The structural and magnetic properties of Bi${}_6$Fe${}_2$Ti${}_3$O${}_{18}$ and Bi${}_6$FeCrTi${}_3$O${}_{18}$ are investigated. The samples can be indexed by an orthorhombic lattice with the space group B2cb and the valence state of Fe and Cr ions is suggested to be $+$3 based on the x-ray photoelectron spectroscopy spectra. The sample Bi${}_6$Fe${}_2$Ti${}_3$O${}_{18}$ is paramagnetic with the presence of a short-range antiferromagnetic ordering; whereas Bi${}_6$FeCrTi${}_3$O${}_{18}$ undergoes a short-range ferrimagnetic-like transition at 106 K originating from the spin canting of the antiferromagnetic coupling of Fe-based and Cr-based sublattices via the Dzyaloshinskii-Moriya interaction due to the distorted crystal structure of Bi${}_6$FeCrTi${}_3$O${}_{18}$, which are in agreement with the EPR results. The magnetic structure and EPR spectra of Bi${}_6$Fe${}_2$Ti${}_3$O${}_{18}$ and Bi${}_6$FeCrTi${}_3$O${}_{18}$ can be understood by the presence of a small amount of isolated Fe${}^{3+}$ ions in an octahedral ligand field with a low symmetry and the coupling between the random occupancy of Fe${}^{3+}$ and/or Cr${}^{3+}$ ions in other two octahedral sites in the layered structure of Aurivilius phase.

• Received 19 April 2012

DOI:https://doi.org/10.1103/PhysRevB.86.104410