H. L. Tsay

Effects of Ca substitution on the structural, electrical and magnetic properties of PrBa2Cu3O7−y

Abstract The nominal (Pr 1− x Ca x )Ba 2 Cu 3 O 7− y and Pr(Ba 1− x Ca x ) 2 Cu 3 O 7− y (0⩽ x ⩽0.2) systems are studied for the effects of Ca substitution on the structural, electrical and magnetic properties of PrBa 2 Cu 3 O 7− y . The X-ray data show that the former system retains its orthorhombic structure, whereas the latter system undergoes an orthorhombic-to-tetragonal transition at about x = 0.1. The room-temperature electrical resistivity of (Pr 1− x Ca x )Ba 2 Cu 3 O 7− y decreases monotonically with increasing Ca concentration, while it exhibits a minimum at x ∼ 0.15 in Pr(Ba 1− x Ca x ) 2 Cu 3 O 7− y . Meanwhile, the substitution of Ca in Pr sites is less effective than in Ba sites in reducing the antiferromagnetic ordering temperature T N of PrBa 2 Cu 3 O 7− y . These results are discussed in conjunction with the roles of hole-compensation, chemical pressure effect, hybridization and atomic disorder in the anomalous physical properties of PrBa 2 Cu 3 O 7− y .

Superconductivity and magnetic order in RSr2Cu2.7Mo0.3O7−δ (R = Y, Pr, Gd, and Tb)

Abstract Polycrystalline samples RSr 2 Cu 2.7 Mo 0.3 O 7−δ with R = Y, Pr, Gd, and Tb were synthesized and studied by means of powder X-ray diffraction, electrical resistivity, magnetic susceptibility, and specific-heat measurements. All four samples form a perovskite-layer structure with a tetragonal symmetry. The calculated lattice constants using space group P4/mmm indicate that these rare-earth ions are basically in the 3+ valence state. Samples R = Y, Gd, and Tb are superconducting with transitions at around 30 K, while PrSr 2 Cu 2,7 Mo 0.3 O 7−δ remains semiconducting down to 2 K. Effective magnetic moments μ eff derived from a Curie-Weiss-behavior susceptibility for R = Gd and Tb are 7.92 μ B and 9.75 μ B , respectively, which are close to those of trivalent free ions. While the μ eff = 3.05 μ B for R = Pr is smaller than that expected for a Pr 3+ free ion. A magnetic ordering transition is observed by a λ-type anomaly in specific-heat measurements at 2.27 K and 5.43 K for R = Gd and Tb, respectively. In contrast, no significant feature was observed in specific heat for PrSr 2 Cu 2.7 Mo 0.3 O 7−δ in the temperature range 0.6–40 K. The similarities and differences in superconducting and magnetic properties between RSr 2 Cu 2.7 Mo 0.3 O 7−δ and RBa 2 Cu 3 O 7−δ are discussed.

Superconductivity and magnetism in Y{sub 1{minus}{ital x}}Pr{sub {ital x}}Sr{sub 2}Cu{sub 2.7}Mo{sub 0.3}O{sub 7{minus}{delta}}

No magnetic transition is observed for PrSr{sub 2}Cu{sub 2.7}Mo{sub 0.3}O{sub 7{minus}{delta}} down to 0.6 K. However, a superconducting transition with {ital T}{sub {ital c}}{similar_to}17 K is seen by partially substituting Ca for Pr in Pr{sub 1{minus}{ital x}}Ca{sub {ital x}}Sr{sub 2}Cu{sub 2.7}Mo{sub 0.3}O{sub 7{minus}{delta}}. The similarities and differences in superconductivity and magnetism between Y{sub 1{minus}{ital x}}Pr{sub {ital x}}Sr{sub 2}Cu{sub 2.7}Mo{sub 0.3}O{sub 7{minus}{delta}} and Y{sub 1{minus}{ital x}}Pr{sub {ital x}}Ba{sub 2}Cu{sub 3}O{sub 7{minus}{delta}} are compared and discussed.

Structural, magnetic and electrical studies on Pr(Ba1−xSrx)2Cu2NbO8−y

Abstract Structural, magnetic and electrical properties of Pr(Ba 1- x Sr x ) 2 Cu 2 NbO 8- y , having a similar structure to PrBa 2 Cu 3 O 7- y but with NbO 2 planes replacing the CuO chains, are investigated by means of X-ray diffraction, magnetic susceptibility and electrical resistivity experiments. Only a small diffraction peak was observed to grow when substituting Sr for Ba. The decrease of the lattice parameters a , c and V with x in Pr(Ba 1- x Sr x ) 2 Cu 2 NbO 8- y following Vegards law may indicate the same crystal structure for both end members. The antiferromagnetic-transition temperature T N for PrBa 2 Cu 2 NbO 8- y determined from the peak in the temperature dependence of magnetic susceptibility decreases rapidly with increasing Sr concentration. Also as the Sr concentration is increased, the effective paramagnetic moment μ eff of the Pr ion is enhanced, while the electrical resistivity is reduced. These results are discussed in conjunction with the structural aspect, hybridization and magnetic coupling mechanism for these Pr-based cuprates.

Effects of Ga doping on the magnetic ordering of Pr in PrBa2Cu3O7

Neutron‐diffraction and ac‐susceptibility measurements have been performed to study the effects of Ga doping on the magnetic ordering of the Pr in PrBa2(Cu1−xGax)3O7. The Ga atoms preferentially replace the Cu atoms in the CuO‐chain layers, and this substitution is found to decrease the antiferromagnetic ordering temperature as the Ga concentration is increased. In addition, the spin arrangement along the c axis is found to change from nearest neighbors being antiparallel for x=0 to nearest neighbors being parallel for x=0.08 (24% Cu chain substitution); for an intermediate x of 0.04 a mixture of the two spin structures is observed. The susceptibility results exhibit Curie–Weiss behavior above TN, and departures from this behavior in the ordered state.

Thermal properties of PrBa2Cu4O8 prepared at ambient oxygen pressure

Abstract Polycrystalline PrBa2Cu4O8 (Pr124) has been prepared at ambient oxygen pressure by nitrite pyrolysis method. Powder x-ray-diffraction patterns show a nearly single R124 phase. Thermogravimetric analysis indicates that its thermal stability is distinct from that of Pr123. Specific heat C has been measured from 0.5 to 40 K, and is very similar to that of Pr123. A maximum of C occurs around 17 K which could be due to a magnetic ordering. Entropy difference ΔS has been calculated from ΔC/T between Pr124 and Y124. The possible origins of ΔS and the related magnetic properties are discussed. Resistivity ϱ(T) of Pr124 shows a nearly metallic behavior similar to what was observed in Pr124 made by the O2-HIP technique. The metallic ϱ(T) at low temperatures is of interest and will be discussed.

Structure and superconductivity of Y (Ba1−xRx)2Cu3O7−δ (R=La, Pr, and Nd)

Powder X-ray diffraction and electrical-resistivity measurement were used to study the structure and the Tc variation of Y(Ba1−xRx)2Cu3O7−δ(0≤x≤0.2; R=La, Pr, and Nd). The solubility limit was estimated as x=0.05, 0.075, and >0.2 with R=Nd, Pr, and La, respectively, which depends on the ionic radius of dopants. All three series undergo an orthorhombic-to-tetragonal transition at x≈0.15-0.2. Meanwhile, the Tc increases slightly through a maximum at x≈0.025; then decreases rapidly with x, indicating an overdoped system for YBa2Cu3O7−δ. Whereas, the normal-state resistivity increases with x for all temperature ranges. The similarities and differences with various dopants are discussed in conjunction with ionic size, valence change and hole-filling mechanism.

Magnetic properties of PrSr2-based cuprates

Abstract The magnetic properties of four PrSr 2 -based cuprates PrSr 2 Cu 2.7 Mo 0.3 O 7−δ , PrSr 2 Cu 3 Pb 2 O 8+δ and PrSr 2 Cu 2 MO 8−δ M = Nb, Ta are studied. Values of effective moment μ eff for Pr ions obtained from the Curie-Weiss law at high temperatures are ranging from 3.0 to 3.3 μ B , which are smaller than that of free Pr 3+ moment 3.58 μ B . The magnetic ordering temperature T N determined by the relative minimum in dχ/dT- T curve is 8.5 K for PrSr 2 Cu 3 Pb 2 O 8+δ and

Anomalous magnetic susceptibility of Y2Fe3Si5

Abstract Results of temperature and field dependence of the magnetic susceptibility for Y 2 Fe 3 Si 5 are reported. A maximum in magnetic susceptibility of Y 2 Fe 3 Si 5 is observed at T max ≈110 K. T max decreases with increasing magnetic field and disappears at fields above 2000 Oe. In addition, this anomaly is also suppressed by substituting a small amount of Sc for Y. It is found that these anomalous magnetic properties are strongly correlated with the dramatic pressure effect on T c of Y 2 Fe 3 Si 5 . Several possible mechanisms such as the structural phase transition, a few ppm of iron impurity effect, Fermi surface topology, spin density wave formation and spin-glass state are considered and discussed to be responsible for these unusual phenomena.