxide when heated to high temperatures. Powder X-ray diffraction (XRD) measurements confirmed the purity of the compounds (~97%) and indicate within the instrumental accuracy, that all samples have the same lattice parameters a=3.846(1) Å and c=28.72(1) Å, in excellent agreement with Ref. 3. ZFC and FC dc magnetic measurements in the range of 5-300 K were performed in a commercial (Quantum Design) super-conducting quantum interference device (SQUID) magnetometer. The resistance was measured by a standard four contact probe and the ac susceptibility was measured by a home-made probe with excitation frequency and amplitude of 733 Hz and 30 mOe respectively, both inserted in the SQUID magnetometer.

Experimental results and discussion
Due to the similarity of the ionic radii of Eu 3+ (0.94 Å ) and Ce 4+ (0.87Å ) and within the instrumental accuracy, the lattice parameters of Eu 2-x Ce x RuSr 2 Cu 2 O 10-δ , are independent of Ce content. The detailed crystal structure and the atomic positions Ru-2122 were studied by synchrotron X-rays diffraction 12 and neutron diffraction 13 experiments, which show that the RuO 6 octahedra are rotated ~ 14° around the c-axis and that this rotation is essentially the same for x=1 and x=0.6 as well as for Ru-1212. There is no evidence for super-cell peaks in the Ru-2122 samples.
(a) Superconductivity in the Eu 2-x Ce x RuSr 2 Cu 2 O 10-δ δ δ δ system The temperature dependence of the normalized ac susceptibility curves (at H dc =0) for the Eu 2-x Ce x RuSr 2 Cu 2 O 10-δ system are presented in Fig. 1. It is readily observed that the x=1 and 0.9 are not SC and that SC occurs for Ce contents of x=0.8-0.4. Fig 2. exhibits the onset of SC deduced from these ac plots which exhibit a bell shape behavior with a peak at 35 K for Ce=0. 6. Similar values are obtained by our resistivity measurements (not shown). A similar bell shape behavior was observed in the Eu 2-x Ce x NbSr 2 Cu 2 O 10-δ (x=0.4-1) system, which serve as reference materials. It is apparent in Fig. 1 that the SC transitions, are much broader than those observed in many other HTSC materials. This transition width is comparable to our previous resistivity data 3,6 and with Ref. The Eu, Ce and Ru ion valencies in Eu 1.5 Ce 0.5 RuSr 2 Cu 2 O 10-δ have been studied by Mossbauer spectroscopy (MS) and X-ray-absorption spectroscopy (XAS) techniques [14][15] . MS performed at 90 and 300 K on 151 Eu show a single narrow line with an isomer shift of 0.69(2) mm/s, indicating that the Eu ions are trivalent with a nonmagnetic J=0 ground state. XAS taken at L III edges of Ce shows also that Ce is tetravalent. XAS taken at the K edge of Ru, at room temperature indicates clearly that the Ru ions are pentavalent [14][15] . It is also apparent, that bulk SC in the M-2122 system appears only for pentavalent M ions and both Nb-2122 and Ta-2122 materials are SC with T C 28-30 K 1 .
We argue that the RCeMSr 2 Cu 2 O 10 (R=Eu or Gd and R/Ce=1) samples serve as the parent stoichiometric insulator compounds (similar to La 2 CuO 4 ). Since the valence of R 3+ , Ce 4+ , Ru 5+ Sr 2+ Cu 2+ and O 2are conclusive, a straightforward valence counting yields a fixed oxygen concentration of 10. Hole doping of the Cu-O planes, which results in metallic behavior and SC, can be optimized with appropriate variation of the R 3+ / Ce 4+ ratio (trivalent R 3+ ions are replaced for Ce 4+ ). SC occurs for Ce contents of 0.4-0.8, and the optimally doped sample is obtained for Ce=0.6. In fact, unlike the La 2-x Sr x CuO 4 system, this substitution does not appear to significantly alter the hole concentration (p) on the Cu-O planes. This is apparent in Fig. 2  EuCeRuSr 2 Cu 2 O 10 is not SC and is only magnetically ordered (Fig.1). ZFC and FC dc magnetic measurements for the parent as-prepared (asp) sample, were performed over a broad range of applied magnetic fields and typical M/H curves measured at 50 Oe, are shown in sake of brevity, we display the data obtained for x=0.1 and x=0.5 (Fig. 7). It is readily observed, that for x=1, the magnetic properties due to the Ru are all enhanced, as compared to the x=0.5. The latter sample is SC and its ZFC branch starts from negative moments. The inflection in the FC branch agrees well with T c determined from the ac curve (Fig.1). The absence of a complete Meissner effect in the Ru-2122 system is discussed in length elsewhere.
However this is of little interest in the present discussion. In the paramagnetic range, the parameters extracted using the CW law, have been obtained by subtracting the paramagnetic moment of the relevant Nb-2122 compound, as described above.
It appears that the C and θ parameters for all materials, are very close to those of  (Fig. 5), the Ru sub lattice becomes AFM ordered. (iii) At T irr < T M , which is Ce dependent (Fig. 8) and also varies with the external field, a weak ferromagnetism is induced, which originates from canting of the Ru moments. T irr is defined as the merging point of the low field ZFC and FC branches, or alternatively, at the temperature in which the remanent moment disappears. This canting arises from the DM anti-symmetric super-exchange interaction, which by symmetry, follows from the fact that the RuO 6 octahedra tilt away from the crystallographic c axis 3,11 . At high magnetic field (H>3000 Oe) the irreversibility is washed out and the M(T) curves exhibit a ferromagnetic-like behavior.(iv) At lower temperatures SC is induced. Fig. 2 shows that T C depends strongly on the R 3+ /Ce 4+ (as hole carriers) and on oxygen concentrations 6