M-Type Barium Hexagonal Ferrite Films (2012)

IntroductionMagnetic garnet materials such as yttrium iron garnet (YIG) have been widely used as active components in many microwave devices. 1,2,3These devices include resonators, filters, circulators, isolators, and phase shifters.They have had a major impact on the advancement of microwave technology.The underlying physical effects in microwave magnetic devices include ferromagnetic resonance (FMR), magnetostatic wave (MSW) propagation, Faraday rotation, and field displacement.Whatever the basis for a given device, the operation frequency is determined essentially by the FMR frequency of the garnet material.The magnetic garnets are low-magnetization, low-magnetocrystalline-anisotropy materials and, therefore, typically have a low FMR frequency in the GHz range.This imposes an upper limit on the practical operation frequency of compact YIG-based devices in the 10-18 GHz frequency range.Presently, there is a critical need for millimeter (mm) wave devices which operate in the frequency range from about 30 GHz to 100 GHz. 4,5,6This need is critical for three reasons.(1) Millimeter waves are recognized as a broadband frequency resource that can offer various wireless access applications.(2) The need for broadband telecommunication capabilities will mandate the use of mm-wave frequencies in next-generation satellite systems.(3) Electromagnetic radiation at mm-wave frequencies can penetrate clouds, fog, and many kinds of smoke, all of which are generally opaque to visible or infrared light.In principle, one can extend the operation frequency of current microwave magnetic devices to the mm-wave frequency range through the use of high external magnetic bias fields.In practical terms, however, the use of high external fields is usually impractical because of the increased device size and weight, as well as incompatibility with monolithic integrated circuit technology.One important strategy for the above-described frequency extension is to use M-type barium hexagonal ferrite BaFe 12 O 19 (BaM) films as a replacement for those magnetic garnets.BaM films can have a very high magnetocrystalline anisotropy field.This high internal field can facilitate ferromagnetic resonance and hence device operation at mm-wave frequencies.The films can also have high remanent magnetization that can allow for device operation in absence of external magnetic fields, namely, self-biased operation, and frequency tuning using very low external fields.To this end, significant efforts have been made in recent years that range from material preparations to structure and property characterizations and also to device applications. www.intechopen.comAdvanced Magnetic Materials 34 Emphasis has been placed on the optimization of deposition processes for low-loss, self-biased BaM thin films, 7,8,9,10 the deposition of BaM thin films on "non-conventional" substrates, such as semiconductor substrates 11,12,13,14 and metallic substrates, 15 the fabrication of BaM thick films on semiconductor substrates, 16,17 the demonstration of BaM-based planar mm-wave devices, 18,19,20,21,22,23,10,24 the development of BaM-based ferromagnetic/ferroelectric heterostructures, 25,26,27,28,29 and the study of multiferroic effects in single-phase BaM materials. 30A variety of different techniques have been used to fabricate BaM film materials.These include pulsed laser deposition (PLD), 7,8,9,10,11,12,28,31 liquid phase epitaxy (LPE), 32,33,34,35 RF magnetron sputtering, 36,37,19,38,39,40 molecular beam epitaxy (MBE), 14 metallo-organic decomposition (MOD), 15 chemical vapor deposition (CVD), 41 and screen printing. 16,17The device demonstration includes both numerical 20,21,22 and experimental efforts. 18,21,22,23,10,24The devices demonstrated include phase shifters, 21 filters, 22,23,10 ,24 circulators, 18 and isolators. 19 This chapter reviews the main advances made in the field of BaM materials and devices over the past five years.Section 2 gives a brief introduction to hexagonal ferrites first and then describes in detail the structure and properties of BaM materials.This section serves to provide a background for the discussions in the following sections.Section 3 reviews the advances made in the development of BaM film materials.Section 3.1 describes the deposition of low-loss, high-remanent-magnetization BaM thin films on sapphire substrates by PLD techniques. 10Section 3.2 discusses the deposition of BaM thin films on metallic substrates by the MOD method. 15Section 3.3 reviews the deposition of BaM thin films on semiconductor substrates by PLD and MBE techniques. 13,14Section 3.4 describes the fabrication of BaM thick films on semiconductor substrates by screen printing. 16,17Section 4 reviews the demonstration of BaM thin film-based mm-wave notch filters 10,24 and phase shifters. 21Finally, Section 5 discusses future work in the field of BaM materials and devices. Structure and properties of M-type barium hexagonal ferrites (BaM) Building blocks of hexagonal ferritesIn many solids, the atoms look like attracting hard spheres and are packed as closely as possible. 42,43Figure 1 shows a close-packed layer of identical spheres which occupy positions A. This layer is formed by placing each sphere in contact with six others in a plane.A second and identical layer of spheres can be placed on top of this layer and occupy positions B. Each sphere in the second layer is in contact with three spheres in the first layer.A third layer of spheres may be added in two ways: they can occupy either positions A or positions C. In principle, there are an infinite number of ways of stacking the close-packed layers.Two very common stacking sequences are "ABAB…" and "ABCABC…".The first one gives a hexagonal close-packed (hcp) structure.The second one gives a structure known as facecentred cubic (fcc).