CRYSTAL-FIELD SPLITTING AND TEMPERATURE-DEPENDENCE OF 2-DIMENSIONAL ANTIFERROMAGNETISM IN THE HIGH-T(C) COMPOUND DYBA2CU4O8

By means of a quantitative analysis of extensive elastic neutron scatt ering experiments performed on the T(c) = 78 K superconductor DyBa2Cu4 O8 it is shown that two-dimensional (2D) antiferromagnetic Dy ordering in the (a,b)-plane remains stable in the whole temperature range from T(N) = (1.10 +/- 0.02) K down to 7 mK. The magnetic difference patter n with good angular resolution fits well to the Warren equation for sc attering on 2D systems with powder averaging. Consequently, the fitted sublattice magnetisation is found to have a 2D Ising character. Moreo ver, the Dy3+ crystal-field levels were determined by inelastic neutro n scattering. From the resulting crystal-field parameters we calculate the sublattice magnetisation of Dy3+ at saturation to be mu(Dy,CEF) = 6.0 muB in good agreement with mu(Dy,obs) = (5.9 +/- 0.5) mu(B) as me asured by neutron diffraction.