Temperature dependence of the hyperfine parameters of maghemite and al-substituted maghemites |
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Authors: | G M da Costa E De Grave L H Bowen P M A de Bakker R E Vandenberghe |
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Institution: | 1. Laboratory of Magnetism, Department of Subatomic and Radiation Physics, University of Gent, Proeftuinstraat 86, B-9000, Gent, Belgium 2. Departamento de Química, Universidade Federal de Ouro Preto, Ouro Preto, MG, Brazil 3. Department of Chemistry, North Carolina State University, 27695, Raleigh, NC, USA
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Abstract: | Synthetic aluminum-substituted maghemite samples, -(Al
y
Fe1-y
)2O3 with y=0, 0.032, 0.058, 0.084, 0.106 and 0.151 have been studied by Mössbauer spectroscopy at 8 K and in the range 80 K to 475 K at steps of 25 K. The spectra have been analysed as a superposition of two sextets composed of asymmetrical Lorentzians. The A- and B-site isomer shifts were constrained as: A=B-0.12 mm/s. From the temperature dependence of B it was possible to determine the characteristic Mössbauer temperature and the intrinsic shift. Both quantities clearly increase with increasing Al content, at least up to 10 mole%. The temperature dependence of the A-and B-sites hyperfine fields could be satisfactorily reproduced using the molecular-field theory assuming an antiparallel spin configuration. The exchange integrals were found as: J
AB
=-25 K; J
AA
=-18 K and J
BB
= -3 K. The hyperfine fields show a crossing in the vicinity of 300 K as a result of the relatively strong A-A interaction. The Curie temperature for the non-substituted sample was calculated to be 930 K and decreases to 765 K for the sample with 15 mol% Al. The gradual decrease of the saturation value of the A-site hyperfine field with increasing Al substitution and the constancy of this quantity for the B sites, suggest that the Al cations occupy the B sites.Research Director, National Fund for Scientific Research, Belgium |
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