Cation distribution and helimagnetic structure of the Ba-2(Zn1-xMgx)(2)Fe12O22 system as revealed by magnetization measurements and neutron diffraction

Magnetization measurements and neutron diffraction experiments were carried out on single crystals of Ba-2(Zn1-xMgx)(2)Fe12O22, where x is the Mg conc entration ranging from x = 0 to 1.0. The Fe ions present in the octahedral situs have a tendency to decrease with increasing x. The configuration of F e magnetic moments in the Mg-rich crystal is a distorted helimagnetic one c onsisting of large and small ferrimagnetic bunches. We have found that ther e are competing exchange interactions among Fe magnetic moments lying on th e forth, fifth and eighth layers of Mg-rich crystal. The exchange integrals J(45), J(48) and J(58) were determined using the molecular field approxima tion: J(45)/J(58) = 0.365 +/- 0.008 and J(48)/J(58) = 0.503 +/- 0.016 at 9 K. These values provide a clear interpretation of the turn angle of the hel ix as well as the helimagnetic structure for the Ba-2(Zn1-xMgx)(2)Fe12O22 s ystem.