Magnetic properties of polycrystalline Sm2−xCexCuO4−y at high magnetic fields

Abstract

We have performed measurements of ac magnetic susceptibility χac as a function of temperature and magnetic field on polycrystalline specimens of Sm2−xCexCuO4−y (0.15≤x≤0.18) prepared from a sol-gel precursor. All the samples studied show a striking double resistive superconducting transition. One of the resistive transitions occurs at a higher temperature Tci and the other one at a lower Josephson-coupling temperature Tcj. Magnetic measurements reveal the appearance of superconductivity below Tcj which is evident from the resistive component χ″ which peaks only at Tcj. Measurements made on pellets and crushed samples with average grain size ∼1 μm show essentially the same features. This suggests that Josephson coupling occurs inside physical grains with dimensions close to 5 μm. We have also utilized the behavior of the peak in the χ″ component as a function of applied magnetic fields to build a H vs T phase diagram for these compounds. Some interesting features were observed in this H vs T phase diagram: (1) a Hα, α=2/3, dependence at low applied magnetic fields (H≤2 kOe) and high temperatures, and (2) an abrupt increase in the magnitude of the exponent at high fields and low temperatures. The behavior at low fields and high temperatures suggests that the magnetic response of the system is mainly controlled by a disordered Josephson network. Possible connections between these two different behaviors are discussed.