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 b y Mossbauer spectroscopy at 8 K and in the range 80 K to 475 K at step s of 25 K. The spectra have been analysed as a superposition of two se xtets composed of asymmetrical Lorentzians. The A- and B-site isomer s hifts were constrained as: delta(A)=delta(B)-0.12 mm/s. From the tempe rature dependence of delta(B) it was possible to determine the charact eristic 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 coul d be satisfactorily reproduced using the molecular-field theory assumi ng an antiparallel spin configuration. The exchange integrals were fou nd as: J(AB)=-25 K; J(AA)=-18 K and J(BB)=-3 K. The hyperfine fields s how 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 sampl e with 15 mol% Al. The gradual decrease of the saturation value of the A-site hyperfine field with increasing Al substitution and the consta ncy of this quantity for the B sites, suggest that the Al cations occu py the B sites.