UMn2Al20: Small Itinerant Moment or Induced Local Moment Ferromagnetism?

Abstract. The magnetic susceptibility, specific heat, resistivity and inelastic neutron scattering spectrum were measured on the cubic heavy Fermion compound UMn2Al20. A ferromagnetic transition is observed in the magnetic susceptibility at Tc=20 K, but no anomaly is seen in the specific heat or resistivity at Tc. This behaviour is similar to that of the single-triplet induced moment ferromagnet Pr3Tl. Since the 5f electrons of uranium have the possibility of being in the 5f electron configuration with a nonmagnetic J=0 ground state and a triplet excited state, such induced moment behaviour is a possibility in UMn2Al20. Since the linear coefficient of specific heat at T = 0 is large (0.3 J/mol-K) a second possibility is moderately small (~ 0.9 B) moment itinerant ferromagnetism in a heavy Fermion system. In the energy transfer range 5 < E < 60 meV the inelastic neutron scattering spectra appear to be dominated by phonon scattering, with no obvious magnetic scattering. Hence, if either scenario is correct, there must be magnetic excitations at lower (1-5 meV) energies.