GENERAL PROPERTIES AND APPLICATIONS OF FERRITES

Transcription

1 CHAPTER I GENERAL PROPERTIES AND APPLICATIONS OF FERRITES 1.1 Introduction 1.2 Ferrite Materials and Properties 1.3 YIG Film Fabrication and Effect of Doping 1.4 High Frequency Integrated Circuits 1.5 Statement of the Problem and Organization of the Thesis 1.6 Summary References

2 1.1 INTRODUCTION Ferrites are magnetic insulators that have ferromagnetic properties and at the same time, high resistance to electric current. Ferrite is a magnetic dielectric that allows an electromagnetic wave to penetrate in it, thereby permitting an interaction between the wave and magnetization with in the medium. A ferrite medium under external magnetization possesses gyromagnetic property, which leads to nonreciprocal wave propagation effects. Ferrites find very important applications in the field of microwave and optical communications. The primary material in ferrites is normally a compound of iron oxide with impurities of magnetic insulator oxides added. The compound of iron oxides provides the ferromagnetic properties and impurities of other oxides increase the resistance to the current flow. This combination of properties is not found in conventional magnetic materials. Iron for example, has good magnetic properties but a relatively low resistance to current flow. The low resistance causes eddy currents and significant power losses at high frequencies. Ferrites, on the other hand, have sufficient resistance to prevent such current. Since ferrites are highly insulating and have significant amount of anisotropy at microwave frequencies, useful interaction between the magnetic properties of the material and electromagnetic waves can be 2

3 expected. The anisotropic properties of ferrites can be studied by treating the spinning electron as a gyroscope. If a static magnetic field is applied to the ferrite medium, then the gyroelectrons will precess around the direction of the applied field and the precession is controlled by the strength of the applied field. If a circularly polarized electromagnetic wave, polarized in the same direction of precession of gyroelectrons, is entering into the ferrite medium, then it will interact strongly with the gyroelectrons. But an oppositely polarized wave will interact less strongly with them. Since for a given direction of rotation, the sense of polarization changes with the direction of propagation, an electromagnetic wave will propagate through ferrites differently in different directions or the ferrites are nonreciprocal. This effect is utilized to fabricate directional devices such as isolators, circulators and gyrators. Also the interaction of ferrimagnetic material with the propagating fields can be controlled by adjusting the strength of the bias field. This effect leads to variety of control devices such as phase shifters, switches and tunable resonators and filters [1-3]. With the advent of high temperature superconductors and with the reports of fabrication of artificial nonlinear dielectric materials with desired nonlinearity, intense researches, theoretical as well as experimental, are taking place to make use of their advantages. From the very beginning, efforts were there to incorporate these new developments in to high frequency integrated circuits. Active research is also there for the effective 3

4 coupling of the special properties of ferrimagnetic materials with the salient features of high temperature superconductors [4-7] and nonlinear materials [8-10]. The advantage of their coupling is that even after the fabrication of the devices and circuits, their performance can be externally controlled, because the permeability of ferrite can be controlled by changing the external biasing field. The work reported in this thesis is in tune with this aspect of effective coupling of properties of ferrite with those of high temperature superconductors and nonlinear materials of general nonlinearity. In the thesis, the electromagnetic propagation characteristics of several structures are undertaken. The structures studied are 1. Standalone Ferrite film 2. Ferrite/Dielectric/Ferrite F/D/F 3. Ferrite/Superconductor F/S 4. Ferrite/Superconductor/Ferrite F/S/F 5. Superconductor/Ferrite/Superconductor S/F/S 6. Ferrite/Dielectric/Superconductor F/D/S 7. Nonlinear substrate/ferrite/nonlinear cladding N/F/N 8. Ferrite substrate/nonlinear cladding F/N 9. Ferrite substrate/ferroelectric cladding F/FE 10. Microstrip circuits with ferrite substrates 4

5 In section 1.2 of this chapter, a review of ferrite materials and properties is given. The method of preparation of good quality film of yttrium iron garnet, Y 3 Fe 5 O 12, which is the most standard example of ferrite, is outlined in section 1.3. The studies undertaken have meaning only when the structures proposed are able to be integrated into microchips. Therefore in section 1.4 a review of high frequency integrated circuits is given. The section 1.5 of this chapter figures out and defines the tasks undertaken in this study. 1.2 FERRITE MATERIALS AND PROPERTIES Ferrites having the spinel structure have the general formula MFe 2 O 4, where M is any divalent metal. They all have the crystal structure of the mineral spinel, MgAl 2 O 3. The only naturally occurring ferrite is the magnetic iron ore, magnetite or ferrous ferrite FeFe 2 O 4. The spinel ferrites usually used for microwave applications are magnesium-manganese ferrite, Mn x Mg y Fe z O 4, with x + y + z = 3, Nickel ferrite and Lithium ferrite [1-4]. The magnetic properties of ferrites arise mainly from the magnetic dipole moment associated with the electron spin. Even though every electron has a magnetic moment, due to mutual cancellation, the net magnetic moment of a full shell is zero. But for transition elements like Cr, Mn, Fe, Co, Ni, etc. there are unfilled electronic shells and such atoms 5

6 have large net magnetic moment and this feature is responsible for ferrimagnetic behaviour of ferrites [3]. Ferrites are prepared by thoroughly mixing iron oxide Fe 2 O 3 and the metallic oxide, MO. The mixture is then fired at a high temperature which is less than melting point of either oxide. In the firing process, the oxides sinter together into a spinel crystal structure. The oxygen atoms are distributed in parallel layers and the metallic atoms are distributed at the octahedral ( a ) sites and the tetrahedral ( d ) sites in between the layers [3, 5]. It is the metal atoms which contribute to the magnetization of the ferrite, but the sum of all the spins in the ferrite adds up to a value considerably more than what is measured. This means that some spins are spinning up and other spins are spinning down. In ferrite materials, the iron atoms are normally distributed so that half of them are in the a sites and half are in the d sites while all of the M atoms are in the a sites. All of the metallic atoms in the a and d sites will arrange themselves to form a system which has a minimum amount of energy. From the Fig.1.1, it is clear that metal ions alone contribute to the magnetization of the ferrite. It is important to note that the spins on the two sites are oppositely directed and that by mixing various metals, the saturation magnetization can be varied within certain limits. 6

7 Fe +++ spins Tetrahedral site Octahedral site Fe +++ spins M ++ spins Fig.1.1 Distribution of spins in ferrites. Ferrites have long been used at conventional frequencies in CRT display and magnetic recording systems. The use of ferrites at microwave frequencies revolutionalised the design of microwave systems. In the past, the microwave equipment was made to conform to the frequency of the system and the design possibilities were limited. But the unique properties of ferrites provide a variable reactance by which microwave energy can be manipulated to conform to the microwave system. Circulators, load isolators, phase shifters, variable attenuators, modulators and switches etc are few useful microwave devices employing ferrite materials [5, 6]. Historically, the spinel ferrites are the first to be used for microwave applications. However they are found to have unacceptably high absorption 7

8 loss at microwave frequencies. In 1956 magnetic garnets were discovered. They are ceramic solids. The best known magnetic garnet is yttrium iron garnet or YIG with chemical formula Y 3 Fe 5 O 12 and its structure is given in Fig.1.2. Low loss, low magnetization, good nonreciprocal property, good magnetic tunability, high resistivity and excellent crystal quality are the hallmarks of YIG. Its dominance is so high that nowadays ferrite means YIG. In YIG, only the ferrite ions are magnetic and all the metal ions are in 3 + valance state. The Fe ions provide only very weak coupling between excitations of the spin lattice (magnons) and excitation of the crystal lattice (phonons). Thus the direct relaxation loss mechanisms are negligible. Similarly since all metal ions are 3 +, electron hopping between Fe 3+ and Fe 2+ is not possible and hence the resistivity is high [7]. A wide range of metallic cations may be substituted for the cations in YIG and a wide range of magnetic garnets can be tailored for particular purposes [1]. The table 1 compares the properties of YIG with other ferrites 8

9 Fig.1.2. YIG structure; c - dodecahedral sites for Y, a - octahedral sites for Fe 2 and d - tetrahedral sites for Fe 3 YIG films are perfect candidates for various applications in microwave electronic devices and optical devices [5]. It has got small ferromagnetic resonance (FMR) linewidth and hence low losses. A linewidth of 0.2 Oe at frequencies of about 10 GHz is common. To obtain such a small linewidth, it is necessary to avoid impurities, defects and inhomogeneities. It is also necessary to prepare the YIG as a single crystal, so that crystal anisotropy effects will not introduce broadening of the resonance peak. 9

10 Table.1. Comparison of properties of some ferrite materials Material Typical composition Characteristics YIG Y 3 Fe 5 O 12 Low magnetization in the range Tesla. Less with Ga substitution. Low curie temperature; 280 o C. Less with Ga substitution. Excellent Crystal quality. Low linewidth and low losses. Spinels Hexagonal ferrites LiFe 2 O 4 (Ni,Zn)Fe 2 O 4 BaFe 12 O 19 High magnetization; Tesla for Li ferrite and 0.5 Tesla for (Ni-Zn)ferrite. High curie temperature; 640 o C for Li ferrite and 375 o C for (Ni-Zn) ferrite. Losses and linewidths higher than garnets. Crystal quality needs improvement High magnetization, 0.45 Tesla. Fairly high Curie temperature, 450 o C. Fairly low losses. Crystal quality Needs improvement. High magnetocrystalline anisotropy For some devices, single crystal films are preferred or even essential. Notable examples are the magnetostatic wave (MSW) devices [5, 8]. Propagation characteristics are influenced by the uniformity of the effective internal field. Films provide greater uniformity than bulk crystals. MSW propagation characteristics are directly related to crystal quality. 10

11 1.3 YIG FILM FABRICATION AND EFFECT OF DOPPING Even though there are so many methods like chemical vapor deposition (CVD), sputter deposition, laser ablation technique, the modern technology is Liquid-Phase Epitaxy (LPE) [7, 9, 10]. Better quality YIG films can be prepared by this method and an added advantage of this method is that it provides superior control over film composition, a factor very significant with bubble materials containing five or six metallic constituents. In LPE, the YIG solution in the solvent (it can be a mixture of PbO and B 2 O 3 ) is kept in a platinum crucible at a temperature about 50 o C below it s saturation temperature 1000 o C. This solution has got a peculiarity that it will be in the metastable state even if it is in the supersaturated state. Now if a well prepared gadolinium gallium garnet (GGG) wafer (which is a nearly perfect commercially available crystal), heated to the saturation temperature is introduced into the YIG solution, crystallization will take place. By rotating the wafer at the proper speed, one can ensure the uniformity of the YIG film. The thickness of the YIG film can be controlled by controlling the time of deposition. YIG films may be grown from sub micron thickness and as thick as 100 micrometer. The lateral dimension can reach a few inches. For the growth of thick YIG films, it requires a reasonably high growth rate and a suitably long growth time. Growth rate in LPE are about 11

12 1000 nm/minute. This is a definite advantage over the CVD and sputter deposition techniques which have lower rates. The LPE melt is sufficiently metastable in the supersaturated state to permit the growth time needed for films to attain 50 or 100 micrometer thickness. However there is evidence that film quality can deteriorate at these thicknesses and growth conditions must be chosen to avoid this [10-12]. In order to improve different film properties in magnetostatic or magneto-optical applications, various ions (Ba, Sc, Bi, La) can be doped into YIG during its film fabrication. For instance the Bi or Sc substitution of some Y ions can increase the Faraday rotation effect. Another important point to be remembered is that steps should be taken to control lattice misfit. The lattice constant of YIG is A o and that of GGG is A o Although the misfit is very small compared to misfit in many epitaxial semiconductor materials, this misfit can have significant effect. In particular, since garnets are free of dislocations and other misfit-accommodating defects at the temperatures used in LPE, the YIG film behaves elastically and experiences an in-plane tensile stress. Defect-free films can be grown even for misfits greater than However at a critical thickness, it becomes energetically favourable for the strain to be relieved by fracture [13]. The critical thickness observed experimentally is about 15 micrometer for YIG on GGG. The misfit can be 12

13 controlled by changing the lattice constant of the film or the substrate. Altering the film composition is the easier way and this can be done by substituting Lanthanum for a small amount of yttrium or by incorporating lead as an impurity from the LPE solvent [14]. Since La is nonmagnetic and is isovalent with the yttrium it replaces, there should be no appreciable effects other than the slight increase in the lattice constant. But the use of La sources that are not contaminated with other rare earth elements is a must since these can introduce direct relaxation loss mechanisms. Due to alteration in the composition, the magnetic properties of YIG films may be modified. Ga may be substituted for some of the Fe to reduce the magnetization. This is commonly done in YIG spheres so that resonance at lower microwave frequencies can occur at fields that are sufficient to produce magnetic saturation. Lattice mismatching due to substitution can be countered by the combined application of La and Ga. It may be noted that for devices such as tunable filters, YIG films without Ga substitution can be used even at low microwave frequencies because the thin film geometry allows the use of an external field that is sufficient for magnetic saturation [15]. In short, the liquid phase epitaxial method is good enough to produce quality YIG films required for various microwave and millimeter wave applications. Microwave integrated 13

14 technology is grown to such an extent that all the good qualities of YIG can be made use of in modern devices. 1.4 HIGH FREQUENCY INTEGRATED CIRCUITS The microwave and optical integrated circuits represent an extension of integrated circuit technology to extremely high frequencies (uhf and higher) and has brought drastic revolution in the field of electronics. Microwave integrated circuits (MIC), can incorporate innumerable components of different types, passive and active, made of YIG and other materials, into a small chip of few square millimeters of size to form a complete microwave subsystem so that size, weight and cost are wonderfully reduced [2]. There are two types of MICs, the hybrid MICs developed first in 1960s and the monolithic MICs (MMICs) fabricated in the late 1970s [16-18]. Hybrid MICs have only one layer of metallization for conductors and transmission lines and discrete components like resistors, capacitors, diodes and transistors, etc. are bonded to the substrate. Alumina, quartz and teflon fiber are commonly used substrates. Alumina is a rigid ceramic-like material with dielectric constant of A high dielectric constant is often desirable for lower frequency circuits because it results in a smaller circuit size. At higher frequencies, however, the substrate thickness must be decreased to prevent radiation loss and other spurious effects; then the 14

15 transmission lines like microstrip, slot line, or coplanar waveguide may become too narrow to be practical. Quartz has a lower dielectric constant (~ 4) which, with it s rigidity, makes it useful for higher frequency circuits (> 20GHz). Teflon and similar types of soft plastic substrates have dielectric constants ranging from 2 to 10 and can provide a large substrate area at a lower cost, as long as rigidity and good thermal transfer are not required. Transmission line conductors for hybrid MICs are typically copper or gold. The substrate of an MMIC must be a semiconductor material to accommodate the fabrication of active devices and devices consisting, several layers of metal, dielectric and resistive films. The frequency range dictate the type of substrate material. GaAs is probably the most common substrate for MMICs suitable for frequencies up to 60 GHz and finds applications in low-noise amplifiers, high gain amplifiers, broadband amplifiers, mixers, oscillators, phase shifters and switches. Sometimes silicon, silicon-on-sapphire and indium phosphide are also used. Potentially, the MMIC can be made at low cost because the manual labour in the fabrication of hybrid MICs is eliminated and that a single wafer can contain a large number of circuits, all of which can be processed and fabricated simultaneously. 15

16 Both monolithic and hybrid MIC synthesis has to make extensive use of efficient computer aided design (CAD) procedures [19]. With the stringent conditions on propagation characteristics of the system in hand, designing an integrated circuit component to be suitable for a specified frequency range is a very difficult task. As far as the waveguiding systems are concerned, this is comparatively easy because after fabrication of the circuit, adjustments are possible. This is impossible in the case of ICs and if the system doesn t meet the expected performance after fabrication, the entire process will become futile. Therefore efficient CAD is a must for MIC fabrication. The integration technology is developed to such an extent that microwave materials and components can be used even in the field of optical communications [20]. Optical communication through glass fiber is developed in the near infrared region 1.3 and 1.55 μ m. Semiconductor lasers are used as light sources. To protect these lasers from backward light, optical isolators are needed. Such nonreciprocal devices can be realized with magneto-optical materials [6, 21]. Magnetic garnets like YIG, which have low optical losses in the infrared window between 1.2 and 5 μ m wavelength, are perfect candidates for these applications. The Faraday rotation, which is the basis for the nonreciprocal effects, can be strongly enhanced by bismuth substitution in YIG. 16

17 1.5 STATEMENT OF THE PROBLEM AND ORGANIZATION OF THE THESIS The first step in investigating the electromagnetic wave propagation in ferrite based devices is the study of wave propagation in layered structures containing ferrites and other materials of interest. The thesis presents a systematic study of electromagnetic wave propagation in multilayered structures of which ferrite layer is an integral part. There are four sections in this study and they are 1. Ferrite and dielectrics 2. Ferrite and superconductors 3. Ferrite and nonlinear dielectrics 4. Inhomogeneous structures like ferrite based microstrip line and slot line. A complete list of structures studied is given in section 1. These studies are distributed in various chapters as follows. As a preliminary to the detailed study of wave propagation in different structures mentioned above, the electromagnetic wave propagation in a ferrite film, an inevitable component of all the structures studied, is formulated in the second chapter. It is followed by the discussion of propagation in ferrite/dielectric/ferrite hybrid structure. The 17

18 dispersion relation is derived from Maxwell s equations and different aspects of propagation are discussed in detail. The detailed study of propagation characteristics of various ferrite superconducting structures are undertaken in the third chapter. Dispersion relations corresponding to the magnetostatic type of electromagnetic wave propagation in all the structures are derived using Maxwell s equations, with main thrust on TE wave propagation. The various aspects of propagations like nonreciprocal effect and tunabilty etc. are thoroughly discussed. In the fourth chapter, nonlinear wave propagation in nonlinear multilayered structures with general nonlinearity are intensively discussed with special attention to tunability, ie., the dependence of propagation on magnetic field and nonreciprocal effect of power, after formulating the analytical field equations, dispersion equations and power expressions. Here also focus is given to lowest mode of transverse electric wave. In chapter V, propagation characteristics of a microstrip line and a slot line printed on magnetized ferrite substrates are analyzed using spectral domain approach. In spectral domain method, the dispersion relation is derived in the Galerkin s procedure, in the Fourier transform domain. 18

19 The main intention of this study has been to bring out the nonreciprocal and field dependent properties of propagation in ferrite loaded linear as well as nonlinear multilayered structures. 1.6 SUMMARY This chapter outlined the work undertaken in the thesis. A short review of ferrite materials and properties is given in the second section followed by a discussion on YIG film fabrication. In the fourth section a brief discussion on high frequency integrated circuits is given as it being the field where the structures studied are to be tested and put into practice. In section 1.5 of this chapter, the works undertaken in the forthcoming chapters are discussed along with the organization of the thesis. REFERENCES 1. A. J. BADAN FULLER, Ferrites at microwave frequencies, Peter Peregrinus Ltd, D. M POZAR, Microwave engineering, Addison- Wesley, New York, K. C. GUPTA, Microwaves, Artech House, Norwood, MA, B. LAX and K. J. BUTTON, Microwave ferrites and ferrimagnetics, McGraw-Hill, WAGUIH S. ISHAK, Magnetostatic wave technology: a review, IEEE Proce. Vol.76,

20 6. J. D. ADAM, LIONEL E.DAVIS, GERALD F.DIONNE, ERNST F. SCHLOEMANN, and STEVEN N. STITZER, Ferrite devices and materials, IEEE Trans. on microwave theory and techniques, Vol. 50, H. L. GLASS, Ferrite films for microwave and millimeter wave devices, IEEE Proce. Vol.76, W. S. ISHAK and K. W. CHANG, Magnetostatic wave devices for microwave signal processing, Hwelett-Packard, D. M. HEINZ, P. J. BESSER, J. M. OWENS and G. R. PULLIAM, Mobile cylindrical magnetic domain in epitaxial garnet films, J. Appl. Phys., Vol.42, H. L. GLASS, Growth of thick single crystal layers of yttrium iron garnet by liquid phase epitaxy, J. Crystal growth, Vol.33, J. E. DAVIES and E. A. D. WHITE, Interface breakdown in garnet liquid phase epitaxy, J. Crystal growth, Vol.27, T. HIBIYA, Surface morphologies and quality of thick liquid phase epitaxial garnet films for magneto-optic devices, J. Crystal growth, Vol.62, J. W. MATHEWS, E. KLOKHOLM and L. S. PLASKET, Defects in magnetic garnet films, Amer. Inst. Phys. Cont. Proc. No.10, M. NEMIROFF and h. YUE, La:YIG discs on GGG substrates for microwave applications, IEEE Trans. Magn.,Vol.18, MURAKAMI, T. OGIHARA and L.OKAMOTO, Tunable bandpass filter using YIG film grown by LPE, IEEE, MTT-S Int. Microwave Symp

21 16. D. N. McQUIDDY, Jr. J. W. Wassel, J. B. LAGRANGE and W. R. WISSEMAN, Monolithic Microwave Integrated Circuits: A Historical Perspective, IEEE Trans. Microwave Theory and Techniques, Vol. MTT-3, H. SOBOL, Applications of integrated circuit technology to microwave frequencies, Proc. IEEE, Vol.59, S. Y. LIAO, Microwave devices and circuits, Prentice Hall of India, F. GARDIOL, Microstrip circuits, John Wiley and & Sons, New York, WALLENHORST, M. NIEMOLLER, H. DOTSCH, P. HERTAL, R. GERHARDT and B. GATHER, Enhancement of the nonreciprocal magneto optic effect of TM modes using iron garnet double layers with opposite Faraday rotation, J. Appl. Phys., Vol.77, D. WEBB, Status of microwave technology in the United States, IEEE MTT-S. Int. Microwave Symp. Atlanta,