Refinement of Crystal Structure of a Magnetoelectric U-Type Hexaferrite Sr4Co2Fe36O60

Abstract

Hexaferrites have long been used in technological applications such as permanent magnets and microwave applications because they retain their ferromagnetic and insulating properties up to temperatures much higher than room temperature. It is known that there are six main types of hexaferrites: M-type (Ba,Sr)Fe12O19, Y-type (Ba,Sr)2M2Fe12O22, W-type (Ba,Sr)M2Fe16O27, X-type (Ba,Sr)2M2Fe28O46, Z-type (Ba,Sr)3M2Fe24O41, and U-type (Ba,Sr)4M2Fe36O60, where M denotes a divalent metal ion. The crystal structures of these hexaferrites can be distinguished by stacking sequences of three basic blocks: S (spinel block), R [(Ba,Sr)Fe6O11] 2 , and T (Ba,Sr)2Fe8O14 (see the left panels of Fig. 1). After the discovery of a magnetoelectric (ME) effect in Y-type Ba0:5Sr1:5Zn2Fe12O22 showing a spin spiral magnetic order, hexaferrites have been attracting renewed attention as a new class of ME multiferroics which operate by applying low magnetic fields at high temperatures. Recently, a U-type hexaferrite, Sr4Co2Fe36O60, has been found to exhibit a low-field ME effect at room temperature. It is known that the U-type structure with the space group R 3m can be described as the sequence (RSR S TS )3, where the ( ) symbol means that the corresponding block turns 180 about the hexagonal c-axis. However, no literature which reports on its detailed crystal structure is available to date. Thus, to determine the detailed crystal structure of the U-type hexaferrite, we grew single crystals of the ME U-type hexaferrite Sr4Co2Fe36O60 and performed a single-crystal x-ray structure analysis. Single crystals of a U-type hexaferrite were grown by the Na2O–Fe2O3 flux method similar to that described in Refs. 8 and 16. We crushed one of the crystals into small pieces, and used one of the pieces (50 50 50 m) for the present single-crystal x-ray crystal structure analysis. To obtain the data for the crystal structure analysis, synchrotron diffraction measurements were performed at BL-8B of the Photon Factory, KEK. Oscillation photographs using an 18 keV x-ray were captured at room temperature by an imaging plate (IP) Weissenberg camera. For the image data processing of digital IP data and structure refinements, the programs RAPID and Crystal Structure (Rigaku) were used. We constructed a starting structural model of a U-type hexaferrite (RSR S TS stacking) by using known atomic coordinates of R, S, and T blocks in a Z-type hexaferrite (RSTSR S T S stacking) (Ref. 17). For the refinement, we regarded Co (atomic number = 27) as Fe (1⁄4 26) because their atomic form factors are practically indistinguishable. The obtained crystal data, the experimental condition, and the structural parameters are listed in Table I. The atomic coordinates in a unit cell and the thermal parameters are also listed in Table II. The atomic displacement parameters B of Fe(10) and O(1) sites are relatively large. In the case of Fe(10) site which is located at the center of a trigonal bipyramid sublattice, its anisotropic atomic displacement parameters remarkably extend along the c-axis (see Ref. 18). This implies that Fe(10) splits along this direction. A similar behavior of anisotropic atomic displacement parameters is reported for the Fe site in a trigonal bipyramid sublattice of an M-type hexaferrite BaFe12O19 (Ref. 19). In the case of the O(1) site, the large B is attributable to the nearest neighbor Sr(2) site which also has relatively large B. Based on the obtained structural data, we calculated valences of the respective Fe sites by using the bond valence sum (BVS) method. The obtained valences of the respective Fe sites are shown in Table II. As a result, the valences of six Fe sites: Me(1), Me(3), Me(4), Me(5), Me(7), and Me(13) are a b c S block