Investigation for the pressure-induced anomalous magnetism of the low dimensional organic quantum spin systems

自旋能隙系统由于其显著的量子效应而被观测到单重基态，这在低维量子自旋材料和有机磁体领域引起了人们的关注。我们精确测量了有机低维量子自旋系统在压力下的磁化强度和磁化率，发现了反常的磁性行为。典型的自旋Peierls物质MEM-[TCNQ]_2在压力下的磁化率在低温下随温度的降低而增加，大致符合Cuire定律，我们对这种反常的压力下的磁性行为进行了两年的研究。本研究共处理了三种自旋能隙体系：（1）卤素物质Ni（<14>C_5H_NQ_2）_2N_3PF_6，（2）量子梯物质：Cu_2（C_6H_N_2<12>）_2Cl_4及相关物质，（3）自旋Peierls物质：MEM-[TCNQ]_2及相关物质。制造出了良好的单晶，并精确测量了它们的磁性能作为压力的函数。在单轴压力下对单晶的磁性能进行了测量，发现MEM-[TCNQ]_2的磁性能表现出明显的各向异性。为了弄清MEM-[TCNQ]_2在压力下磁化率增加的原因，我们用自制的X波段电子自旋共振（ESR）压力池测量了单轴压力下的ESR谱：ESR谱的温度依赖性表明，在压力下有两种不同的自旋，这是通过磁化强度和磁化率等宏观测量所假设的。

Spin-gap systems where singlet-ground states are observed because of significant quantum effect have attracted attention in the field of low-dimensional quantum spin materials and organic magnets. We have precisely measured the magnetization and magnetic susceptibility for an organic low-dimensional quantum spin system under pressure and found the anomalous magnetic behavior. The magnetic susceptibility of a typical spin-Peierls substance, MEM-[TCNQ]_2, under pressure increases with decreasing temperature at low temperatures and it is roughly described by Cuire's law.We have studied to clarify such anomalous magnetic behavior under pressure for two years. Three kinds of spin-gap systems, (1)Haldane substance Ni(C_5H_<14>NQ_2)_2N_3PF_6, (2)quantum ladder substances : Cu_2(C_6H_<12>N_2)_2Cl_4 and related substances, (3)spin-Peierls substances : MEM-[TCNQ]_2 and a related substance, were treated in this study. Good single crystals were made and their magnetic properties were measured precisely as a function of pressure. As a result of the measurement for a single crystal under uniaxial pressure, we found that the magnetic properties of MEM-[TCNQ]_2 show significant anisotropy. In order to clarify the origin of the increase of the magnetic susceptibility under pressure for MEM-[TCNQ]_2, we have measured the electron-spin-resonance (ESR) spectra under uniaxial pressure by using a hand-made pressure cell for a conventional X-band ESR instrument : The temperature dependence of the ESR spectra show that there are two different spins under pressure, which has been supposed by the macroscopic measurement such as the magnetization and magnetic susceptibility.