Observation of a Secondary Slow Relaxation Process for the Field-Induced Single-Molecule Magnet U(H₂BPz₂)₃

The trigonal prismatic complex U(H(2)BPz(2))(3) is characterized by single crystal X-ray diffraction and ac magnetic susceptibility measurements. The ac susceptibility data demonstrate the presence of multiple processes responsible for slow magnetic relaxation. Out-of-phase signals observed at ac switching frequencies between 1 and 1500 Hz in dc fields of 500-5000 Oe indicate a thermal relaxation barrier of ca. 8 cm(-1) for the molecule, with a temperature-independent process taking over at the lowest temperatures probed. Significantly, an unprecedented, slower relaxation process becomes apparent for ac switching frequencies between 0.06 and 1 Hz, for which a monotonic increase of the relaxation time with an applied dc field suggests a direct relaxation pathway.