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Peculiarities of the crystal growth in the system of hexaferrites Sr3Co2Fe24O41 and SrCoxTixFe12-xO19 by floating zone melting
|Anatoly M. Balbashov 1, Marya E. Voronchikhina 1, Alexander A. Mukhin 2, Vsevolod Y. Ivanov 2, Ludmila D. Iskhakova 3|
1. Moscow power engineering institute (MPEI), Moscow 111250, Russian Federation
Hexaferrites with a cone magnetic structure attract recently a significant attention as new high (room) temperature magnetoelectric (ME) materials [1, 2]. We have attempted to grow single crystals of Sr3Co2Fe24O41 (Z-type) and SrCoxTixFe12-xO19, x =1.5- 2 (M-type) hexaferrites.
The single crystals have been grown by floating zone (FZ) melting with light heating  from feed rods of stoichiometric compositions. All crystal growth processes were performed in pure oxygen under high pressure up to 80 atm in order to (a) prevent of Fe2+ ions arising due to high melting points of these compositions and (b) suppress of peritectic phase transformation . Typical growth speed was 5 mm/h. Investigations of chemical composition were carried out by scanning electron microscope JSM 5910-LV (JEOL) with INCA ENERGY (Oxford Instruments). The XRD measurements were carried out using DRON-4-13 diffractometer (CuKα-radiation, graphite monochromator).
Obtained crystals are the cylindrical rods about 6 mm in diameter and up to 60 mm in length. The true phase diagram of these compositions is not known, but one can assume the congruent melting and freezing are not take place. Therefore the seed crystallization is possible from float zone melt composition conformed to crystallization field of needed compounds. Really mentioned hexaferrites were grown using polycrystalline seeds, however, after beginning of crystal growth process the change of the melt composition occurred. Then the crystal growth of stoichiometric composition started as the growth from a high temperature solution like that for iron garnets grown by FZ . After growth of initial part (~10 mm) of the crystal concurrent crystal blocks disappear and only single block crystal survives with hexagonal plane oriented along of growth axis.
Digital X-ray Laue analysis confirmed the good crystal quality of SrCoTi M-hexaferrite and X-ray microprobe did not reveal any other phases. The magnetic measurements revealed a strong anisotropy of magnetization along and perpendicular hexagonal c-axis and change of the easy direction from the c-axis to the basal plane with increasing x from 1.5 to 2.
For SrCo Z-hexaferrite the picture is different. On the microprobe image it was observed two phase structures: the composition of the main (dark) phase was identified close to SrCoFe11O19 and the minor number of precipitations of second (white) phase was identified as Sr4Fe6O13. The main phase had the shape of rather large single crystalline platelets with a mirror surface looked on the boule cleavage. The platelets were directed along boule axis. Laue X-ray measurements revealed the hexagonal structure of the platelets with c-axis perpendicular to plane of platelets. A noticeable magnetic anisotropy confirmed the anisotropic contribution of the M - hexaferrite. The growth of these crystals evidently proceeds from the melt of non- stoichiometric composition. Therefore the crystal growth of this material on the crystalline seed is difficult and requires of a small growth speed.
1. Y. Kitagawa, et al., Nature Mater. 9, 797 (2010).
2. L. Wang, et al., Sci. Rep. 2, 223 (2012).
3. A.M. Balbashov, S.K. Egorov, J. Cryst. Growth, 52, 498-504 (1981).
4. H.J. Van Hook, J.Am.Ceram. Soc. 47, 579 (1964).
Presentation: Poster at 17th International Conference on Crystal Growth and Epitaxy - ICCGE-17, Topical Session 6, by Anatoly M. Balbashov
See On-line Journal of 17th International Conference on Crystal Growth and Epitaxy - ICCGE-17
Submitted: 2013-03-23 14:02 Revised: 2013-04-13 11:00