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Jörg Lehmann «  Arafa Hassen »  Veronika Fritsch
Arafa Hassen
Superconductivity and magnetism of rutheno-cuprates
Supervisor: Prof. Dr. Alois Loidl [Experimental physics V]
Date of oral examination: 06/04/2003
135 pages, english , ISBN 3-8322-1673-1
We studied the structural properties, the electrical resistivity and the magnetic properties of the ''ferromagnetic'' RuSr2RECu2O8 (Ru-1212RE) superconductor, where RE = Gd and Eu. Pure Ru-1212Gd compounds are characterized by superconductivity (Tc = 45 K) in the CuO planes coexisting with weak ferromagnetism in the RuO planes. The isostructure Ru-1212Eu compounds have nearly the same magnetic ordering temperature and lower Tc = 33 K. Pure Ru-1212RE reveals properties similar to those observed in heavily underdoped high-Tc materials. The properties of Ru[Sr(1-x)L(x)]2GdCu2O8 compounds, where L = La and Na are systematically investigated as a function of x, temperature and of external magnetic field. Doping with La or Na gives no significant structural changes but enhances the the magnetic ordering temperature Tm. Already at x = 0.03La, the superconductivity is completely suppressed due to the reduction of the charge carrier density. On the other hand, doping with Na at Sr site in Ru-1212Gd can be considered as a hole doping and one would expect that Tc increases but due to disordering or trapping the charge carriers, the superconducting transition decreases with increasing Na content. On increasing x the charge carriers are localized at low temperatures and for x = 0.10 La or Na, semiconducting transport properties dominate below room temperature. Similarly, Ru(1-x)M(x)Sr2GdCu2O8, with M = Ti and Rh for 0 < x < 0.20 were studied. Both ferromagnetic (FM) and superconducting (SC) transition temperatures are reduced with increasing doping concentration. The degree of suppression superconducting transition depends on the electronic configuration of the substituting cation. The superconductivity becomes fully suppressed at x = 0.10 and 0.20 for Ti and Rh, respectively. A generalized phase diagram for Ru-1212Gd system is suggested. Moreover, we studied the effect of substituted cations at the Cu site in Ru-1212RE and compared our results to other high-Tc superconductors.