Cation distribution and helimagnetic structure of the Ba2(Zn1-xMgx)2Fe12O22 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 concentration 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 Fe magnetic moments in the Mg-rich crystal is a distorted helimagnetic one consisting of large and small ferrimagnetic bunches. We have found that there are competing exchange interactions among Fe magnetic moments lying on the 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 approximation: 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 helix as well as the helimagnetic structure for the Ba-2(Zn1-xMgx)(2)Fe12O22 system.