High quality single crystals of BaFe12O19 were grown at the Oak Ridge National Laboratory, Tennessee, USA, using the ScIDre HKZ floating zone furnace with 100 atm of flowing oxygen. 

In a paper which was published end of June 2015 and selected as an Featured Article in APL Materials (APL Materials 3, 062512, 2015), the authors  H. B. Cao, Z. Y. Zhao, M. Lee, E. S. Choi, M. A. McGuire, B. C. Sales, H. D. Zhou, J.-Q. Yan and D. G. Mandrus report the growth procedure and a detailed neutron diffraction study of BaFe12O19 single crystals. This compound is particularly interesting because it exhibits magnetic and quantum paraelectric behavior at the same time. For the first time high quality single crystals of BaFe12O19 could be obtained by the floating zone method, applying high oxygen pressure. Concerning the growth of the samples, the authors write:

“Under one atmosphere of oxygen, BaFe12O19 decomposes into BaFe18O27 (the so-called W-phase) and other oxides before melting. This incongruent melting can be suppressed by high pressure oxygen, and congruently melting behavior was reported above 50 atm of oxygen. Since the capability of maintaining 50 atm oxygen pressure is absent in most growth laboratories, BaFe12O19 crystals and doped variants have been mainly grown with flux techniques. The Model HKZ high pressure image furnace enables crystal growth in controlled atmosphere at pressures up to 150 atm. The high pressure capability can be used to suppress the evaporation loss of volatile components and tune the oxygen content in oxides. The floating zone growth of BaFe12O19 in high oxygen pressure provides a good example that high oxygen pressure can be used to modify the phase stability in P − T − x space.”

The detailed analysis of the obtained crystals with neutron diffraction and other methods gave strong evidence that BaFe12O19 is a ferrimagnetic quantum paraelectric compound. Thus, it provides another materials platform which can be tuned to optimize the cross-coupling between magnetism and ferroelectricity. Finally, the authors summarize:

“The successful growth ofBaFe12O19 single crystals demonstrates that floating zone growth under high pressure oxygen can extend the region of P − T − x in which large, high quality crystals of certain materials can be grown. … For the growth of BaFe12O19, the applied high oxygen pressure prevents decomposition at high temperatures. This makes possible a quick floating zone growth of large single crystals without using solvent or crucibles. Together with the recent report of successful growth of ordered anion-deficient Ca2Co2O5 in flowing oxygen atmosphere of 145 atm, our growth of BaFe12O19 in 100 atm oxygen suggests that the high pressure floating zone technique will be a useful tool for new and metastable materials in single crystal form.”

Cao et al., APL Materials (2015)