Superexchange Interactions of (Ba_1−xSr_x)₂Zn₂Fe₁₂O₂₂ System Studied by Neutron Diffraction

抄録

Neutron diffraction experiments were carried out on single crystals of (Ba_1−xSr_x)₂Zn₂Fe₁₂O₂₂ 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 J₄₅, J₄₈, and J₅₈ are determined by using the molecular field approximation as J₄₅⁄J₅₈=0.365₇+0.019₀x+0.061₇x² and J₄₈⁄J₅₈=0.504₇+0.010₄x+0.011₆x² at 8 K. These relations provide a clear interpretation for the magnetic structure of the (Ba_1−xSr_x)₂Zn₂Fe₁₂O₂₂ system. Zn ion distributions in the tetrahedral sites and a local lattice deformation around Sr ions are also discussed in detail.