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挫自旋系统的完美原型，由于晶格几何的限制，两个磁矩不能彼此反平行。由于磁挫折导致大量简并自旋构型，这些材料的基本性质仍未完全了解。因此，根据简单最近邻相互作用经典模型，烧绿石晶格上的自旋不会发展到0 K以下的任何长程阶。一般来说，这些体系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，这些受挫系统显示出许多新的奇异态，如：新的非共线序、自旋液体、自旋玻璃、自旋冰和量子自旋冰。

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.