Interaction of superconductivity and magnetism in borocarbide superconductors

The interaction of rare-earth magnetism and superconductivity has been a topic of interest for many years. In classical magnetic superconductors (Chevrel phases, ternary rhodium borides, etc) as well as in the high-T-c cuprates the superconducting state usually coexists with antiferromagnetic order on the rare-earth sublattice. In these compounds the magnetic ordering temperature T-N is much below the superconducting transition temperature T-c. The discovery of superconducting borocarbides RT2B2C with R = Sc, Y, La, Th, Dy, Ho, Er, Tm or Lu and T = Ni, Ru, 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 ThCr2Si2 type. Contrary to the behaviour of Cu in the cuprates Ni does not carry a magnetic moment in the borocarbides. Various types of antiferromagnetic structures on the rare-earth sublattice have been found to coexist with superconductivity in RNi2B2C for R = Tm, Er, Ho and Dy. Particularly of interest is the case of HoNi2B2C for which three different types of antiferromagnetic structures have been observed: (i) a commensurate one with Ho moments aligned ferromagnetically within layers perpendicular to the tetragonal c axis where consecutive layers are aligned in opposite directions, (ii) an incommensurate spiral along the c axis and (iii) an incommensurate a-axis-modulated structure with a modulation vector tau approximate to (0.55, 0, 0). This wave vector emerges in various RNi2B2C compounds with magnetic as well as nonmagnetic R elements and is connected with Fermi surface nesting. Both incommensurate magnetization structures have been shown to be related to the near-reentrant behaviour observed in HoNi2B2C whereas the commensurate structure coexists well with the superconducting state in this compound. The variation of T-N and T-c with the de Gennes factor can roughly be drawn on straight lines from Lu to Gd and from Lu to Tb, respectively, with the exception of Yb. Consequently, T-c > T-N holds for Tm, Er, Ho and T-c < T-N 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 details of the electron structure, crystalline electric fields, the difference in the R and R' ionic radii or to the effect of nonmagnetic impurities in an antiferromagnetic superconductor. In an external magnetic field some of the RNi2B2C compounds show metamagnetic transitions combined with a large negative magnetoresistance. A small net magnetization found at low temperatures and zero magnetic field in ThNi2B2C has been interpreted as weak ferromagnetism of Dzyaloshinsky-Moriya type. A similar phenomenon observed for superconducting ErNi2B2C is still under discussion.