IEEE TRANSACTIONS ON MAGNETICS, VOL. 42, NO. 10, OCTOBER 2006 3353

Textured Sc-Doped Barium–Ferrite Compacts forMicrowave Applications Below 20 GHz

Tomokazu Sakai1, Yajie Chen2, C. N. Chinnasamy2, C. Vittoria2, Fellow, IEEE, and V. G. Harris2

Electrical and Computer Engineering Department, Northeastern University, Boston, MA 02115-5000 USACenter for Magnetic and Microwave Materials Laboratory, Department of Electrical and Computer Engineering,

Northeastern University, Boston, MA 02115 USA

Sc is substituted for Fe in Ba M-type hexaferrite to reduce the anisotropy field and shift the ferromagnetic resonance (FMR) frequencyto below 20 GHz. We have produced pure phase BaFe11 ScO19 powders having a platelet microstructure. These materials are pressedunder an applied magnetic field of 95.5 105 A/m. Resulting compacts are sintered and are shown to have a clear uniaxial anisotropywith the -axis perpendicular to the surface plane. Hysteresis loops display a squareness of 0.83 with a coercivity of 186 kA/m. The

-band FMR power derivative linewidth is measured to be 79.6 kA/m.

Index Terms—BaFe11ScO19 hexaferrite, ferromagnetic resonance (FMR), textured compacts.

I. INTRODUCTION

BARIUM ferrite doped by scandium [1] has great poten-tial as a self-biased material for nonreciprocal passive de-

vice applications below 18 GHz [2]–[4]. This frequency rangeis heavily utilized in industries for applications in radar andwireless communication technologies [5]-[6]. The goal in pro-cessing this material BaFe ScO for circulator device appli-cations is to maintain high saturation and remanent magnetiza-tions with the -axis aligned perpendicular to the plane of thecompact. In addition to these magnetic and structural properties,the materials should have a minimal microwave loss, i.e., a fer-romagnetic resonance (FMR) linewidth of 39.8 kA/m, at thetargeted frequencies. Here, we describe the processing of purephase powders, compaction within a high magnetic field, andoptimal sintering conditions to obtain highly textured compactshaving uniaxial magnetic properties. Hysteresis loop squarenessis measured to be 0.83 and the FMR derivative linewidth was79.6 kA/m at 9.5 GHz.

II. PROCESSING of BaFe ScO POWDER

We first obtained a pure phase Sc-doped barium hexaferritepowder using conventional ceramic processing techniques.Specifically, starting chemicals of BaCO , Fe O , and Sc O ,having the molar ratio of 2:11:1 (BaCO :Fe O :Sc O ),were mixed with alcohol in a mortar. After evaporation ofthe alcohol, the mixture was pressed to make a solid compactusing a uniaxial pressure of 145 10 N m . The compactwas sintered in air at 1523 K for 15 h and then pulverizedin a planetary mill using steel balls at 300 r/min for 1 h. Theball to powder mass charge ratio was 7:1. To obtain a singlephase BaFe ScO powder this procedure was repeated threetimes. The crystal structure was characterized by using theX-ray diffraction method (Philips Xpert, Cu-K radiation).The morphology of BaFe ScO was examined by directobservation via scanning electron microscope (SEM, Hitachi

Digital Object Identifier 10.1109/TMAG.2006.879639

Fig. 1. X-ray diffraction data of the BaFe ScO powder sintered at 1523 K.

S-4800 with EDX). The magnetization and coercivity weremeasured at room temperature in a maximum applied fieldof 995 kA/m using a vibrating sample magnetometer (VSM,ADE Technologies). Room-temperature FMR spectra weretaken at -band frequency using differential power absorptiontechnique operating Varian microwave bridge for microwavesource and Varian E-line console which included built-inlock-in amplifier and 100 kHz modulation signal for detectionof absorption signal.

III. FIELD COMPACTION OF POWDERS

Fig. 1 shows representative X-ray diffraction (XRD) patternfor single phase BaFe ScO . All peaks have been indexed tothe Miller indices of the magnetoplumbite crystal structure. Fur-thermore, the as-prepared single-phase powders are, next, ballmilled using agate balls to reduce the average particle size to2–3 m. The single domain size of BaFe ScO is about 1 m[7], and we have found in previous studies [8] that particles

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3354 IEEE TRANSACTIONS ON MAGNETICS, VOL. 42, NO. 10, OCTOBER 2006

Fig. 2. SEM micrograph of BaFe ScO powder after 10 h of ball millingusing agate balls.

having diameters in the range 2–3 m allow for effective crys-tallographic orientation in an applied magnetic field. In this pro-cedure a variety of agate balls, having diameters ranging from5 to 12 mm, were used. The ball to powder mass ratio was 7:1.Alcohol was used in the milling operation to moderate the tem-perature of the powder. The agate ball mill was operated at aspeed of 400 r\min for a period of 10 h. Every 30 min the rota-tion of the mill was reversed to ensure a homogenous distribu-tion of the powder. Fig. 2 is a scanning electron micrograph ofthe powder after ball milling for 10 h. As seen in Fig. 2, the sizeof the particles varied between 0.5 and 3 m. It is common forball milled powders to exhibit a wide range of particle sizes.

The 10-h milled powder was mixed with alcohol and ultra-sonically dispersed to minimize the agglomeration of the par-ticles. The paste consisting of alcohol and particles was thenintroduced to a die set in preparation of field alignment andcompaction.

In the field compaction operation, the plunger and seat aremade of an FeCo alloy that allows for the focusing of the mag-netic field through the sample. The die is wrapped with coppercoils that allow a field of 955 kA/m during compaction. A uni-axial pressure of 145 10 N m was applied during field com-paction. The oriented specimen was then annealed at a range ofsintering temperatures from 1000 to 1473 K and at soak timesof 1–5 h. For these studies the heating rate was 673.15 K/h andsamples were allowed to cool in air.

IV. STRUCTURE OF OPTIMALLY SINTERED COMPACTS

In the pursuit of highly textured compacts, the platelet par-ticles shown in Fig. 2 must align with respect to the appliedmagnetic field direction. Specifically, the -axis (i.e., the mag-netic easy axis) of the platelet particles must align parallel to

and orthogonal to the compact surface. Such an alignmentof particles has the advantage of providing high hysteresis loopsquareness needed for self-biased circulator applications. Fig. 3is a scanning electron micrograph of the surface of a compactafter sintering at 1373 K for 2 h. This image shows a denselypacked arrangement of particles having hexagonal shapes with

Fig. 3. SEM image of the surface of an aligned BaFe ScO compact aftersintering 1100 C for 2 h.

TABLE IMAGNETIC PROPERTIES FOR SAMPLES PREPARED UNDER

VARIOUS CONDITIONS

the basal plane of crystals aligned parallel to the plane of thecompact surface. This is reminiscent of Ba-hexaferrite samplesprepared under similar conditions. [9]

We believe that the small particles, 1 m seen in Fig. 2,play a valuable role in the further processing of the compact inprohibiting the growth of platelet particles and improving thedensity of the finished compact.

V. MAGNETIC PROPERTIES OF BaFe ScO

The magnetic properties of the oriented specimens sinteredat various temperatures and hours are shown in Table I. As canbe seen in Table I, the maximum hysteresis loop squareness,necessary for self-biased material applications, is obtained forthe sample sintered at 1473 K for 2 h. For these conditions, thecoercivity and squareness values are 0.83 and 186.8 kA/m, re-spectively. Fig. 4 is a plot of hysteresis loops collected for themagnetic field applied in-plane and out of the plane of the com-pact. These data illustrate a clear uniaxial anisotropy with theeasy magnetic axis aligned out of the plane. The high hysteresisloop squareness value of 0.83 provides a high remanence mag-netization and may be suitable for self-biased applications.

In-plane FMR measurements with an applied magnetic fieldof 620.7 kA/m, indicate that for the optimally prepared samplethe resonant frequency was 9.53 GHz. The FMR derivative

SAKAI et al.: TEXTURED Sc-DOPED BARIUM FERRITE COMPACTS FOR MICROWAVE APPLICATIONS BELOW 20 GHz 3355

Fig. 4. Magnetic hysteresis loops acquired with the applied magnetic fieldaligned parallel (dashed) and perpendicular to the sample plane (solid). Thesample was sintered at 1473 K for 2 h.

power linewidth , was measured to be 79.6 kA/m. Althoughthis value is relatively high in comparison to the samples pre-pared by pulsed laser deposition and liquid phase epitaxy[10], however, it is low in comparison to the polycrystalline(multidomain) compacts. Further work is in progress to achievethe FMR linewidth of about 16 kA/m.

VI. CONCLUSION

Sc-doped Ba-hexaferrite powders were synthesized by a con-ventional ceramic method and subjected to field compaction andsintering. The resulting samples had a hysteresis loop square-ness of 0.83, a value that is favorable for self-biased applicationsat frequencies below 20 GHz. The high remanence arises from

well-oriented platelet particles, while the high coercivity arisesfrom small particles, copious grain boundaries, and porosity.

-band FMR measurements show a derivative power linewidthof 79.6 kA/m.

ACKNOWLEDGMENT

This work was supported by the Defense Advanced ResearchProjects Agency under Grant HR0011-05-1-0011 and by the Of-fice of Naval Research under Grant N00014-05-10349.

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Manuscript received March 13, 2006 (e-mail: sakai.tomokazu@gmail.com).