Two-dimensional behavior of the rare earth ordering in oxide superconductors

doi:10.1016/0925-8388(92)90337-9

Journal of Alloys and Compounds

Volume 181, Issues 1–2, 3 April 1992, Pages 419-429

https://doi.org/10.1016/0925-8388(92)90337-9 Get rights and content

Abstract

Neutron scattering has been used to reveal the nature of the magnetic ordering of the rare earth ions in 1–2–3, 2–4–8, 2–4–7 and 2-1–4 oxide superconductors. The interactions are found to be antiferromagnetic in nature and quite weak, leading to ordering temperatures which are a few degrees Kelvin or less. In the first three systems the separation of the rare earth ions is much larger along the c axis than along the ab directions, which renders these materials prototypical two-dimensional (2D) magnets. In the ErBa₂Cu₃O₇ and DyBa₂Cu₃O₇ systems, for example, a rod of scattering characteristic of 2D behavior is readily observed, while the order parameter obeys the exact solution of the $S 12$ , 2D Ising model. An extreme case of 2D behavior is found for the DyBa₂Cu₄O₈ material, where a geometric cancellation of the already weak interactions occurs along the c axis, effectively decoupling the rare earth ab layers. The system thus exhibits no crossover to the 3D behavior usually found well below the ordering temperature, making it the best example of a 2D magnet known to date.

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Most work has been done in replacing Ni in RNi2B2C by its neighbors in the periodic table i.e. Co, Cu, Pd, Pt, but also by other transition metals, see e.g. Hilscher and Michor (1999). The transition temperature Tc is reduced by Ni → Cu (R = Y, Choi et al. 1998) as well as Ni Co (R = Y: Schmidt et al. 1994; Hoellwarth et al. 1996; R = Dy, Ho, Er, Tm, Lu: Schmidt and Braun 1997; R = Ce: El Massalami et al. 1997; R = Gd: Bud'ko et al. 1995b, 1995c; R = Ho: Lynn et al. 1996; R = Er: Felner et al. 1997a and Bud'ko and Canfield 2000b; R = Lu: Cheon et al. 1998). This can be qualitatively understood within the framework of a simple rigid-band picture assuming a more or less rigid band structure across the substitutional series and a varying degree of band filling due to the different number of conduction electrons in Co, Ni, Cu.
The rapid progress in the study of borocarbides is closely connected with the availability of high quality polycrystalline samples over a wide range of compositions as well as single crystals early in the development of the field. The main trends of the superconducting properties and ordering of the rare earth's magnetic moments have been elucidated. One of the most interesting aspects of borocarbides is the possibility to observe coexistence and competition between superconductivity and magnetic order. Most of the zero-field magnetic structures have been obtained, but a number of key questions remain to be clarified. A variety of both commensurate and incommensurate magnetic structures was observed. To explain these structures theoretically a detailed understanding of the role of super exchange versus indirect exchange, combined with single-ion crystal field and hybridization effects, is needed. Inelastic neutron scattering experiments giving the form of magnetic excitation spectra can provide a detailed picture of these interactions.
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The magnetic ordering of the lanthanide ions in superconducting systems has been a topic of active interest for many years. In conventional “magnetic superconductors,” such as the Chevrel phase, and related materials, the lanthanide moments are coupled very weakly both to the metallic electrons and to each other, resulting in very low magnetic ordering temperatures and a delicate energetic balance with superconductivity. A similar situation occurs for the layered cuprates in which the development of long range lanthanide magnetic order also occurs at very low temperatures. In contrast to the earlier systems, the superconductivity in the cuprates typically occurs at much higher temperatures, and the antiferromagnetic order that develops on the lanthanide sublattice coexists with the superconducting state. In addition, the 2-1-4 class of materials provides the first examples of superconductors where the exchange interactions unambiguously dominate the lanthanide energetics, such as Sm₂CuO₄ has a dipole ordering temperature that is two orders of magnitude lower than the observed ordering temperature of 6 K.
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The iron oxycarbonate Sr₄Fe₂O₆CO₃ is reported. The crystal structure and magnetic order were determined by magnetic susceptibility, powder neutron diffraction, and neutron scattering studies. The structure is tetragonal between 10 and 400 K (I4/mmm, a=3.88907(4) Å and c=27.9818(4) Å at 295 K), and is related to a 4:3 type Ruddlesden–Popper phase with CO₃ plaques in place of the middle layer of transition metal octahedra. The three-dimensional magnetic ordering transition temperature is 361 K. The magnetic order is antiferromagnetic within the FeO₂/SrO/SrO/FeO₂ blocks and ferromagnetic between blocks.

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Two-dimensional behavior of the rare earth ordering in oxide superconductors

Abstract

Solid State Commun.

Solid State Commun.

Phys. Rev.

Solid State Commun.

Phys. Rev. B

Phys. Rev. B

A review of both theory and experiment which pertain to the oxide superconductors is given

Phys. Rev. B

Phys. Rev. B