Polarized-neutron study of TmCo2

Diffraction experiments with polarized neutrons have been carried out on a Tm${\mathrm{Co}}_2$ single crystal, at various temperatures, in a field of 57.2 kOe applied parallel to the [$01\overline{1}$] direction. The localized moments on Tm and Co sites have been determined at 4.2 and 100 K from the magnetic densities obtained by Fourier projections. The Tm moment ($5.4{\mu }_B$ at 4.2 K) is reduced by crystal-field effects. A $3d$-type Co moment is coupled antiparallel to the Tm one. The magnetic scattering amplitudes measured for the reflections to which the $4f$ Tm moment does not contribute give direct evidence for the existence of a diffuse magnetic density. This diffuse density is oscillatory with distance and remains similar at all temperatures. The $4f$ Tm and the $3d$ Co moments have been evaluated at eight temperatures from 4.2 to 250 K. The Co susceptibility above 25 K is temperature independent. The Co behaves as a Pauli paramagnet under the action of two opposite fields: the molecular field due to Tm atoms and the applied field. A collective-electron model is therefore appropriate for the description of paramagnetic susceptibility. At 4.2 K a large increase of the Co susceptibility in the molecular field leads to a value of the Co moment of $0.8{\mu }_B$. This result can be related to the increase of the susceptibility observed in Y${\mathrm{Co}}_2$ in high applied fields and to the first-order ferri-paramagnetic transition of some $R{\mathrm{Co}}_2$ compounds, where $R$ stands for rare-earth metals.