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-Tc cuprates the
superconducting state usually coexists with antiferromagnetic
order on the rare-earth sublattice. In these compounds the
magnetic ordering temperature TN is much below the
superconducting transition temperature Tc. 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 τ≈(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 TN and Tc 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, Tc>TN holds for Tm, Er, Ho and
Tc<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 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 TbNi2B2C has been interpreted as weak
ferromagnetism of Dzyaloshinsky-Moriya type. A similar
phenomenon observed for superconducting ErNi2B2C is still
under discussion.