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Abstract
A series of Dy(III) mononuclear complexes [DyA2L]+ (L denotes Schiff base N5 ligand that occupies equatorial positions and A− denotes bidentate anionic O-donor ligands such as NO3− (1), AcO− (2), and acac− (3)) were synthesized to investigate the correlation between the slow magnetic relaxation phenomena and the coordination structures around Dy(III). The Dy(III) ion in each complex is in a nona-coordination with the anionic O-donor ligand occupying up- and down-side positions of the N5 equatorial plane. 2 and 3 show slow magnetic relaxation phenomena under a zero bias-field condition, and all complexes showed slow magnetic relaxation under the applied 1000-Oe bias-field conditions. Arrhenius analyses revealed that the ΔE/kB, the barrier height for magnetization flipping, increases in this order, with the values of 24.1(6), 85(3), and 140(15) K. The effects of the exchanging axial ligands on the magnetic anisotropy were discussed together with the DFT calculations.
Highlights
Single-molecule magnets (SMMs) are fascinating molecule-based nanomaterials, which are characterized by slow relaxation of magnetization at low temperatures [1–10]
Magnetic anisotropy plays an essential role in preventing the magnetization flipping; as the orbital angular momentum of 4f electrons is unquenched in the complex formations, each lanthanide(III) (Ln(III)) ion possesses a large magnetic moment correlated with the total angular momentum J, which is defined by the length of the vector summation of the spin angular momentum S and the orbital angular momentum L
Equatorial positions of Dy(III) ion were occupied by five N-donor atoms from L, and the up- and down-sides were occupied by anionic O-donor atoms aiming to achieve the easy-axis magnetic anisotropy of an oblate type Dy(III) ion
Summary
Single-molecule magnets (SMMs) are fascinating molecule-based nanomaterials, which are characterized by slow relaxation of magnetization at low temperatures [1–10]. The larger negative charges and the shorter coordination distances of O-donors lead to a larger stress along z-axis, and this is likely to enhance the easy-axial magnetic anisotropy for Dy(III) as an oblate ion. To reveal the effects of the bias field on slow magnetic relaxations, the bias field dependences of the out-of-phase susceptibilities were measured under the DC field in the ranges of 0 to 3000 Oe and 0 to 5000 Oe for several temperatures (Figure 4). The crystal field anisotropy as well as the magnetic anisotropy is enhanced in the order of 1 to 3, and the kinetic parameters in Table 1 clearly support the idea This result indicates that magnetic relaxation phenomena are strongly dependent on the nature of axial ligands, and that the increase of negative charge distributions on O-donor atoms plays a primitive role in enhancing the magnetic anisotropy. This may suggest the presence of other factors enhancing the barrier height of the magnetization flipping; we examined the electronic structures of the complexes with the DFT technique shown below
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