Magnetic structure and crystal field states of the heavy fermion system YbPt₅B₂

Abstract

Monoclinic compounds YbPt5B2 and LuPt5B2 [space group C2/m (No. 12), own structure type] have been studied by means of elastic and inelastic neutron scattering experiments. The heavy fermion system YbPt5B2 orders antiferromagnetically below TN1 = 7.8 K and exhibits an incommensurate magnetic structure with a temperature-dependent propagation vector, k1 = (0.194, 0, −0.045; T = 6 K). Below TN2 = 4.7 K, a transition into a commensurate structure, k2 = (0, 0, 0), takes place. The transition at TN2 turns out to be of first order, as obvious from commensurate and incommensurate contributions to the magnetic moments around T = TN2 . Rather large Yb moments, m(Yb) > 3μB , yet reduced due to the Kondo effect, were obtained as a result of the splitting of the Yb 3+ eightfold-degenerate ground state into four doublets, owing to crystalline electric field (CEF) effects. Such unusually large moments in Yb systems can be explained only by an appropriate wave function ∑i=7/2 i=−7/2 ±αi|i/2〉, ∑i αi^2 = 1, constituting the CEF ground state doublet, which indeed was concluded from the present inelastic neutron data, considering the crystalline electric field theory. The overall CEF splitting is on the order of 44 meV, while the first excited level is located ≈25 meV above the ground state.