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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 – Ce 3 + 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 4 f 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 CeO 2 samples that is spontaneously ferromagnetic. Detailed analysis of the magnetic properties suggests that the conventional explanations of the magnetism of CeO 2 are untenable, and directions for further research are suggested. [ABSTRACT FROM AUTHOR]

Polyaniline (PANI)-coated iron-based nanocomposites with a range of magnetisation and coercivity values were produced by in situ polymerisation of aniline and chemical reduction of Fe by BH. We have varied the ratio of aniline to iron, keeping the amount of BH added constant, and vice versa. Room-temperature magnetisation loops, X-ray diffraction patterns, Mössbauer spectra and transmission electron microscopy images show that PANI plays two distinct roles: on the one hand, it coats the iron particle surface, preventing oxidation of the Fe cores and, on the other, it enhances the formation of an FeB alloy. The incorporation of boron in the alloy is facilitated by the polymer entrapment of small Fe nuclei. The mean particle size determined for composites obtained with low PANI content is (50 ± 20) nm, while for PANI-rich composites (30 ± 10) nm particles are formed. [ABSTRACT FROM AUTHOR]

Cerium oxide nanoparticles and nanoporous thin films exhibiting room temperature magnetism have been synthesized by two different methods. The magnetization curves for both particles and films are almost anhysteretic; they fit the empirical expression M\approx Ms\tanh(H/H0), where Ms and H0 are the saturation magnetization and extrapolated saturation field respectively. It is estimated that about 0.1–0.5% of the volume of the CeO2 samples are magnetic. This is consistent with models where the magnetism is associated with grain boundaries, surfaces or interfaces which occupy only a small fraction of the entire sample volume. The effect in the electrodeposited films depends on film morphology, which is controlled by the electrochemical variables. The results are discussed in terms of current models for d^0 magnetism. [ABSTRACT FROM AUTHOR]