Studies on Quantum Spin Clusters with Competing Interactions

我们研究了一种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端。该化合物中存在三种不同的交换作用（J_1, J_2, J_3）。当这些是反铁磁性时，系统有几何挫折。我们对该化合物的单晶进行了磁化率和磁化率测量。磁化强度随磁场的增大而单调增加，呈现1/2的磁化平台。在40 T和65 T附近，磁化强度逐渐增大，40 T和65 T之间的磁化平台强度为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，并计算了交换常数。此外，我们在35 GHz至1.6 THz的频率范围内进行了高达55 T的ESR实验。我们观察到的ESR信号为g ~ 2.2和g ~ 4.4，分别对应于最低五重奏中Δm=±1和Δm=±2的跃迁。除此之外，我们还对S=1键交替链进行了比热实验，并观察到在磁场作用下，当能量间隙闭合时，由于长范围有序，出现了一个尖峰。我们发现峰的温度依赖性与霍尔丹材料NDMAP非常接近。

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.