L. Badía-Romano

The very low-temperature relaxation processes of the magnetic moment in the “butterfly” molecule [Fe3Tb(μ3–O)2(CCl3COO)8(H2O)(THF)3], {Fe3TbO2}, have been investigated by a combination of heat capacity and ac susceptibility measurements, both at zero magnetic field and under an applied magnetic field. The heat capacity has been interpreted within an approximation in which the Fe3 subcluster has a total spin SFe3 = 5/2, the Tb3+ (J = 6) ligand field interaction is described in terms of Stevens operators, and an Fe3–Tb antiferromagnetic Heisenberg interaction is considered, with JTbFe3/kB = –0.23 K. The coupled Fe3 (Kramers) and Tb (non-Kramers) yield to the cluster to behave as a Kramers electronic spin system. At zero applied field (H = 0), the uniaxial anisotropy barrier introduced by the magnetic lanthanide ion (Tb3+) gives rise to two thermally activated quantum tunneling of magnetization (TAQTM) Orbach processes. The first relaxation pathway predominates at the lowest range of temperatures with a TAQTM1 process with an activation energy of E1/kB = 1.35(1) K. Since the Tb3+ ion consists entirely of isotopes with non-zero nuclear spin (I = 3/2), the tunneling processes are assisted via electron–nuclear hyperfine pathways. As the system is cooled below T = 0.17 K, a slow direct acoustic phonon assisted relaxation process between the nuclear split ground states is dominant. This latter process is affected by phonon-bottleneck effect. A second, TAQTM2 faster process is observed at the highest measured temperatures, with activation energy of E2/kB = 3.70(1) K. Both activations may be related to the first and second excited energy levels of the magnetic cluster. Upon cooling, the relaxation time crosses over to a temperature-independent quantum tunneling of magnetization regime via electron–nuclear hyperfine pathways. At temperatures above 2 K, even under the application of an external magnetic field, the relaxation time decreases below the experimental time window, resulting in too fast relaxation processes. Consequently, the {Fe3TbO2} molecule can be classified as a nanomagnet exhibiting single-molecule magnet behavior at sub-K temperatures.