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 Physics , 2001, Abstract: We present the results from a magnetization, structural, and nuclear quadrupole resonance study of the ruthenate-cuprate, RuSr2R2-xCexCu2O10+d with x=1 and R=Eu. This compound is a superconductor for Ce doping in the range 0.4<=x<=0.8 and displays ferromagnetic order for 0.4<=x<=1.0. We show that the tilting and rotation of the RuO6 octahedra are essentially the same for x=1, x=0.6 and the superconducting antiferromagnet, RuSr2GdCu2O8. However, the moment per Ru in RuSr2EuCeCu2O10+d is comparable to that observed in RuSr2EuCu2O8. These results indicate that it is unlikely that the different magnetic order found in RuSr2R2-xCexCu2O10+d and RuSr2RCu2O8 (R=Y,Gd,Eu) is due to changes in structurally induced distortions of the RuO6 octahedra as found in the ruthenate compounds (e.g. Sr1-xCaxRuO3). We show that the Cu nuclear quadrupole resonance (NQR) data are similar to those observed in the single CuO2 layer superconductor, La1-xSrxCuO4, where the Cu spin-lattice relaxation rate is dominated by hyperfine coupling within the CuO2 layers and any additional hyperfine coupling from spin fluctuations in the RuO2 layers is small.
 Physics , 1999, DOI: 10.1103/PhysRevLett.83.3713 Abstract: Relying on the inhomogeneous (layered) crystal, electronic, and magnetic structure, we show how superconductivity can coexist with the ferromagnetic phase of RuSr2GdCu2O8 as observed by Tallon and coworkers. Since the Cu d_{x^2-y^2} orbitals couple only to apical O p_x, p_y orbitals (and only weakly), which also couple only weakly to the magnetic Ru t_{2g} orbitals, there is sufficiently weak exchange splitting, especially of the symmetric CuO2 bilayer Fermi surface, to allow singlet pairing. The exchange splitting is calculated to be large enough that the superconducting order parameter may be of the Fulde-Ferrell-Larkin-Ovchinnikov type. We also note that \pi-phase formation is preferred by the magnetic characteristics of RuSr2GdCu2O8.
 Physics , 1999, Abstract: Resistivity, thermoelectric power and heat capacity are investigated in the ferromagnetic superconductor RuSr2GdCu2O8 with and without Zn substitution. The thermodynamic signatures of the ordering of the Ru moments and the paramagnetic Gd moments as well as the onset of superconductivity are all clearly seen and quantified. The materials are shown to exhibit bulk superconductivity which coexists with spatially-uniform ferromagnetism. They appear to be typical underdoped superconducting cuprates in which the pseudogap dominates normal-state transport, thermodynamic and substitutional properties. However, a field-induced increase in Tc could suggest some degree of triplet pairing.
 Physics , 2005, DOI: 10.1103/PhysRevB.72.174508 Abstract: A spontaneous vortex state (SVS) between 30 K and 56 K was observed for the weak-ferromagnetic superconductor RuSr2GdCu2O8 with ferromagnetic Curie temperature TC = 131 K and superconducting transition temperature Tc = 56 K. The low field (20 G) Superconducting hysteresis loop indicates a narrow Meissner state region within average lower critical field Bc1(T)= Bc1(0)[1 - (T/T0)2], with average Bave c1 (0) = 12 G and T0 = 30 K. Full Meissner shielding signal in very low applied field indicates an ab-plane Bab c1(0) ~ 4 G with an estimated anisotropic parameter g ~ 7 for this layered system. The existence of a spontaneous vortex state between 30 K and 56 K is the result of weak-ferromagnetic order with a net spontaneous magnetic moment of ~ 0.1 muB/Ru, which generates a weak magnetic dipole field around 10 G in the CuO2 bi-layers. The upper critical field Bc2 varies linearly as (1 - T/Tc) up to 7-T field. The vortex melting line Bm varies as (1 - T/Tm)3.5 with melting transition temperature Tm = 39 K and a very broad vortex liquid region due to the coexistence and the interplay between superconductivity and weak-ferromagnetic order.
 Iranian Journal of Physics Research , 2010, Abstract: We have performed a first-principle calculation of electronic structure of RuSr2GdCu2O8, a ferromagnetic-superconductor, by employing a full-potential linearized augmented plane-wave method within the density functional theory. Hydrostatic pressure applied up to 6 GPa by varying the volume of the unit cell with constant a:b:c ratio. Optimization of internal parameters showed that there exists shear stress due to the residual forces in Ru-O layers which leads to anti-phase rotation of RuO6 octahedra in experimental structure of this compound. Partial charge analysis showed that applying pressure leads to hole injection in Cu-O sheets and by applying charge transfer model, we obtained 1.9 K/GPa for the rate of increase in superconducting transition temperature with pressure. The exchange coupling interaction J between the adjacent Ru atoms was calculated by energy difference between the AFM and FM configuration of magnetic structure of Ru atoms. According to the result of the calculations, the magnetic moment of Ru atoms decreased and exchange coupling parameter J increased by applying pressure.
 Physics , 2003, Abstract: The properties of the MoSr2RCu2O8 (R=rare earth) system are found to systematically change with the contraction of the R ions. For the light R ions (La-Nd) the samples are paramagnetic down to 5 K, whereas in the intermediate range (Sm-Tb), the Mo sublattice orders antiferromagnetically at TN, ranging from 11 to 24 K. For the heavy R ions, Ho-Tm and Y, superconductivity appears at TC in the range 19-27 K and antiferromagnetism sets in at TN < TC. This latter behavior resembles most of the magneto-superconductors, but is in sharp contrast to the iso-structural RuSr2RCu2O8 system where TN > TC.
 Physics , 2000, DOI: 10.1103/PhysRevB.62.11369 Abstract: The phase diagram of temperature versus exchange field is obtained within a BCS model for d-wave superconductivity in CuO_2 layers which is coupled to ferromagnetic RuO_2 layers in RuSr_2GdCu_2O_8. It is found that the Fulde-Ferrell-Larkin-Ovchinnikov state is very sensitive to the band filling factor. For strong exchange field, we point out that superconductivity could only exist in the interfaces between ferromagnetic domains. The magnetization curve is calculated and its comparison with experiment is discussed. We also propose the measuring of tunneling conductance near a single unitary impurity to detect the strength of the exchange interaction.
 Physics , 2007, DOI: 10.1016/j.physb.2007.10.350 Abstract: Similar to the optimal-doped, weak-ferromagnetic (WFM induced by canted antiferromagnetism, T$_{Curie}$ = 131 K) and superconducting (T$_{c}$ = 56 K) RuSr$_{2}$GdCu$_{2}$O$_{8}$, the underdoped RuSr$_{2}$EuCu$_{2}$O$_{8}$ (T$_{Curie}$ = 133 K, T$_{c}$ = 36 K) also exhibited a spontaneous vortex state (SVS) between 16 K and 36 K. The low field ($\pm$20 G) superconducting hysteresis loop indicates a weak and narrow Meissner state region of average lower critical field B$_{c1}^{ave}$(T) = B$_{c1}^{ave}$(0)[1 - (T/T$_{SVS}$)$^{2}$], with B$_{c1}^{ave}$(0) = 7 G and T$_{SVS}$ = 16 K. The vortex melting transition (T$_{melting}$ = 21 K) below T$_{c}$ obtained from the broad resistivity drop and the onset of diamagnetic signal indicates a vortex liquid region due to the coexistence and interplay between superconductivity and WFM order. No visible jump in specific heat was observed near T$_{c}$ for Eu- and Gd-compound. This is not surprising, since the electronic specific heat is easily overshadowed by the large phonon and weak-ferromagnetic contributions. Furthermore, a broad resistivity transition due to low vortex melting temperature would also lead to a correspondingly reduced height of any specific heat jump. Finally, with the baseline from the nonmagnetic Eu-compound, specific heat data analysis confirms the magnetic entropy associated with antiferromagnetic ordering of Gd$^{3+}$ (J = S = 7/2) at 2.5 K to be close to $\it{N_{A}k}$ ln8 as expected.
 Physics , 2002, DOI: 10.1103/PhysRevB.67.184507 Abstract: The macroscopic magnetizations of a RuSr2(Eu0.7Ce0.3)2Cu2O10+d sample were investigated. A ferromagnet-like transition occurs around T_M in the low-field magnetization. Highly nonlinear M(H), non-Curie-Weiss susceptibility, and slow spin-dynamics, however, were observed up to T_1 approx 2-3 T_M. In addition, an antiferromagnet-like differential-susceptibility maximum of Ru appears around a separate temperature T_AM between T_1 and T_M. The data are therefore consistent with a phase-separation model: superparamagnetic clusters (or short-range spin-orders) are first precipitated from the paramagnetic matrix below T_1, followed by an antiferromagnetic transition of the matrix at T_AM and an apparent ferromagnetic (FM) transition around T_M, where the long-range spin-order is established in the FM species imbedded in the matrix.
 Physics , 2005, DOI: 10.1140/epjb/e2006-00317-4 Abstract: Magnetic-field changes $<$ 0.2 Oe over the scan length in magnetometers that necessitate sample movement are enough to create artifacts in the dc magnetization measurements of the weakly ferromagnetic superconductor RuSr$_{2}$GdCu$_{2}$O$_{8}$ (Ru1212) below the superconducting transition temperature $T_{c} \approx$ 30 K. The observed features depend on the specific magnetic-field profile in the sample chamber and this explains the variety of reported behaviors for this compound below $T_{c}$. An experimental procedure that combines improvement of the magnetic-field homogeneity with very small scan lengths and leads to artifact-free measurements similar to those on a stationary sample has been developed. This procedure was used to measure the mass magnetization of Ru1212 as a function of the applied magnetic field H (-20 Oe $\le$ H $\le$ 20 Oe) at $T < T_{c}$ and discuss, in conjunction with resistance and ac susceptibility measurements, the possibility of a spontaneous vortex state (SVS) for this compound. Although the existence of a SVS can not be excluded, an alternative interpretation of the results based on the granular nature of the investigated sample is also possible. Specific-heat measurements of Sr$_{2}$GdRuO$_{6}$ (Sr2116), the precursor for the preparation of Ru1212 and thus a possible impurity phase, show that it is unlikely that Sr2116 is responsible for the specific-heat features observed for Ru1212 at $T_{c}$.
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