Two-dimensional frustrated magnetic state in superconducting RuSr2 Eu1.5 Ce0.5 Cu2 O10

In this paper we investigate the magnetic state and the role of the crystalline structure in RuSr2 Eu1.5 Ce0.5 Cu2 O10 (Ru-1222). Measurements were made in the isomorphic series (Nb1-x Rux) Sr2 Eu1.5 Ce0.5 Cu2 O10 [(Nb,Ru)-1222], with 0≤x≤1. Three-dimensional (3D) XY fluctuations above the magnetic transition were not observed in Ru-1222, suggesting a weak interplane coupling between the RuO2 layers. The compositional dependence of the magnetic susceptibility shows a rapid broadening with increasing Nb content, explained in terms of a cluster-glass state. The variation in several superconducting parameters as a function of Ru content is linear in the whole concentration range, with no jumps at the critical concentration for which percolation of long-range order is expected. 3D Arrhenius- and Vogel-Fulcher-type dependencies fail to describe the dynamic properties. Fitting of a generalized Vogel-Fulcher-type dependence, with ln (τ/ τ0) =A (T- T0) -B, yields B=2.0, in excellent agreement with Monte Carlo simulations for two-dimensional (2D) systems. The value deduced for T0 agrees well with the reopening of hysteresis in the M (H) curves. The observed superconducting and magnetic features are explained in terms of a scenario of 2D magnetic islands at the RuO2 layers, with no long-range magnetic order.