MAGNETIC-PROPERTIES AND SPONTANEOUS DISTORTION IN TBCO2

The authors present magnetisation measurements performed on a single crystal of TbCo2 as well as spontaneous distortion studied by X-ray powder diffraction. At any temperature below Tc (240K) the easy magnetisation direction is (111) while the most difficult is (100). A huge rhombohedral distortion appears below Tc; 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 7F6 ground state multiplet of the Tb3+ 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 TbFe2, 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 Jz=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.