Superexchange interactions of (Ba1-xSrx)2Zn2Fe12O22 system studied by neutron diffraction

Neutron diffraction experiments were carried out on single crystals of (Ba1-xSrx)(2)Zn2Fe12O22 at 8 K, where x is the Sr concentration ranging from 0 to 1.0. Competition of three superexchange interactions exists among Fe magnetic moments present in the 4th, 5th, and 8th layers of Sr-rich crystals. This competition leads to a distorted helimagnetic structure consisting of large and small ferrimagnetic bunches. The relations between exchange integrals J(45), J(4)8(,) and J(58) are determined by using the molecular field approximation as J(45)/J(58) = 0.365(7) + 0.019(0)x + 0.061(7)x(2) and J(48)/J(58) = 0.504(7) + 0.010(4)x + 0.011(6)x(2) at 8 K. These relations provide a clear interpretation for the magnetic structure of the (Ba1-xSr,)(2)Zn2Fe12O22 system. Zn ion distributions in the tetrahedral sites and a local lattice deformation around Sr ions are also discussed in detail.