Slow relaxation in a one-dimensional rational assembly of antiferromagnetically coupled [Mn4] single-molecule magnets.
Slow relaxation in a one-dimensional rational assembly of antiferromagnetically coupled [Mn4] single-molecule magnets.
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DOI：
10.1021/ja0551685
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发表时间：
2005-11
影响因子：
15
通讯作者：
L. Lecren;O. Roubeau;C. Coulon;Yang-guang Li;Xavier F. Le Goff;W. Wernsdorfer;H. Miyasaka;R. Clérac-R.-Cl
中科院分区：
文献类型：
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作者：
L. Lecren;O. Roubeau;C. Coulon;Yang-guang Li;Xavier F. Le Goff;W. Wernsdorfer;H. Miyasaka;R. Clérac-R.-Cl
四个离散的Mn(III)/Mn（II）四核配合物，具有双立方核，[Mn(4)(hmp)(6)(CH(3)CN)(2)(H(2)O)(4)](ClO(4))(4)。2 ch (3) CN (1), (Mn (4) (hmp) (6) (H (2) O) (4)] (ClO)(4)(4)。2 H (2) O (2), (Mn (4) (hmp) (6) (H (2) O)(2)(不)(3)(2)(2)(ClO(4))。4 h (2) O(3)和(Mn (4) (hmp) (6) (Hhmp) (2)] (ClO)(4)(4)。以Hhmp（2-羟甲基吡啶）和Mn(ClO(4))(2)为原料合成了2CH(3)CN(4)。6H(2)O在四乙基氢氧化铵存在下，随后加入纳米(3)(3)或过量的Hhmp(4)。直流电（dc）磁测量表明，Mn(2+)-Mn（3+）和Mn(3+)-Mn（3+）磁相互作用在1-3中都是铁磁性的，导致Mn(4)单元的基态为S(T) = 9。此外，这些配合物是单分子磁体（SMMs），清楚地显示出热激活和基态隧道机制。[Mn(4)]磁芯几何形状的微小变化导致4中的S(T) = 1基态。在相同的合成条件下，通过添加NaN(3)，得到了[Mn(4)]单元的一维组装体catena-{[Mn(4)(hmp)(6)(N(3))(2)](ClO(4))(2)}(5)。双椅状的N(3)（-）桥将相同的[Mn(4)]单元连接成链式排列。该材料表现为S(T) = 9四聚体弱反铁磁耦合的Ising组装体。在低温下，首次在反铁磁链中观察到磁化的慢弛豫，其激活行为为δ (tau)/k(B) = 47 k和tau(0) = 7 x 10(-)(11) s。这种原始的热激活弛豫过程的观察是由有限尺寸效应引起的，特别是由奇数单元段中自旋的非补偿引起的。推广了已知的反铁磁耦合Ising自旋多分散有限段动力学性质的理论，估计了特征能隙Delta（xi）和Delta（tau）的理论表达式，并成功地与实验值进行了比较。
Four discrete Mn(III)/Mn(II) tetranuclear complexes with a double-cuboidal core, [Mn(4)(hmp)(6)(CH(3)CN)(2)(H(2)O)(4)](ClO(4))(4).2CH(3)CN (1), [Mn(4)(hmp)(6)(H(2)O)(4)](ClO(4))(4).2H(2)O (2), [Mn(4)(hmp)(6)(H(2)O)(2)(NO(3))(2)](ClO(4))(2).4H(2)O (3), and [Mn(4)(hmp)(6)(Hhmp)(2)](ClO(4))(4).2CH(3)CN (4), were synthesized by reaction of Hhmp (2-hydroxymethylpyridine) with Mn(ClO(4))(2).6H(2)O in the presence of tetraethylammonium hydroxide and subsequent addition of NaNO(3) (3) or an excess of Hhmp (4). Direct current (dc) magnetic measurements show that both Mn(2+)-Mn(3+) and Mn(3+)-Mn(3+) magnetic interactions are ferromagnetic in 1-3 leading to an S(T) = 9 ground state for the Mn(4) unit. Furthermore, these complexes are single-molecule magnets (SMMs) clearly showing both thermally activated and ground-state tunneling regimes. Slight changes in the [Mn(4)] core geometry result in an S(T) = 1 ground state in 4. A one-dimensional assembly of [Mn(4)] units, catena-{[Mn(4)(hmp)(6)(N(3))(2)](ClO(4))(2)} (5), was obtained in the same synthetic conditions with the subsequent addition of NaN(3). Double chairlike N(3)(-) bridges connect identical [Mn(4)] units into a chain arrangement. This material behaves as an Ising assembly of S(T) = 9 tetramers weakly antiferromagnetically coupled. Slow relaxation of the magnetization is observed at low temperature for the first time in an antiferromagnetic chain, following an activated behavior with Delta(tau)/k(B) = 47 K and tau(0) = 7 x 10(-)(11) s. The observation of this original thermally activated relaxation process is induced by finite-size effects and in particular by the noncompensation of spins in segments of odd-number units. Generalizing the known theories on the dynamic properties of polydisperse finite segments of antiferromagnetically coupled Ising spins, the theoretical expressions of the characteristic energy gaps Delta(xi) and Delta(tau) were estimated and successfully compared to the experimental values.