Magnetosuperconductivity in ruthenocuprates RuSr2GdCu2O8(Ru(2)

We reviewed our various results on rutheno-cuprate magneto-superconductors RuSr2GdCu2O8(Ru-1212) and RuSr2(Gd0.75Ce0.25)2Cu2O10(Ru-1222). It is observed, that it is difficult to control the oxygen content of Ru-1212, though the same is possible up to some

Ru-1212 [4]. Note that according to this general categorization scheme, the standard CuBa2RCu2O7-δ phase is referred to as Cu-1212.

Although bulk magnetism due to ordering of the Ru moments was confirmed for Ru-1212 from µSR (muon spin rotation/relaxation/resonance) and ESR (electron spin resonance) studies [2,5], the exact type of ordering is still debated. In particular, the results revealed from neutron scattering experiments [6-9] and magnetization studies [10-13] do not agree with each other. While the former concludes the ordering to be of antiferromagnetic nature, the latter indicates some ferromagnetic ordering. The appearance of bulk superconductivity at low temperatures was initially criticised [14]. However, a recent report argues that bulk superconductivity exists in this compound within a magnetically ordered state [15]. Appearance of bulk superconductivity in Ru-1212 is also confirmed from specific heat (CP) measurements [16,17], though the existing reports do not agree with each other in terms of CP measurements under magnetic field [Cp(H,T)]. In particular, the superconducting transition temperature (Tc) as viewed from the CP peak increases with field in one report [16] but decreases with field in another [17]. While the former indicates towards the triplet pairing, the latter suggests a normal underdoped high-Tc superconductor (HTSC) case. It is also suggested that small impurity of GdSr2RuO6 presumably present in the samples of Ref. 16 is responsible for the CP(H,T) behaviour being different from that of Ref. 17. Also worth noting is the fact that, though µSR and ESR studies indicate towards bulk nature of magnetic ordering [2,5], the Cp measurements done by the same group of authors do not reveal a distinct peak at the ordering temperature [16]. A hump in Cp seen at the magnetic ordering temperature is rather indicative of either short-range magnetic correlations or low-dimensional magnetic ordering of Ru spins.

Even though “magneto-superconductivity” was first realised in Ru-1222, a lot still remains about the physical characterization of the phase. However the main features are the same for both Ru-1212 and Ru-1222. The magnetic structure of Ru-1222 has been studied by neutron powder diffraction [18]. Despite the fact that various physical-property measurements have been carried out on Ru-1212 [2,6-17] and Ru-1222 [1,18-23], no final consensus has been reached, i.e. discussion on their basic characteristics in terms of the oxygen stoichiometry, valence state of Ru, carrier concentration and doping mechanism has not been completed yet. This becomes more important in the event when contradictory experimental results are obtained on different samples [2,6,8,10,11,14,17,21-23]. Also, it has been reported [24] that solid solutions of composition (Ru1-xCux)Sr2GdCu2O8-δ can form within 0 < x < 0.75 with Tc up to 74 K. Interestingly with the higher x values in the above composition the Ru spins do not order magnetically down to 5 K. Henceforth to conclude the coexistence of long-range magnetic ordering of Ru spins with superconductivity in the CuO2 plane, one should strictly avoid the formation of (Ru1-xCux)-1212 solid solutions in pristine Ru-1212. Worth mentioning is the fact that Ru/Cu intermixing becomes more complicated as the two elements cannot be distinguished without ambiguity by neutron diffraction, a technique commonly used for fixing various cation occupancies in inorganic solids. Both Ru and Cu do have nearly the same scattering cross-sections for thermal neutrons.

Here it is important to note that the possible coexistence of superconductivity and magnetism (in particular ferromagnetism) has been a topic of debate for decades even in the case of more homogeneous intermetallic compounds. (For a recent update of the topic, see /article/world/15/19.) In particular the microscopic length scales

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