ic050093r_si_003.cif (12.42 kB)

Fast Magnetization Tunneling in Tetranickel(II) Single-Molecule Magnets

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posted on 23.01.2006, 00:00 by En-Che Yang, Wolfgang Wernsdorfer, Lev N. Zakharov, Yoshitomo Karaki, Akira Yamaguchi, Rose M. Isidro, Guo-Di Lu, Samuel A. Wilson, Arnold L. Rheingold, Hidehiko Ishimoto, David N. Hendrickson
A series of Ni4 cubane complexes with the composition [Ni(hmp)(ROH)Cl]4 complexes 1−4 where R= −CH3 (complex 1), −CH2CH3 (complex 2), −CH2CH2(C4H9) (complex 3), −CH2CH2CH2(C6H11) (complex 4), hmp- is the anion of 2-hydroxymethylpyridine, t-Buhmp- is the anion of 4-tert-butyl-2-hydroxymethylpyridine, and dmb is 3,3-dimethyl-1-butanol] and [Ni(hmp)(dmb)Br]4 (complex 5) and [Ni(t-Buhmp)(dmb)Cl]4 (complex 6) were prepared. All six complexes were characterized by dc magnetic susceptibility data to be ferromagnetically coupled to give an S = 4 ground state with significant magnetoanisotropy (D ≈ −0.6 cm-1). Magnetization hysteresis measurements carried out on single crystals of complexes 16 establish the single-molecule magnet (SMM) behavior of these complexes. The exchange bias observed in the magnetization hysteresis loops of complexes 1 and 2 is dramatically decreased to zero in complex 3, where the bulky dmb ligand is employed. Fast tunneling of magnetization is observed for the high-symmetry (S4 site symmetry) Ni4 complexes in the crystal of complex 3, and the tunneling rate can even be enhanced by destroying the S4 site symmetry, as is the case for complex 4, where there are two crystallographically different Ni4 molecules, one with C2 and the other with C1 site symmetry. Magnetic ordering temperatures due to intermolecular dipolar and magnetic exchange interactions were determined by means of very low-temperature ac susceptibility measurements; complex 1 orders at 1100 mK, complex 3 at 290 mK, complex 4 at ∼80 mK, and complex 6 at <50 mK. This confirms that bulkier ligands correspond to more isolated molecules, and therefore, magnetic ordering occurs at lower temperatures for those complexes with the bulkiest ligands.