Pump-probe experiments on the single-molecule magnet Fe8: measurement of excited level lifetimes.

We present magnetization measurements on the single-molecule magnet Fe8 in the presence of pulsed microwave radiation. A pump-probe technique is used with two microwave pulses with frequencies of 107 and 118 GHz and pulse lengths of several nanoseconds to study the spin dynamics via time-resolved magnetization measurements using a Hall probe magnetometer. We find evidence for short spin-phonon relaxation times of the order of 1 micros. The temperature dependence of the spin-phonon relaxation time in our experiments is in good agreement with previously published theoretical results. We also established the presence of very short energy diffusion times that act on a time scale of about 70 ns.

Nature of heavy quasiparticles in magnetically ordered heavy fermions UPd(2)Al(3) and UPt(3).

The optical conductivity of the heavy fermions UPd(2)Al(3) and UPt(3) has been measured in the energy range from 0.04 to 5 meV. In both compounds a well pronounced pseudogap of less than 1 meV develops in the optical response at low temperatures; we relate this to the antiferromagnetic ordering. From the energy dependence of the effective mass and scattering rate we conclude that the enhancement of the mass mainly occurs below the energy which is related to magnetic correlations between the local magnetic moments and the itinerant electrons. This implies that the magnetic order in these compounds is the prerequisite to the formation of the heavy quasiparticles and eventually to superconductivity.