Model explaining magnetic phases and behavior in Ruthenium-based superconducting ferromagnets A.V. Pan * , R. Nigam, S. Dou Institute for Superconducting and Electronic Materials, University of Wollongong, Northfields Avenue, Wollongong, NSW 2522, Australia article info Article history: Accepted 7 December 2009 Available online 12 January 2010 Keywords: Superconducting ferromagnets Magnetic behavior Spin glass abstract A complete model has been developed to explain the sophisticated magnetic behavior in Ruthenium (Ru) based superconducting ferromagnets. In the complex system of Ru-materials, always consisting of at least two interacting magnetic species, multiple magnetic transitions have been observed. The superpo- sition of corresponding ferromagnetic signals has been proposed to be responsible for producing features compatible with spin glasses, superparamagnetism, ferromagnetism, and antiferromagnetism. Slight dif- ferences in the temperature dependence of the Ru-species and the similarity in their magnetic behavior induce the rich phase diagram, which is enabled by the structural (hence, magnetic) inhomogenity and clustering. The combination of microstructural, magnetic and transport measurements along with careful analysis of the nonlinear ac susceptibility has allowed us to understand the spin ordering in these cluster systems, and establish the cohesive model working in the entire temperature and field ranges. Ó 2009 Elsevier B.V. All rights reserved. Phenomena arising due to interaction between magnetic phases and superconductivity are one of the most intensively studied and intriguing field of research due to its relevancy to the superconduc- ting and spin electronics, colossal magneto-resistance, and mecha- nisms of superconductivity [1,2]. The interaction between magnetic and superconducting materials have been shown to unexpectedly result in enhancement of superconducting charac- teristics, such as critical current density [3,4]. Moreover, the poten- tial for use of superconductors for electronic applications can be significantly enhanced if superconductors are combined with mag- netic materials to form hybrid structures, such as junctions and multilayered systems [2,5]. This drives the interest to natural materials combining magnetic ordering properties with supercon- ducting, such as Ru-based superconducting ferromagnets. Three Ru-systems have been most intensively studied: Sr–Ru–O [1,11,12], RuSr 2 RE 2x Ce x Cu 2 O 10 (Ru-1222) [6–8], and RuSr 2 RE- Cu 2 O 8 (Ru-1212) [8–10], where RE can be Eu, Gd or Sm. In this work, we present the superposition model explaining the behavior of Ru-based 1212 and 1222 systems over the entire tem- perature range. Moreover, the model incorporates the vast major- ity of existing inconsistent explanations [9,13–16] into one cohesive scenario. To a large extent, the model is based on inhomo- geneity in the systems investigated. Indeed, inhomogeneous ex- change fields are known to result in novel spin related phenomena particularly in hybrid structures [1]. X-ray diffraction (XRD) patterns of RuSr 2 Eu 1.5 Ce 0.5 Cu 2 O 10 pre- pared at slightly different conditions indicated that the amount of the secondary phases (in particular Ru-1212) in the Ru-1222 system can be minimized [8], so that the corresponding DC magne- tization [M(T)] evolves from a double-peak curve in its zero-field cooled (ZFC) part to a single peak curve for the optimal sample with the marginal amount of secondary phases (Fig. 1). Taking into account corresponding results obtained by transport measure- ments, showing a single-step superconducting transition for the Ru-1222 sample with the single peak in the M(T) curve as oppose to a double-step resistivity transition for the sample with the dou- ble-peak in the M(T) curve [17], the following explanation is proposed. Similarly to Ru-1212 system whose ZFC M(T) curve is also shown in Fig. 1, the optimal Ru-1222 system is paramagnetic down to Curie temperature of about 120 K, where ferromagnetic (FM) transition occurs. At about 25 K, the superconducting transition oc- curs marked by a change in the slope of ZFC and field cooled (FC) parts. The additional peak arising in the non-optimised sample is due to the FM transition of the secondary Ru-1212 phase occurring at around 140 K. Thus, for the non-optimal samples, the multiple transitions observed are due to the superposition of two different magnetic signals from two Ru-phases present in the system. According to our XRD results [8], which are consistent with the re- sults prepared at similar conditions [13], the Ru-1222 non-optimal sample contains about 15% of the Ru-1212 phase. Therefore, we have summed the ZFC parts of the optimal Ru-1222 sample and the Ru-1212 sample shown in Fig. 1 with the corresponding weighting of 85% for the Ru-1222 signal and 15% for the Ru-1212 0921-4534/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2009.12.010 * Corresponding author. E-mail address: pan@uow.edu.au (A.V. Pan). Physica C 470 (2010) S707–S709 Contents lists available at ScienceDirect Physica C journal homepage: www.elsevier.com/locate/physc