Exchange Interactions at the Origin of Slow Relaxation of the Magnetization in {TbCu3} and {DyCu3} Single-Molecule Magnets

New {TbCu3} and {DyCu3} single-molecule magnets (SMMs) containing a low-symmetry LnIII center (shape measurements relative to a trigonal dodecahedron and biaugmented trigonal prism are 2.2–2.3) surrounded by three CuII metalloligands are reported. SMM behavior is confirmed by frequency-dependent out-of-phase ac susceptibility signals and single-crystal temperature and sweep rate dependent hysteresis loops. The ferromagnetic exchange interactions between the central LnIII ion and the three CuII ions could be accurately measured by inelastic neutron scattering (INS) spectroscopy and modeled effectively. The excitations observed by INS correspond to flipping of CuII spins and appear at energies similar to the thermodynamic barrier for relaxation of the magnetization, ∼15–20 K, and are thus at the origin of the SMM behavior. The magnetic quantum number Mtot of the cluster ground state of {DyCu3} is an integer, whereas it is a half-integer for {TbCu3}, which explains their vastly different quantum tunneling of the magnetization behavior despite similar energy barriers.

本文报道了两类新型{TbCu3}与{DyCu3}单分子磁体（single-molecule magnets, SMMs），其中心为低对称性三价镧系离子（LnIII），相对于三角十二面体和双加三角棱柱的配位多面体畸变参数为2.2–2.3，周围由三个二价铜金属配体配位环绕。该类单分子磁体的磁行为通过频率依赖的异相交流磁化率信号，以及单晶温度与扫速依赖的磁滞回线得以验证。中心LnIII离子与三个CuII离子之间的铁磁交换相互作用可通过非弹性中子散射（inelastic neutron scattering, INS）光谱精准测定，并得到有效建模。非弹性中子散射观测到的激发对应于CuII自旋的翻转，其对应能量与磁化弛豫的热力学势垒（约15–20 K）相近，因此是该类单分子磁体行为的起源。{DyCu3}团簇基态的总磁量子数Mtot为整数，而{TbCu3}的总磁量子数为半整数，这解释了尽管二者势垒相近，但磁化量子隧穿行为却存在显著差异的核心原因。