Institut Laue Langevin

报告人简介：

Tim Ziman attended university in Auckland, New Zealand before preparing a thesis in at Oxford with Roger Elliott, in the theory disordered magnetism, localization and critical phenomena. During subsequent work in the U.S. (Cornell, Rutgers, with Elihu Abrahams, and Delaware) and then in France (especially at the ILL, where theory related to neutron scattering was championed by Philippe Nozières) his interests have broadened to include low-dimensional magnetism and quantum phase transitions, magnetic oscillations and frustrated magnetism. Collaborations with Sadamichi Maekawa and colleagues have encouraged him to look at the interface between that subject and studies of materials of interest via neutron or X-ray scattering.

报告摘要：

The Rare Earth Garnets have been known since the 1950’s and ferrimagnetic Yttrium Iron Garnet is widely used in magnetism and magneto-optics. Substituting other rare earths, with their own magnetic ions, for Yttrium gives a variety of different materials with rich physical behaviour, including magnetic compensation and non-collinear magnetic structures. These materials are now experiencing a renewal in interest. I will discuss the particular case of Terbium Gallium Garnet (TbIG), where there have been recent advances in the study of its non-collinear magnetism and related dynamics. When TbIG is substituted for a ferromagnet in a device for measuring the Spin Seebeck effect, this has given puzzling results at low temperatures, it goes to zero in zero field but increases to positive values with the application of a magnetic field. This is opposite to expectations that the SSE should be suppressed by a magnetic field. We present calculations of the crystal-field excitations exploiting previously measured Stevens parameters taken for the Terbium sites in Terbium Gallium Garnet from neutron-scattering data and magnetic exchange from the Fe moments, included as local molecular fields, which leads to ‘umbrella structures’ on local triples of Tb moments. We show how this leads naturally to the `double umbrella' magnetic structure observed in magnetic diffraction and implies crystal field excitations with characteristic dependence on external magnetic fields. By looking at the lowest predicted crystal field level we can suggest an explanation for the Spin Seebeck behaviour observed.

邀请人：周 毅（8264 9925）

联系人：胡 颖（8264 9361）