Metastable Crystalline Cobalt Iron Oxide Nano-Flakes with Antiferromagnetic/Ferrimagnetic Composition Mosaicity

Abstract

By thermal decomposition of a crystalline hydroxycarbonate precursor with a Co:Fe ratio of 2:1, crystals with alternating ferrimagnetic and antiferromagnetic nano-domains were synthesized using a facile synthetic approach that combined bottom-up co-precipitation of the precursor with a self-assembled top-down nano-structuring during spinel formation. Due to the miscibility gap of the spinel phase diagram at this composition, a topotactic segregation into CoFe₂O₄-like and Co₃O₄-like domains takes place at 400 °C, giving rise to porous crystalline nano-flakes with spatial compositional fluctuations on a scale of approximately 5 nm. Experimental methods and density functional theory showed that the metastable nature of this interface-rich material is manifested in the unexpectedly low lattice parameter of the iron-rich domains, which can be explained by the compressive strain executed on this phase due to mosaicity. Investigations of the magnetic properties revealed an exchange bias effect, due to this unique microstructure, which is typically known for thin films or core/shell nanoparticles. Treatment at temperatures higher than 450 °C causes this microstructure to break down, the lattice strain to relax, and finally leads to properties expected for the thermodynamically stable phases according to the phase diagram.

Keywords: Exchange bias; Magnetic properties; Phase diagrams; Spinel phases; Synthetic methods.

Figure 1

a) Schematic representation of the Fe─Co─O phase diagram, adapted from Zhang et al.,^{[ 20 ]} The positions of CoFe₂O₄, Co₂FeO₄, and Co₃O₄ in the phase diagram were added.^{[ 21} , ^{22 ]} b) The crystalline precursor decomposition approach leading to single‐crystalline nano‐flakes and c) thermogravimetric analysis for identifying feasible temperature ranges for spinel formation.

Figure 2

SEM micrographs a)–c), X‐ray diffraction patterns d)–f) and PDF analysis g)–i) of the samples after thermal treatment at three different calcination temperatures: 400 °C (left), 800 °C (center), 900 °C (right). PXRD refinements (red curve) with overlapping experimental data (black curve) are shown together with the difference curve (blue, in offset) and are additionally indexed with Bragg reflections of the cobalt‐rich phase (green) and iron‐rich phase (brown) for 400 and 800 °C samples, and with Co₂FeO₄ for 900 °C. PDF refinements are shown with experimental PDF (black) and fit (red), as well as in offset the difference (blue), cobalt‐rich phase (green) and iron‐rich phase (brown).

Figure 3

STEM images of all three samples (upper row), EDX maps with Fe represented in blue and cobalt represented in yellow (middle row) and the cobalt to iron atomic ratio derived from EDX spectra extracted from the two different areas 1 and 2 (lower row).

Figure 4

HR‐TEM images with superimposed iFFT image of the 400 °C sample a), FFT pattern b), zoomed in areas 1 and 2 c), and determination of the lattice parameter in real space d), extracted from line profiles from region 1 and 2, as indicated by the white arrows in (c).

Figure 5

a) Lattice parameter of the cobalt (green)‐ and iron (brown)‐ rich phases for the whole calcination series were determined by Rietveld refinement and are represented by squares. The lattice parameter for the 400 °C sample was also obtained from PDF data (open circles) and HR‐TEM (triangles). b) Magnetic hysteresis loops of the sample calcined at 400 °C, measured subsequent to field cooling (FC) from 390 K at 14 T (red) and ‐14 T (blue).

Figure 6

a) Top and b) side view of the structure and spin density of (111)‐oriented heterostructure of CoFe₂O₄ and Co₃O₄ with an interface along the hexagonal a direction. Yellow and cyan colors represent the majority and minority spin density, respectively. M _IF presents the total magnetic moment of the interface layer in the heterostructure in μ_B. a and a _bulk denote the lattice parameter of the CoFe₂O₄ and Co₃O₄ part of the heterostructure and the related bulk lattice constants, respectively.

Figure 7

Mössbauer spectra recorded at 4.3 K and an applied field of 10 T parallel to the γ‐ray propagation direction on the representative samples calcinated at 400, 800, and 900 °C. Experimental data (black dots) is shown together with a theoretical data fit (red line) composed of several superimposed subspectra, representing Fe atoms residing on B‐ (green) and A‐sites of the spinel lattice, with latter being assigned to contributions from Fe³⁺ ions in Fe‐rich (blue) and Co‐rich (violet) environments.