Temperature dependence of the hyperfine parameters of maghemite and Al-substituted maghemites

Synthetic aluminum-substituted maghemite samples, gamma-(AlyFe1-y)(2)O-3, with y=0, 0.032, 0.058, 0.084, 0.106 and 0.151 have been studied by Mossbauer 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: delta(A)=delta(B)-0.12 mm/s. From the temperature dependence of delta(B) it was possible to determine the characteristic Mossbauer 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