Michihito Muroi

Evolution of ferromagnetism in LaMnO 3+d

An extensive study has been made of the magnetic properties of LaMnO3+δ with finely tuned hole concentrations, x (=2δ), ranging between 0·08 and 0·15. As x increases in the range studied, the spontaneous magnetisation at low temperature increases from only about 20% to almost 100% of the value for full polarisation of Mn spins, and the coercivity Hc decreases by more than an order of magnitude, e.g. from 4·2 kOe to 120 Oe at 5 K. The magnetic ordering temperature Tc takes a minimum at x = xc ~ 0·12. For x < xc, the inverse susceptibility shows a marked decrease as Tc is approached from above. Measurements of minor hysteresis loops for the sample with x = 0·08 show that magnetisation is reversible with respect to field changes except near Hc. These observations are discussed in terms of a two-phase exchange coupling (TPEC) model in which it is assumed that at low temperature the system separates into two phases, a hole-free antiferromagnetic phase and an optimally doped ferromagnetic phase, and the two phases are coupled through superexchange interactions at the interface.

Crystal structure and Tc in copper oxides : a clue to the mechanism of high-temperature superconductivity

Abstract The critical temperature T c of cuprate superconductors has been mapped onto a Δ V M - d p plane where Δ V M is the difference between the Madelung site potential for a hole on a Cu site and that on an O site and d p is the Cu-O bond length within the CuO 2 plane. It is found that the materials with a larger Δ V M tend to have a smaller d p , and that T c increases either with the decrease of Δ V M or with the decrease of d p . These observations are discussed in terms of a local pair resonance model in which high- T c superconductivity is viewed as macroscopic resonance involving coherent motion of paired holes in real space. Structural features affecting T c are also discussed.

On the Rare-earth Ionic Size Effect in the R1-xPrxBa2Cu3Oy system (R=Rare-earth element)

Abstract Superconducting and magnetic properties of the R 1− x Pr x Ba 2 Cu 3 O y system can be consistently explained within the framework of the percolation model in which localization of holes by two neighbouring Pr ions and the resultant local destruction of the superconductivity are assumed. A tendency of the system to phase-separate into RBa 2 Cu 3 O y and PrBa 2 Cu 3 O y , assumed to be stronger the smaller the difference in the ionic size, shifts the percolation threshold at which Pr ions form an infinite cluster and macroscopic superconductivity gives way to long-range antiferromagnetic ordering of Pr moments. The enhanced Neel temperature T N for lighter R ions is ascribed to the increase in the number of neighbouring Pr-Pr pairs due to the clustering tendency. Results of Monte Carlo simulations are presented to support these arguments.

Diamagnetic properties of Y1−xPrxBa2Cu3Oy: Evidence for inhomogeneous superconductivity

Abstract Magnetization measurements have been made on polycrystalline samples of Y 1− x Pr x Ba 2 Cu 3 O y ( y ∼7, 0.4⩽ x ⩽0.6) at 5 K. Two threshold Pr concentrations are identified: x c1 ∼0.46 and x c2 ∼0.56. As x increases towards x c1 , the initial susceptibility ∥ d M d H ∥ H=0 decreases sharply and irreversible entry of magnetic flux into the grains begins to take place at a much lower field. The critical current density J c , derived using the Bean model, decreases monotonically with x but the slope of the J c versus x curve is much larger for x x c1 than for x > x c1 . The diamagnetic signals almost disappear at x c2 . These observations are discussed in terms of a 2D percolation model in which it is assumed that two Pr ions, when they occupy neighbouring rare earth sites, localize a hole on the oxygen site between them, locally destroying the superconductivity.

On the Sr substitution effect in the R1−xPrxBa2−ySryCu3O7 system (R=rare-earth element)

Abstract The variation of the superconducting properties with the Sr concentration y in R 1− x Pr x Ba 2− y Sr y Cu 3 O 7 can be accounted for by taking the following two factors into consideration: mutual substitution of the Sr and Pr ions on each others sites and local structural changes arising from the substitution. The former decreases the Pr concentration on the R/Pr site, while the latter modify the energy of a hole on the Cu, O and Pr sites (through the changes in the Madelung potential) and the hopping interaction between them (through the changes in the bond length), thereby affecting the ability of the Pr ions to localize a hole. It is argued that the 2D percolation model we proposed to explain the properties of R 1− x Pr x Ba 2 Cu 3 O 7 is applicable to the Sr-doped system. Possible explanations are also given of several problems regarding the behaviour of Pr ions in high- T c , as well as insulating, cuprates, including the ionic size effect on T N (the magnetic ordering temperature of Pr moments) in the related systems, e.g. Tl(Ba 1− x Sr x ) 2 PrCu 2 O 7 , Nb(Ba 1− x Sr x ) 2 PrCu 2 O 8 and PrBa 2− x Pr x Cu 3 O 7 .

Charge transfer, phase separation and percolative superconductivity in YBa2Cu3O6+y

Abstract The dependence of the superconducting properties of YBa 2 Cu 3 O 6+ y on the oxygen content y is discussed in terms of a two-phase model in which it is assumed that the optimal hole concentration, p opt = 0.25 per CuO 2 unit, is a local condition and that the average hole concentration p smaller than p opt results in phase separation into superconducting and insulating phases. For y = 1, p = p opt is satisfied everywhere in the CuO 2 plane and the material is a homogeneous bulk superconductor. As p decreases with decreasing y , the fraction of the insulating phase increases and superconductivity takes place through a 2D percolative network. Macroscopic superconductivity is lost when the insulating phase forms an infinite cluster. It is argued that the oxygen ions in the CuO y plane affect the cuperconducting properties not only by controlling p but also by modifying the hole distribution in the CuO 2 plane through local structural changes and long-range Coulomb interaction. Explanations are given of the two-plateau structure in the T c versus y curve and of the appearance of the seemingly homogeneous “60 K phase” at y ∼0.5-0.6. The relevance of the two-phase model to other systems and the origin of the phase separation are also discussed.

Sputter deposition of YBa2Cu3O7-x thin films with low gas pressure

Thin films of YBa2Cu3O7-x have been prepared by rf-magnetron sputtering using a single oxide target with a low operating gas pressure of 0.5 Pa at which almost all the sputtered atoms reach the substrate without colliding with the gas. In spite of the low oxygen partial pressure of 0.25 Pa, sufficient oxygen was supplied to the film during the deposition to form the superconducting phase on a MgO(100) substrate. The Tc of the as-grown film rose with the increase of the substrate temperature and reached 83 K at 650°C. Thanks to the high reactivity and high kinetic energy of the depositing atoms, superconducting films with excellent crystallinity were obtained over a wide range of the substrate temperature between 525 and 650°C despite the high deposition rate of 2 A/s, and the 123phase was formed even at 500°C.

Defect-induced superconductivity in PrBa2Cu3O7

Abstract The appearance of superconductivity in some PrBa 2 Cu 3 O y ( y ∼7) single crystals and thin films can be accounted for on the assumption of the presence of disorder in the Ba/Pr site occupancy. It is argued that Ba ions occupying the Pr site and Pr ions occupying the Ba site both weaken Pr–O hybridisation through local structural changes and lower the Madelung potential at the Cu site relative to that at the Pr site, thus favouring the Cu III oxidation state over the Pr IV oxidation state. Bulk superconductivity shows up when local superconducting regions, created near the defects, percolate through the sample. It is concluded that stoichiometric, defect-free PrBa 2 Cu 3 O 7 is insulating.

Low temperature synthesis of YBa sub 2 Cu sub 3 O sub 7 minus x thin films

Thin films of YBa 2 Cu 3 O 7− x (YBCO) have been prepared on MgO substrates at 650 °C by rf-planar magnetron sputtering using a single oxide target. Depositions at two different substrate positions were examined: (A) above the inner boundary of the erosion area of a target (position A), (B) right above the center of a target (position B). The films deposited at position A suffered a serious deficiency in the contents of Ba and Cu, especially in the low gas pressure region due to the bombardment of the growing films by high-energy particles. Though we could obtain almost stoichiometric films by raising the gas pressure to 15 Pa to reduce the bombardment, high temperature annealing at 950 °C was required to improve their crystallinity and to achieve high critical temperatures ( T c ). On the other hand, those deposited at position B were more excellent in crystallinity than those at position A and showed high T c above 80 K without any annealing. Critical current density C/r) at 77 K was improved from about 10 3 A/cm 2 to about 10 4 A/cm 2 by annealing at low temperature of 500 °C. It is concluded that the deposition with reduced bombardment by high-energy particles enables the fabrication of high T c YBCO films without a high temperature process.

Inhomogeneous superconductivity in Y1-xPrxBa2-ySryCu3O7

Abstract The effect of heat treatment on the superconducting properties of Y 1− x Pr x Ba 2− y Sr y Cu 3 O 7 is accounted for within the framework of the percolation model. High-temperature heat treatment in an inert gas atmosphere increases the difference in the Madelung potential between the Y/Pr and Ba/Sr sites by reducing the oxygen content in the Cu–O basal plane, thereby facilitating the ordering of the A-site cations (Y/Pr and Ba/Sr). The resultant smaller content of trivalent ions on the Ba/Sr site allows long Cu–O chains to develop during the subsequent low-temperature annealing in O 2 , thus increasing the number of holes transferred to the CuO 2 plane and hence, the fraction of the superconducting phase. The coupling between strongly superconducting regions through weak links improves, as a result, and T c increases. It is argued that the formation and growth of nonsuperconducting regions within the CuO 2 plane are the universal origin of the decrease in T c and eventual transition to an insulating state observed in the whole range of high- T c cuprates when the mobile hole concentration is reduced from the optimal value by various types of chemical substitution