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

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 T_N1 = 7.8 K and exhibits an incommensurate magnetic structure with a temperature-dependent propagation vector, (Formula presented.) = (0.194, 0, −0.045; T = 6 K). Below T_N2 = 4.7 K, a transition into a commensurate structure, (Formula presented.) = (0, 0, 0), takes place. The transition at T_N2 turns out to be of first order, as obvious from commensurate and incommensurate contributions to the magnetic moments around T = T_N2. Rather large Yb moments, m(Yb) > 3μ_B, yet reduced due to the Kondo effect, were obtained as a result of the splitting of the Yb³⁺ 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 (Formula presented.), 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.