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

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