ANISOTROPIC EXCHANGE INTERACTIONS IN UNIGE

UNiGe crystallizing in the orthorhombic TiNiSi-type of structure order s magnetically below T-N = 50 K with an additional magnetic phase tran sition at 42 K. Both structures (below 42 K commensurate antiferromagn etic, between 42 K and T-N incommensurate) are non-collinear with a si gnificant a-axis component (mu(x) = 0.35 mu(B)/U at 20 K). The magneti c properties are highly anisotropic both in the ordered and the parama gnetic state. There are two metamagnetic transitions both with the fie ld applied along the b- and the c-axis. While the magnetic structure a bove the second metamagnetic transition is forced ferromagnetic for bo th field orientations, for the field applied along the a-axis the magn etization curve at 4.2 K is linear up to 38 T and no change in magneti c structure is observed. UNiGe provides strong evidence for anisotropi c exchange interaction. The fact that the a-axis component cannot be a ligned by the highest magnetic field used indicates that the antiferro magnetic interaction is much stronger between the a-axis components th an between the others. This behaviour cannot be due to single-ion anis otropy because in that case the free energy does not depend on the sig n of the a-axis component. (C) 1998 by Elsevier Science B.V. All right s reserved.