Magnetic properties and spontaneous distortion in TbCo₂

The authors present magnetisation measurements performed on a single crystal of TbCo 2 as well as spontaneous distortion studied by X-ray powder diffraction. At any temperature below T c (240K) the easy magnetisation direction is (111) while the most difficult is (100). A huge rhombohedral distortion appears below T c ; at 4.2K the angular variation between the cube edges is epsilon =4.4*10 -3 rad. The anisotropy due to cobalt being negligible, the authors have analyzed the results in terms of the three contributions acting on the 7 F 6 ground state multiplet of the Tb 3+ ions: crystal field, exchange field and magnetoelastic energy. At 4.2K, as the anisotropy of Tb is very high, a 135 kOe field applied along the (100) or the (110) is not strong enough to align the magnetisation parallel to these axes. As in TbFe 2 , the large spontaneous rhombohedral distortion results from a strong molecular field associated with strong crystal field effects. The molecular field leads to almost the fundamental level J z =6 ( mu Tb approximately=9 mu B ) that is a very anisotropic 4f distribution and the crystalline field essentially arises, as generally observed, from the d character of the conduction band.