The magnetization density in a UCu₂P₂ single crystal

Using polarized neutrons we have studied the magnetization density arising from unpaired electrons in the compound UCu₂P₂. This phosphide crystallizes in the hexagonal CaAl₂Si₂-type structure (P3m1) and is a ferromagnet with the highest Curie temperature (T_c=216 K) among all known 5f-electron compounds. Analysis of the experimentally derived form-factor function f(sin theta / lambda ) indicates an admixture of the 6d states with the 5f states due to the overlapping and covalency effects. In addition, we have found that this form factor is affected by the crystal-field effect occurring either for the U⁴⁺ or U³⁺ ion. In agreement with the molecular approach developed by Watson and Freeman to correlate covalency with magnetization density, we found a real occupation of the 6d states with the spatial symmetry of the U site, which explains an important anisotropy of the magnetic form factor at low values of sin theta / lambda .