Order-disorder and magnetic exchange interactions in substituted strontium hexaferrite SrAxFe12−xO19(A=Ga, In)

Abstract

The unusual magnetic behavior of the 12k site in SrGaxFe12−xO19 (0.3≤x≤6.0) and SrInxFe12−xO19 (0.3≤x≤1.4) has been investigated by means of 57Fe Mössbauer spectroscopy at 298 K. The bifurcation of the 12k Mössbauer subspectrum into 12k1 and 12k2 components of widely differing intensities has been confirmed for both Ga3+ and In3+. The bifurcation is apparent at x values as low as 0.3, and the less intense 12k2 pattern corresponds to an abrupt drop in the hyperfine magnetic field Heff, of 50 and 75 kOe for Ga3+ and In3+, respectively. Heff for the more intense 12k1 pattern is virtually unchanged from that of pure SrFe12O19 for x values as high as 1.0. Room-temperature Mössbauer spectra confirm that the Ga3+ and In3+ ions preferentially occupy the 4f2 octahedral site. There are indications, however, that the dopants also enter the 2b trigonal-bipyramidal site. The change in Heff at the 12k sites indicates that complex and competing exchange interactions are present, and that the splitting of the 12k magnetic hyperfine spectrum is not due simply to random disruptions of the exchange interactions. The intensities of the 12k1 and 12k2 subspectra have been used to model the exchange interactions at the 12k site.