  Single crystals of the heavy-fermion compound YbAgGe have been studied by
neutron scattering. The magnetic ordering occurring below T1=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 Gamma of the order of 1 meV. The spins fluctuate
predominantly in the basal plane. No spin-wave excitations are observed in the
magnetically ordered phase. Below the Kondo temperature, TK=20 K, Gamma shows a
strong q dependence for wave vectors along the c* direction, but is
q-independent in the basal plane. Gamma shows initially a rapid increase with
temperature T at the antiferromagnetic zone center, but follows a standard
sqrt(T) 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.

Strongly Correlated Electrons