The Hexagonal Ferrites

The structure of these oxides consists of layers of either close-packed oxygen atoms or else close-packed layers of barium and oxygen in a ratio of 1 to 3 of the type found in a 111 sheet of a cubic barium perovskite. In this latter sheet each barium atom is isolated from direct contact with any other barium atom by oxygen neighbours which act as an effective screen. These layers are stacked one on top of the other in a way which preserves the overall hexagonal symmetry of each layer, and the hexagonal 

The results described were the product of F-ray diffraction analysis and it is natural to turn to electron microscopy to obtain more precise information on these complex phases. Two groups have carried out such investigations, McConnel, Hutchison, and Anderson and Amelinckx et al.f while recently Hirotsu and Sato have also studied defects in these materials.  

These studies have produced a number of results of note. In some ways the most important of these is that ordered ferrite structures with long unit cells do exist. 

The notes here are only a brief summary of the studies made. Despite the efforts so far put in, none of these studies is exhaustive and it is certain that other electron-microscope investigations will lead to the discovery of other phases or vestiges of phases in this system. 

---

The hexagonal ferrites have a structure related to the spinel structure but with hexagonal close-packed oxygen ions and a unit cell made up of two formulae of MN12O19, where M is divalent (Ba, Sr, or Pb) and N is trivalent (Al, Ga, Cr, or Fe),... 

These requirement.s are fulfilled by the hexagonal ferrites (hexaferrites) BaFei20i9 and SrFe]20m. 

Lotgering, F. K., Vromans, P. H. G. M. Huyberts, M. A. H. (1980). Permanent-magnet material obtained by sintering the hexagonal ferrite W = BaFeig027. Journal of Applied Physics, 51, 5913-18. 

Hard magnetic materials have rather rectangular, broad, hysteresis curves (Figure 12.16). These materials are used in permanent magnets, with applications from door catches to electric motors. The hexagonal ferrites are hard magnetic materials, as are alloys such as SmCos and Nd2Fei4B. 

Estimate (a) the saturation magnetisation and (b) the magnetic inductance for the hexagonal ferrite ferroxdur , BaFei20i9. The hexagonal unit cell has parameters flo = 0.58836 nm, cq = 2.30376 nm and volume 7.128 x 10 m. There are two formula units in the unit cell. Assmne that the orbital angular momentum is completely quenched in this material. 

More properly tetracalcium aluminoferrite. This must be differentiated from ferrites, which are spinels of general formula AFe O, and from the hexagonal ferrites, that adopt structures related to that of magnetophimbite, E Fej O,. ... 

Ara] Arakcheeva, A.V., Karpinskii, O.G., Crystal Structure of the Hexagonal Ferrite... 

Magnetic ceramics may be divided into one of three different classes depending upon the type of crystal structure and magnetic properties that they display. The three classes of magnetic ceramics include the spinel ferrites, the hexagonal ferrites... 

The mineral barium magnetoplumbite has the chemical formula BaFe Oig or BaO -bFeiOs and is perhaps the most important of the hexagonal ferrite since it is a hard magnet with the spins all aligned along the c-axis. This oxide is used in the magnetic stripe on credit cards. 

---