Multifrequency EMR and Magnetic Characterization of Synthetic Powdered Hematite

The present study reports the results of a combined approach which uses morphological characterization, magnetic measurements, and multifrequency electron magnetic resonance (EMR; 9-285 GHz) to identify and characterize both the bulk and the nanoscale magnetic properties of a sample of synthetic hematite, alpha-Fe2O3. We show here that the heterogeneous nature of the investigated sample is reflected in some peculiarities of the EMR spectra. The use of multifrequency EMR spectroscopy allowed us to assign, for the first time, the signals due to different magnetic modes of the bulk hematite on a powdered sample, indicating a large predominance of this behavior in the considered sample. At the same time, however, we observed a large decrease of the Morin transition temperature for a part of the sample, which we attributed to the presence of a fraction of smaller particles. Moreover, the presence of a single domain superparamagnetic phase was evidenced, due to the fraction of nanometer-size particles. These results are discussed with relation to the magnetic and morphological characterization undertaken by TEM showing the complementarity of the different techniques.