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Magnetic structure of strontium ferrite Sr4Fe4O11

The crystal and magnetic structure of strontium ferrite Sr4Fe4O11 (SrFeO2.75) has been investigated using neutron powder diffraction and magnetic susceptibility measurements over the temperature range 1.5–293 K and by Mössbauer spectroscopy at room temperature. Sr4Fe4O11 is orthorhombic (symmetry group Cmmm) over the temperature range 1.5–293 K. Its crystal structure contains two iron sites occupied by trivalent and tetravalent iron. The trivalent magnetic moments order antiferromagnetically below TN = 232(4) K and the magnetic unit cell coincides with the chemical cell. The ordered moments are parallel to the b axis and the tetravalent iron moments form a magnetically frustrated sublattice. The extrapolated value of the Fe3+ moment is 3.55(5) μB at 0 K. The antiferromagnetic coupling occurs by the superexchange mechanism and is mediated by oxygen ions. There are no signs of long-range magnetic order in the tetravalent sublattice down to 1.5 K.