Structural and magnetic characterization of maghemites prepared from Al-substituted magnetites

Abstract

SUMMARYSynthetic aluminum-substituted maghemites were characterized by total chemical analysis, powder X-ray diffraction (XRD), Mössbauer spectroscopy (ME), and vibrating sample magnetometry (VSM). The aim was to determine the structural, magnetic, and hyperfine properties of γ γ γ γ γ-Fe 2-x Al x O 3 as the Al concentration is varied. The XRD results of the synthetic products were indexed exclusively as maghemite. Increasing Al for Fe substitution decreased the mean crystalline dimension and shifted all diffraction peaks to higher o 2θ θ θ θ θ angles. The a 0 dimension of the cubic unit cell decreased with increasing Al according to the equation a o = 0.8385 -3.63 x 10 -5 Al (R 2 = 0.94). Most Mössbauer spectra were composed of one sextet, but at the highest substitution rate of 142.5 mmol mol -1 Al, both a doublet and sextet were obtained at ≈ 300 K. All hyperfine parameters from the sub-spectra were consistent with high-spin Fe 3+ (0.2 a 0.7 mms -1 ) and suggested a strong superparamagnetic component associated with the doublet. The magnetic hyperfine field of the sextets decreased with the amount of Al-substitution [B hf (T) = 49.751 -0.1202Al; R 2 = 0.94] while the linewidth increased linearly. The saturation magnetization also decreased with increasing isomorphous substitution.Index terms: isomorphous substitution, hyperfine parameters, spinel, ferromagnetic.( RESUMO: CARACTERIZAÇÃO ESTRUTURAL E MAGNÉTICA DE MAGHEMITAS PREPARADAS A PARTIR DE MAGNETITAS SUBSTITUÍDAS COM AlMaghemitas sintéticas substituídas com Al foram caracterizadas por meio de análise química total, difratometria de raios-X (DRX), espectroscopia Mössbauer (EM) e magnetometria de amostra vibrante (MAV). Objetivou-se com este trabalho determinar as propriedades estruturais, magnéticas e hiperfinas de γ-Fe 2-x Al x O 3, conforme a variação da concentração de Al.Os resultados de DRX dos produtos sintéticos foram indexados somente para maghemita. Com o aumento da substituição de Fe por Al, o diâmetro médio do cristalito diminuiu e levou todos os picos de difração para maiores ângulos o 2θ. A dimensão a 0 da cela unitária cúbica decresceu com o aumento de Al, de acordo com a equação a o = 0,8385 -3,63 x 10 -5 Al (R 2 = 0,94). A maioria dos espectros Mössbauer foi composta de um sexteto, mas na taxa mais alta de substituição de 142,5 mmol mol -1 Al um doubleto e sexteto foram obtidos a ≈ 300 K. Todos os parâmetros hiperfinos do subespectro foram consistentes com o alto spin Fe 3+ (0,2 a 0,7 mms -1 ) e sugeriram um forte componente superparamagnético associado com o dubleto. O campo magnético hiperfino dos sextetos decresceram com a substituição de Al [B hf (T) = 49,751 -0,1202Al; R 2 = 0,94], enquanto as larguras da linha aumentaram de forma linear. A magnetização de saturação também diminuiu com o aumento da substituição isomórfica.Termos de indexação: substituição isomórfica, parâmetros hiperfinos, espinélio, ferrimagnético.

Mineralogical Evolution of Magnetic Rhodic Oxisols under Different Lithological Influences in Brazil

“…Dimension a values were close to the expected for cubic Mh (a = 0.8350 nm) (Eggleton, 1988), consistent with values estimated after Rietveld refinement by Poggere et al (2018). The values indicate heterogeneity in Mh particle size, which is mainly conditioned by the IS process in the crystal structure (Schulze and Schwertmann, 1984;Batista et al, 2013). According to the values of dimension a (<0.8329 nm) and MCS Mh220 (<31 nm), the Mh presents high IS (Batista et al, 2013).…”

“…The values indicate heterogeneity in Mh particle size, which is mainly conditioned by the IS process in the crystal structure (Schulze and Schwertmann, 1984;Batista et al, 2013). According to the values of dimension a (<0.8329 nm) and MCS Mh220 (<31 nm), the Mh presents high IS (Batista et al, 2013). Since ferrimagnetic particle size determines the type of magnetic behavior, the Mh showed superparamagnetic behavior (<30 nm), which explains the high values of dependent frequency χ fd (>14 %) (Dearing, 1999).…”