Based on magnetization, specific heat, magnetostriction and neutron-diffraction studies on single-crystal TbCo(2)B(2)C, 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 T(c) = 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-orthorhombic distortion. The manifestation of an FM state in TbCo(2)B(2)C is unique among all other isomorphous borocarbides, in particular TbNi(2)B(2)C (T(N) = 15 K, incommensurate modulated magnetic state) even though the Tb ions in both isomorphs have almost the same crystalline electric field properties. The difference among the magnetic modes of these Tb-based isomorphs is attributed to a difference in their exchange couplings which are in turn caused by a variation in their lattice parameters and in the position of their Fermi levels.

a b s t r a c tThe borocarbides RNi 2 B 2 C (R ¼ magnetic rare earth) exhibit rich H-T magnetic phase diagrams. Using field-dependent specific heat measurements on single-crystals of RNi 2 B 2 C (R ¼ Gd, Er), this work investigated the magnetic contribution to the specific heat when T and H are varied across these H-T phase diagrams. These measurements, together with the ones reported on HoNi 2 B 2 C, confirm that the overall evolution of each C mag ðT; HÞ curve is a faithful reflection of the features observed in the corresponding phase diagram: in particular the successive field-induced metamagnetic modes, appearing in the reported magnetization MðT; HÞ curves, are also manifested in these C mag ðT; HÞ curves, just as required by the Maxwell identity ð@C mag =@HÞ T ¼ Tð@ 2 M=@T 2 Þ H . Within the lower ranges of temperature and fields, the calculations based on linearized field-dependent spin-wave theory are found to reproduce satisfactorily the measured C mag ðT; HÞ curves: accordingly, within these ranges, the thermodynamical properties of these compounds can be rationalized in terms of only two parameters, namely, the spin-wave energy gap and the stiffness coefficient. Based on the satisfactory agreement between theory and experiment, we are able to provide an explanation for the plateau like behavior observed in, say, the MðT; HÞ isotherms. Finally, for the particular case of GdNi 2 B 2 C wherein the anisotropy is dictated solely by the classical dipole interactions, the main features of its C mag ðT; HÞ are found to be reproduced by numerical calculations based on the model of Jensen and Rotter [Phys. Rev. B 77 (2008) 134408].