报告题目：Spontaneous ferromagnetic order at high temperatures without d-electrons? — The CeO₂conundrum

报 告 人：J. M.D. Coey, School of Physics and CRANN, Trinity College, Dublin, Irelnd

报告时间：2017-10-25 10:30

报告地点：物理系理科楼C302

报告摘要： Cerium dioxide raises the question of whether spontaneous ferromagnetic order is possible at high temperatures withoutd-electrons. There are many reports in the literature of a ferromagnetic-like response at room temperature to an applied magnetic field, for bulk, nanocrystalline or thin film samples, with or without cation doping. Typical values of the saturation magnetization are very small, of order 100 Am^-1, but reports range from zero up to 1,000 kAm^-1. The effect is somehow related to lattice defects — Ce³⁺cations or oxygen vacancies — but it is a challenge to understand how electrons associated with these defects could order ferromagnetically at room temperature and above. Straightforward impurity effects are considered, and models based on conventional ferromagnetic superexchange or double exchange are discussed, as is superparamagnetism or exchange splitting of the 4fband or a defect-related impurity band. Results then are compared with a new model of athermal giant orbital paramagnetism, which does not involve spontaneous magnetic order, but is related to the zero-point fluctuations of the vacuum. A key issue is the fraction, if any, of the volume of the CeO₂samples that is spontaneously ferromagnetic. Detailed analysis of the magnetic properties suggests that the conventional explanations of the magnetism of CeO₂(and other systems including nanoporous alumina membranes and SrTiO₃surfaces) untenable, and directions for further research are suggested.