Crystalline and magnetic structure–property relationship in spinel ferrite nanoparticles

Abstract

Magnetic spinel ferrite MFe2O4 (M = Mn, Co, Ni, Zn) nanoparticles have been prepared via simple, green and scalable hydrothermal synthesis pathways utilizing sub- and supercritical conditions to attain specific product characteristics. The crystal-, magnetic- and micro-structures of the prepared crystallites have been elucidated through meticulous characterization employing several complementary techniques. Analysis of energy dispersive X-ray spectroscopy (EDS) and X-ray absorption near edge structure (XANES) data verifies the desired stoichiometries with divalent M and trivalent Fe ions. Robust structural characterization is carried out by simultaneous Rietveld refinement of a constrained structural model to powder X-ray diffraction (PXRD) and high-resolution neutron powder diffraction (NPD) data. The structural modeling reveals different affinities of the 3d transition metal ions for the specific crystallographic sites in the nanocrystallites, characterized by the spinel inversion degree, x, [M2+1-xFe3+x]tet[M2+xFe3+2-x]octO4, compared to the well-established bulk structures. The MnFe2O4 and CoFe2O4 nanocrystallites exhibit random disordered spinel structures (x = 0.643(3) and 0.660(6)), while NiFe2O4 is a completely inverse spinel (x = 1.00) and ZnFe2O4 is close to a normal spinel (x = 0.166(10)). Furthermore, the size, size distribution and morphology of the nanoparticles have been assessed by peak profile analysis of the diffraction data, transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM). The differences in nanostructure, spinel inversion and distinct magnetic nature of the M2+ ions directly alter the magnetic structures of the crystallites at the atomic-scale and consequently the macroscopic magnetic properties of the materials. The present study serves as an important structural benchmark for the rapidly expanding field of spinel ferrite nanoparticle research.

Structure and magnetic properties of W-type hexaferrites

“…Introduction of the additional S layer necessitates two Me 2+ ions in the structure, as in spinel-ferrites, which enable the direct substitution of magnetic or nonmagnetic Me 2+ ions without the need for additional substitutions to maintain the charge balance (Tokunaga et al, 2010;Wang et al, 2012). This enhances the tuneability of WHF in comparison with MHF (Gorter, 1950;Andersen et al, 2018). By partially substituting nonmagnetic species into specific sites, which align opposite to the net magnetization of the unit cell, i.e.…”

Local and long-range atomic/magnetic structure of non-stoichiometric spinel iron oxide nanocrystallites

“…Consequently, very high quality diffraction data and/or complementary characterization techniques are needed in order to reliably determine the atomic structure of spinel iron oxide nanoparticles (Holder & Schaak, 2019). Indeed, our recent studies of the atomic structures of related nanocrystalline compounds in the spinel ferrite family have revealed substantial differences compared with the bulk equivalents (Andersen, Saura-Mú zquiz et al, 2018;Andersen et al, 2019;Hö lscher et al, 2020). In addition, the surface of Fe 3 O 4 particles is known to oxidize in air to form an Fe-deficient outer layer, commonly assumed to be -Fe 2 O 3 .…”

Expanding the tunability and applicability of exchange-coupled/decoupled magnetic nanocomposites

“…13 Ferrites are among the most used magnetic materials, owing to their good magnetic properties, chemical and mechanical stability, and the availability of elements they are based on. Especially interesting are the spinel ferrites (SFs), as they allow easy tunability of the magnetic properties with small changes on the chemical composition, [14][15][16] thus increasing their versatility towards different applications. SFs have been widely used in the electronic industry, for high-density data storage and spintronic devices.…”