学术报告（Gang Cao, Lance E. De Long）

报告时间：2011年6月17日  下午2:00

报告地点: 强磁场中心技术装备厅201会议室

主持人：田明亮   研究员

学术报告一

报告题目：Spin-Orbit Interaction Rediscovered in Transition Metal Oxides

报告人：Prof. Gang Cao

 (Center for Advanced Materials and Department of Physics and Astronomy University of Kentucky, Lexington, Kentucky)

报告摘要：

The 4d- and 5d-transition metal oxides exhibit nearly every collective state known for solids. It is commonly expected that ruthenium oxides and iridium oxides are more metallic and less magnetic than their 3d and 4f counterparts because of the extended nature of the 4d and 5d orbitals. In marked contrast, many ruthenates and iridates are magnetic insulators that exhibit a large array of phenomena seldom or never seen in other materials. In this talk, we review the anomalous physical properties of the layered ruthenates and iridates, and then focus on the novel physics of the newly discovered Jeff = Mott insulator Sr₂IrO₄ and its derivatives. The critical underlying mechanism for these phenomena is the spin-orbit interaction, which strongly competes with other interactions to create an unusual balance between relevant degrees of freedom in this class of materials.

学术报告二

报告题目：R-Type Ferrites: Authentic High-TC Ferromagnetic Semiconductors

报告人：Prof. Lance E. De Long

（Department of Physics and Astronomy University of Kentucky）

报告摘要：

There is an ongoing, intensive effort to discover new materials for spin-transport electronics (“spintronics”), in which the spin of charge carriers provides enhanced functionality for nano-scale devices. One class of devices involves the injection of spin-polarized electrons into commercial semiconductors such as doped Si or GaAs, which requires a highly transmissive interface with a room-temperature, ferromagnetic (FM) semiconductor. However, identification of suitable narrow-gap semiconductors that are also FM above room temperature has proven elusive.

We have conducted x-ray and neutron diffraction, magnetotransport, magnetic moment, and soft x-ray scattering measurements on single crystals of hexagonal R-type ferrites . These materials exhibit long-range FM order at temperatures as high as 490 K, accompanied by practical narrow-gap semiconducting properties that include a large anomalous Hall conductance, low resistivity, high carrier concentration and low coercive field.  Alterations of the 3d-element (M = Fe,Co,Mn,Ti) composition, or variations of the relative concentration x of 3d-solutes and 4-d Ru induce negligible changes of the basal plane lattice parameters, but nevertheless yield a wide range of physical properties desired in practical epitaxial heterostructures.

The R-type ferrites also exhibit fundamentally interesting phenomena driven by magnetic frustration of antiferromagnetic (AFM) interactions on a disordered Kagomé lattice. Remarkably, simple substitution of M = Fe for Co or Mn results in an evolution from metallic to semiconducting behavior, and a shift from frustrated “all-in/all-out” AFM order that sets in well below 200 K, to collinear FM states ordering well above room temperature. This evolution is accompanied by giant “topological” Hall effect anomalies that are driven by complex changes of spin chirality and Berry phase with applied magnetic fields.