Consequences of magnetic anisotropy in realizing practical microwave hexaferrite devices Anton Geiler a,n , Andrew Daigle a , Jianwei Wang b , Yajie Chen b,c , Carmine Vittoria b,c , Vince Harris b,c a Metamagnetics, Inc., Sharon, MA 02067, USA b Center for Microwave Magnetic Materials and Integrated Circuits (CM3IC), Northeastern University, Boston, MA 02115, USA c Department of Electrical and Computer Engineering, Northeastern University, Boston, MA 02115-5000, USA article info Available online 28 February 2012 Keywords: Ferrite Spinel Hexaferrite Ferromagnetic resonance Magnetocrystalline anisotropy Antenna Electromagnetic bandgap metamaterial Circulator Isolator Inductor Electromagnetic interference device abstract With the rapidly growing demand for bandwidth in wireless communication systems and increasing frequencies of operation of electronic devices, in recent years resurgence in scientific interest in highly anisotropic hexagonal ferrites has been noted. This interest stems in part from a number of emerging applications that pose significant materials challenges that cannot be addressed using traditional rf and microwave ferrite materials. In this manuscript, several specific applications that could benefit from the unique properties of hexagonal ferrites are discussed. Fundamental principles of operation, materials requirements, as well as unique device design and modeling challenges are reviewed. Applications of textured magnetically uniaxial hexagonal ferrite composites in microwave and mm-wave non- reciprocal ferrite control devices are discussed. Applications of textured magnetically planar hexagonal ferrite composites as antenna and electronic bandgap metamaterial substrates, rf inductors and transformers, and electromagnetic interference suppression devices are reviewed. Suggestions on directions for future research and development are provided. & 2012 Elsevier B.V. All rights reserved. 1. Introduction Hexagonal M-type barium ferrite (BaM) belongs to a family of ferrimagnetic oxides possessing closely related hexagonal crystal structures [1]. One of the most practical and important magnetic properties of hexagonal ferrites is the high crystalline anisotropy field. The immediate outcome of the anisotropy field is the existence of ‘‘easy’’ and ‘‘hard’’ magnetization directions. In M-type materials, the high uniaxial anisotropy field is along the c-axis of the crystal. There is also basal plane anisotropy (in the plane perpendicular to the c-axis) that exhibits six-fold symmetry. Hexagonal Y- and Z-type ferrites are closely related to the M-type structure. In Y-type ferrites, for most compositions, the magnetocrystalline anisotropy field H y aligns the magnetization perpendicular to the crystallographic c-axis, in the basal plane of the unit cell. There exists an additional, weaker anisotropy field H j in the basal plane that possesses six-fold symmetry [2]. The competition between uniaxial and planar magnetocrystalline anisotropy fields in both Y- and Z-type hexaferrite allows for materials exhibiting easy magnetization direction, cone, or plane, depending on the species and amounts of substitutional cations in the unit cell. This inherent flexibility of the hexagonal ferrite family, combined with the relatively high magnitudes of magne- tocrystalline anisotropy fields, allows for unique opportunities in device design. The magnetic structure of all hexaferrite materials is deter- mined by superexchange interactions among the various mag- netic iron sublattices in the unit cell. For M-type hexaferrite, for example, there are five different sublattices of iron cations of either tetrahedral, octahedral, or bipyramidal symmetry. For barium M-type hexaferrite (BaM) the net ferrimagnetic moment is 20 m B . The saturation magnetization of this material at room temperature is typically in the 4500–4800 G range [1]. For Y- and Z-type hexaferrites, the saturation is typically somewhat lower. For zinc-doped barium Y-type hexaferrite (ZnY), for example, the saturation magnetization is approximately 2000 G [2]. For cobalt- doped barium Z-type hexaferrite (CoZ) the corresponding value is approximately 3300 G [3]. The uniaxial anisotropy field H A of BaM is typically in the 16,500–17,500 Oe range [1]. In comparison, the planar anisotropy field H y of ZnY is approximately 10,000 Oe and that of CoZ is in the 9000–12,000 Oe range [3]. All of the above compounds are characterized by high electric resistivity, which makes them well-suited for applications in rf, microwave, and mm-wave devices. The magnetic and electric properties of hex- agonal ferrites are a strong function of the composition, with a wide variety of cation substitutions in the unit cell leading to unparalleled tunability of the frequency response [2–4]. These Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/jmmm Journal of Magnetism and Magnetic Materials 0304-8853/$ - see front matter & 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2012.02.050 n Corresponding author. Tel.: þ1 781 562 0756; fax: þ1 781 253 3913. E-mail address: geiler@metamagneticsinc.com (A. Geiler). Journal of Magnetism and Magnetic Materials 324 (2012) 3393–3397