Magnetic ordering of Fe atoms in YBa2(Cu1−x Fe x )3O3 in superand semiconducting states

Magnetic behavior of Fe atoms in YBa₂(Cu_1−x Fe _x )₃O _y system in superconducting (SC) and non-SC states is studied. In SC region one magnetic ordering point T_m1 is observed for all Fe atoms in Cu1 sites: T_m1=10 K and 16 K for x=0.05 and x=0.10, respectively. In the non-SC state for x=0.05 Cu1 and Cu2 sublattices are magnetically independent and two magnetic ordering points are found: T_m1=20 K and T_m2=405K. For x=0.10 a strong magnetic interaction between Cu1 and Cu2 sublattices appears and only one ordering point T_m2=435 K is observed.     

Three-dimensional antiferromagnetic ordering in the light rare-earth high-T c superconductor NdBa2Cu3O6.86

Powder neutron diffraction investigations performed in the temperature range from 20 mK to 300K prove that the magnetic Nd moments of the superconductor NdBa₂Cu₃O_6.86 withT _c=88 K order three-dimensionally (3D) antiferromagnetic belowT _N=(551±10) mK. As in similar Gd and Dy compounds the corresponding wave vector isk=[1/2, 1/2, 1/2]. In approximate agreement with crystal field calculations the ordered magnetic moment of Nd amounts at saturation to (1.14±0.06) _B and at 25 mK the magnetic moments are oriented parallel [0,0,1]. The transition to the magnetically ordered state corresponds to the Landau type critical exponent 0.5, in contrast to the predominant 2D character of such heavy rare-earth systems. The crystal structure of NdBa₂Cu₃O_6.86 is orthorhombic similar to the one of YBa₂Cu₃O_7–x.     

Pressure-induced superconductivity in tetragonal NdBa2Cu3O6+x

The appearance of superconductivity and relaxation of the transition temperature to its equilibrium value T _c≈30 K over the course of five days have been observed in a tetragonal sample of NdBa₂Cu₃O_6.67 under 1 GPa pressure. The time dependence T _c(t) correlates with a decrease of the room-temperature electrical resistance R(t). The superconducting phase disappears 1.3 h after the pressure is removed. This behavior is explained by a redistribution of charge as a result of pressure-induced oxygen reordering in the CuO_x planes. A large effect of oxygen ordering on the transition temperature under pressure has been observed near the metal-insulator transition (the largest of all those measured in the 1-2-3 system), along with a nonlinear temperature dependence of T _c. Fiz. Tverd. Tela (St. Petersburg) 39, 1328–1334 (August 1997)     

Antiferromagnetic order in PrBa2Cu3O x (x≈6, 7)

Zero and weak transverse fieldμ ⁺ SR measurements on PrBa₂Cu₃O _x samples withx≈6 andx≈7 show ordered magnetism in both oxygen concentration limits. As expected for equivalent doping (nominally PrBa₂Cu₃O _x ∼YBa₂Cu₃O _x −0.5), neither sample is superconducting. Two muon signals are observed in thex≈6 sample, as in YBa₂Cu₃O₆, but only one (the weaker) has the same local magnetic field as in YBa₂Cu₃O₆. In thex≈7 sample, only one site is observed; its local field is reduced with respect to that of the primary site in YBa₂Cu₃O₆ by a factor roughly consistent with the carrier-density dependence of the Cu ion moment in antiferromagnetic YBa₂Cu₃O _x .     

Superconductivity and magnetic order in CaLaBaCu3O z

X-ray diffraction, thermogravimetric analysis, magnetization and Mossbauer studies of a⁵⁷Fe probe, in CaLaBa(Cu_1−x Fe _x )₃O _z withz=7, 6.5 andx=0.00 to 0.05 have been performed. Forz=7,T _c drops from 74 K forx=0.00 to 35 K forx=0.05. In CaLaBa(Cu_1−x Fe _x )₃O _z withz=7, 10% of the iron is magnetically ordered withH _eff=530 K kOe andT _N=400 K, even though the sample is superconducting. In the oxygen poor, non-superconducting samples (z=6.5) 20% of the iron is magnetically ordered withH _eff=340 kOe andT _N=340 K. Since the iron reflects the magnetic order of the Cu(2) ions, this may show that perhaps two inequivalent Cu(2) sites exist in CaLaBa(Cu_1−x Fe _x )₃O₇, ones of which is magnetically ordered. The experimental observations may be interpreted in terms of the special crystal structure which can allow superconductivity and magnetism to coexist, probably in separate Cu(2) planes.     

Mössbauer absorption in some high Tc superconductors:170Yb in YbBa2Cu3O7−x,166Er in ErBa2Cu3O7−x,and57Fe in YBa2Cu3O7−x

For the high T_c compound YbBa₂Cu₃O_7−x, Mossbauer absorption measurements show that magnetic ordering occurs within the Yb³⁺ sublattice at 0.35K (μ_sat=1.7μ_B). For ErBa₂Cu₃O_7−x the Er³⁺ sublattice orders at 0.7K (μ_sat=4μ_B). Measurements on⁵⁷Fe diluted into YBa₂Cu₃O_7−x shows that no ordered magnetic moments exist within the Cu sublattices down to at least 4.2K. Instituto de Fisica, Porto-Alegre, Brasil (supported by CNPq)     

A muon spin rotation study of hydrogen in YBa2Cu3O y

Superconducting YBa₂Cu₃O _y samples were doped with hydrogen and investigated by the muon spin rotation technique. For hydrogen concentrations above a thresholdx, which depends on the oxygen stoichiometry, we find a well defined precession signal in zero external field. This is a clear indication of magnetic ordering in these samples. In samples with a smaller hydrogen content no magnetic ordering was found. For these samples however, the depolarization rate σ(T→0) as measured in a transverse external field depends strongly on the hydrogen content. Our data are consistent with the assumption that hydrogen acts as an electron donor, filling the hole states in YBa₂Cu₃O _y .     

Local magnetism in superconducting YBa2Cu3O6+x

Oxygen-deficient YBa₂Cu₃O_6+x samples were investigated by the muon spin rotation technique in zero external field. In an intermediate oxygen concentration region we find strongly relaxing μSR signals indicating the existence of internal magnetic fields. The strongly relaxing signals were observed only in samples with superconducting transition temperatures below 50 K. As an interpretation of the data we suggest that magnetic ordering and superconductivity coexist in these samples within regions of extensions of a few lattice constants, but not necessarily on the same sites.     

Magnetic phase present in YBa2(Cu1−x Fe x )3O6+δ oxides

The Mössbauer study of⁵⁷Fe: YBa₂Cu₃O_6+δ oxides was very important to establish the preferential occupation of Cu(1) site by Fe at very low concentrations. Recent determination of antiferromagnetic ordering for Cu moments in Cu(2) sites (T≈450 K) and our early observation of a small proportion of a magnetic phase at room temperature for Fe:YBa₂Cu₃O₆ lead us to perform systematic studies of YBa₂(Cu_1?x Fe _x )₃O₆ withx=0.005, 0.03, 0.05, 0.10 and 0.15 in order to obtain information about the iron occupation of Cu(2) sites.     

Superconductivity and magnetic ordering in YBa2(Cu1−x Fe x )3O7

Transverse-and zero-field μSR measurements have been made for YBa₂(Cu_1−x Fe _x )₃O₇ withx=0.04, 0.08 and 0.12. The temperature range studied was from approximately 7.5 K to 100 K. The onset of magnetic ordering commences at about 7.5 K forx=0.04, 10 K forx=0.08 and 20 K forx=0.12. The Gaussian depolarization parameter, σ ofG _x (t) = exp(−σ² t ²/2), is depressed by a factor of about 0.6 forx=0.04, but for thex=0.08 sample σ is depressed by a factor of 10 and increasing suppression is seen as the temperature is lowered below 45 K. This decrease in σ is interpreted in terms of decreasing electronic mean free paths.     

Magnetic ordering of YBa2(Cu1−x Fe x )3O7

The Mössbauer spectra of YBa₂(Cu_1?x Fe _x )₃O₇ at room temperature show several doublets attributed to Fe in Cu(1) sites with different oxygen configurations. Here we present a systematic study performed at 4.2 K forx=0.005, 0.01, 0.03, 0.05, 0.10, 0.15. To obtain information about the magnetic ordered state two samples, withx=0.005 andx=0.15, have been studied at 4.2 K underB _ext=5 T. The Mössbauer spectra indicate that the iron moments are polarized forx=0.005, while in the ordered state (x=0.15) they have an antiferromagnetic or spin-glass-like arrangement with high anisotropy.     

Positive muon sites and possibility of anyons in the YBa2Cu3O x system

Possible positive muon sites in YBa₂Cu₃O _x were determined from the observedμ ⁺ hyperfine fields in antiferromagnetically ordered YBa₂Cu₃O _x and GdBa₂Cu₃O₇. After determining theμ ⁺ sites, the possibility of anyons or chiral spin ordering in the superconducting YBa₂Cu₃O₇ is discussed. Positive muon implanted in YBa₂Cu₃O₇ feel static magnetic fields of average 1.4G, which are explicable in terms of nuclear magnetic dipolar fields. Non observation of static local magnetic fields of electronic origin (the upper limit is the order of 0.1 G) means that anyons or chiral spin ordering might not exist in superconducting YBa₂Cu₃O₇.     

Study of highT c superconductor YBa2Cu3O x by TDPAC

We report in this paper Time Differential Perturbed Angular Correlation studies in YBa₂Cu₃O _x wherex=6.93 and 6.78. We show by the measurements presented herein that the results depend crucially on the oxygen content of the sample which was independently checked to affect also theT _c and the width of the superconducting transition.     

Magnetism and candidate muon-probe sites in RBa2Cu3O y

Key results of zero-field (ZF) and transverse-field (TF) muon-spin-relaxation (μSR) experiments on superconducting and insulating RBa₂Cu₃O _y (R123 _y , with R=Eu, Gd, Pr and Pr/Y:y=6, 7) are examined. The chemical behavior of the positive muon probe is addressed, and muon-oxygen bonding is shown to occur in all these cuprates. To explain magnetic fields at muon-probe sites in Pr _x Y_1−x Ba₂Cu₃O _y (0<=x<0.5,y=7 andx=0,y=6) samples, improvements on the reported magnetic structures from neutron diffraction are necessary. Cu magnetism in Pr123y (y=6,7) is observed belowT _N1, which is near RT. The magnetism seen belowT _N2 can be interpreted assuming an additional ordering in the Cutt-O chain layers. Alternatively, Pr ordering is also considered as the cause of the second phase transition. Considering the specific muon-probe location, a more detailed interpretation can be provided for the μSR parameters, measured in the normal and mixed states of these unconventional superconductors.     

μ+SR studies on superconducting YBa2(Cu1−xFex)3Oy system

Spin-glass like magnetic ordering of iron moments was observed in both orthorhombic and tetragonal YBa₂(Cu_1−xFe_x)₃O_y (x=0.08) by μ⁺SR measurements. In a “Tetra” sample, all the muons sense the superconducting transition at 60 K and magnetic ordering at around 15 K, while in an “Ortho” sample they reveal that two magnetically different parts exist in the sample: about 40% of the sample is superconducting withT _c ≈90K and the remaining part is magnetic withT _M≈33K. These phenomena can be explained in terms of clustering of the Fe atoms in the “Ortho” sample.     

161Dy Mössbauer measurements in DyBa2Cu3O7−x

Superconducting and non-superconducting samples of DyBa₂Cu₃O_7?x have been examined by Mössbauer spectroscopy down to 0.05 K. The saturated Dy³⁺ sublattice 4f shell magnetic moments and quadrupole interactions show fairly wide distributions in both samples. These are attributed to crystal field inhomogeneities. Neither the Dy³⁺ paramagnetic relaxation rates nor the isomer shifts show any evidence of coupling between Dy³⁺ and a possible conduction band.     

Investigation of superconductivity and magnetism in ceramic YBa2Cu3O6+x

In this work the magnetic and superconducting properties of YBa₂Cu₃O_6+x , ceramic samples are studied with regard to there preparations technology and oxygen content. It is shown that at 0.41≤x≤0.45 the magnetic and superconducting states coexist.     

Systematic Mössbauer study of YBa2Cu3−x Fe x Oδ and PrBa2Cu3−x Fe x Oδ as a function of temperature

We study the similitudes and differences between YBa₂Cu_3?x Fe _x O_δ and PrBa₂Cu_3?x Fe _x O_δ systems, using Mössbauer spectroscopy. Both systems, withz=0.04, were studied at several temperatures. The spectra of PrBa₂Cu_3?x Fe _x O_δ showed four asymmetric quadrupole doublets with apparently different line widths, which were fitted with four symmetric superimposed quadrupole doublets with the same line width, but assuming a small (～4–6 kOe) residual magnetic field in the Fe sites, which are mainly the Cu(1) sites of the 1–2–3 structure. On the other hand, the PrBa₂Cu_3?x Fe _x O_δ spectra showed only three asymmetric quadrupole doublets which can be fitted with three superimposed symmetric doublets and a residual magnetic field. In this case, the Fe atoms also occupy the Cu(1) sites of the 1–2–3 structure. The temperature variations of the Mössbauer spectra and their parameters seem to indicate that, in the Pr-system, a structural change takes place between 12 K and 40 K.     

Magnetic interactions between Copper and RE in RE Ba2Cu3O7-δ

The antiferromagnetic ordering temperatures of the rare earth (RE) moments in RE Ba₂ Cu₃O_7-gd, obtained from specific heat measurements, are roughly in agreement with the de Gennes factors, i.e., the ordering mechanism can be mainly seen as an indirect spin-spin exchange. However, the oxygen dependence of T _N is found to be reversed for the light rare earths compared to the heavy rare earths. As origin for this systematic observation an indirect interaction between the 3d-moments of copper and the 4f-moments of rare earths is discussed as a second order effect. Such an interaction is supported by measurements of the 4f relaxation behavior on the Nd 1: 2: 3 cuprates by inelastic magnetic neutron scattering. Here, the usual thermally driven increase of the magnetic relaxation rate is suppressed up to about 80 K. This correlates with the appearance of a spin gap found by Rossat- Mignod in YBa₂Cu₃O₇ and therefore the 3d-4f coupling can be understood as an interaction of the 4f moments with a spin-fluctuation exchange in the CuO₂ planes. Furthermore, the quasielastic magnetic response has a Gaussian contribution at temperatures below 100 K, i.e., much above the long ranged ordering temperatures T _N. Magnon-like excitations appear already at slightly larger temperatures than T _N. In addition the paramagnetic inelastic spectra show only little dependence of the crystal field scheme on the oxygen concentration.     

μSR in oxygen deficient YBa2Cu3O x (6.5⩽x⩽7.0)

A systematic μSR study of the local magnetic field distribution in a series of oxygen deficient YBa₂Cu₃O _x samples with 6.5⩽x⩽7.0 is reported. Special attention was given to perform the experiments under the same conditions, so that the oxygen content of the measured samples was the only parameter varied. The behavior of the depolarization rate σ as a function of the oxygen contentx was found to have strong similarities with the behavior of the critical temperatureT _c as a function ofx. In particular, two step-like increases of σ were observed abovex=6.7 and 6.9. The temperature dependence of the normalized depolarization rate σ(T)/σ(0) is well described by the two-fluid model forx⩾6.781(1) and clearly deviates from this behavior forx⩽6.704(1). Our results are compared to those obtained by other groups.