Evolution of the Magnetic and Structural Properties with the Chemical Composition in Oleate-Capped MnxCo1- xFe2O4Nanoparticles

Understanding the complex link among composition, microstructure, and magnetic properties paves the way to the rational design of well-defined magnetic materials. In this context, the evolution of the magnetic and structural properties in a series of oleate-capped manganese-substituted cobalt ferrites (MnxCo1-xFe2O4) with variable Co/Mn molar ratios is deeply discussed. Single-phase ferrites with similar crystallite and particle sizes (about 10 nm), size dispersity (14%), and weight percentage of capping oleate molecules (17%) were obtained by an oleate-based solvothermal approach. The similarities among the samples permitted the interpretation of the results exclusively on the basis of the actual composition, beyond the other parameters. The temperature and magnetic field dependences of the magnetization were studied together with the interparticle interactions by DC magnetometry. Characteristic temperatures (Tmax, Tdiff, and Tb), coercivity, anisotropy field, and reduced remanence were found to be affected by the Co/Mn ratio, mainly due to the magnetic anisotropy, interparticle interactions, and particle volume distribution. In addition, the cobalt and manganese distributions were hypothesized on the basis of the chemical composition, the inversion degree obtained by 57Fe Mössbauer spectroscopy, the anisotropy constant, and the saturation magnetization

57^{57}Fe-enriched perovskites M(Fe0.5Nb0.5)O3(M–Pb,Ba)M(Fe_{0.5}Nb_{0.5})O_{3} (M – Pb, Ba) studied by Mössbauer spectroscopy, NMR and XRD in the wide temperature range 4.2–533 K

The 57Fe enriched almost single-phase perovskites Pb(Fe0.5Nb0.5)O3 (PFN) and Ba(Fe0.5Nb0.5)O3 (BFN), prepared by a ceramic method (solid-state synthesis), were studied by Mössbauer spectroscopy, nuclear magnetic resonance (NMR), conventional and synchrotron X-ray powder diffraction (XRD). The temperature dependences of hyperfine and structural parameters of PFN, BFN from 4.2 K to temperatures above ferroelectric ordering of PFN (TS ∼ 375 K), with attention to the values of magnetic and structural transitions, were obtained. The antiferromagnetic magnetic ordering transitions were found under the Néel temperatures TN ∼ 167(3) K and 32(2) K for PFN and BFN, respectively. The spin-glass transition was at TG ∼ 10(3) K and 20(5) K for PFN and BFN, respectively. In PFN sample a small change of structural parameters around TN and structural change from trigonal to cubic structure at T ∼ 400 K was observed by XRD. The temperature dependence of XRD shows stable cubic structure in the temperature range from 4.2 K to 530 K for BFN. From Mössbauer and NMR spectroscopies it is found that both structures have perturbed environments for Nb and Fe. However, in case of PFN the low values of transferred hyperfine fields disfavour random Fe/Nb arrangement and allow proposing a picture of Fe/Nb arrangement