第一过渡系金属-镝单分子磁体研究进展与展望

Research Progress and Prospect on the First Series Transition Metal‑Dy Single Molecule Magnets

第一过渡系中的顺磁性离子Cr^Ⅲ、Mn^Ⅱ/Mn^Ⅲ、Fe^Ⅱ/Fe^Ⅲ、Co^Ⅱ、Ni^Ⅱ和Cu^Ⅱ及抗磁性离子CoⅢ和Zn^Ⅱ均可与Dy^Ⅲ在多齿螯合配体配位下形成单分子磁体配合物。在本文中,我们阐述或汇总了几乎所有的第一过渡系金属-镝单分子磁体。对于由顺磁性第一过渡金属离子和Dy^Ⅲ离子形成的配合物,有2个有趣的现象需要引起人们的注意:一是一些Cr-Dy配合物具有较高的阻塞温度和较大的矫顽场,这可归功于配合物内Cr^Ⅲ离子和Dy^Ⅲ离子之间较强的磁耦合作用(|J|>10 cm^-1)。二是报道的Fe2^Ⅱ-Dy配合物的能垒可达到319 cm^-1(459 K),这在第一过渡系金属-镝单分子磁体中也是比较高的。这可能与Fe2^Ⅱ-Dy中Dy^Ⅲ具有较高的轴向对称性(D5h)有关,且从头计算表明该配合物中Dy的第一激发态也具有较高的轴向对称性。除了部分Cr-Dy和Fe^Ⅱ-Dy配合物外,其他顺磁性第一过渡金属-Dy的能垒较低,这可能由配合物内顺磁离子间弱的磁耦合造成的。为了消除磁耦合对磁弛豫行为影响,近年来人们关注于使用抗磁性第一过渡金属离子与DyⅢ构建单分子磁体配合物。相比其他核数的Zn-Dy配合物,三核Zn2Dy配合物被报道的数目最多且研究得最为深入,这可能与较易调控Zn2Dy中Dy配位几何对称性有关。最后,我们提出了几点关于进一步提升第一过渡系金属-镝单分子磁体的磁性能的建议,其中最为重要的是控制Dy配位几何的轴向对称性及Dy的基态mJ的电荷分布。对于第一过渡系金属-镝单分子磁体中的Dy^Ⅲ离子,Dy^Ⅲ基态mJ的电荷与配体的电荷之间的静电排斥应该降到最低。

Among the first series transition metal(TM)ions,not only the paramagnetic TM ions(such as Cr^Ⅲ ,Mn^Ⅱ/Mn^Ⅲ ,Fe^Ⅱ/Fe^Ⅲ ,Co^Ⅱ,Ni^Ⅱ and Cu^Ⅱ)but also diamagnetic TM ions(such as Co^Ⅲ and Zn^Ⅱ)can assemble with Dy^Ⅲ to form the TM-Dy single molecule magnets(TM-Dy SMMs).In this paper,we survey most of the TM-Dy complexes with SMM behavior.For the TM-Dy SMMs,two interesting phenomena should be emphasized on.One is some Cr-Dy complexes possessing relatively high blocking temperature(TB)and large coercivities,which could be ascribed to the strong magnetic couplings between Cr^Ⅲ and Dy^Ⅲ ions(|J|>10 cm^-1).Another is the Fe^Ⅱ2-Dy complex with the energy barrier of 319 cm^-1(459 K),which is high among the TM-Dy SMMs.The reason might be that the axial symmetry of the coordination of Dy^Ⅲ ion is high(D5h).Additionally,ab initio calculations indicate the excited state also possess high axial symmetry.Besides the Cr^Ⅲ-Dy and Fe^Ⅱ2-Dy complexes,other TM-Dy SMMs usually exhibits relatively low energy barriers,which may be due to the weak magnetic interactions between paramagnetic TM ions and Dy^Ⅲ ions.To elimate the effect of the weak magnetic couplings on the relaxation behavior,the study of the DTM-Dy SMMs(DTM=diamagnetic the first series transition metal ions such as Co^Ⅲ and Zn^Ⅱ)has been received wide attention and their number is dramatically increasing.For the Zn-Dy SMMs,the number of the Zn2Dy trinuclear complexes is larger than that of other Zn-Dy complexes,indicating that the Zn2Dy received more attention.The reason might be that the coordination geometry of the Dy^Ⅲ in the Zn2Dy complexes can be more easily modulated.Finally,we provided some reseach thoughts for further improving the SMM performance of the TM-Dy complexes;the most important thoughts are that the axial symmetry of the coordination geometries of Dy^Ⅲ ions and the distribution of charge density of Dy^Ⅲ ions in TM-Dy complexes should be controlled.For Dy^Ⅲ in the TM-Dy complexes,the electrostatic repulsion between the Dy^Ⅲ ground-mJ charge density and the ligand charge density should be minimized.