Coexistence of Superconductivity and Magnetism in Borocarbides

Abstract

The interplay of rare-earth magnetism and superconductivity has been a topic of interest for many years. In cuprates as well as in the classical magnetic superconductors the superconducting state usually coexists with antiferromagnetic order on the rare-earth sublattice. Usually, their magnetic ordering temperature TN is much below the superconducting transition temperature T„ The discovery of superconducting borocarbides RT2B2C with R = Sc, Y, La, Th, U, Dy, Ho, Er, Tm or Lu and T = Ni, Pd or Pt (where not all of these combinations of R and T result in superconductivity) has reanimated the research on the coexistence of superconductivity and magnetic order. Most of these borocarbides crystallize in the tetragonal LuNi2B2C type structure which is an interstitial modification of the ThCr2Si2type. Contrary to the behavior of Cu in the cuprates Ni does not carry a magnetic moment in the borocarbides. Various types of antiferromagnetic structures have been found to coexist with superconductivity for R = Tm, Er, Ho and Dy in RNi2B2C. In the case of HoNi2B2C three different types of antiferromagnetic structures have been observed (i) a commensurate one with Ho moments aligned ferromagnetically within layers perpendicular to the tetragonalcaxis where consecutive layers are aligned in opposite directions, (ii) an incommensurate spiral along thecaxis and (iii) an incommensurate a-axis modulated structure that was shown to be important for the reentrant behavior whereas the other two structures coexist with the superconducting state. The variation of TN and Tcwith the de Gennes factor can well be drawn on straight lines from Lu to Gd and from Lu to Tb, respectively. Consequently, Tc> TN holds for Tm, Er, Ho and T, < TN for Dy. However, the study of various pseudoquaternary (R,R’)Ni2B2C compounds has shown that this so called de Gennes scaling is not universal for the borocarbides and it breaks down in some cases which is attributed to effects of crystalline electric fields, the difference in the R ionic radii or the effect of non-magnetic impurities in an antiferromagnetic superconductor. In an external magnetic field some of the RNi2B2C compounds show metamagnetic transitions combined with a large negative magnetoresistance.