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首页> 外文期刊>Coordination chemistry reviews >Quantum tunneling of the magnetization in [(Mn6M)-M-III](3+) (M = Cr-III, Mn-III) SMMs: Impact of molecular and crystal symmetry
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Quantum tunneling of the magnetization in [(Mn6M)-M-III](3+) (M = Cr-III, Mn-III) SMMs: Impact of molecular and crystal symmetry

机译：[（Mn6M）-M-III]（3+）（M = Cr-III，Mn-III）SMM中磁化的量子隧穿：分子和晶体对称性的影响

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摘要

Single-molecule magnets (SMMs) are compounds that exhibit a hysteresis in the magnetization of pure molecular origin and that stay magnetized for a certain time without applied magnetic field. This behavior is associated with an energy barrier for magnetization reversal resulting in a slow relaxation of the magnetization at low temperature. The energy barrier can be overcome by a thermal pathway over the top of the barrier and by a quantum tunneling through the barrier. In order to slow down the magnetization reversal, the probability for both pathways must be minimized. We evaluate the influence of the molecular and crystal symmetry on the quantum tunneling for a family of heptanuclear SMMs [(Mn6MIII)-M-III](3+)(M-III = Cr-III, Mn-III) synthesized with the triplesalen ligand (talen(t-Bu2))(6-) using different salts and solvates. [(Mn6CrIII)-Cr-III](3+) SMMs have only a moderate height of the energy barrier of 15-19 cm(-1). Single-crystal magnetization measurements of a [(Mn6CrIII)-Cr-III](3+) SMM of low molecular and crystal symmetry exhibit strong zero-field tunneling due to transversal components by dipolar stray fields and rhombic E-st terms. Crystallization of [(Mn6CrIII)-Cr-III](3+) in a high-symmetry trigonal space group using the rod-shaped anion lactate results in a high molecular and crystal symmetry. This almost completely suppresses zero-field tunneling due to vanishing transversal components. [(Mn6MIII)-M-III](3+) SMMs have a diamagnetic Mn-III l.s. ion in the center due to efficient spin-orbit coupling. The high molecular and crystal symmetry of the [(Mn6MIII)-M-III](3+) lactate salt results in a blocking temperature T-B approximate to 2 K despite of only a S-t,S-1 = 4 spin ground state with an energy barrier of 0.18 cm(-1). This demonstrates the strong impact of a high molecular and crystal symmetry on the suppression of QTM. At fields above 2T, a S-t,S-2 = 12 state becomes ground state, which even exhibits an open hysteresis up to +/- 10 T. We propose five requirements, which must be simultaneously fulfilled, to make 'better' SMMs with higher blocking temperatures: (1) a high magnetic ground state (either S-t, L-t, or J(t)), (2) a strong magnetic anisotropy, (3) a control of the molecular topology with symmetry of at least C-3 but less than cubic and collinearly aligned local anisotropy tensors, (4) a control of the crystal structure with collinear aligned molecular easy axes and symmetric environments, and (5) isolated ground state multipletts, i.e. strong exchange couplings in polynuclear SMMs. (C) 2014 Elsevier B.V. All rights reserved.
机译：单分子磁体（SMM）是在纯分子起源的磁化过程中表现出磁滞现象并且在未施加磁场的情况下保持磁化一定时间的化合物。此行为与磁化反转的能垒相关，从而导致低温下磁化的缓慢松弛。可以通过在势垒顶部上方的热路径和通过势垒的量子隧道来克服能垒。为了减慢磁化反转，必须将两个路径的可能性减至最小。我们评估了分子和晶体对称性对用三链烯合成的七核SMMs [（Mn6MIII）-M-III]（3 +）（M-III = Cr-III，Mn-III）量子隧穿的影响。配体（talen（t-Bu2））（6-），使用不同的盐和溶剂化物。 [（Mn6CrIII）-Cr-III]（3+）SMM仅具有15-19 cm（-1）的中等能垒高度。低分子和晶体对称性的[（Mn6CrIII）-Cr-III]（3+）SMM的单晶磁化测量显示，由于偶极杂散场和菱形E-st项引起的横向分量，导致强零场隧穿。使用棒状阴离子乳酸在高对称三角空间基团中[（Mn6CrIII）-Cr-III]（3+）的结晶会导致分子和晶体对称。这几乎完全抑制了由于横向分量消失而引起的零场隧穿。 [（Mn6MIII）-M-III]（3+）SMM具有抗磁性Mn-IIIl.s。由于有效的自旋轨道耦合，离子在中心。尽管仅具有能量的St，S-1 = 4自旋基态，[（Mn6MIII）-M-III]（3+）乳酸盐的高分子和晶体对称性仍导致约2 K的封闭温度TB阻值为0.18 cm（-1）。这证明了高分子和晶体对称性对抑制QTM的强大影响。在高于2T的磁场中，St，S-2 = 12状态变为基态，甚至表现出高达+/- 10 T的开放磁滞。我们提出了五个要求，必须同时满足这些要求，以制造出“更好”的SMM。更高的阻断温度：（1）高磁性基态（St，Lt或J（t）），（2）强磁性各向异性，（3）对称性至少为C-3的分子拓扑结构的控制但小于立方和共线排列的局部各向异性张量，（4）通过共线排列的分子易轴和对称环境控制晶体结构，以及（5）孤立的基态多重态，即多核SMM中的强交换耦合。 （C）2014 Elsevier B.V.保留所有权利。

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