Neutron scattering and specific-heat studies of the crystal fields in the rare-earth monophosphides are reported. It is found that the fourth-order crystal-field parameters for the phosphides and for other rare-earth pnictides fall on a universal curve which is close to that predicted by an effective-point-charge model for the light rare earths but deviates markedly for the heavy rare earths.Crystal-field effects in insulators have been the subject of intense research for more than four decades. The corresponding problem in metals, however, has only been seriously studied in the last few years. This recent interest in crystal fields in metals has been stimulated especially by two important experimental advances: (i) it has been found that inelastic-neutron-scattering techniques can yield detailed information about the crystal fields in a large class of rare-earth metallic compounds, ' 5 (ii} electron-paramagnetic-resonance studies of very dilute noble-metal-rareearth and transition-metal-rare-earth
Antiferromagnetic spin waves have been observed in a single crystal of MnF2 at 4.2°K by the inelastic scattering of neutrons. Measurements were carried out using a twin rotor time-of-flight spectrometer having a mean incident neutron wavelength of 3.00 Å and a resolution (half-width at half-height) of 0.06 Å. Spin waves were excited in the {010} plane of the crystal and the dispersion plotted for propagation in the 〈100〉 and 〈001〉 directions. The observed dispersion curves can be accurately described by spin-wave theory using the following values for the parameters concerned: J1=0.32°K (ferromagnetic), J2=−1.76°K (antiferromagnetic), |J3|<0.05°K, and an anisotropy field at zero wave vector HA=1.06°K, where J1 is the exchange coupling between nearest neighbours (in the 〈001〉 direction), J2 acts between next nearest neighbours (〈111〉 directions), and J3 refers to third-neighbour coupling (along the 〈100〉 and 〈010〉 directions). A small effect arising from the wave vector dependence of HA has been calculated and taken into account in the evaluation of these parameters. The values quoted are in excellent agreement with those predicted from measurements of perpendicular susceptibility, spin-wave resonance, paramagnetic resonance in (Mn, Zn)F2, and from paramagnetic neutron scattering at room temperature.
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