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

在低维量子自旋材料和有机磁体领域，由于量子效应而观察到单重态的自旋能隙系统引起了人们的关注。我们精确测量了有机低维量子自旋系统在压力作用下的磁化强度和磁化率，发现了系统的反常磁行为。典型的自旋-Peierls物质MEM-[TCNQ]_2在压强下的磁化率随温度的降低而增加，大致符合Cuire定律.我们研究了两年的压强下的这种反常磁性行为.研究了三种自旋能隙体系：(1)Haldane物质Ni(C_5H_&lt；14&gt；NQ_2)_2N_3PF_6；(2)量子阶梯物质：Cu2(C_6H_&lt；12&gt；N_2)_2Cl4及相关物质；(3)自旋-Peierls物质：MEM-[TCNQ]_2及相关物质。得到了良好的单晶，并精确地测量了它们的磁性能随压力的变化关系。对单晶在单轴压力下的测量结果表明，MEM-[TCNQ]_2的磁特性表现出明显的各向异性。为了阐明MEM-[TCNQ]_2的磁化率在压力下增加的原因，我们用自制的压力盒测量了MEM-[TCNQ]_2在单轴压力下的电子自旋共振(ESR)谱：ESR谱的温度依赖关系表明，在压力下存在两种不同的自旋，这是由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.