Crystallographic Texture and Magnetic Anisotropyand Their Influence Upon Microwave Devices

V. G. HARRIS,1,3 Y. CHEN,1,4 and Z. CHEN2,5

1.—Department of Electrical and Computer Engineering and Center for Microwave MagneticMaterials and Integrated Circuits, Northeastern University, Boston, MA 02115, USA. 2.—IntelLabs, Santa Clara, CA 95054, USA. 3.—harris@ece.neu.edu. 4.—e-mail: y.chen@neu.edu.5.—e-mail: zhaohui.chen@intel.com

The role of magnetic and structural anisotropies in determining the radio-frequency properties of ferrites and their performance characteristics whenintegrated within high-frequency devices are explored. Both thin-film epitaxyand bulk polycrystalline texture are discussed, defining viable paths to real-izing the crystallographic texture required for device integration.

INTRODUCTION

The focus of this article is on the role of magneticand structural anisotropies in determining theradio-frequency (rf) properties of ferrites and theirperformance characteristics when they are inte-grated within high-frequency devices.

Let us first consider ferrite materials as magneticceramics that possess both permittivity and perme-ability concomitant with low-microwave dielectricand magnetic losses. They exist in a variety of crystalstructures that include the cubic garnets (spacegroup: Ia3d, Fig. 1a) and spinels (space group: Fd3m,Fig. 1b), and the hexaferrite magnetoplumbite (spacegroup: Pb3/mmc, Fig. 1c) phases.1 It is noteworthythat the hexaferrite structure and its many variantsare constructed in part of an assembly of spinel sub-units that are rotated about a common axis as shownin Fig. 1c. Crystal symmetry is broken in this struc-ture by the inclusion of the barium ion on the low-symmetry trigonal bipyramidal site giving rise to theoverall hexagonal symmetry of the unit cell.

The ferrite structure, be it spinel, garnet, orhexaferrite, has as its structural backbone a close-packed arrangement of oxygen anions. Metalliccations, magnetic and nonmagnetic, and typicallydivalent and trivalent, reside on the interstices ofthe close-packed oxygen lattice in some cases fillingall available sites, while in others preferentiallyfilling select sites.

Ferrites find a wide variety of uses in military andcommercial radar and communication electronics asintegral materials in traveling wave tubes,switch-mode power supplies, transformers, power

converters, inductor cores, filters, etc., as well aspassive electronics used in transmit and receivemodules as isolators, circulators, phase shifters,filters, directional couplers, power limiters, and soon.

Radar applications have obvious military appli-cations but may also be used in systems for com-mercial aviation, severe weather warning radar,oceanographic satellite radar, and automobile anti-collision radar, to name a few. Other applicationsinclude satellite communication uplinks and down-links over a broad range of frequencies.

What we will show in this article is how crystallo-graphic texture in ferrites gives rise to unique mag-netic anisotropies that can then be used to selectoperating frequencies, bandwidth, and to somedegree loss characteristics. This unique interrela-tionship will be explored in greater detail subse-quently.

FERRITE MAGNETISM

Magnetism in the ferrites derives from the indi-rect exchange energy J between spins of neighbor-ing metallic ions via oxygen ions. Because J isnegative, this results in the antiparallel alignmentof spins as the lowest energy configuration. Becausethe distance between metal ions is too great tosupport direct exchange, such as that experiencedin most magnetic metals, the exchange is mediatedby the oxygen anion that resides between the twocations and is thus considered an indirect exchange,i.e., superexchange. Three factors principally affectthe strength of the superexchange; these include the

JOM, Vol. 65, No. 7, 2013

DOI: 10.1007/s11837-013-0622-3� 2013 TMS

(Published online May 31, 2013) 883

distance, direction, and angle of the cations withrespect to the anion. While the direction and angleare defined by the relationship between Me-O-Me,the critical distance is between Me-O (not Me-Me)has impact on J. All three factors determine thedegree of orbital overlap between the extending 2porbitals of oxygen and the 3d orbitals of the cationsand, hence, the magnitude of J. By examiningp- and d-orbital alignments, one can readily con-clude that larger Me-O-Me angles produce strongernegative exchange, while angles that approach 90�produce weak exchange. This is seen in the ferritesystems where, for example, in spinel ferrites theexchange, JAB, corresponds to the largest A-O-Bangle, �154�, which is far larger than that of theB-O-B JBB, �125�. The A-O-A correlation forms anangle less than 80� and the distance between A-O iscomparatively larger, �3.5 A, leading to the small-est exchange JAA. JAA is so comparatively small thatmany report this to be zero or ignore it altogether.Magnetization results from an imbalance of spins onthe ferrimagnetically coupled sublattices. As men-tioned previously, one approach to increasing thenet magnetization is to populate one sublattice

preferential to the other with nonmagnetic cationswhile concomitantly maintaining a strong exchangeenergy J to minimize Yafet and Kittel2 spin canting.Alternatively, one may populate one sublattice rel-ative to the other with high spin moments. For athorough treatment of molecular fields in ferrites,the reader is directed to a recent text by Dionne.3

Because the hexaferrites are the only ferritespossessing strong magnetocrystalline anisotropy,we focus the remainder of this article to the engi-neering of both magnetic and structural anisotro-pies in this class of materials. Structural anisotropycorresponds to crystallographic texture.

Importantly, when made with strong anisotropy inboth magnetic polarization and structure, hexafer-rites experience a magnetic easy axis along thecrystallographic c-axis and find unique applicationvalue in passive rf devices such as circulator/isola-tors, phase shifters, filters, and antenna substrates.Importantly, they are the select materials for useabove X-band frequencies because their high mag-netic anisotropy leads to high ferromagnetic reso-nance (FMR) frequencies. Of course hexaferrites alsofind applications as permanent magnets,4 magnetic

Fig. 1. Crystal structure of the (a) yttrium iron garnet (YIG), (b) spinel ferrite with the magnetic moment direction of each cation indicated, and(c) barium M-type hexaferrite unit cell structure (adapted from Ref. 1).

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recording,5 and electromagnetic absorbers6 to namebut three other applications. We will not focus onthese material variants and those specific applica-tions, but instead we limit the scope of this article toferrite systems used for off-resonance rf deviceapplications where it is very important to minimizedielectric and magnetic losses.

THE EFFECT OF CATION SUBSTITUTIONON MAGNETIC ANISOTROPY

It has been established in careful experimentalstudies by numerous researchers that certain sub-stitutions for the Fe cation lead to dramatic changesin magnetic anisotropy energy, Ku, and the mag-netic anisotropy field, HA. In order to appreciate theinterrelationship between HA and fFMR, we willprovide background information.

The imaginary component of the FMR signal isillustrated as the frequency dependence of l¢¢. Themaximum in l¢¢ coincides with the resonance fre-quency of l¢.

It is essential to operate a passive rf device farfrom the peak in l¢¢ while at the same time closeenough to benefit from the high differential realpermeability. Operating too close to the peak in l¢¢leads to high device insertion loss. Any insertionloss greater than 1 dB is likely to be seen as toolossy and thus will greatly limit wide scale use. Wehave shown by iterative finite-element method(FEM) analysis of designs, which the FMR loss ofthe magnetic material (as FMR linewidth) is ad-justed, that a 1-dB insertion loss roughly correlatesto an FMR linewidth of nearly 600 Oe (at X-band).This rather crude demonstration depends on othercritical factors such as the operational frequencyrelative to FMR, impedance matching, and conduc-tion losses (among others) that were not be includedin the linewidth calculation.

The FMR linewidth in turn depends on the intrinsicmagnetic anisotropy and extrinsic parameters such ascrystal quality, grain orientation, porosity, etc.7 Thelatter properties are often reflected in the coercivity ofthe ferrite that in turn has been shown to determine inpart the remanent magnetization leading to self-biasedproperties. Both the magnetization and anisotropyfield are determined from atomic-level parameters andinteractions. For example, fundamental sources ofanisotropy include magnetocrystalline, dipolar, spin–orbit coupling, single ion, pair-order, etc. These in turnare affected by the proportion, type, and valence ofcations, as well as the bond angle and distance betweencations and anions. Point, line, and volume defects alsocontribute to changes in exchange that in turn affectmagnetic anisotropy, magnetization, thermomagneticbehavior, and FMR linewidth. In this way, one canappreciate the care in preparation that must be affor-ded magnetic materials for use at rf frequencies.

The FMR condition depends on, among otherparameters, the strength of HA (i.e., the magneticanisotropy field), intrinsic magnetization (Ms), and

the applied field (H). Variations in applied field andHA act to shift the FMR frequency allowing thetailoring of device operational frequency and per-formance for applications over a wide range of fre-quencies. For example, the work by Albanese andDeriu8 showed an unambiguous trend in which Feion substitutions with In and Ga lead to FMR fre-quencies as high as W-band. Similarly, it was shownby these same researchers that substitutions of Inand Sc can shift the FMR frequency to X-band. Acombination of cobalt and titanium (as Cox/Tix) hasalso been shown to reduce the magnetic anisotropyenergy, thererby reducing the FMR frequency. Athigh substitution levels, the c-axis alignmentweakens HA to the point of the ferrite becomingplanar with the magnetic easy axis aligning withinthe basal plane. Similarly, the addition of Ir and Co(as Cox/Irx) reveals clear trends of increasing xreducing HA from >15 kOe to<1.2 kOe to the pointwhere for x > 0.65, the hexaferrite is poorly definedand possibly planar.9 Chen et al.10 in substitutingZn and Zr (as Znx/2/Zrx/2) also found a pronouncedreduction in HA from �16 kOe to <10 kOe. Daigleet al.11 has experimented with the substitution of Coions in both Y and Z hexaferrites as choice materialsas antenna substrates for UHF to S band applica-tions.

It is important to realize the essential role ofmagnetic anisotropy in establishing the strengthand direction of easy magnetic polarization. In thepresence of the large magnetocrystalline anisotropyenergy of the hexaferrite structure, the magneticeasy polarization aligns along the crystallographicc-axis. Hence, often the problem reduces to thesuccess of manipulating the c-axis orientation.

THE EFFECT OF CRYSTALLOGRAPHICTEXTURE ON DC AND RF MAGNETIC

PROPERTIES

Because it is known that the magnetocrystallineanisotropy energy, Ku, closely couples to the crys-tallographic lattice, the ability to tailor structureand structural texture can be of great value. Hence,we pose the question of ‘‘How does one go aboutcontrolling crystallographic texture in the ferrites?’’

There is more than one answer to this question,and they depend on the type of ferrite material andimportantly the magnetic anisotropy energy. Fur-thermore, the approach taken to thin/thick filmfabrication and bulk compacts and engineeredcomponents take on very different approaches.

The principal motivation for the growth of lowloss ferrites as thin and thick films is their use inmicrowave control devices such as phase shifters,filters, and circulators/isolators. Further motivationstems from a long-time goal of using the stronguniaxial magnetic anisotropy to stabilize highmagnetic polarization in the remanent state. Such asystem would eliminate need for bulky and expen-sive biasing magnets such as those used in circula-

Crystallographic Texture and Magnetic Anisotropy in Microwave Ferrites 885

tors and isolators. The most direct way of realizingmagnetic anisotropy is through crystallographictexture, especially for the hexaferrites, stemmingfrom crystallographic anisotropy. For thin-film fer-rites, the goal is to grow films as either seed crystalsor engineered films ready for further processing onlattice-matched substrates.

The epitaxial growth of ferrites on these sub-strates typically leads to low dislocation densitiesand other point and volume defects that leads to lowmicrowave loss. As a result, microwave losses tendto be low, for spinel and garnets in the 1 s of Oe andfor the hexaferrites in the 10 s of Oe. Zuo et al.12

showed that the growth of Mn-ferrite spinel filmscan experience an enhanced magnetic anisotropy ofmore than �5000 Oe when epitaxially grown on(100) MgO substrates. (100) MgO substrates expe-rience a 0.94% (2 9 1) lattice mismatch with a<5%mismatch in thermal expansion coefficients withrespect to Mn ferrite and many other spinel ferrites.This enhancement in HA was attributed to bothanion defects due to growth pressure conditions andcation inversion (or disorder), as well as their con-tributions to magnetic anisotropy while maintain-ing high crystalline quality.

Most studies involving the epitaxial growth ofspinels and garnets involve their growth on micro-wave oxide substrates including MgO, STO,13 andGGG,14 etc. However, since the 1980s, the integra-tion of thin and thick film ferrites with semicon-ductor substrates has been a motivating goal. Thisgoal of integration stems from the advances made inactive semiconductor devices such as those em-ployed in RADAR transmit and receive modules(TRMs) in amplifier and signal processing devices.

This pursuit has largely resulted in frustrationfrom the incompatibility between thermal andchemical instability of semiconductor substrates,

such as Si, Ge, and GaAs, at the high temperaturesrequired for the processing of low-loss ferrites (i.e.,‡800�C). This situation changed in c. 2000 with thedevelopment and availability of high-quality single-crystal wafers of SiC and GaN. In recent years, GaNhas become the choice material for use in TRM high-frequency amplifiers while SiC has taken on adominant role in high-frequency–high-power ther-mal management. It is noteworthy that these wide-band-gap semiconductors not only provide chemicaland structural stability at the very high processtemperatures required by the ferrites, but alsopossess the same hexagonal symmetry with anapproximate lattice match to the hexaferrites.

A recent review by Chen and Harris15 examined thehistorical development of ferrite thin-film growth onsemiconductors and other functional substrates. Inthat review, the work of Chen et al.,16 was reviewed inwhich hexagonal ferrites were epitaxially grown onSiC and GaN.17 In an impressive demonstration, Chenet al. showed via pole figure analysis the epitaxialgrowth of the barium hexaferrite on GaN (see Fig. 2).The cross-section transmission electron microscopyimage of the hexaferrite/MgO/GaN/sapphire struc-ture is shown in Fig. 3. The right-hand side depicts anexpanded view of the interface where one can see alimited degree of intermixing. MgO is used in this in-stance to relieve strain at the interface as well as tohelp reduce chemical interdiffusion between thehexaferrite and substrate. The arrows denote regionsof interdiffusion from GaN to MgO. At the MgO-ferriteinterface at this region, we see evidence of the forma-tion of a soft phase—likely a spinel ferrite of MgFe2O4.Limited structural and magnetic characterizationdata support this assertion. The growth of high-qual-ity hexagonal ferrite is further affirmed by the FMRpeak-to-peak linewidth of less than 100 Oe. A line-width that is measured to be less than 100 Oe is

Fig. 2. Pole figure obtained for (008) reflection, and the corresponding 2D projections, with 2h value fixed at 30.30�. The single dominant peakcorresponds to u = n = 0� for the BaM (008) reflection. The weaker peaks exhibiting sixfold symmetry correspond to closely spaced BaM (104)reflections. The hysteresis loops depict both in-plane alignment (magnetic hard direction) and out-of-plane alignment (magnetic easy direction)(adapted from Ref. 1).

Harris, Y. Chen, and Z. Chen886

consistent with microwave insertion losses well below1 dB required in such devices as circulators and iso-lators. This development provides a pathway to real-izing the integration of active and passive devices on asingle wafer: a long-sought goal. The remaining chal-

lenges to realizing wide-scale integration are thedevelopment of large diameter GaN substrate mate-rials (i.e., ‡4-in diameter) together with the transitionfrom research-scale pulsed laser deposition to anindustrial acceptable deposition technology.

Fig. 3. Transmission electron microscopy images of the BaM film grown on MgO/GaN/Al2O3 in cross section. The image (b) is an expanded viewof the section denoted in (a). The regions denoted by arrows signify intermixing (used with the permission of the author).

Fig. 4. Orientation map along the [0001] direction measured by EBSD for the BaM ferrite sintered by the topochemical growth process. The colorcode for the crystallographic orientations is given in the stereographic triangle below the map. Pole figure along the [0001] direction for the BaMferrite sintered by the topochemical growth process. Magnetic hysteresis loops and SEM image of the surface morphology for the BaM compactsintered at 1350�C for 10 h. Perpendicular and parallel refer to the orientation of the applied magnetic field with respect to the sample surface(adapted from Ref. 1).

Crystallographic Texture and Magnetic Anisotropy in Microwave Ferrites 887

The processing of crystallographically texturedferrite polycrystalline bulk samples requires a muchdifferent approach that is equally challenging.

In order to align polycrystals, one must eitherseed the growth of crystals, in essence takingadvantage of epitaxy between the seed and theferrite, or in the case with magnetic systems, suchas ferrites, a magnetic field may be used to producea torque on particles to lend themselves to align-ment and subsequent texture.

Chen et al.18 demonstrated the earlier approachof seeding in the growth of highly textured bariumhexaferrite. In that example, goethite, i.e., a-FeO-OH, nanorods mixed with BaCO3 were dispersed ina polymer solution and oriented under a 90-kOemagnetic field during polymerization. The magneticorientation arose principally from the interaction ofthe magnetic field with the anisotropic antiferro-magnetism of the goethite particles. The orientedantiferromagnetic particles result in the topochem-ical growth of BaFe12O19 ferrite grains along the[0001] direction by a one-step sintering process. Thedegree of grain orientation was determined usingmagnetic measurements as well as orientation dis-tribution functions and pole figures deduced byelectron backscatter diffraction analysis (see Fig. 4).

Subsequently, these same authors demonstratedthe crystallographic texture of polycrystalline screenprinted barium hexaferrite films by applying mag-netic fields to align particles of a green body and forma highly textured sample.19 BaFe12O19 hexaferritethick film samples, processed through the use of thescreen-printing technique, coupled with hot-presssintering, have high hysteresis loop squareness (Mr/Ms) and low FMR linewidths. Scanning electronmicroscopy (SEM) and x-ray diffraction (XRD) mea-surements exhibit strong crystallographic c-axisalignment of crystals perpendicular to the film plane.Static magnetic and FMR measurements were per-formed to determine the effect of the preparationtechnique on magnetic hysteresis and microwaveproperties. Hot pressing during sintering produceddense thick films having high squareness (>0.95) andreduced coercivity (�1900 Oe). Of greater impor-tance was the measurement of a minimum peak-to-peak FMR linewidth, 320 Oe at U-band, for filmsranging in thickness from 100 lm to 500 lm. Theo-retic estimates suggest that such narrow linewidthscan be attributed to the reduction in porosity and theimprovement in c-axis orientation of crystallites inpolycrystalline barium ferrite films. As such, hexa-ferrite materials prepared using this technique offernew opportunities in the design and processing ofnext generation self-biased planar microwave de-vices (Fig. 5).

IMPACT ON MICROWAVE AND MILLIME-TER-WAVE APPLICATIONS

As discussed, the role of crystallographic textureis often a vehicle to manipulation of magnetic

anisotropy that in turn allows for the tuning of theoperating frequency, bandwidth, and loss, amongother properties. Figure 6 is a plot of ferrite opera-tional bands versus frequency. In Fig. 6, we see theimpact of cation dopants, anisotropy energy, andtexture in the choice of materials for certain fre-quency bands. This qualitative depiction reflects theflexibility of the hexagonal ferrite and the otherspinels and garnets in covering large swaths offrequency space by adjusting Ku via cation substi-tution. One notices that the garnets and spinels fallin the low-frequency side of the spectrum. This isdue to the lack of strong magnetocrystallineanisotropy in cubic systems. The NiZn-ferrites areseen to possess utility beyond that of the MnZn-ferrites in the range of operational frequencies dueto their far superior insulating properties. Alterna-tively, the hexaferrite systems cover many decadesof frequency space depending on the nature of cat-ion substitution and the intrinsic magnetocrystal-line anisotropy energy. Different variants of the

Fig. 5. (a) Scanning electron microscopy image of the cross sectionof a 200 lm screen-printed film after magnetic field alignment andheat treatment procedures. (b) Magnetic hysteresis loops acquiredwith the applied magnetic field aligned along the in-plane sampledirection (open squares) and perpendicular to the sample plane(solid squares) (adapted from Ref. 1)

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magnetoplumbite structure are listed includingM, Y, and Z. Each find utility for a wide range ofapplications and over a broad range of frequencies.

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Fig. 6. Frequency spectrum with available ferrites listed as viable materials for off resonance device consideration.

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