IEEE TRANSACTIONS ON MAGNETICS, VOL. 41, NO. 11, NOVEMBER 2005 4353

Synthesis of Nano-Sized Spherical Barium–StrontiumFerrite Particles

S.-H. Gee1, Y.-K. Hong1, Member, IEEE, F. J. Jeffers2, Fellow, IEEE, M.-H. Park1, J. C. Sur3, C. Weatherspoon1,and I. T. Nam4

Department of Materials Science and Engineering, University of Idaho, Moscow, ID 83844-3024 USAAdvanced R&D Department, Iomega Corporation, Escondido, CA 92029 USA

Department of Physics, Wonkwang University, Iksan, Jurabook-do, 570-749 South KoreaDepartment of Advance Materials Engineering, Kangwon National University, Chooncheon, Kangwon-do, 200-701 South Korea

Magnetic recording media requires good particle dispersion, a smooth surface, and small interparticle interaction to make an ade-quate signal-to-noise ratio (SNR). Well dispersed 50–60 nm sized spherical barium–strontium ferrite (S-Ba/Sr-Fe) nanoparticles weresuccessfully prepared with 40 nm sized hematite precursor particles and BaCO3/SrCO3 colloid. The coercivity and saturation magne-tizations of S-Ba/Sr-Fe nano-particles were 1568 Oe and 48.6 emu/g, respectively. In order to evaluate magnetic interaction, magnetictape was prepared using an Eiger mill with binder and organic solvent.� measurement showed the S-Ba/Sr-Fe nanoparticles in thetape had negative magnetic particle-to-particle interaction.

Index Terms—Barium ferrite, magnetic interaction, nanoparticles, particulate recording media.

I. INTRODUCTION

H IGH-DENSITY particulate digital recording media is vastand critical to the information data storage industry. Par-

ticulate recording media is the most dominant and cost-effec-tive technology used for mass media and archival storage. Withan increase in areal density and a decrease in bit size, the par-ticle size must decrease to improve the recording media capa-bility and to give an adequate signal-to-noise ratio (SNR). Metalparticles (MP ; size nm) [1]–[3] have been used forparticulate recording media due to its high-saturation magneticmoment and good thermal stability. However, the metal parti-cles are intrinsically susceptible to oxidation and consequentlydegradation of saturation magnetization. MP needs a non-magnetic passivation layer to prevent the particle surface fromcorrosion, thereby limiting further reduction of MP particlesize due to its lower effective magnetic moment.

On the other hand, barium ferrite (BaFe O , magneto-plumbite structure; BaFe) is a good candidate for high-densityparticulate media because it has no passivation layer, excellentchemical stability, high-uniaxial magnetic anisotropy, narrowswitching field distribution (SFD), and weak intergranularinteraction [4]–[7]. Hexagonal platelet barium ferrite (H-BaFe)particles, however, form poker-chip-like stacks due to mu-tual magnetic interaction during the dispersion process whenmaking magnetic tape. Stacked H-BaFe particles deterioraterecording capability because of recording media noise, surfaceroughness, and poor dispersion [5]–[7]. In order to solve theseproblems, the substantially spherical shaped and the associatedlow aspect ratio (approaching 1:1) of the BaFe particles wereintroduced to prevent particle stacking as compared to H-BaFe.Spherical barium ferrite (S-BaFe) particles can form onlypoint-to-point contact; thus, the recording capability will beincreased by improving dispersion stability and decreasing the

Digital Object Identifier 10.1109/TMAG.2005.855248

degree of magnetic interaction in a magnetic paint or otherrecording media [5]–[7].

The authors have previously developed 0.3 m sized S-BaFeparticles with dopants, and the curve showed that mag-netic particle-to-particle interaction in the tape of S-BaFeparticles is weaker than in H-BaFe [5]–[9]. To our knowledge,there have been no attempts to synthesize nano-sized spher-ical barium–strontium ferrite particles (Ba Sr Fe O ;S-Ba/Sr-Fe), In this paper, we report 50–60 nm sized S-Ba/Sr-Feparticles and the magnetic properties of the particles and mag-netic tape.

II. EXPERIMENT

Colloidal 0.5 mol BaCO and 0.5 mol SrCO were syn-thesized by adding NaOH and Na CO to the Ba(NO )and Sr(NO ) solution. Well-dispersed 40 nm sized sphericalhematite precursors were synthesized by forced hydrolysis ofacid Fe [10] and put into the solution with vigorous stirringto improve dispersion.

As-produced samples were decanted and dried at 105 C for6 h in a dryer oven and heat-treated at various temperatures inthe range of 700 C–900 C for 1–36 h in air. Heat-treatmentconditions were optimized by considering both magnetic andphysical properties of S-Ba/Sr-Fe particles. An important factorin this study is the conversion of spherical hematite particles toS-Ba/Sr-Fe nanoparticles at a lower temperature while retaininggood particle shape.

Microstructure, particle size, and particle size distributionof S-Ba/Sr-Fe nanoparticles were observed by X-ray diffrac-tion pattern (XRD) and transmission electron microscope(TEM) at room temperature. Magnetic properties of nano-sizedS-Ba/Sr-Fe particles were determined by vibrating samplemagnetometer (VSM) and Mössbauer spectra. Sextets nor-mally arise from the Mössbauer transition of the nucleus Fein a magnetic field. A small distribution of magnetic fields

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4354 IEEE TRANSACTIONS ON MAGNETICS, VOL. 41, NO. 11, NOVEMBER 2005

Fig. 1. X-ray patterns of spherical barium–strontium ferrite nanoparticles asa function of various heat-treated temperatures; (a) 800 C for 2 h, (b) 735 Cfor 12 h, (c) 720 C for 24 h, and (d) 700 C for 36 h.

contributing to one sextet can be taken into account by a corre-lated widening of the Lorentzian lines. The relative intensitiesof the Lorentzian’s were calculated under the assumption of apolycrystalline specimen.

III. RESULTS AND DISCUSSION

Heat-treatment temperature was optimized by consideringmagnetic properties, dispersion, particles shape, and size ofthe spherical barium–strontium ferrite (Ba Sr Fe O ;S-Ba/Sr-Fe) nanoparticles. Fig. 1 shows X-ray diffraction pat-terns of S-Ba/Sr-Fe heat-treated at various temperatures rangingfrom 700 C to 800 C in a time period of 2 to 36 h in air. Theamount of hematite precursor phase decreased as the heat-treat-ment temperature increased. During the heat-treatment process,strontium ion (Sr ) and barium ion (Ba ) in colloidal formdiffuse into spherical hematite nanoparticles and the hematiteparticles are converted to the S-Ba/Sr-Fe nanoparticles.

The hysteresis loops of S-Ba/Sr-Fe particles, as shown inFig. 2 were taken at room temperature. The saturation magne-tizations with 10 kOe maximum applied field in 800 C-2 h,735 C-12 h, 720 C-24 h, and 700 C-36 h are 51.0, 48.7,48.6, and 41.7 emu/g, respectively. The saturation magnetiza-tion value of S-BaFe is smaller than the barium ferrite bulk valuebecause of the remaining hematite phase in the powder after theconversion process to S-Ba/Sr-Fe from hematite precursor withcolloidal BaCO and SrCO . The coercivities of S-Ba/Sr-Fenanoparticles with different heat-treatment temperatures are inthe range of 1567 to 1812 Oe. Optimal magnetic properties ofS-Ba/Sr-Fe require a relatively higher heat-treatment tempera-ture to increase the crystallinity. However, the spherical shapeof nanoparticles provides better particle dispersion and requiresa relatively lower heat-treatment temperature.

Other research groups investigated an initial crystallizationtemperature of 687 C. The particles shape was remarkablychanged to hexagonal plated shape during annealing at 800 Cand 1000 C [11], [12]. Fig. 3 shows TEM micrographs ofS-Ba/Sr-Fe nanoparticles at various heat-treatment temper-atures. Fig. 3(c) and (d) shows well-dispersed S-Ba/Sr-Fe

Fig. 2. Magnetic properties of spherical barium–strontium ferritenanoparticles as a function of various heat-treated temperatures.

Fig. 3. TEM micrographs of spherical barium–strontium ferrite nanoparticlesas a function of various heat-treated temperatures; (a) 800 C for 2 h, (b) 735 Cfor 12 h, (c) 720 C for 24 h, and (d) 700 C for 36 h.

with sizes ranging from 50 to 60 nm after being heat treatedat 720 C for 24 h and 700 C for 36 h, but agglomeratednanoparticles shapes are observed under TEM for particlesheat-treated at a temperature higher than 800 C. Such agglom-erated S-Ba/Sr-Fe particles do not disperse during the millingprocess for the preparation of magnetic paint. The nondispersedparticles can contribute to defects; pinholes, stains, and a roughsurface on the magnetic film. These defects cause media noise,collision between head and media, etc. Therefore, the particularheat-treatment temperature is chosen on the basis of targetedproperties of the particles.

Fig. 4 shows room-temperature Mössbauer spectra as afunction of heat-treatment temperature. Mössbauer spectrawere contributed by the magnetic hyperfine field from Fe infive distinct crystallographic sites in the hexa-ferrite structure.Four of these sites, which are 12 k, , 2a, and 2b, maybe identified, but the week 2a spectrum is buried under the

spectrum. The spectrum of a heated 800 C-2 h sampleshows the same properties as the bulk value spectrum; howeverthe magnetic hyperfine field (T) decreases with decreasingheat-treatment temperature. This is partially attributed to poor

GEE et al.: SYNTHESIS OF NANO-SIZED SPHERICAL BARIUM–STRONTIUM FERRITE PARTICLES 4355

Fig. 4. Mössbauer spectra of spherical barium–strontium ferrite nanoparticlesas a function of various heat-treated temperatures.

crystallinity and/or the existence of a small amount of othermagnetic phases in the powder.

In order to make magnetic paint for evaluating of the mag-netic interaction, binder (MR-110, Nippon Zeon Co., Japan) andorganic solvent (cyclohexanone), which was prepared by usingan Eiger mill, were used to make magnetic tape. It was longi-tudinally oriented at an applied field of 3000 Oe strength. TheS-Ba/Sr-Fe tape shows a negative curve, as shown in Fig. 5.Magnetic particle interaction in the tape of S-Ba/Sr-Fe particlesis weaker than in H-BaFe. The result agrees with the authors’previous reports [5]–[7]. The broad peak comes from a largecoercivity distribution in S-Ba/Sr-Fe nanoparticles due to theexistence of hematite, other magnetic phase in the particles, notperfectly oriented, and/or poor crystallinity.

IV. CONCLUSION

Well-dispersed spherical barium–strontium ferrite (BaSr Fe O ; S-Ba/Sr-Fe) nanoparticles were successfullysynthesized using 40 nm sized hematite precursor particleswith BaCO /SrCO colloid. After heat-treatment at 720 Cfor 24 h, the average particle size was in the range of 50 and60 nm. The coercivity, saturation magnetization, and remanentmagnetic moment of S-Ba/Sr-Fe are 1568 Oe, 48.6 emu/g, and28.9 emu/g, respectively. The S-Ba/Sr-Fe tape had negativemagnetic particle-to-particle interaction. It is applicable tohigh-density particulate recording media. However, it hada broad curve due to the existence of hematite, othermagnetic phase in the powder, and/or poor crystallinity. Further

Fig. 5. �M curve of spherical barium–strontium ferrite nanoparticlesheat-treated at 720 C for 24 h.

detailed study of the narrow of S-Ba/Sr-Fe tape is inprogress.

ACKNOWLEDGMENT

This work was supported by Iomega Corporation. Thispaper was presented at the International Magnetics Conference(Intermag), Nagoya, Japan, April 4–8, 2005. See IEEE Trans.Magn., vol. 41, October 2005.

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Manuscript received February 7, 2005.