受限低维量子磁性材料的强磁场ESR研究

Low-dimensional quantum magnetism is an important topic of condensed matter physics. With the deficiencies and opportunities in present studies, we will try to introduce new degree of freedom-nanoscale in the bulk low-dimensional quantum magnetic materials and perform pulsed high-field ESR study on the novel ground state of these materials. First, nano-grain samples with various sizes will be synthesized and their morphology and crystal structure will be characterized. Second, by means of temperature- and frequency-dependent ESR measurements, we will study the magnetic ordering, magnetic excitation and spin gap of nano-grain samples. The relationship between the ground-state properties and magnetic field as well as nano-grain size will be constructed. The mechanism will be understood by analyzing the topological structure, magnetic anisotropy and exchange interaction. Much attention will be paid on the S=1/2 Cu and V oxides and the low-dimensional frustrated systems. In particular, we will focus on those materials which have been close to quantum critical point. It is expected to introduce spin gap or long-range magnetic order in these materials and find new quantum phenomena and transitions.

低维量子磁性是凝聚态物理研究的热点。本项目针对当前研究的不足和蕴涵的研究机遇，尝试在块体低维量子磁性材料中引入新的自由度- - -纳米受限，并利用脉冲强磁场ESR研究其异常基态。首先制备系列不同晶粒尺寸的纳米晶样品，表征其形貌和晶体结构；然后通过变温和变频ESR测量，研究纳米晶样品的磁有序、磁激发和自旋隙等，建立它们与强磁场和纳米晶粒尺寸之间的关联，并通过分析拓扑结构、各向异性和磁交换相互作用，揭示其微观起源。重点研究S=1/2小自旋Cu、V氧化物和自旋阻挫的材料体系，特别关注已接近量子临界点的材料，探索在这些材料中引入自旋隙或长程有序，并发现新的量子效应和量子相变的可能性。

低维量子磁性材料中的量子效应一直是凝聚态物理研究的热点。依照项目计划书，本项目首先利用脉冲强磁场ESR系统地研究了三类材料的纳米受限效应：（1）具有1/3量子磁化平台的自旋链α-CoV2O6，（2）处于量子临界点的Haldane链Y2BaNiO5和SrNi2V2O8以及（3）Gd基自旋链Gd2BaNiO5。我们的研究发现，低维量子磁性材料纳米尺度效应的一般规律是：当晶粒尺寸降低到纳米尺度（几十纳米量级）时，体系反铁磁区域的磁性会得到加强，磁有序温度降低，原有的量子效应如磁化平台、Haldane自旋隙等会逐渐被抑制。根据“核壳”模型，“核”由反铁磁自旋构成，而“壳”由未补偿的、无序的自旋组成。当样品进入纳米尺度时，核内反铁磁相关长度变小，而粒子表面形成大量的顺磁性离子，从而抑制了原来的Haldane自旋隙和反铁磁有序。我们尚未发现因纳米受限而诱导的自旋隙（有序-无序）和长程有序（无序-有序）效应。其次，我们还利用助溶剂和水热法合成了系列新型的低维量子磁性材料，如具有Kagome结构的量子磁体Cu4(OH)6FCl、五聚体自旋团簇Na3Cu5(PO4)4F•4H2O、自旋链化合物R2V2O7 (R = Ni,Co, Mn)和β-Cu2V2O7，并利用强磁场ESR和磁化研究了它们的磁性，发现了分数量子磁化平台等量子效应。