Magnetism and Superconductivity in RuM1.5Ce0.5Sr2Cu2O10 (M=Eu and Y)

RuY1.5Ce0.5Sr2Cu2O10, is not superconducting (SC) and orders magnetically at TM =152 K. The Ru moment at 5K is 0.75 mB. Ferromagnetic hysteresis loops are observed below Tirr= 100 K. At 100 K, the remanent magnetization (Mrem) and the coercive field (HC) become zero. Surprisingly, at Tirr <T< TM a reappearance of antiferromagnetic (AFM) hysteresis loops are observed with a peak at 120 K for both Mrem and HC. The paramagnetic constants are: Peff =2.06 mB (greater than the 1.73 mB expected for low spin Ru5+) and q= 136 K which agrees well with TM. Substitution of Zn (2.5% atm.) for Cu in RuEu1.5Ce0.5Sr2Cu2O10 (TC=38 K), suppresses TC but does not affect the magnetic features such as: TM and Tirr, Mrem and HC. This indicates that both SC and magnetic states are practically decoupled. We argue that the magnetic structure of this system becomes (i) AFM ordered at TM. (ii) Due to the tilting of the RuO6 octahedra away from the crystallographic c axis, at Tirr < TM, weak-ferromagnetic (W-FM) order is induced by the canting of the Ru moments. (iii) At TC (TM/TC~4), the specific materials (M=Eu, Gd) become SC. Below TC, both SC and W-FM states coexist.