Room temperature magnetism in CeO2-A review

Cerium dioxide clearly 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 to an applied magnetic field at room temperature for bulk, nanocrystalline or thin film samples, with or without cation doping. Typical values of the saturation magnetization are very small, of order 0.1 kAm(-1), but reports range from zero up to 1000 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 exchange splitting of the 4f band or a defect related impurity band. Results are also compared with a new model of athermal giant orbital paramagnetism that involves no spontaneous ferromagnetic order. 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 are untenable, and directions for further research are suggested. (C) 2018 Elsevier B.V. All rights reserved.