Investigation of frustrated antiferromagnetic compounds using ultrasonic velocity measurements

三角形晶格和焦绿石反铁磁体是二维和三维受阻自旋系统的完美原型，其中由于晶格几何的限制，两个磁矩不能反平行。由于磁性受挫导致了大量的简并自旋组态，这些材料的基本性质仍然没有完全被理解。因此，根据简单的最近邻相互作用经典模型，焦绿石晶格上的自旋不会发展到0K以下的任何长程有序。一般而言，这些系统的物理性质与反对称交换相互作用(Dzyaloshinskii-Moriya)、偶极相互作用、各向异性、磁弹性耦合，甚至低温下的量子效应有关。因此，这些受挫系统表现出许多新的奇异状态，如：新的非共线有序、自旋液体、自旋玻璃、自旋冰和量子自旋冰。*拟议的研究计划还专注于多铁磁性材料的研究，多铁磁性材料是同时表现出反铁磁、铁电和铁弹性性质的化合物。除了基本的科学兴趣外，我们主要对多铁化合物感兴趣，这些化合物显示出磁性和铁电性之间的强烈相互作用。这种耦合被称为磁电效应，可能被用作未来电子存储设备、致动器、开关和磁场传感器中控制磁化的替代手段。最近的研究结果再次引起了人们对这一领域的兴趣，这些结果表明，具有非共线自旋构型(三角形晶格反铁磁体)的受挫磁体子集可能是观察强磁电效应的良好候选者。*这类材料的基本弹性性质是通过高分辨率的超声波速度测量获得的。我们的实验室提供了一种灵活的方法来研究单晶在液压作用下的弹性性质随温度和磁场的变化。我们是世界上少数几个能够在很大范围内(50MK-300K，0-18Tesla和0-15kbar)测量弹性性质和作为三个外部参数的函数的磁性相图之一。我们的方法有可能获得重要的结果，以便澄清在这类化合物中观察到的弹性、电和磁性之间耦合的性质。这些结果也将为我们课题组开发的理论模型提供指导。因此，这项工作应该有助于更好地理解受挫反铁磁体的性质，并可能导致用于技术应用的新材料的开发。

Triangular lattice and pyrochlore antiferromagnets are perfect prototype of 2D and 3D frustrated spin systems where two magnetic moments cannot be anti-parallel to each other owing to constraints imposed by the lattice geometry. Due to the magnetic frustration, which leads to a large number of degenerate spin configurations, the fundamental properties of these materials are still not fully understood. As a result, according to simple nearest neighbor interaction classical models, spins on the pyrochlore lattice are not expected to develop any kind of long range order down to 0 K. In general, the physical properties of these systems are rather associated with antisymmetric exchange interaction (Dzyaloshinskii-Moriya), dipolar interaction, anisotropy, magnetoelastic couplings, and even quantum effects at low temperatures. Consequently, these frustrated systems display many new exotic states such as: new non-collinear orders, spin liquids, spin glasses, spin ices, and quantum spin ices.******The proposed research program also focuses on the study of multiferroic magnetic materials, which are compounds that simultaneously show antiferromagnetic, ferroelectric, and ferroelastic properties. Besides the fundamental scientific interest, we are principally interested in multiferroic compounds which show a strong interplay between magnetism and ferroelectricity. This coupling, referred to as the magnetoelectric effect, can potentially be used as an alternative means for controlling the magnetization in future electronic memory devices, actuators, switches, and magnetic field sensors. A renewed interest in this field has been triggered by recent results which show that a subset of frustrated magnets, which favor non-collinear spin configuration (triangular lattice antiferomagnets), are potentially good candidates for the observation of strong magnetoelectric effects.******The fundamental elastic properties of this class of materials are accessed using high resolution ultrasonic velocity measurements. Our laboratory offers a flexible way to investigate the elastic properties of single crystals under hydraulic pressure as a function of temperature and magnetic field. We are one of the few groups in the world capable of measuring elastic properties and magnetic phase diagrams as a function of three external parameters over a wide range (50 mK - 300 K, 0 - 18 Tesla, and 0 - 15 kbar). Our approach holds potential for the acquisition of significant results in order to clarify the nature of the coupling between elastic, electric, and magnetic properties observed in this family of compounds. These results will also provide guidance for theoretical models developed by our group. This work should therefore contribute to a better understanding of the properties of frustrated antiferromagnets and could lead to the development of new materials for technological applications.*****