Magic-angle effects in a trigonal Mn3III cluster: Deconstruction of a single-molecule magnet

Abstract

We present angle-dependent high-frequency EPR studies on a single-crystal of a trigonal Mn3 cluster with an unusual structure in which the local magnetic easy-axes of the constituent Mn(III) ions are tilted significantly away from the molecular C3 axis towards the magic-angle of 54.7 degrees, resulting in an almost complete cancelation of the 2nd-order axial magnetic anisotropy associated with the ferromagnetically coupled total spin ST = 6 ground state. This contrasts the situation in many related Mn3 single-molecule magnets (SMMs) that have been studied intensively in the past, for which the local MnIII anisotropy tensors are reasonably parallel, resulting in substantial barriers to magnetization relaxation (Ueff = 30 to 35 cm 1) and magnetization blocking below about 2.5 K. The suppression of the 2nd-order anisotropy in the present case results in a situation in which the zero-field splitting (ZFS) of the ST = 6 ground state is dominated by 4th- and higher-order interactions. This provides a unique opportunity to study in depth how molecular geometry influences these interactions that are responsible for quantum tunneling of magnetization in high-symmetry SMMs. Angle-dependent EPR measurements provide a full mapping of the molecular magneto-anisotropy. Meanwhile, irreducible tensor operator (ITO) methods are employed in order to obtain analytic expressions that directly relate molecular anisotropy to the microscopic physics, i.e., the ZFS tensors associated with the individual MnIII ions, their orientations, and the exchange coupling between the three spins. We find that the magic-angle tilting leads to a massive compression of the ST = 6 ground state energy level diagram and strong mixing between spin projection states. Although these characteristics are antagonistic to SMM behavior, they provide important insights into the physics of polynuclear molecular nanomagnets.