We discuss several classes of conventional magnetic superconductors including the ternary rhodium borides and molybdenum chalcogenides (or Chevrel phases), and the quaternary nickel-borocarbides. These materials exhibit some exotic phenomena related to the interplay between superconductivity and long-range magnetic order including: the coexistence of superconductivity and antiferromagnetic order; reentrant and double reentrant superconductivity, magnetic field induced superconductivity, and the formation of a sinusoidally-modulated magnetic state that coexists with superconductivity. We introduce the article with a discussion of the binary and pseudobinary superconducting materials containing magnetic impurities which at best exhibit short-range “glassy” magnetic order. Early experiments on these materials led to the idea of a magnetic exchange interaction between the localized spins of magnetic impurity ions and the spins of the conduction electrons which plays an important role in understanding conventional magnetic superconductors. These advances provide a natural foundation for investigating unconventional superconductivity in heavy-fermion compounds, cuprates, and other classes of materials in which superconductivity coexists with, or is in proximity to, a magnetically-ordered phase.

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Conventional magnetic superconductors

The first neutron diffraction results obtained on single crystals of HoMo 6 S 8 are reported. At temperatures below T m =0.65 K, HoMo 6 S 8 develops a magnetic ordering of long period; the magnetic moments, aligned along the [111] direction, are perpendicular to the modulation wave vector k=[k, − k, 0] (rhombohedral cell). This modulated phase can be quenched at T=0.1 K (k=0.008). However, by slow cooling, the modulated phase transforms into a ferromagnetic ordering with microdomains of size 1000 A to 1500 A Presence d'un ordre magnetique a longue periode a T m <0,65 K, avec des moments magnetiques alignes selon la direction [111] et perpendiculaires au vecteur d'onde de la modulation k=[k, − k, 0] (maille rhomboedrique). Trempe de cette phase modulee a 0,1 K, mais formation d'une phase a ordre ferromagnetique avec microdomaines de dimension 1000-1500 A par refroidissement lent

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Neutron diffraction study of HoMo6S8 single crystal

Publisher Summary This chapter describes the interplay between magnetism and superconductivity in rare earth compounds, where the magnetic moments are produced by the 4f-electrons of the rare earths and where the conduction electrons in most cases are d-electrons, well-separated from the magnetic 4f-electrons. A key element of these compounds is that the exchange interaction between the conduction electrons and the magnetic moments is weak. This assures that the unavoidable pairbreaking caused by exchange scattering can be kept on a low level, and, thus, make the confrontation of the two phenomena possible. In the two most prominent classes of materials, the Chevrel phases (REMo 6 S 8 ) and the ternary borides (RERh 4 B 4 ) this weak exchange interaction can be related to the particular cluster-type crystal structure, which provides a spatial separation of the two kinds of electrons. A similar, but extreme situation is found in the oxide superconductors, REBa 2 Cu 3 O 7 , where the coupling is practically absent between the RE and the conduction electrons. On the other extreme, compounds with magnetic 3d-atoms have, so far, turned out to have too strong exchange interactions, so that scattering effects destroy superconductivity in compounds able to develop long-range order.

Chapter 6 Magnetic superconductors

Along with the rare-earth molybdenum chalcogenides RExMo6X8 (X = S or Se), the rare-earth rhodium borides RERh4B4 have been studied extensively in recent years in connection with the interplay between superconductivity and magnetism. The purpose of this chapter is to review the experiments that have been carried out on the RERh4B4, compounds since their discovery some five years ago. The experimental situation for the RExMo6X8 compounds is discussed in Chap.5 by Ishikawa, Fischer and Muller, while the theoretical aspects of magnetic superconductors are considered in Chap.9 by Fulde and Keller.

Superconductivity, Magnetism and Their Mutual Interaction in Ternary Rare Earth Rhodium Borides and Some Ternary Rare Earth Transition Metal Stannides

For pt.I see ibid, vol.19, no.33, p.6505 (1986). The authors use a generalised Ginzburg-Landau mean-field model to examine possible structures of the intermediate state of ferromagnetic superconductors which combine magnetisation and superconductivity. A new alternating vortex lattice structure is proposed for materials with an easy-plane magnetic anisotropy. One component of the magnetisation is periodically divided into large oppositely polarised domains. Each domain contains one row of vortex (anti-vortex) lines. The magnetisation does not vanish in the Bloch walls, instead it is perpendicular to the magnetisation in the domains and has an oscillating structure because of the interaction with superconductivity. It is sinusoidally oscillating and not a spiral wave independent of any anisotropy in the easy plane. The fraction of the magnetisation which oscillates is substantial. This new structure agrees better with the properties of the intermediate state of ErRh4B4 than previous structures.

https://iopscience.iop.org/article/10.1088/0022-3719/19/33/005/pdf

Spatial structures of the intermediate state between superconductivity and ferromagnetism. II. Easy-plane magnetic anisotropy

Neutron Scattering Studies of Magnetic Superconductors

Results obtained in the last few years obtained by neutron diffraction on the nature of the magnetic ordering in magnetic superconductors are reviewed. Emphasis is given to studies of the complex intermediate phase in ferromagnetic superconductors where both superconductivity and ferromagnetism appear to coexist.

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David G. Hinks

Papers

Neutron Scattering Studies of Magnetic Superconductors

Neutron-scattering studies of magnetic superconductors

Neutron Scattering Studies of the Magnetic Ordering in Ternary Rare Earth Compounds