Temperature dependence of the hyperfine parameters of maghemite and Al-substituted maghemites.
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1995
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Synthetic aluminum-substituted maghemite
samples, 7-(AlyFel_y)203, with y = 0, 0.032, 0.058, 0.084,
0.106 and 0.151 have been studied by M6ssbauer 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:
6A---- 6B-- 0.12 mm/s. From the temperature dependence
of 6B it was possible to determine the characteristic
M6ssbauer temperature and the intrinsic shift. Both quantities
clearly increase with increasing A1 content, at least
up to 10 mole%. The temperature dependence of the Aand
B-sites hyperfine fields could be satisfactorily reproduced
using the molecular-field theory assuming an antiparallel
spin configuration. The exchange integrals were
found as: JAB = --25 K; JAA = --18 K and JBB= --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% A1. The gradual decrease of the
saturation value of the A-site hyperfine field with increasing
A1 substitution and the constancy of this quantity for
the B sites, suggest that the A1 cations occupy the B sites.
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COSTA, G. M. da et al. Temperature dependence of the hyperfine parameters of maghemite and Al-substituted maghemites. Physics and Chemistry of Minerals, v. 22, p. 178-185, 1995. Disponível em: <https://link.springer.com/article/10.1007/BF00202298>. Acesso em: 20 abr. 2017.