Role of Magnetic Exchange Interactions in the Magnetization Relaxation of {3d-4f} Single-Molecule Magnets: A Theoretical Perspective

摘要

Combined density functional and ab initio calculations are performed on two isomorphous tetranuclear {Ni(3)(III)Ln(III)} star-type complexes [Ln=Gd (1), Dy (2)] to shed light on the mechanism of magnetic exchange in 1 and the origin of the slow magnetization relaxation in complex 2. DFT calculations correctly reproduce the sign and magnitude of the J values compared to the experiments for complex 1. Acute (sic)Ni-O-Gd bond angles present in 1 instigate a significant interaction between the 4f(xyz) orbital of the Gd-III ion and 3d(x2-y2) orbital of the Ni-II ions, leading to rare and strong anti-ferromagnetic Ni center dot center dot center dot Gd interactions. Calculations reveal the presence of a strong next-nearest-neighbour Ni center dot center dot center dot Ni anti-ferromagnetic interaction in complex 1 leading to spin frustration behavior. CASSCF+RASSI-SO calculations performed on complex 2 suggest that the octahedral environment around the Dy-III ion is neither strong enough to stabilize the m(J) vertical bar +/- 15/2 > as the ground state nor able to achieve a large ground-state-first-excited-state gap. The ground-state Kramers doublet for the Dy-III ion is found to be the m(J) vertical bar +/- 13/2 > state with a significant transverse anisotropy, leading to very strong quantum tunneling of magnetization (QTM). Using the POLY_ANISO program, we have extracted the J(NiDy) interaction as -1.45 cm(-1). The strong Ni center dot center dot center dot Dy and next-nearest-neighbour Ni center dot center dot center dot center dot Ni interactions are found to quench the QTM to a certain extent, resulting in zero-field SMM behavior for complex 2. The absence of any ac signals at zero field for the structurally similar [Dy(AlMe4)(3)] highlights the importance of both the Ni center dot center dot center dot Dy and the Ni center dot center dot center dot Ni interactions in the magnetization relaxation of complex 2. To the best of our knowledge, this is the first time that the roles of both the Ni center dot center dot center dot Dy and Ni center dot center dot center dot Ni interactions in magnetization relaxation of a {3d-4f} molecular magnet have been established.