Abstract

Pure and chromium substituted lanthanum orthoferrites LaFe_1 − xCr_xO₃ (x = 0, 0.1, 0.25, 0.4, 0.55, 0.7, 0.85, 1) were synthesised in air by self-propagating high-temperature synthesis (SHS): a combustion process involving the reaction of lanthanum(III) oxide, iron(III) oxide, chromium(III) oxide and iron (or chromium) metal powder. Two series of samples were produced: series 1 – SHS in the absence of an external magnetic field; and series 2 – SHS in a magnetic field of 1.1 T; both processes were followed by sintering at 1400 °C for 36 h with intermediate grinding. Scanning electron microscopy, energy dispersive analysis of X-rays, X-ray powder diffraction, infra-red and ultra-violet/visible spectroscopy experiments were carried out on both series of samples. X-Ray diffraction data showed that single phase orthorhombic ferrites were produced. The ferrites showed a systematic decrease in lattice parameters and unit cell volume with chromium content (e.g. for series 1: x = 0, V = 242.4 Å³; x = 1, V = 234.0 Å³). ⁵⁷Fe Mössbauer spectroscopy indicated that at low Cr concentration (x = 0.1 and 0.25) more than 96% of the Fe atoms experienced a perturbative disruption to their interatomic exchange interactions, consistent with the random distribution of Cr³⁺ ions on the B sublattice. The remaining 4% or less of Fe atoms were observed in a paramagnetic state, indicating a small degree of inhomogeneous Cr clustering in the lattice. The room temperature exchange interaction percolation limit for Cr substitution of the Fe lattice was found to lie between x = 0.4 and 0.55, and the Mössbauer spectra for the x ≥ 0.55 samples showed paramagnetic singlets. Room temperature magnetisation versus field curves showed hysteresis loops for x ≤ 0.7 (series 1) and x ≤ 0.55 (series 2), and anhysteretic curves for higher Cr contents. After an initial decrease with increasing Cr, the maximum magnetisations of the samples peaked at x = 0.25 (σ_max = 2.1 emu g⁻¹, series 1) and x = 0.4 (σ_max = 1.7 emu g⁻¹, series 2), before falling to near zero at higher concentrations. These features are consistent with a more homogeneous Cr substitution of the Fe sublattice in the series 2 samples, related to higher diffusion rates during the hotter combustion reactions achieved in the applied field SHS reactions. These results indicate that the SHS route is a potentially useful means of synthesising orthoferrites with desirable properties.