Design of Single-Molecule Magnets: Insufficiency of the Anisotropy Barrier as the Sole Criterion
Design of Single-Molecule Magnets: Insufficiency of the Anisotropy Barrier as the Sole Criterion
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DOI：
10.1021/acs.inorgchem.5b01209
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发表时间：
2015-08-03
影响因子：
4.6
通讯作者：
Piligkos, Stergios
中科院分区：
文献类型：
--
作者：
Pedersen, Kasper S.;Dreiser, Jan;Piligkos, Stergios
利用高分辨率吸收光谱和荧光光谱对三角对称单核Yb3+单分子磁体（SMM）的电子能谱进行了测定，发现第一个被激发的电子偶极体位于距地面偶极体近500 cm（-1）的位置。将该- SMM的顺磁弛豫时间拟合到热激活（Orbach）模型{tau = tau(0) X exp [Delta(Orbach)/(k(B)T)]}}得到的激活势垒Delta（Orbach）仅为38 cm（-1）。这一结果与光谱观测不符。因此，我们明确地证明，仅根据实验数据，不能先验地认为奥巴赫弛豫是决定SM.Ms自旋动力学的主要机制。这项研究强调了这样一个事实，即通过最大化与配体场密切相关的各向异性势垒作为唯一需要调整的参数来优化SMM行为的一般合成方法是不够的，因为完全忽略了分子的磁矩与其环境的相互作用。预计奥尔巴赫机制仅在激发配体场态的能量低于德拜温度的情况下占主导地位，这对于分子晶体来说是典型的低温度，因此，阻止使用各向异性势垒作为设计标准来实现高温SM.Ms。因此，需要考虑额外的设计标准，以解决传统双井图之外存在的替代松弛过程。
Determination of the electronic energy spectrum of a trigonalsymtnetry mononuclear Yb3+ single-molecule magnet (SMM) by highresolution absorption and luniinescente spectroscopies reveals that the first excited electronic doublet is placed nearly 500 cm(-1) above the ground one. Fitting of the paramagnetic relaxation times of this- SMM to a thermally activated (Orbach) model {tau = tau(0) X exp [Delta(Orbach)/(k(B)T)]} affords an activation barrier, Delta(Orbach), of only 38 cm(-1). This result is incompatible with the spectroscopic observations. Thus, we unambiguously demonstrate, solely on the basis Of experimental data, that Orbach relaxation cannot ci priori be considered as the main mechanism determining the spin dynamics of SM.Ms. This study highlights the fact that the general:synthetic approach of optimizing SMM behavior by maximization of the anisotropy barrier, intimately linked to the ligand field, 'as the sole paratneter to be tuned, is insufficient because of the complete neglect of the interaction of the magnetic moment of the molecule with its environment. The Orbach mechanism is expected dominant only in the cases in which the energy of the excited ligand field state is below the Debye temperature, which is typically low for molecular crystals and, thus, prevents the use of the anisotropy barrier as a design criterion, for the realization of high-temperature SM.Ms. Therefcire, consideration of additional design criteria that address the presence of alternative relaxation processes beyond the traditional double-well picture is required.