RESUMEN
Detailed EPR investigations on as-grown and annealed TiO2 nanoparticles in the anatase and rutile phases were carried out at X-band (9.6 GHz) at 77, 120-300 K and at 236 GHz at 292 K. The analysis of EPR data for as-grown and annealed anatase and rutile samples revealed the presence of several paramagnetic centers: Ti3+, O-, adsorbed oxygen (O2-) and oxygen vacancies. On the other hand, in as-grown rutile samples, there were observed EPR lines due to adsorbed oxygen (O2-) and the Fe3+ ions in both Ti4+ substitutional positions, with and without coupling to an oxygen vacancy in the near neighborhood. Anatase nanoparticles were completely converted to rutile phase when annealed at 1000° C, exhibiting EPR spectra similar to those exhibited by the as-grown rutile nanoparticles. The high-frequency (236 GHz) EPR data on anatase and rutile samples, recorded in the region about g = 2.0 exhibit resolved EPR lines, due to O- and O2- ions enabling determination of their g-values with higher precision, as well as observation of hyperfine sextets due to Mn2+ and Mn4+ ions in anatase.
RESUMEN
This paper reports an investigation on the role of transition-metal ions in producing ferromagnetism in CeO2 nanoparticles by electron paramagnetic resonance (EPR). Several samples of CeO2 nanoparticles annealed at 200, 300, 400, and 500 degrees C, doped with 5% Ni and 5% Co ions, characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), thermogravimetry analysis (TGA) and mass spectroscopy (MS), were investigated by X-band EPR at 4, 10 and 300 K, and by magnetometry at 300 K. Magnetic properties and EPR/FMR (Ferromagnetic Resonance) spectra of these nanoparticle samples were found to depend strongly on the annealing temperature (T(A)), oxygen stoichiometry, and dopant-ion species. Different behavior of saturation magnetization in the samples with the dopants, Co and Ni, is found to be due to different-inward and outward-surface diffusion of these impurity ions, respectively, during annealing. A detailed simulation of EPR/FMR spectra of isolated Co and Ni ions carried out here provides in-depth details on the role of the doped ions and oxygen (O-) defects played in the observed magnetic properties.
RESUMEN
High frequency (236 GHz) electron paramagnetic resonance (EPR) studies of Fe(3+) ions at 255 K are reported in a Sn(1-x)Fe(x)O(2) powder with x = 0.005 which is a ferromagnetic semiconductor at room temperature. The observed EPR spectrum can be simulated reasonably well as overlap of spectra due to four magnetically inequivalent high-spin (HS) Fe(3+) ions (S = 5/2). The spectrum intensity is calculated, using the overlap I(BL) + (I(HS1)+I(HS2)+I(HS3)+I(HS4))×e(-0.00001×B), where B is the magnetic field intensity in Gauss, I represents the intensity of an EPR line (HS1, HS2, HS3, HS4), and BL stands for the base line. (The exponential factor, as found by fitting to the experimental spectrum, is related to the Boltzmann population distribution of energy levels at 255 K, which is the temperature of the sample in the spectrometer.) These high-frequency EPR results are significantly different from those at X-band. The large values of the zero-field splitting parameter (D) observed here for the four centers at the high frequency of 236 GHz are beyond the capability of X-band, which can only record spectra of ions only with much smaller D values than those reported here.