  Based on magnetization, specific heat, magnetostriction, and neutron
diffraction studies on single-crystal TbCo2B2C, it is found out that the
paramagnetic properties, down to liquid nitrogen temperatures, are well
described by a Curie-Weiss behavior of the Tb+3 moments. Furthermore, below Tc=
6.3 K, the Tb-sublattice undergoes a ferromagnetic (FM) phase transition with
the easy axis being along the (100) direction and, concomitantly, the unit cell
undergoes a tetragonal-to-orthorhhombic distortion. For fields up to 90 kOe, no
field-induced splitting of the Co 3d orbitals was observed; as such the
internal field must be well below the critical value needed to polarize the Co
3d subsystem. The manifestation of a FM state in TbCo2B2C is unique among all
other isomorphous borocarbides, in particular TbNi2B2C (Tn=15 K, incommensurate
modulated magnetic state) even though the Tb-ions in both isomorphs have almost
the same crystalline electric field properties. The difference in the magnetic
modes of these Tb-based isomorphs is attributed to a difference in their
exchange couplings caused by a variation in their lattice parameters and in the
position of their Fermi levels.
