Studies on Quantum Spin Clusters with Competing Interactions

我们研究了一个具有蝴蝶结构的镍四核配合物，其中镍离子位于钻石形状的顶点。该化合物中存在三种不同的交换作用（J_1，J_2，J_3）。当它们是反铁磁时，系统具有几何阻挫。我们对这种化合物的单晶进行了磁化率和磁化强度测量。磁化强度随着磁场的增加而单调增加，并显示出1/2磁化平台。然后，在40 T和65 T附近观察到磁化强度的逐步增加。在40 T和65 T之间的磁化平台的大小是饱和磁化强度的四分之三。利用上述模型对实验结果进行了分析，并求出了交换常数。此外，我们进行了ESR实验高达55 T的频率覆盖从35 GHz到1.6太赫兹。我们观察到g = 2.2和g = 4.4的ESR信号，分别对应于最低五重态中Δm=±1和Δm=±2的跃迁.此外，我们还对S=1的交替键链进行了比热实验，观察到在磁场作用下能隙闭合时，长程有序的尖锐峰.我们已经发现，该峰的温度依赖性是非常接近的Haloxane材料NDMAP。

We investigated a nickel four nuclear complex with a butterfly structure in which the nickel ions are located at the apexes of a diamond shape. Three different exchange interactions (J_1, J_2, J_3) exist in this compound. When these are antiferromagnetic, the system has a geometrical frustration.We performed susceptibility and magnetization measurements on single crystals of this compound. The magnetization increases monotonically with increasing field and shows the 1/2 magnetization plateau. Then, the stepwise magnetization increases were observed around 40 T and 65 T. The magnitude of the magnetization plateau between 40 T and 65 T is three fourths of the saturation magnetization. The experimental results were analyzed by the above-mentioned model and evaluated the exchange constants. Moreover, we carried out the ESR experiments up to 55 T for the frequencies covering from 35 GHz to 1.6 THz. We observed ESR signals with g〜2.2 and g〜4.4 which correspond to the Δm=±1 and Δm=±2 transitions within Ithe lowest quintet, respectively.In addition to this work, we performed specific heat experiments on an S=1 bond alternating chain and we observed a sharp peak attributed to the long range ordering when the energy gap closes by application of magnetic fields. We have found the temperature dependence of the peak is very close to that of the Haldane material NDMAP.