Two magnetic transitions at TM~160 K and at TM2~80-100 K, are observed in the magneto-superconducting RuEu2-xCexSr2Cu2O10-δ (TC = 32-50 K). Below TM2, the Ru5+ moments are weakly-ferromagnetically ordered and wide ferromagnetic hysteresis loops are observed, they become narrow and disappear at ~ 60-70 K. Dc magnetic studies on c-axis oriented epitaxial thin films of Ru-1222 on (100) SrTiO3 wafers show that the easy axis of the magnetization is in the basal plane and the Ru5+ ions are in their high-spin state. Above TM2, (i) small antiferromagnetic-like hysteresis loops reappear with a peak in the coercive fields around 120 K. (ii) A small peak at ~120 K is also observed in the dc and ac susceptibility curves. The two phenomena are absent in the non-superconducting RuEuCeSr2Cu2O10-δ (x=1) samples. Two scenarios are suggested for the phenomena at TM2M. (i) For x<1, the decrease of the Ce4+ content, is compensated by non-homogeneous oxygen depletion, which may induce a reduction of Ru5+ ions to Ru4+. The higher ordering temperature, TM, which does not change with x, may result from Ru4+ nano-size rich clusters, in which the Ru4+-Ru4+ exchange interactions are stronger than the Ru5+-Ru5+ interactions. Or alternatively,(ii) The presence of nanoparticles of a minor foreign extra Ru4+ magnetic phase, such as Sr-Cu-Ru-O3, in which Cu is distributed inhomogeneously in both the Ru and Sr sites.
Eu2-xCexRuSr2Cu2O10-δ(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 sublattice coexist. The hole doping in the CuO2 planes, is controlled by appropriate variation of the Ce concentration and/or increasing the oxygen concentration. SC occurs for Ce contents of 0.4-0.8, with the highest TC=35 K for Ce=0.6. Due to the granular nature of the materials the magneto-resistance R(H) below TC is positive and unexpected hysteresis loops are observed. The R(H) curve on decreasing the applied field (Hext) is much smaller than the R(H) curve for increasing (Hext). The width of the loops depends strongly on the weak-link properties. 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. Systematic magnetic studies on Eu2-xCexRuSr2Cu2O10-δ show that TM, Tirr and Msat decrease with x, and the full Ce dependent magnetic-SC phase diagram is presented. A simple model for the SC state is proposed. The interpretation of the magnetic behavior is: (i) the system becomes antiferromagnetically (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.
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