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

• Published in: Chemistry : a European journal 2016-01, Vol.22 (2), p.672-680
• Main Authors: Singh, Saurabh Kumar, Beg, Mohammad Faizan, Rajaraman, Gopalan
• Format: Article
• Language: English
• Subjects: ab initio calculations; Alternating current; Anisotropy; Antiferromagnetism; CASSCF calculations; Chemistry; Density; density functional calculations; Exchange; Frustration; Functional anatomy; Ground state; Ions; magnetic properties; Magnetization; Magnets; Mathematical analysis; Orbitals; Quantum tunnelling; {3d-4f} SMMs; {Ni-Ln} SMMs; {Ni3-Ln} complexes
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.201503102

Combined density functional and ab initio calculations are performed on two isomorphous tetranuclear {Ni3IIILnIII} 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 ∢NiOGd bond angles present in 1 instigate a significant interaction between the 4fxyz orbital of the GdIII ion and 3d${{_{x{^{2}}- y{^{2}}}$ orbital of the NiII ions, leading to rare and strong antiferromagnetic Ni⋅⋅⋅Gd interactions. Calculations reveal the presence of a strong next‐nearest‐neighbour Ni⋅⋅⋅Ni antiferromagnetic interaction in complex 1 leading to spin frustration behavior. CASSCF+RASSI‐SO calculations performed on complex 2 suggest that the octahedral environment around the DyIII ion is neither strong enough to stabilize the mJ |±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 DyIII ion is found to be the mJ |±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 JNiDy interaction as −1.45 cm−1. The strong Ni⋅⋅⋅Dy and next‐nearest‐neighbour Ni⋅⋅⋅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⋅⋅⋅Dy and the Ni⋅⋅⋅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⋅⋅⋅Dy and Ni⋅⋅⋅Ni interactions in magnetization relaxation of a {3d–4f} molecular magnet have been established. Quantum tunneling: DFT and ab initio calculations suggest that both Ni⋅⋅⋅Dy and 1, 3 Ni⋅⋅⋅Ni (see figure) interactions help to quench the QTM behavior in {3d–4f} single‐molecule magnets.