We report by the first time a high pressure X-ray diffraction and Raman spectroscopy study of cobalt ferrite (CoFe₂O₄) nanoparticles carried out at room temperature up to 17 GPa. In contrast with previous studies of nanoparticles, which proposed the transition pressure to be reduced from 20–27 GPa to 7.5–12.5 GPa (depending on particle size), we found that cobalt ferrite nanoparticles remain in the spinel structure up to the highest pressure covered by our experiments. In addition, we report the pressure dependence of the unit-cell parameter and Raman modes of the studied sample. We found that under quasi-hydrostatic conditions, the bulk modulus of the nanoparticles (B₀ = 204 GPa) is considerably larger than the value previously reported for bulk CoFe₂O₄ (B₀ = 172 GPa). In addition, when the pressure medium becomes non-hydrostatic and deviatoric stresses affect the experiments, there is a noticeable decrease of the compressibility of the studied sample (B₀ = 284 GPa). After decompression, the cobalt ferrite lattice parameter does not revert to its initial value, evidencing a unit cell contraction after pressure was removed. Finally, Raman spectroscopy provides information on the pressure dependence of all Raman-active modes and evidences that cation inversion is enhanced by pressure under non-hydrostatic conditions, being this effect not fully reversible.

The influence of Nb doping was studied on ribbons of nominal composition Nd_yFe_86-x-yB₁₄Nb_x (y = 7, 8; x = 0, 2, 4). The alloys were prepared by melt-spinning and then annealed at 973 K for 20 minutes. The magnetic characterization revealed that niobium addition improves substantially the magnetic properties of the studied alloys. The structural characterization showed that, in most alloys, the magnetically hard phase Nd₂Fe₁₄B precipitated as the main phase. This was not the case for the ternaries (x = 0) and for the Nd₇Fe₇₇B₁₄Nb₂ alloys. Furthermore, the addition of Nb inhibited the precipitation of α-Fe, independently of the Nd content. From the complete set of samples studied in this work, we found that the sample with nominal composition Nd₈Fe₇₄B₁₄Nb₄ exhibited the highest coercive field H_C = 575 kA/m while Nd₇Fe₇₅B₁₄Nb₄ showed the highest maximum energy product, (BH)_max = 73 kJ/m³.

This article introduces the main characteristics of PyMossFit, a software for Mössbauer spectra fitting. It is explained how each aspect of the code works. Based on the Lmfit Python package, it is a robust data fitting tool. Designed to run through Jupyter Notebook in the Google Colab cloud, it also allows one to work via multiple devices and operating systems. In addition, it allows the fitting procedure to be performed collaboratively among researchers. The software performs the folding of raw data with a discrete Fourier transform. Data smoothing is available with the use of a Savitzky–Golay algorithm. Moreover, a K-nearest neighbor algorithm enables users to determine the present phases by matching the correlations of hyperfine parameters from a local database.

CoFe₂O₄ films were deposited on Si [(100) or (111)] substrates by pulsed laser deposition, varying substrate temperature and deposition time. All films showed highly (111)-preferred orientation. Magnetic measurements (hysteresis loops) at room temperature indicated that the films have in-plane magnetic anisotropy. By atomic force microscopy, the surface topology revealed an average roughness of approximately 15 nm due to the presence of droplets. To define the relevance of droplets on the films' magnetic properties, we applied a nanopolishing technique after which the surfaces became notoriously smoother. As a consequence of this surface modification, we found that the hysteresis loops (measured again after nanopolishing) revealed changes in the magnetic response of the samples. The thinner films ((t ~50) nm) revealed waist-type hysteresis loops with diminished values of the coercive field, while the thickest film ((t ~100) nm) increased its coercive field without qualitative change in its loop shape. We attributed the altered magnetic response to different mechanisms that depend on sample thickness. For the thinner films, an additional anisotropy (to that existing in the plane) was induced after the nanopolishing procedure. For the thickest film, it was observed that differences are present after the elimination of the droplets, showing their important role in the magnetic response of the films.