An inelastic neutron scattering study of single-crystal heavy-fermion YbAgGe

Single crystals of the heavy-fermion compound YbAgGe have been studied by neutron scattering. The magnetic ordering occurring below T, 0.5 K is characterized by a commensurate propagation vector k = (1/3, 0, 1/3) and the moments in the basal plane of the hexagonal structure. The dynamic magnetic susceptibility is dominated by quasielastic spin fluctuations with a characteristic energy F-q(T) of the order of I meV. The spins fluctuate predominantly in the basal plane. No spin-wave excitations are observed in the magnetically ordered phase. Below the Kondo temperature, T-K similar to 20 K, 17,(T) shows a strong q-dependence for wavevectors along the c* direction, but is q-independent in the basal plane. Gamma(q)(T) initially shows a rapid increase with temperature T at the antiferromagnetic zone centre, but follows a standard rootT law for other q values and for T > TK in general. These observations classify YbAgGe as a well-behaved heavy-fermion compound with a particular q-dependence of the antiferromagnetic spin fluctuations, possibly related to the geometrical frustration of the Yb3+ ions.