ORAL SESSION 5A7: Non-Cuprates II

Friday, Feb. 25, 9:00 a.m. – 11:00 a.m., Imperial Ballroom-Center (Hyatt)

Chairs: J. Tallon (New Zealand Inst.for Ind. Res., Ltd.), A. Balatsky (Los Alamos Nat’l. Lab)

5A7.1 Colossal Magnetoresistance Manganites: Electron and Mn-site Doping

B. Raveau, C. Martin, A. Maignan, and M. Hervieu, Laboratoire CRISMAT, UMR 6508 associated to the CNRS, ISMRA, 6 boulevard du Maréchal Juin, 14050 Caen Cedex, France

Presenting Author: B. Raveau

The magnetic and transport properties of Mn⁴⁺ - rich manganites (electron doped) Ln_1-xCa_xMnO₃, with 0.50 < x < 1, and for which colossal magnetoresistance (CMR) has recently been evidenced, are reviewed. Relationships between those properties and the crystal chemistry of these oxides are established. It is shown that these manganites differ dramatically from the hole doped manganites by the following points:

(i) They exhibit a transition from metal (M) to insulator (I) at decreasing temperature.

(ii) CMR effect appears for small A-site cations and is very sensitive to the electron concentration.

(iii) There is evidence for a metallic cluster glass region (x » 0.90) and CMR appears at the boundary between this region and the charge ordered region.

Electron microscopy and neutron diffraction show that the magnetotransport transition of x = 0.85 sample is correlated to a structural transition from orthorhombic to monoclinic, whereas short range ordering appears for x = 0.80, and long range charge ordering appears for x < 0.80, with a modulation vector q » 1-x.

Complete magnetic phase diagrams are established, and the issues of charge ordering and of phase separation are discussed.

The doping of Mn sites by various elements is studied. The spectacular effect of chromium doping, which induces a M - I transition and consequently CMR in half doped manganites, is explained in terms of double exchange mechanism.

5A7.4 Physical Properties of the Double Perovskite RA₂Ru_1-xCu_xO₆ (R=Y,Ho,Gd; A=Ba, Sr)

M.K. Wu 1, D.Y. Chen 1, F.Z. Chien 2, D.C. Ling 2, and Y.Y. Chen 3. 1 Physics Dept., National Tsing Hua University, Hsinchu, Taiwan. 2 Physics Dept., Tamkang University, Taipei, Taiwan. 3 Institute of Physics, Academia Sinica, Taipei, Taiwan.

Presenting Author: M.K. Wu

We have achieved in the synthesis of a series of new superconducting compounds A₂RRu_1-xCu_xO₆, where A = Ba or Sr, R = Y, Ho, or Gd, and x < 0.2. Strong evidences based on magnetic susceptibility, neutron diffraction and mSR measurements, show that these compounds exhibit unusual coexistence of magnetism and superconductivity at low temperature. We also observed unambiguously two pronounced specific jumps at about the same temperatures as those for superconducting and magnetic order temperatures, observed by magnetic susceptibility measurements, in the samples that doped with Cu. On the other hand, only a rather broad peak was seen in the parent compound, which is an antiferromagnetic insulator. These results are consistent, semi-quantitatively, with a detailed theoretical calculation of the magnetic specific heat based on the double exchange model. Detailed experimental results and plausible theoretical model will be presented and discussed.

5A7.5 Spontaneous Vortex Phase in the Magneto- superconductor Gd_1.5Ce_0.5RuSr₂Cu₂O_10-d (Ru-2122)

I. Felner 1, E.B. Sonin 1, T. Machi 2. and N. Koshizuka 2. 1 Racah Institute of Physics, The Hebrew University, Jerusalem, 91904, Israel. 2 Superconductivity Research Laboratories, ISTEC, 10-13, Shinonome, Koto-Ku, Tokyo 135, Japan.

Presenting Author: I. Felner

The superconducting ceramic material Gd_1.5Ce_0.5RuSr₂Cu₂O_10-d is also ferro-magnetically ordered at T_M>>T_c (T_c~42 and T_M~180 K), thus, superconductivity (SC) coexist with ferromagnetism. SC is confined to the CuO₂ planes, whereas the ferromagnetic state is due to the Ru sublattice [1]. The observed saturation moment of the Ru sublattice is about 1m B per unit cell, which translated into an internal field B_int= 4p M (higher than H_c1) of a few hundreds of G. This internal field induces vortices in the SC planes which have been observed in ceramic material for the first time, by means of magneto-optical garnet indicators. It is shown, that below T_c, at zero applied field, vortices are present in the sample in equilibrium, and disappear above T_c. On the other hand, under an external field of 50 G, the flux density persists up to T_M. These measurements confirm our previous prediction that in SC-ferromagnetic materials vortices are present in the sample in equilibrium without an external field. [2].

[1] I. Felner, U. Asaf, Y. Levi and O. Millo, Phys. Rev. B R3374 (1997).

[2] E.B. Sonin and I. Felner, Phys. Rev. B R14000 (1998).

5A7.6 Evidence for coexistence of ferromagnetism and superconductivity in the ruthenate-cuprate hybrid compound RuSr₂RECu₂O_8+d obtained from muon-spin-rotation experiments

C. Bernhard 1, J.L. Tallon 2, A. Golnik 1, Th. Blasius 3, and Ch. Niedermayer 3. 1 Max-Planck-Institut FKF, Heisenbergstrasse 1, D-70569 Stuttgart, Germany. 2 Industrial Research Laboratories, P.O. Box 31310, Lower Hutt, New Zealand. 3 Univerty of Konstanz, Faculty of Physics, D-78434 Konstanz, Germany.

Presenting Author: C. Bernhard

We report the result of muon-spin-rotation (mSR) measurements on polycrystalline samples of the hybrid ruthenate-cuprate compound RuSr₂RECu₂O_8+d (RE=Gd,Eu) for which superconductivity in the CuO₂ planes (T_c=45 K) and ferromagnetic order of the Ru-moments (m(Ru5+)»1 mB) in the RuO layers (TM=135 K) seem to coexist on an atomic scale [1,2]. Our ZF-measurements establish that the magnetic phase at T<TM is microscopically homogeneous and accounts for the entire sample volume, i.e., there is no non-magnetic fraction [2]. Our mSR measurements have been complemented by specific heat measurements which establish a sizeable specific heat jump at T_c and thus provide strong indication for bulk superconductivity [3]. Rather surprisingly, we find no clear indication that the static magnetic order is affected by the onset of superconductivity at T_c=45 K << TM=135 K. As shown in Fig. 1, neither the local magnetic field at the muon site Bm nor the static field distribution <(D B_m)²>~1/T₂ exhibit any clear anomaly at T_c. To the contrary, the dynamic relaxation rate 1/T₁, as studied by longitudinal-field measurements, exhibits a strong anomaly below T_c=45 K. In fact, the observed 1/T1 anomaly provides the first indication that the superconducting charge carriers interact strongly with the Ru-moments. We discuss the possible implications of our data in terms of coexistence of superconducting charge carriers in the CuO₂ planes and magnetically ordered Ru-moments (with at least a sizeable ferromagnetic compoment) in the RuO layers.

[1] J.L. Tallon et al., IEEE Trans. Appl. Supercon. (in press).

[2] C. Bernhard et al., Phys. Rev. B 59, 14099 (1999).

[3] J.L. Tallon , J.W. Loram and C. Bernhard, to be published.

5A7.7 Possible Nucleation of a 2D Superconducting Phase on WO₃ Single Crystals Surface Doped with Na⁺

S. Reich and Y. Tsabba, Department of Materials and Interfaces, The Weizmann Institute of Science, Rehovot 76100, Israel

Presenting Author: S. Riech

WO₃ crystals with a surface composition of Na_0.05WO₃ were grown. These crystals exhibit a sharp diamagnetic step in magnetization at 91K, and a magnetic hysteresis below this temperature. As the temperature is lowered below 100K in transport measurements, a sharp metal to insulator transition is observed, this is followed by a sharp decrease in the resistivity when the temperature is lowered to about 90K. When the surface of the crystals was covered by gold the depth of the diamagnetic step had decreased considerably. These results indicate a possible nucleation of a superconducting phase on the surface of these crystals. This is a non cuprate system exhibiting a critical temperature in the HTS range, also WO₃ does not have an antiferromagnetic background when undoped.