YBNISN, A FERROMAGNETIC KONDO LATTICE

We present an investigation of the low temperature properties of the ternary intermetallic alloy YbNiSn by X-ray and neutron diffraction, resistivity, magnetisation and Yb-170 Mossbauer spectroscopy measurements. It is shown that YbNiSn is a ferromagnet (T(c) = 5.65 K) with the Yb3+ magnetic moments lying along the orthorhombic c axis. At low temperatures a strong anisotropy of the interionic exchange coupling is evidenced (lambda(c) congruent-to 100 kOe/mu(B), lambda(a) congruent-to 15 kOe/mu(B) which competes with the Crystal Electric Field anisotropy (g(c) congruent-to 1.67, g(a) congruent-to 3.72), leading to the unusual situation where the ferromagnetic moments do not lie along the easy magnetisation axis a. Hybridisation of the 4f electrons with band electrons leads to a reduction of the spontaneous moment (saturated value 0.85-mu(B)) and is likely to be the origin of the large value of the molecular field constant lambda(c) as well as of the anisotropy of the exchange interaction and of the resistivity.