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 RT 2 B 2 C 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 LuNi 2 B 2 C type structure which is an interstitial modification of the ThCr 2 Si 2 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 RNi 2 B 2 C for R = Tm, Er, Ho and Dy. Particularly of interest is the case of HoNi 2 B 2 C 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-axismodulated structure with a modulation vector τ ≈ (0.55, 0, 0). This wave vector emerges in various RNi 2 B 2 C 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 HoNi 2 B 2 C 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 )Ni 2 B 2 C 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

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Abstract.The upper critical field H c2 (T) of sintered pellets of the recently discovered MgB 2 superconductor was investigated by transport, ac susceptibility and dc magnetization measurements in magnetic fields up to 16 T covering a temperature range between T c ∼ 39 K and T = 3 K ∼ 0.1T c . The temperature dependence of the upper critical field, H c2 (T), shows a positive curvature near T c similar to that found for the borocarbides YNi 2 B 2 C and LuNi 2 B 2 C indicating that MgB 2 is in the clean limit. The irreversibility line was consistently determined from dc magnetization measurements and from the imaginary component of ac susceptibility. The irreversibility field was found to increase up to 8.5 T at 10 K.