Publications


Observation of a magnetic transition in para-pyridyl nitronyl nitroxide using zero-field μSR

Solid State Communications 92 (1994) 569-572

SJ Blundell, PA Pattenden, RM Valladares, FL Pratt, T Sugano, W Hayes

We report the observation of a magnetic transition at around 90 mK in the nitronyl nitroxide radical system para-pyridyl nitronyl nitroxide (p-PYNN) in zero applied field using the muon-spin rotation (μSR) technique. © 1994.


Magnetoresistance and magnetization in submicron ferromagnetic gratings

Journal of Applied Physics 75 (1994) 5249-5256

C Shearwood, SJ Blundell, MJ Baird, JAC Bland, M Gester, H Ahmed, HP Hughes

A technique for engineering micron and submicron scale structures from magnetic films of transition metals has been developed using a combination of electron- and ion-beam lithography enabling high-quality arrays of submicron magnetic Fe wires to be fabricated. This process can be used to fabricate novel devices from a variety of metal combinations which would not be possible by the usual liftoff metallization method. The structure and magnetic properties are reported of an epitaxial 25 nm Fe(001)/GaAs(001) film and the wire gratings which are fabricated from it. The width of the wires in the grating is 0.5 μm for all structures studied, but the separation of each wire is varied in the range 0.5 to 16 μm. An artificially induced shape anisotropy field of around 1 kG, consistent with a magnetostatic calculation, was observed for all separations studied. The field dependence of the magneto-optic Kerr effect and magnetoresistance (MR) data is consistent with a twisted magnetization configuration across the width of the sample beneath saturation for transverse applied fields. In this case, the detailed form of the field dependence of the MR is strikingly modified from that observed in the continuous film and is consistent with coherent rotation of the magnetization.


CRYSTAL-FIELD EXCITATION LINEWIDTHS IN (LN(X)Y(1-X))BA2CU3O7 (LN=ER, HO, TM)

PHYSICA C 235-240 (1994) 1707-1708

A MUKHERJEE, AT BOOTHROYD, JS GARDNER

We have studied the temperature dependence of crystal field (CF) excitation linewidths of lanthanide ions (Er and Ho) in YBa(2)Cu(3)0(7) by neutron scattering. The Ho data reveal an anomaly in the CF damping around T-c with no evidence of a coherence peak below T-c. We have applied the theory of Becker, Fulde and Keller to derive the reduced s-f coupling constants from the CF linewidth data for the ions Er, Ho and Tm.


Identity of planar defects in the 'infinite-layer' copper oxide superconductor

Nature 370 (1994) 352-354

H Zhang, YY Wang, H Zhang, VP Dravid, LD Marks, PD Han, DA Payne, PG Radaelli, JD Jorgensen

The 'infinite-layer' compound1-3AcuO2(where A stands for cations such as strontium or calcium), has the simplest structure of all superconducting copper oxides, with only bare cations separating the CuO2planes. Accordingly, an understanding of the doping mechanism(s) that lead to superconductivity in this compound may facilitate the elucidation of the same phenomenon in the other copper oxide superconductors. Recently, Azuma and co-workers2,4observed planar defects in an infinite-layer phase synthesized at high oxygen pressure, and proposed that the defects are A-cation deficient, and lead to superconductivity (with transition temperature Tc≈ 100-110 K) in this compound. Here, based on quantitative X-ray and high-resolution electron-microscopic analysis of the planar defects in (Sr, Ca)CuO2, we propose that the defects consist of a corrugated Sr-O layer substituted for a CuO2layer, with the incorporation of apical oxygen atoms (which are absent in the parent structure) at roughly half the available sites in the neighbouring Sr layers. This is equivalent to an insertion of a Sr3O2 ± xblock in an otherwise infinite-layer sequence. The variable oxygen stoichiometry of our defect model can account for the occurrence of p-type superconductivity (following high-pressure oxygenation), n-type superconductivity (high-pressure reduction) or lack of superconductivity (high-pressure neutral-atmosphere annealing) in this system, depending on the synthesis conditions4. © 1994 Nature Publishing Group.


Phase diagram and phase separation in La<inf>2-x</inf>Sr<inf>x</inf>CuO<inf>4+δ</inf>

Journal of Alloys and Compounds 207-208 (1994) 206-212

DC Johnston, F Borsa, JH Cho, FC Chou, DR Torgeson, D Vaknin, JL Zarestky, J Ziolo, JD Jorgensen, PG Radaelli, AJ Schultz, JL Wagner, SW Cheong

The phase diagram of the title system is remarkably rich, exhibiting regions of long-range antiferromagnetic order, short-range (spin-glass-like) order, high temperature superconductivity, or none of these. Here, the phase diagram is reviewed, with an emphasis on our recent studies using139La NQR, powder and single crystal neutron diffraction, and magnetization measurements for 0≤χ≤0.15 and 0≤δ≤0.1. © 1994.


Roles of oxygen defects in copper oxide superconductors

Journal of Superconductivity 7 (1994) 145-149

JD Jorgensen, PG Radaelli, H Shaked, JL Wagner, BA Hunter, JF Mitchell, RL Hitterman, DG Hinks

Oxygen vacancy and interstitial defects can have a profound effect on the superconducting properties of copper oxide compounds. Recent work on compounds such as La2CuO4+xand HgBa2CuO4+xhas provided new insight into the role of interstitial oxygen defects as a doping mechanism. The number of carriers created by each interstitial defect depends on the local defect structure. Studies of (La, Sr, Ca)3Cu2O6+xwith various metal compositions and metalsite ordering show that interstitial oxygen defects that form between the CuO2layers in this structure systematically lower Tcand eventually destroy superconductivity. Conversely, oxygen vacancies in the CuO2planes have surprisingly little effect at concentrations below 3%. The infinite-layer compounds, ACuO2, where A=La, Sr, Ca, Nd, etc., in solid-solution combinations, could offer a similar environment for the formation of interstitial oxygen defects between the CuO2planes, allowing interstitial oxygen defects to contribute to the doping of these compounds. However, neutron diffraction experiments on Sr0.9La0.1CuO2(Tc= 42 K) have not found any interstitial oxygen. © 1994 Plenum Publishing Corporation.


Pressure-induced structural changes in superconducting HgBa<inf>2</inf>Ca<inf>n-1</inf>Cu<inf>n</inf>O<inf>2n+2+δ</inf>(n = 1, 2, 3) compounds

Physica C: Superconductivity and its applications 221 (1994) 1-10

BA Hunter, JD Jorgensen, JL Wagner, PG Radaelli, DG Hinks, H Shaked, RL Hitterman, RB Von Dreele

The crystal structures of superconducting HgBa2CuO4+δand HgBa2CaCu2O6+δhave been investigated with a pressure up to 0.6 GPa and HgBa2Ca2Cu3O8+δto 9.2 GPa by neutron powder diffraction. The compressibility along the c-axis is nearly the same for the three compounds and up to two times larger than the compressibility along the a-axis. The one-layer compound, HgBa2CuO4+δ, shows the largest a-axis compressibility, while HgBa2Ca2Cu3O4+δshows the smallest compressibility. The bond compressibilities of HgBa2CuO4+δand HgBa2CaCu2O6+δare significantly different from HgBa2Ca2Cu3O8+δbond compressibilities. In the one- and two-layer compounds the largest bond compressibility was the Cu-O2 (apical) bond distance, while for the three-layer compound it was the Hg-O2 bond distance. © 1994.


TUNNEL-JUNCTIONS ON 1/2/4-Y-BA-CU-O SINGLE-CRYSTALS

PHYSICA C 235 (1994) 1913-1914

P ROMANO, R DILEO, AM CUCOLO, A NIGRO, B DABROWSKI, PG RADAELLI


Synthesis, crystal structure and properties of Hg<inf>2</inf>Ba<inf>2</inf>(Y, Ca) Cu<inf>2</inf>O<inf>8-δ</inf>: the first cuprate superconductor containing a double mercury-oxygen layer

Physica C: Superconductivity and its applications 235-240 (1994) 925-926

PG Radaelli, M Marezio, JL Tholence, S de Brion, S Loureiro, A Santoro, Q Huang, JJ Capponi, M Alario-Franco, C Chaillout

The first cuprate superconductor containing double (HgO)2layers was synthesized using a high-pressure, high-temperature technique. The compound, with chemical formula Hg2Ba2Y1-xCaxCu2O8-δ, contains a rock salt-type double HgO layer. The prototype compound Hg2Ba2YCu2O8-δis an insulator. Superconductivity is induced in the system by partially replacing yttrium with calcium. © 1994.


Phase separation and doped-hole segregation in La<inf>2</inf>CuO<inf>4+δ</inf>and La<inf>2-x</inf>Sr<inf>x</inf>CuO<inf>4+δ</inf>

Physica C: Superconductivity and its applications 235-240 (1994) 257-260

DC Johnston, WR Bayless, F Borsa, PC Canfield, SW Cheong, JH Cho, FC Chou, Z Fisk, JD Jorgensen, A Lascialfari, LL Miller, PG Radaelli, JE Schirber, AJ Schultz, DR Torgeson, D Vaknin, JL Wagner, J Zarestky, J Ziolo

Herein, we review the magnetic, superconducting and structural phase diagrams of the title systems, with an emphasis on our recent results form magnetic and structural neutron diffraction, magnetic susceptibility and system, and indicate the occurrence of frustrated phase separation on a nanoscopic length scale in the La2-xSrxCuO4system with O < x ≲ 0.05.139La nuclear quadrupole resonance measurements. The results clarify the miscibility gap in the La2CuO4+δ. © 1994.


LOCAL CONFIGURATIONS IN ERBA2CU3O6+X FOR 0.15-LESS-THAN-OR-EQUAL-TO-X-LESS-THAN-OR-EQUAL-TO-0.9

PHYSICA C 235 (1994) 1261-1262

M VAROTTO, G AMORETTI, C BUCCI, R DERENZI, G GUIDI, S JANG, P RADAELLI, C SEGRE


Suppression of superconductivity in orthorhombic La<inf>2-x</inf>M<inf>x</inf>CuO<inf>4</inf>(M=Ca,Sr)

Physica C: Superconductivity and its applications 235-240 (1994) 339-340

B Dabrowski, JD Jorgensen, RL Hitterman, JL Wagner, Z Wang, BA Hunter, PG Radaelli, DG Hinks

We have extended the calcium solid-solubility limit in La2-xCaxCuO4to x = 0.2 using synthesis at high oxygen pressure. The superconducting transition temperature increases with doping to 34 K at x = 0.15 and is suppressed for x > 0.15, similar to the Sr-substituted material. At a fixed doping level, the optimum superconducting properties (the highest Tc) are found for flat and square CuO2planes. © 1994.


High pressure synthesis and properties of the HgBa<inf>2</inf>Ca<inf>n-1</inf>Cu<inf>n</inf>O<inf>2n+2+δ</inf>(n=1-6) superconductors

Physica C: Superconductivity and its applications 235-240 (1994) 146-149

JJ Capponi, JL Tholence, C Chaillout, M Marezio, P Bordet, J Chenavas, SM Loureiro, EV Antipov, E Kopnine, MF Gorius, M Nunez-Regueiro, B Souletie, P Radaelli, F Gerhards

High-pressure high temperature synthesis has allowed us to prepare members of the new superconducting mercury cuprates series with n = 1-6. The variation of the structural and magnetic properties, oxygen content and Tcacross the series, which appears to be the richest among the different superconducting cuprates, will be presented and discussed. Tcand the oxygen content δ increases with n from Hg-1201 (Tc=95K, δ=0.06) to a maximum of 135 K with δ=0.40 for Hg-1223. Structural refinement from neutron diffraction data on Hg-1245 showed a further decrease of δ = 0.30 in the high pressure as prepared compound (Tc= 101K). Tentatives to increase δ by a treatment under 200 bar of oxygen in Hg-1234, Hg-1245 and Hg-1256 showed instead a loss of oxygen from the overdoped as prepared samples. © 1994.


THE HG-BEARING CUPRATE SUPERCONDUCTORS - THE RELATIONSHIP BETWEEN THE OXYGEN-CONTENT AND T-C IN HG-1212 AND THE EFFECT OF PRESSURE ON THE STRUCTURE AND T-C IN HG-1223

EUROPEAN JOURNAL OF SOLID STATE AND INORGANIC CHEMISTRY 31 (1994) 843-854

M MAREZIO, JJ CAPPONI, PG RADAELLI, PP EDWARDS, AR ARMSTRONG, WIF DAVID


Low-energy spin fluctuations in YBa2Cu3O6.1 and ErBa2Cu3O6+x A Cu (1) T1 NQR study

Physica C: Superconductivity and its applications 226 (1994) 301-310

SG Jang, C Bucci, R De Renzi, G Guidi, M Varotto, C Segre, P Radaelli

63Cu(1) NQR signals and their relaxation have been measured in YBa2Cu3O6.1and ErBa2Cu3O6+xat temperatures between 10 K and 300 K. In YBa2Cu3O6.1a peak is observed in the spin-lattice relaxation, T1-1, around 140 K. An explanation in terms of doped-hole motion is presented and alternative O(4)-O(5) diffusion mechanisms are critically discussed. In ErBa2Cu3O6+xthe low-temparature T1-1is dominated by magnetic fluctuations of Er; quantitative agreement is found when the correct ground state and magnetic dynamic properties of the rare earth are taken into account. © 1994.


Magnetic-induction profile in a type-I superconductor by polarized-neutron reflectometry.

Phys Rev B Condens Matter 49 (1994) 15789-15798

MP Nutley, AT Boothroyd, CR Staddon, DM Paul, J Penfold


The magnetoresistance of sub-micron Fe wires

Journal of Magnetism and Magnetic Materials 135 (1994)

SJ Blundell, C Shearwood, M Gester, MJ Baird, JAC Bland, H Ahmed

A novel combination of electron- and ion-beam lithography has been used to prepare Fe gratings with wire widths of 0.5 μm and wire separations in the range 0.5-4 μm from an Fe/GaAs (001) film of thickness 25 nm. With an in-plane magnetic field applied perpendicular to the length of the wires, a harder magnetisation loop is observed using the magneto-optic Kerr effect (MOKE), compared with that observed in the unprocessed film. We observe a strong effect in the magnetoresistance (MR) when the magnetic field is applied transverse to the wires. It is believed that this effect originates from the highly non-uniform demagnetising field in each wire of the grating. These results demonstrate that the combination of MOKE and MR measurements can provide important information about the magnetisation reversal processes in magnetic gratings and can be used to understand the effect of shape anisotropy on magnetic properties. © 1994.


CRYSTAL FIELDS IN RBA(2)CU(4)O(8) SUPERCONDUCTORS STUDIED BY INELASTIC NEUTRON-SCATTERING

PHYSICA C 235-240 (1994) 1665-1666

EA GOREMYCHKIN, BD RAINFORD, DB CURRIE, MT WELLER, AT BOOTHROYD

Neutron spectroscopy has been used to study the crystal field excitation spectra in high quality samples of RBa(2)Cu(4)O(8) high temperature superconductors, with R = Dy, Ho and Er. The samples were prepared under high oxygen pressure. Analysis of the spectra included treatment of the spin-orbit and Coulomb interactions, sad yielded crystal field parameters which showed a high degree of consistency from one rare earth to another.


Synthesis and properties of a cuprate superconductor containing double mercury-oxygen layers.

Science 265 (1994) 380-383

PG Radaelli, M Perroux, M Marezio, S de Brion, JL Tholence, Q Huang, A Santoro

A cuprate superconductor containing double mercury layers was synthesized with a high-pressure, high-temperature technique. The compound, with chemical formula Hg(2)Ba(2)-Y1-xCaxCu(2)O(8-delta), contains a double HgO layer with structure similar to that of rock salt. The prototype compound Hg(2)Ba(2)YCu(2)O(8-delta) is an insulator. Superconductivity is induced in the system by partially replacing yttrium with calcium.


Tunneling spectroscopy into YBa2Cu4O8: Intralayer and interlayer analysis.

Phys Rev B Condens Matter 50 (1994) 10397-10400

AM Cucolo, RD Leo, P Romano, B Dabrowski, DG Hinks, PG Radaelli

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