Lippert Kai-Alexander, Mukherjee Chandan, Broschinski Jan-Philipp, Lippert Yvonne, Walleck Stephan, Stammler Anja, Bögge Hartmut, Schnack Jürgen, Glaser Thorsten
Lehrstuhl für Anorganische Chemie I, Fakultät für Chemie, Universität Bielefeld , Universitätsstrasse 25, D-33615 Bielefeld, Germany.
Fakultät für Physik, Universität Bielefeld , Universitätsstrasse 25, D-33615 Bielefeld, Germany.
Inorg Chem. 2017 Dec 18;56(24):15119-15129. doi: 10.1021/acs.inorgchem.7b02453. Epub 2017 Nov 30.
Single-molecule magnets (SMMs) retain a magnetization without applied magnetic field for a decent time due to an energy barrier U for spin-reversal. Despite the success to increase U, the difficult to control magnetic quantum tunneling often leads to a decreased effective barrier U and a fast relaxation. Here, we demonstrate the influence of the exchange coupling on the tunneling probability in two heptanuclear SMMs hosting the same spin-system with the same high spin ground state S = 21/2. A chirality-induced symmetry reduction leads to a switch of the Mn-Mn exchange from antiferromagnetic in the achiral SMM [MnCr] to ferromagnetic in the new chiral SMM [MnCr]. Multispin Hamiltonian analysis by full-matrix diagonalization demonstrates that the ferromagnetic interactions in [MnCr] enforce a well-defined S = 21/2 ground state with substantially less mixing of M substates in contrast to [MnCr] and no tunneling pathways below the top of the energy barrier. This is experimentally verified as U is smaller than the calculated energy barrier U in [MnCr] due to tunneling pathways, whereas U equals U in [MnCr] demonstrating the absence of quantum tunneling.
单分子磁体（SMMs）由于自旋反转的能垒U，在没有外加磁场的情况下能在相当长的时间内保持磁化。尽管在提高U方面取得了成功，但难以控制的磁量子隧穿常常导致有效势垒U降低和快速弛豫。在此，我们展示了交换耦合对两个具有相同自旋系统且基态高自旋S = 21/2的七核单分子磁体中隧穿概率的影响。手性诱导的对称性降低导致Mn - Mn交换从非手性单分子磁体[MnCr]中的反铁磁转变为新的手性单分子磁体[MnCr]中的铁磁。通过全矩阵对角化进行的多自旋哈密顿量分析表明，与[MnCr]相比，[MnCr]中的铁磁相互作用使基态明确为S = 21/2，M子态的混合显著减少，并且在能垒顶部以下没有隧穿路径。这在实验中得到了验证，因为由于隧穿路径，[MnCr]中的U小于计算出的能垒U，而[MnCr]中的U等于U，表明不存在量子隧穿。
