Origin of magnetization tunneling in single-molecule magnets as determined by single-crystal high-frequency EPR

We review an extensive body of single-crystal high-frequency electron paramagnetic resonance (HFEPR) data in order to determine the transverse spin Hamiltonian parameters that control the tunneling of the direction of magnetization in a variety of integer and half-integer-spin single-molecule magnets (SMMs). The SMMs studied are members of the following families: S = 9/2 [Mn4O3Cl]6+; S = 5 [Mn3NiO4]6+; S = 6 [Mn3ZnO4]6+; and S = 4 [Ni4(OR)4]4+. HFEPR spectra for the half-integer S = 9/2 Mn4 complexes that have C3 symmetry do not provide measurable evidence for transverse spin Hamiltonian terms. This finding is consistent with the relatively large coercive field seen in the magnetization hysteresis loops for these complexes. On the other hand, a low symmetry S = 9/2 complex exhibits a much faster rate of ground-state magnetization tunneling, in agreement with HFEPR spectra for a powder sample that gives a rhombic zero-field splitting (ZFS) parameter of E = 0.140 cm−1. The S = 5 Mn3Ni systems exhibit magnetization tunneling that is much faster than seen for the high-symmetry S = 9/2 Mn4 complexes. This can be attributed to their integer-spin ground states. Like the C3 symmetry Mn4 SMMs, the HFEPR spectra for high-symmetry Mn3Ni complexes do not provide measurable evidence for transverse ZFS terms. However, the spectra exhibit broad peaks, suggesting distributions in the local molecular environments brought about by disordered solvate molecules. This disorder likely explains the fast tunneling in the high-symmetry S = 5 Mn3Ni systems, though one cannot rule out fourth- (and higher-) order interactions that cannot be detected by HFEPR due to the broad resonances. The one S = 6 Mn3Zn complex shows an even faster rate of tunneling compared to the isostructural S = 5 Mn3Ni complex. Finally, the S = 4 [Ni(hmp)(dmb)Cl]4 complex provides unique insights into the origin of fourth- (and higher-) order interactions found for many SMMs on the basis of analysis using a giant spin Hamiltonian (GSH) approximation. We conclude that the fourth-order anisotropy found for the S = 4 ground state of [Ni(hmp)(dmb)Cl]4 originates from the second-order ZFS interactions associated with the individual NiII ions, but only as a result of higher-order processes that occur via S-mixing between the ground state and higher-lying (S < 4) spin-multiplets. The S-mixing is relatively strong in this system because of comparable exchange and anisotropy energy scales. The relatively fast tunneling is a direct consequence of this S-mixing, as opposed to any intrinsic fourth-order (spin–orbit) anisotropy associated with NiII.