Optical investigation of the antiferromagnetic phase transitions in heavy-electron compounds

We have investigated the electrodynamic response of the heavy-electron compounds UPd2Al3 (T-N = 14.5 K) and UCu5 (T-N = 15 K). The complete excitation spectra were obtained by Kramers-Kronig transformation of the optical reflectivities measured on a fairly broad frequency range (i.e. from 15 up to 10(5) cm (1)), combined with the direct measurement of the conductivity at millimeter and microwave frequencies. In UCu5 we found an absorption in the far infrared below T-N, which is ascribed to excitations across a spin-density-wave-type gap and indicates the itinerant nature of the magnetic phase transition. This feature is absent in UPd2Al3, in accordance with the local magnetic moment character of the antiferromagnetic order.