IEEE MAGNETICS LETTERS, Volume 1 (2010) 4500204

Information Storage

Conversion of Worm-Shaped Antiferromagnetic Hematite to FerrimagneticSpherical Barium-Ferrite Nanoparticles for Particulate Recording Media

Jeevan Jalli1∗, Yang-Ki Hong1, Jae-Jin Lee1∗, Gavin S. Abo1∗, Ji-Hoon Park1∗, Alan M. Lane2,Seong-Gon Kim3, and Steven C. Erwin4

1Department of Electrical and Computer Engineering and Center for Materials for Information Technology, The University of Alabama,Tuscaloosa, AL 35487 USA2Department of Chemical and Biological Engineering and Center for Materials for Information Technology, The University of Alabama,Tuscaloosa, AL 35487 USA3Department of Physics and Astronomy, Mississippi State University, Mississippi State, MS 39762 USA4Center for Computational Materials Science, Naval Research Laboratory, Washington, DC 20375 USA∗Student Member, IEEE

Received August 30, 2010, revised September 25, 2010, accepted October 4, 2010, published November 17, 2010.

Abstract—We used a modified, two-step hydrothermal process to produce worm-shaped antiferromagnetic hematite-barium iron oxide particles and converted them to 25–45 nm spherical ferrimagnetic barium-ferrite (S-BaFe) nanoparticlesfor high-density magnetic recording media application. Saturation magnetization and coercivity of the S-BaFe nanoparticleswere 50.7 emu/g and 4311 Oe, respectively. The thermal stability of KuV/kBT ≈ 81 was estimated for the S-BaFenanoparticles from time-dependent remanent coercivity measurements.

Index Terms—Information storage, barium ferrite, dynamic remanent coercivity, particulate recording media, thermal stability.

I. INTRODUCTION

Hexagonal barium-ferrite (H-BaFe) platelet particles arewidely used in particulate recording media due to their excellentmagnetic properties, such as high uniaxial magnetic anisotropy,high Curie temperature, large saturation magnetization, highcoercivity, and excellent chemical stability [Sharrock 2000]. Re-cently, it has been shown that it is feasible to realize 29.5 and15.0 Gb/in2 recording density employing fine barium-ferrite par-ticles [Cherubini 2010, Matsumoto 2010]. Also, Nagata et al.utilized extremely small 21 nm H-BaFe platelet particles for anadvanced particulate tape having a thermal stability Ku V /kB Tof 82 with a recording density of 7 Gb/in2 [Nagata 2006], whereKu is the first-order anisotropy constant and V is the volumeof magnetic particle. Ku V and kB T correspond to the magneticenergy and the thermal energy, respectively. Berman et al. re-ported that H-BaFe recording medium with a giant magnetore-sistance head is capable of a linear density of 400 kbpi with asoft error rate of 6 × 10−5 approaching equivalent areal densityof 6.7 Gb/in2 [Berman 2007].

Although H-BaFe platelet particles are being used for high-density recording media, their greatest disadvantage is their dis-persibility and high degree of agglomeration in magnetic paint[Hong 2000]. H-BaFe platelet particles form poker-chip-likestacks or clusters due to mutual magnetic interactions, whichlimit the media recording capabilities, such as poor SNR [Shar-rock 1995, Hong 1999, 2000]. To achieve better media record-ing performance, use of nanosized spherical barium-ferrite (S-BaFe) particles have been previously proposed [Hong 1999].Some advantages of using S-BaFe include the low aspect ra-tio of 1:1 and a point-to-point contact between the particles,

Corresponding author: Y.-K. Hong (ykhong@eng.ua.edu).Digital Object Identifer: 10.1109/LMAG.2010.2087315

which enhances the dispersibility of the magnetic nanoparti-cles, and consequently increases the SNR. Previously, we usedthe adsorption–diffusion process to convert spherical magnetite(S-Mag) precursor nanoparticles (12 nm) to 24–30 nm S-BaFeparticles that involves spherical-to-spherical shape transforma-tion [Gee 2005, Jalli 2009]. However, the adsorption–diffusionprocess involves several processing steps as well as heattreatment temperatures higher than 800 ◦C, which causes theS-BaFe particles to agglomerate. In this paper, we report an al-ternative hydrothermal process to synthesize S-BaFe nanopar-ticles. In this process, we first produce intermediate worm-shaped antiferromagnetic hematite particles (60 nm long and 20nm wide) containing a small amount of spinel barium iron oxidethat are, in turn, converted to ultrafine, ferrimagnetic S-BaFenanoparticles (25–45 nm in diameter). The magnetic proper-ties and thermal stability of resulting S-BaFe nanoparticles aresuitable for future high-density magnetic recording media appli-cations.

II. EXPERIMENT

At the beginning of the hydrothermal process, the followingmixture was autoclaved at 180 ◦C for 5–12 h and cooled to theroom temperature: Ba(NO3)2 and Fe(NO3)3·9(H2O) dissolved in60 mL distilled water, and sodium oleate (Fe/sodium oleate =0.15) dissolved in 60 mL ethanol and 10 mL oleic acid. After theautoclaving step is completed, the brownish red precipitates atthe bottom of the Teflon liner of the autoclave were collected andwashed with a combination of ethanol and hexane. The result-ing precipitates were dried in an oven for 8–10 h. The dried fineparticles comprising hematite (α-Fe2O3) coated with barium ironoxide (BaFe2O4) were subjected to heat treatment at varioustemperatures to be converted to ultrafine S-BaFe particles. Thecollected precursor and S-BaFe particles were characterizedby X-ray powder diffraction (XRD) to identify crystalline phases.

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4500204 IEEE MAGNETICS LETTERS, Volume 1 (2010)

Fig. 1. Magnetic properties of as-collected oven-dried nanoparticles.

Fig. 2. XRD pattern of the as-collected oven-dried nanoparticles.

Fig. 3. TEM micrographs of as-collected oven-dried nanoparticles.

Transmission electron microscopy (TEM) analyses were per-formed to assess the particle morphology and crystallinity. Thespecific saturation magnetization, coercivity, and dynamic re-manent coercivity of the S-BaFe nanoparticles were measuredat room temperature by vibrating sample magnetometry (VSM)at a maximum applied field of 10 kOe.

Fig. 4. XRD patterns of S-BaFe as a function of various heat-treatmenttemperatures. (a) 680 ◦C. (b) 700 ◦C. (c) 750 ◦C. (d) 800 ◦C for 2 h.

III. RESULTS AND DISCUSSION

Fig. 1 shows the hysteresis loop of the oven-dried as-collected precursor particles obtained from the autoclave, whichshows typical hematite-like magnetic properties. The satura-tion magnetization of the as-collected particles is 0.78 emu/gat 10 kOe, and the coercivity is 250 Oe. Fig. 2 shows the XRDpattern of the as-collected precursor particles. The pattern con-firms that the precursor particles are composed of hematite(α-Fe2O3) and spinel monoferrite (BaFe2O4). It is understoodthat during the hydrothermal process, iron nitrate reacted withthe barium nitrate to form hematite and monoferrite. The mor-phology of the as-collected precursor particles observed byTEM shows a worm-like particle shape with a length of 60 nmand width of 20 nm, as shown in Fig. 3. There was no signifi-cant change in the shape and size of the particles with longerreaction time. To convert the precursor worm-shaped parti-cles (α-Fe2O3/BaFe2O4) to S-BaFe nanoparticles, the precur-sor particles were heat-treated at various temperatures. Duringthe process of heat-treatment, BaFe2O4 reacted with hematite(α-Fe2O3) to form barium-ferrite (BaFe12O19). Steier et al.showed that this process occurs when barium ions diffuse intothe hematite to form barium ferrite at elevated temperatures[Steier 1999] through the reaction:

BaFe2O4 + 5Fe2 O3 → BaO·6Fe2O3.

Fig. 4 shows the X-ray diffraction patterns of the heat-treatedparticles from 680 ◦C to 800 ◦C for 2 h in air. The intensitiesof X-ray peaks corresponding to the phases of α-Fe2O3 andBaFe2O4 weaken as the temperature increases from 680 ◦Cto 800 ◦C. Barium-ferrite phase with an insignificant amount ofhematite phase was observed from S-BaFe nanoparticles heat-treated at 800 ◦C, as shown in Fig. 4(d). TEM micrographs showthat the shapes of the S-BaFe particles are spherical, and thesize ranged from 25 to 45 nm (see Fig. 5). The magnetic hys-teresis loop of S-BaFe nanoparticles annealed at 800 ◦C for 2 his shown in Fig. 6. Saturation magnetization (σ s) of 50.7 emu/g

IEEE MAGNETICS LETTERS, Volume 1 (2010) 4500204

Fig. 5. TEM micrographs of S-BaFe nanoparticles heat-treated at800 ◦C for 2 h.

Fig. 6. Hysteresis loop of S-BaFe nanoparticles heat-treated at 800 ◦Cfor 2 h.

and a coercivity (Hc) of 4311 Oe at a maximum applied field of 10kOe were obtained. The hysteresis loop resembles the one pro-posed by Stoner et al. for an assembly of noninteracting, singlemagnetic domains, and uniaxial particles having their axes ran-domly oriented [Stoner 1948]. Therefore, we can conclude thatour S-BaFe nanoparticles are well separated, hold their single-domain structure and are not interacting with each other. Thisconfirms again the advantage of our previously proposed S-BaFe [Hong 1999] over the H-BaFe platelets, which have beenused for extremely high-density recording tape. The shape-transformation mechanism, from the worm shape of antiferro-magnetic particles to spherical shape of ferrimagnetic barium-ferrite nanoparticles, is not yet understood. The saturation mag-netization of σ s (50.7 emu/g) is much lower than the theoreticalvalue of 72 emu/g [Smit 1959] for bulk single-crystal bariumferrite. This is attributed to the smaller size of the particles com-pared to the magnetic single domain size of 0.1 μm [Goto 1980]and low degree of crystallinity due to the lower heat-treatmenttemperature. This low-heat-treatment temperature led to the in-complete crystallization of barium-ferrite, leaving behind a smallamount of hematite phase in S-BaFe powder as confirmed bythe XRD pattern in Fig. 4. Moreover, it has been proposed thatthe relatively low magnetization in fine particles arises due tothe surface effects such as spin canting and sample inhomo-geneity [Shafi 1997]. The effect of annealing temperature on themagnetic properties of S-BaFe particles is presented in Fig. 7.

Fig. 7. Magnetic properties as a function of heat-treatment tempera-tures of S-BaFe nanoparticles.

The saturation magnetization enhancement is directly relatedto the higher amount of BaFe12O19 crystallinity as the tempera-ture was increased, which is in good agreement with the XRDpatterns. The magnetic properties of the samples heat-treatedat 680 ◦C, 700 ◦C, 750 ◦C, and 800 ◦C are 39.6, 42.4, 44.1, and50.7 emu/g, respectively, and the coercivity ranges from 3600to 4300 Oe.

In order to evaluate archival stability of information data stor-age media, the thermal stability factor Ku V /kB T for the syn-thesized S-BaFe nanoparticles was determined by performingdynamic remanent coercivity measurements with VSM. DC-demagnetization measurement (DCD) at different time scalesfor an applied field were performed on S-BaFe particles. Due tothermal relaxation of the particles, remanent coercivity (Hc) de-creases with an increase in the duration time of the appliedfield. The thermal stability factor Ku V /kB T can be obtainedfrom Sharrock’s formula for 3-D randomly oriented particles as[Sharrock 1999]

Hc(t) = Ho

[1 −

[kB T

Ku Vln

(At

ln2

)]n](1)

where Ho is the intrinsic coercive field for a random distribu-tion of noninteracting particles with uniaxial anisotropy constantKu and V is the magnetic activation volume. The exponent ndepends on the model of the energy barrier. In this case, n =2/3 is used to account for the 3-D random orientation of theparticles. The attempt frequency A is taken to be 109 s−1. kB isthe Boltzmann constant and T is the absolute temperature. t isthe time needed for a constant field of magnitude Hc to reducethe magnetization from remanent saturation to zero. The valuesof Ho and Ku V /kB T are obtained by fitting the Hc and t valuesto the aforementioned equation, as shown in Fig. 8. From thisanalysis, a stability factor of

Ku V

kB T≈ 81 (2)

is estimated for the S-BaFe nanoparticles, which is highenough to maintain archival property of data storage media. An

4500204 IEEE MAGNETICS LETTERS, Volume 1 (2010)

Fig. 8. Remanent coercivity as a function of time measured by VSMand fitted curve using Sharrock’s equation [Sharrock 1999].

activation volume (V ) of 1961 nm3 is obtained from (2) using Ku

= Hk Ms /2x , where Hk = 7500 Oe and Ms = 228 emu/cm3 (den-sity of barium ferrite = 4.5 g/cm3) at room temperature and x =0.5 [Sharrock 1999]. On the other hand, for 21-nm (physical vol-ume = 2200 nm3) H-BaFe particles, a V of 4559 nm3 is expectedfrom Ku = 7.9 × 105 erg/cm3 and Ku V /kB T = 87 at room tem-perature [Matsumoto 2010]. The difference in volume for the S-BaFe particles can be understood by two reasons. First, our pre-liminary results show that the S-BaFe particle may actually bean H-BaFe particle surrounded by a nonmagnetic Fex Oy shell[Abo 2010], which causes the magnetic volume much smallerthan the geometric volume. Second, it has been previously re-ported that an increase in activation volume is expected if themutual particle-to-particle interaction is greater in the medium[Song 1993]. In our case, due to the spherical nature of the par-ticles, the mutual particle-to-particle interaction is much weaker;therefore, a lower V is expected. This was previously confirmedin our work, which showed a weak particle-to-particle interactioncompared to H-BaFe nanoparticles [Jalli 2009]. Understandingthe mechanisms for the phase and crystallographic transforma-tion from α-Fe2O3 to S-BaFe will give us more information toevaluate these particles; this is currently under investigation.

IV. CONCLUSION

A two-step nanomanufacturing process was developed tosynthesize 25–45 nm S-BaFe particles for high-density mag-netic recording media applications. Hydrothermally processedworm-like fine antiferromagnetic hematite-barium spinel par-ticles were converted to 25–45 nm ferrimagnetic S-BaFenanoparticles by annealing at 800 ◦C for 2 h. The thermal sta-bility (Ku V /kB T ) of the S-BaFe particles was 81. These S-BaFenanoparticles are applicable to high-density particulate record-ing media.

ACKNOWLEDGMENT

This work was supported by Information Storage IndustryConsortium and partly supported by ONR. The authors would

like to thank M. P. Sharrock of Imation for his encouragementand helpful discussions. They would also like to thank S. Baeof MINT Center, University of Alabama.

REFERENCES

Abo G S, Hong Y K, Jalli J, Bae S, Lee J, Park J, Neveu N, Kim S, Sur J(2010), “Modeling of hysteresis loops for spherical barium ferrite (S-BaFe) particles with uniaxial anisotropy,” presented at the 11th JointMMM-Intermag conference, Washington, DC, Jan. 18–22, paper no.DP-06.

Berman D, Biskeborn R, Bui N, Childers E, Cideciyan RD, Dyer W, Eleft-heriou E, Hellman D, Hutchins R, Imaino W, Jaquette G, Jelitto J, Ju-bert P-O, Lo C, McClelland G, Narayan S, Oelcer S, Topuria T, Hara-sawa T, Hashimoto A, Nagata T, Ohtsu H, Saito S (2007), “6.7 Gb/in2

recording areal density on barium ferrite tape,” IEEE Trans. Magn.,vol. 43, no. 8, pp. 3502–3508, doi: 10.1109/TMAG.2007.899634.

Cherubini G, Cideciyan RD, Dellmann L, Eleftheriou E, Haeberle W,Jelitto J, Kartik V, Lantz MA, Oelcer S, Pantazi A, Berman D, ImainoW, Jubert P-O, McClelland G, Koeppe P, Tsuruta K, Harasawa T,Murata Y, Musha A, Noguchi H, Ohtsu H, Shimizu O, Suzuki R(2010), “29.5 Gb/in2 recording areal density on barium ferrite tape,”presented at the 21st magnetic recording conference TMRC—2010,La Jolla, CA, Aug. 16–18, paper no. C3.

Gee S H, Hong Y K, Jeffers F J, Park M H, Sur J C, Weatherspoon C,Nam I T (2005), “Synthesis of nano-sized spherical barium-strontiumferrite particles,” IEEE Trans. Magn., vol. 41, no. 11, pp. 4353–4355,doi: 10.1109/TMAG.2005.855248.

Goto K, Ito M, Sakurai T (1980), “Studies on magnetic domains of smallparticles of barium ferrite by colloid-SEM method,” Jpn. J. Appl.Phys., vol. 19, pp. 1339–1346, doi: 10.1143/JJAP.19.1339.

Hong Y K, Jeffers F J, Park M H (2000), “Magnetic interaction in spher-ical barium ferrite particles tape,” IEEE Trans. Magn., vol. 36, no. 5,pp. 3863–3865, doi: 10.1109/20.908404.

Hong Y K, Jung H S (1999), “New barium ferrite particles: Spher-ical shape,” J. Appl. Phys., vol. 85, pp. 5549–5551, doi:10.1063/1.369891.

Jalli J, Hong Y K, Bae S, Abo G S, Lee J J, Sur J C, Gee S H, Kim S G,Erwin S C, Moitra A (2009), “Conversion of nano-sized sphericalmagnetite to spherical barium ferrite nanoparticles for high densityparticulate recording media,” IEEE Trans. Magn., vol. 45, no. 10,pp. 3590–3593, doi: 10.1109/TMAG.2009.2022496.

Matsumoto A, Murata Y, Musha A, Matsubaguchi S, Shimizu O (2010),“High recording density tape using fine barium-ferrite particles withimproved thermal stability,” IEEE Trans. Magn., vol. 46, no. 10,pp. 1208–1211, doi: 1109/TMAG.2009.2039798.

Nagata T, Harasawa T, Oyanagi M, Abe N, Saito S (2006), “Arecording density study of advanced barium-ferrite particulatetape,” IEEE Trans. Magn., vol. 42, no. 10, pp. 2312–2314, doi:10.1109/TMAG.2006.878675.

Shafi K V P M, Koltypin Y, Gedanken A, Prozorov R, Balogh, Lendvai J,Felner I (1997), “Sonochemical preparation of nanosized amorphousNiFe2O4 particles,” J. Phys. Chem. B, vol. 101, pp. 6409–6414, doi:10.1021/jp970893q.

Sharrock M P (1999), “Measurement and interpretation of magnetictime effects in recording media,” IEEE Trans. Magn., vol. 35, no. 6,pp. 4414–4422, doi: 10.1109/20.809133.

Sharrock M P (2000), “Recent advances in metal particulate recordingmedia: Toward the ultimate particle,” IEEE Trans. Magn., vol. 36,no. 5, pp. 2420–2425, doi: 10.1109/20.908453.

Sharrock M P, Carlson L W (1995), “The application of barium ferriteparticles in advanced recording media,” IEEE Trans. Magn., vol. 31,no. 6, pp. 2871–1873, doi: 10.1109/20.490177.

Smit J, Wijn H P J (1959), “Ferrites,” Philips Techn. Library, Eindhoven.Song K, Parker M R, Harrell J W (1993), “Determination of activation

volumes in BaFe and metal particle coatings,” J. Mag. Mag. Mater.,vol. 120, pp. 180–183, doi: 10.1016/0304-8853(93)91315-X.

Steier H P, Requena J, Moya J S (1999), “Transmission electron mi-croscopy study of barium hexaferrite formation from barium carbon-ate and hematite,” J. Mater. Res., vol. 14, pp. 3647–3652, doi:10.1557/JMR.1999.0492.

Stoner E C, Wohlfarth E P (1948), “A mechanism of magnetic hysteresisin hetrogeneous alloys,” Phil. Trans. Roy. Soc., vol. A 240, pp. 599–642, doi: 10.1098/rsta.1948.0007.