Hyperfine interactions and irreversible magnetic behavior in multiferroic aurivillius compounds

In this work investigations of structure and magnetic properties of conventionally sintered Bim+1Ti3Fem–3O3m+3 compounds with 4 ≤ m ≤ 8 were performed using X-ray diffraction, Mössbauer spectroscopy and vibrating sample magnetometry. Room-temperature Mössbauer spectra of the compounds correspond to a paramagnetic state, however, low temperature measurements (80 K) reveal the antiferromagnetic state with a residual paramagnetic phase. Temperature dependencies of magnetic susceptibility, χσ(T), provided magnetic ordering temperatures and revealed an irreversibility in Aurivillius compounds with m ≥ 5. In the case of Bi5Ti3FeO15 compound the χσ(T) dependence shows a paramagnetic behavior down to 2 K. The Bi6Ti3Fe2O18 compound reveals a magnetic ordering at 11 K. The compounds with m = 6–8 show a magnetic ordering at temperatures higher than 200 K. Highly irreversible character of their temperature dependencies of χσ indicates a spin-glass type disordered magnetism with frustration due to a random distribution of Fe on Ti at their sites