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 SrCo_xTi_xFe_12-xO₁₉, x =1.5- 2 (M-type) hexaferrites.

The single crystals have been grown by floating zone (FZ) melting with light heating [3] 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 [4]. 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 (CuK_α-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 [3]. 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).

• Legal notice:
  

  Copyright (c) Pielaszek Research, all rights reserved.
  The above materials, including auxiliary resources, are subject to Publisher's copyright and the Author(s) intellectual rights. Without limiting Author(s) rights under respective Copyright Transfer Agreement, no part of the above documents may be reproduced without the express written permission of Pielaszek Research, the Publisher. Express permission from the Author(s) is required to use the above materials for academic purposes, such as lectures or scientific presentations.
  In every case, proper references including Author(s) name(s) and URL of this webpage: https://science24.com/paper/28841 must be provided.

1. The influence of atmosphere on β-Ga₂O₃  single crystal growth by floating zone