Magnetic anisotropy and weak ferromagnetism of single-crystal TbNi2B2C

The anisotropic magnetization M(T) of single-crystal TDNi2B2C is reported as a function of applied magnetic field (H) for temperatures T from 2 to 300 K and with H parallel and perpendicular to the c axis (H parallel to c and H perpendicular to c). In low field (H less than or equal to 100 G) TbNi2B2C orders antiferromagnetically at Neel temperature T-N=(14.9 +/- 0.1) K. Weak ferromagnetic (WF) behavior is found for T less than or equal to 8 K and H less than or equal to 10 kc. Both phase transitions are also detected in zero-field in-plane resistivity rho(ab)(T) measurements. In contrast, previous low-field M(T,H) data showed antiferromagnetic ground states for (Gd,Dy,Ho,Er,Tm)Ni2B2C crystals. The origin of the WF ordering, given the point symmetry (4/mmm) of the Tb+3 ions, is addressed together with the observed in-plane anisotropy in the high-field magnetization. Possible reasons for the absence of superconductivity above 2 K in TbNi2B2C are also discussed. Finally, a partial H-T magnetic phase diagram of TbNi2B2C is presented for H parallel to[110] and aspects of the similar H-T diagram for H parallel to[100] are discussed.