Cerium dioxide raises the question of whether spontaneous ferromagnetic order is possible at high temperatures without d-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 4f band 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 CeO2 samples 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 SrTiO3 surfaces) untenable, and directions for further research are suggested.

Bio：Michael Coey is Professor Emeritus at Trinity College Dublin, and author of several books and numerous papers on magnetism and magnetic materials, including the textbook 磁学和磁性材料 (reprint: Peking University Press 2014). His work includes contributions to amorphous and disordered magnetim, permanent magnetism, magnetism of soils and minerals, dilute oxides and magneto-electrochemistry, with more recent work on magneto-microfluidics, spin electronics, d-zero magnetism and half metals. A Fellow of the Royal Society and Foreign Associate of the US National Academy of Science, he has served as Chairman of the IUPAP Magnetism Commission C9, and Divisional Associate Editor of Physical Review Letters. In 2010, he was Albert Einstein Professor of the Chinese Academy of Sciences. He founded Magnetic Solutions Ltd and the Trinity College Science Gallery, and was a promoter of CRANN, Ireland’s nanoscience research centre. His first visit to Peking was as a student in 1963.