Charge and spin states of Ru and Cu in magnetic superconductorRuSr2GdCu2O

Abstract

Electronic and magnetic properties of RuSr 2 GdCu 2 O 8 have been investigated by electric resistivity, magnetization, and Cu-, Ru-NMR measurements. Magnetic order (T M ϭ133 K) and superconductivity ͓T s (onset)ϳ52 K͔ have been confirmed. We observed two kinds of Ru-NMR signals ͑the hyperfine fields of 101 Ru are 590 kOe and 290 kOe͒, suggesting a charge segregation of Ru 5ϩ (Sϭ3/2) and Ru 4ϩ (Sϭ1) with complete replacement of the Cu-O chains by Ru-O layers. The RuO 6 octahedra are connected through apical oxygens to two layers of CuO 5 square pyramids. The magnetic order of Ru ions in RuSr 2 RCu 2 O 8 is believed to be associated with the hybridization of Ru-4d and oxygen 2p orbital in the RuO 2 planes, while superconductivity could occur in the CuO 2 planes, if holes are doped inherently into Cu-O 3d-2p orbitals. In this sense, the valence states of Cu and Ru ions are crucial for understanding the origin of ferromagnetism and superconductivity. It is also quite interesting if RuSr 2 RCu 2 O 8 is a particular ferromagnetic superconductor with a high magnetic ordering temperature, as this would suggest a superconducting order parameter of the FuldeFerrell-Larkin-Ovchinnikov type 9 or a possible triplet superconductivity. 4 An interplay between both the order parameters through electromagnetic interactions would produce an exotic state such as a self-induced vortex state, 10 as investigated previously for a magnetic superconductor, Ce 1Ϫx Gd x Ru 2 . 11 Concerning the magnetic nature, the ferromagnetism is still puzzling. Though large magnetic moments would be expected for the ionic state of Ru 5ϩ (Sϭ3/2) or Ru 4ϩ (S ϭ1), an upper limit of the saturated magnetic moments under applied field are obtained to be ϳ1.05 B by the magnetization measurements. 2-4 Moreover, a weak ferromagnetism is observed by the spontaneous magnetization at zero field cooling, and the ferromagnetic component is estimated to be 0.28 B . 2,3 The Ru moments in the ferromagnetic state are suggested to be perpendicular to the c axis by a SR experiment. 3 However, the recent neutron diffraction measurement on RuSr 2 GdCu 2 O 8 ͑Ref. 12͒ demonstrates that the magnetic order of the Ru moments (ϳ1.18 B ) is antiferromagnetic along the c axis in contrast to the conclusion of the ferromagnetic ordering. The ferromagnetic contribution is presumed to be a canting component in the antiferromagnetic arrangements. A recent band calculation 13 supports the lowest energy for an antiferromagnetic arrangement. Thus, the magnetic nature of RuSr 2 RCu 2 O 8 is still controversial.Magnetic order and superconductivity are believed to be mutually exclusive in general. Therefore, a naive question arises: how does the superconducting order parameter develop under the ferromagnetically ordered state with very high T M ? In order to gain insight into the true origin of the coexistence of the magnetism and superconductivity, further extended experiments, especially from a microscopic point of view, are required. Here, we have carried out nuclear magnetic resonance...

Magnetic transitions and ferromagnetic clusters inRuSr2(Eu,Ce)Xue1, Lorenz2, Cao3 et al. 2003Phys. Rev. B315350View full textAdd to dashboardCite

“…6 While at least two other NPD studies claim similarly negligible FM components, 7,8 both the large FM moment and the perpendicular spin direction were confirmed by independent NMR measurements. 9 These conflicts will be difficult to reconcile, in our opinion, if these ruthenocuprates are microscopically homogeneous.…”

Mössbauer study of the (Ru1-xFex)Sr2GdCu2O8-δ system and two of its possible impurities: SrRuO3 and Gd2CuO4

“…On the other hand, the only surroundings compatible with the high ΔQ (≈ 1.70 mm/s) value of doublet C would turn out to be Fe 3+ in four-fold (planar) coordination in the Ru sites. The assigned Fe coordination is understood from the fact that while the charge character of the Ru ions in the structure is a 4+/5+ mixed-valence state [12,30], that of the Fe atoms tends to be 2+/3+ ionic state. Therefore, in order to keep the charge valence of the system, the oxygen environment around the Fe ions in the Ru sites tends to a five-fold pyramidal coordination with the apical oxygen atoms of the SrO planes (doublet A), or to a four-fold planar coordination with two apical oxygen atoms (doublet C).…”

“…Since the RuSr 2 GdCu 2 O 8 compound is found to be oxygen-stoichiometric, it is then assumed that the oxidation state of Ru should be 5+, and that of copper 2+. However, the proposed self-doping mechanism would imply a 4+/5+ mixed-valence state of Ru (as has been experimentally determined [12,13]) and a 2+/3+ mixed-valence state of Cu. The distribution of holes between the two bands may then be written as Ru 5−2p Sr 2 Gd(Cu 2−p ) 2 O 8 [14].…”

The Superconducting and Magnetic States in RuSr2GdCu2O8, Based on the Magnetic, Transport and Magneto-Caloric Characteristics

“…The observed field dependences of the resonance frequencies led to propose the model structure built of the ferromagnetic a-b planes, with expectance of the field induced spin flop transition to ferromagnetism involving the in-plane components of the magnetisation, likely to occur also in view of expected substantial magnetic anisotropy of the Ru magnetic lattice [13]. Several experiments evidenced presence of the Ru 5+ and Ru 4+ valence states in RuSr 2 GdCu 2 O 8 , [13][14][15][16]. Recently communicated investigation of the magnetic ground state by means of the resonant X-ray diffraction with data collected for the small single crystal of superconducting RuSr 2 GdCu 2 O 8 , has reconciled some of the previous hypotheses with an observation of the G-type structure with the Ru magnetic moments pointing along the low symmetry direction, and with the ferromagnetic in-plane components proposed to align antiparallely in the adjacent Ru-O layers [17].…”