Crystalline electric field effects in PrNi₂B₂C: Inelastic neutron scattering

PrNi₂B₂C as a member of the borocarbide series is characterized by antiferromagnetic order below T_N =4 K and the absence of superconductivity (at least down to 100 mK). There are two effects responsible for the absence of superconductivity in PrNi₂B ₂C. These are the strong conduction electron-Pr moment interaction and a comparatively lower density of states. We studied the crystalline electric field (CEF) excitations and excitons in this compound by inelastic neutron scattering. The CEF level scheme obtained from these data comprises a singlet ground state, a doublet at 1 meV, and further higher levels at 5.2, 24.3 (doublet), 25.1, 29.4, and 31.5 meV. Large dispersion was found for the 1 meV excitation and explained theoretically taking into account magnetic exchange interactions. The calculated crystal-field parameters explain satisfactorily the neutron spectra as well as the heat-capacity and magnetic-susceptibility data. This leads to the conclusion that PrNi₂B₂C can be described by the standard model of rare-earth magnetism. Thus the heavy-fermion concept, suggested by some groups earlier in literature, is not the cause of the suppression of superconductivity. Excitation spectra of the diluted series Pr_1-xY_xNi₂B₂C were also investigated. No drastic changes in the CEF level scheme have been observed in these compounds. Hence the CEF level scheme of the full compound, i.e., PrNi₂B₂C, is reasonably valid for these samples too. The superconducting-transition temperature T_C ∼15.5 K for YNi ₂B₂C decreases linearly with decreasing Y concentration x. Samples with x≤0.65 do not exhibit superconductivity down to 2 K.