• Abstract

# Conversion of Worm-Shaped Antiferromagnetic Hematite to Ferrimagnetic Spherical Barium-Ferrite Nanoparticles for Particulate Recording Media

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) nanoparticles for high-density magnetic recording media application. Saturation magnetization and coercivity of the S-BaFe nanoparticles were 50.7 emu/g and 4311 Oe, respectively. The thermal stability of KuV/k BT ≈ 81 was estimated for the S-BaFe nanoparticles from time-dependent remanent coercivity measurements.

SECTION I

## INTRODUCTION

Hexagonal barium-ferrite (H-BaFe) platelet particles are widely used in particulate recording media due to their excellent magnetic properties, such as high uniaxial magnetic anisotropy, high Curie temperature, large saturation magnetization, high coercivity, and excellent chemical stability [Sharrock2000]. Recently, it has been shown that it is feasible to realize 29.5 and15.0 Gb/in2 recording density employing fine barium-ferrite particles [Cherubini 2010, Matsumoto2010]. Also, Nagata et al. utilized extremely small 21 nm H-BaFe platelet particles for an advanced particulate tape having a thermal stability Ku V/kBT of 82 with a recording density of 7 Gb/in2 [Nagata 2006], where Ku is the first-order anisotropy constant and V is the volume of magnetic particle. Ku V and kBT correspond to the magnetic energy and the thermal energy, respectively. Berman et al. reported that H-BaFe recording medium with a giant magnetoresistance head is capable of a linear density of 400kbpi with a soft error rate of 6 × 10−5 approaching equivalent areal density of 6.7Gb/in2 [Berman 2007].

Although H-BaFe platelet particles are being used for high-density recording media, their greatest disadvantage is their dispersibility and high degree of agglomeration in magnetic paint [Hong 2000]. H-BaFe platelet particles form poker-chip-like stacks or clusters due to mutual magnetic interactions, which limit the media recording capabilities, such as poor SNR [Sharrock1995, Hong 1999, 2000]. To achieve better media recording 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 ratio of 1:1 and a point-to-point contact between the particles, which enhances the dispersibility of the magnetic nanoparticles, and consequently increases the SNR. Previously, we used the adsorption–diffusion process to convert spherical magnetite (S-Mag) precursor nanoparticles (12 nm) to 24–30nm S-BaFe particles that involves spherical-to-spherical shape transformation[Gee 2005, Jalli 2009]. However, the adsorption–diffusion process involves several processing steps as well as heat treatment temperatures higher than800 °C, which causes the S-BaFe particles to agglomerate. In this paper, we report an alternative hydrothermal process to synthesize S-BaFe nanoparticles. In this process, we first produce intermediate worm-shaped antiferromagnetic hematite particles (60 nm long and 20 nm wide) containing a small amount of spinel barium iron oxide that are, in turn, converted to ultrafine, ferrimagnetic S-BaFe nanoparticles (25–45 nm in diameter). The magnetic properties and thermal stability of resulting S-BaFe nanoparticles are suitable for future high-density magnetic recording media applications.

SECTION II

## EXPERIMENT

At the beginning of the hydrothermal process, the following mixture was autoclaved at 180 °C for 5–12 h and cooled to the room temperature: Ba(NO3) 2 and Fe(NO3)3·9(H2O) dissolved in 60 mL distilled water, and sodium oleate (Fe/sodium oleate = 0.15) dissolved in 60 mL ethanol and 10 mL oleic acid. After the autoclaving step is completed, the brownish red precipitates at the bottom of the Teflon liner of the autoclave were collected and washed with a combination of ethanol and hexane. The resulting precipitates were dried in an oven for 8–10 h. The dried fine particles comprising hematite (α-Fe 2O3) coated with barium iron oxide (BaFe2O4) were subjected to heat treatment at various temperatures to be converted to ultrafine S-BaFe particles. The collected precursor and S-BaFe particles were characterized by X-ray powder diffraction (XRD) to identify crystalline phases. Transmission electron microscopy (TEM) analyses were performed to assess the particle morphology and crystallinity. The specific saturation magnetization, coercivity, and dynamic remanent coercivity of the S-BaFe nanoparticles were measured at room temperature by vibrating sample magnetometry (VSM) at a maximum applied field of 10 kOe.

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.
Fig. 4. XRD patterns of S-BaFe as a function of various heat-treatment temperatures.(a) 680 °C. (b) 700 °C. (c) 750 °C. (d)800 °C for 2 h.
SECTION III

## RESULTS AND DISCUSSION

Fig. 1 shows the hysteresis loop of the oven-dried as-collected precursor particles obtained from the autoclave, which shows typical hematite-like magnetic properties. The saturation magnetization of the as-collected particles is 0.78 emu/g at 10 kOe, and the coercivity is 250 Oe. Fig. 2 shows the XRD pattern of the as-collected precursor particles. The pattern confirms that the precursor particles are composed of hematite (α-Fe2O3) and spinel monoferrite (BaFe 2O4). It is understood that during the hydrothermal process, iron nitrate reacted with the barium nitrate to form hematite and monoferrite. The morphology of the as-collected precursor particles observed by TEM shows a worm-like particle shape with a length of 60 nm and width of 20 nm, as shown in Fig. 3. There was no significant change in the shape and size of the particles with longer reaction time. To convert the precursor worm-shaped particles (α-Fe2O3/BaFe2O4) to S-BaFe nanoparticles, the precursor particles were heat-treated at various temperatures. During the process of heat-treatment, BaFe2O 4 reacted with hematite (α-Fe2O3)to form barium-ferrite (BaFe 12O19). Steier et al. showed that this process occurs when barium ions diffuse into the hematite to form barium ferrite at elevated temperatures [Steier 1999] through the reaction: TeX Source $${\rm BaFe}_2 {\rm O}_4 + 5{\rm Fe}_2 \,{\rm O}_3 \to {\rm BaO} {\cdot} 6{\rm Fe}_2 {\rm O}_3.$$

Fig. 5. TEM micrographs of S-BaFe nanoparticles heat-treated at 800 °C for2 h.
Fig. 6. Hysteresis loop of S-BaFe nanoparticles heat-treated at 800 °C for 2 h.

Fig. 4 shows the X-ray diffraction patterns of the heat-treated particles from 680 °C to 800 °C for 2 h in air. The intensities of X-ray peaks corresponding to the phases of α-Fe2O3 and BaFe2O4 weaken as the temperature increases from 680 °C to 800 °C. Barium-ferrite phase with an insignificant amount of hematite phase was observed from S-BaFe nanoparticles heat-treated at 800 °C, as shown in Fig. 4(d). TEM micrographs show that the shapes of the S-BaFe particles are spherical, and the size ranged from 25 to 45 nm (see Fig. 5). The magnetic hysteresis loop of S-BaFe nanoparticles annealed at 800 °C for 2 h is shown in Fig. 6. Saturation magnetization (σs) of 50.7 emu/g and a coercivity (Hc) of 4311 Oe at a maximum applied field of 10 kOe were obtained. The hysteresis loop resembles the one proposed by Stoner et al. for an assembly of noninteracting, single magnetic domains, and uniaxial particles having their axes randomly oriented [Stoner 1948]. Therefore, we can conclude that our S-BaFe nanoparticles are well separated, hold their single-domain structure and are not interacting with each other. This confirms again the advantage of our previously proposed S-BaFe [Hong1999] over the H-BaFe platelets, which have been used for extremely high-density recording tape. The shape-transformation mechanism, from the worm shape of antiferromagnetic particles to spherical shape of ferrimagnetic barium-ferrite nanoparticles, is not yet understood. The saturation magnetization of σs (50.7 emu/g) is much lower than the theoretical value of 72 emu/g [Smit 1959] for bulk single-crystal barium ferrite. This is attributed to the smaller size of the particles compared to the magnetic single domain size of 0.1 μm [Goto 1980] and low degree of crystallinity due to the lower heat-treatment temperature. This low-heat-treatment temperature led to the incomplete crystallization of barium-ferrite, leaving behind a small amount of hematite phase in S-BaFe powder as confirmed by the XRD pattern in Fig. 4. Moreover, it has been proposed that the relatively low magnetization in fine particles arises due to the surface effects such as spin canting and sample inhomogeneity [Shafi 1997]. The effect of annealing temperature on the magnetic properties of S-BaFe particles is presented in Fig. 7. The saturation magnetization enhancement is directly related to the higher amount of BaFe 12O19 crystallinity as the temperature was increased, which is in good agreement with the XRD patterns. The magnetic properties of the samples heat-treated at 680 °C, 700 °C, 750 °C, and 800 °C are 39.6, 42.4, 44.1, and 50.7 emu/g, respectively, and the coercivity ranges from 3600 to 4300 Oe.

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

In order to evaluate archival stability of information data storage media, the thermal stability factor Ku V/ kBT for the synthesized S-BaFe nanoparticles was determined by performing dynamic remanent coercivity measurements with VSM. DC-demagnetization measurement (DCD) at different time scales for an applied field were performed on S-BaFe particles. Due to thermal relaxation of the particles, remanent coercivity (Hc) decreases with an increase in the duration time of the applied field. The thermal stability factor Ku V/ kBT can be obtained from Sharrock's formula for 3-D randomly oriented particles as [Sharrock1999] TeX Source $$H_c (t) = H_o \left[{1- \left[{{{k_B T}\over {K_u V}} {\rm ln}\left({{{At}\over {{\rm ln}2}}} \right)}\right]^n } \right] \eqno{\hbox{(1)}}$$where Ho is the intrinsic coercive field for a random distribution of noninteracting particles with uniaxial anisotropy constant Ku and V is the magnetic activation volume. The exponent n depends on the model of the energy barrier. In this case, n = 2/3 is used to account for the 3-D random orientation of the particles. The attempt frequency A is taken to be 10 9 s−1. kB is the Boltzmann constant and T is the absolute temperature. t is the time needed for a constant field of magnitude Hc to reduce the magnetization from remanent saturation to zero. The values of Ho and Ku V/kBT are obtained by fitting the Hc and t values to the aforementioned equation, as shown in Fig. 8. From this analysis, a stability factor of TeX Source $${{K_u V}\over {k_B T}} \approx 81 \eqno{\hbox{(2)}}$$

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

is estimated for the S-BaFe nanoparticles, which is high enough to maintain archival property of data storage media. An activation volume(V) of 1961 nm 3 is obtained from (2) using Ku = HkMs/2x, where Hk = 7500 Oe and Ms = 228 emu/cm3 (density of barium ferrite= 4.5 g/cm3) at room temperature and x =0.5 [Sharrock 1999]. On the other hand, for 21-nm (physical volume =2200 nm3) H-BaFe particles, a V of 4559 nm3 is expected from Ku = 7.9 × 105 erg/cm3 and Ku V /kBT = 87 at room temperature [Matsumoto2010]. The difference in volume for the S-BaFe particles can be understood by two reasons. First, our preliminary results show that the S-BaFe particle may actually be an H-BaFe particle surrounded by a nonmagnetic Fe xO y shell[Abo 2010], which causes the magnetic volume much smaller than the geometric volume. Second, it has been previously reported that an increase in activation volume is expected if the mutual particle-to-particle interaction is greater in the medium [Song 1993]. In our case, due to the spherical nature of the particles, the mutual particle-to-particle interaction is much weaker; therefore, a lower V is expected. This was previously confirmed in our work, which showed a weak particle-to-particle interaction compared to H-BaFe nanoparticles [Jalli 2009]. Understanding the mechanisms for the phase and crystallographic transformation from α-Fe2O3 to S-BaFe will give us more information to evaluate these particles; this is currently under investigation.

SECTION IV

## CONCLUSION

A two-step nanomanufacturing process was developed to synthesize 25–45nm S-BaFe particles for high-density magnetic recording media applications. Hydrothermally processed worm-like fine antiferromagnetic hematite-barium spinel particles were converted to 25–45 nm ferrimagnetic S-BaFe nanoparticles by annealing at 800 °C for 2 h. The thermal stability (Ku V/kBT) of the S-BaFe particles was 81. These S-BaFe nanoparticles are applicable to high-density particulate recording media.

### Acknowledgment

This work was supported by Information Storage Industry Consortium and partly supported by ONR. The authors would like to thank M. P. Sharrock of Imation for his encouragement and helpful discussions. They would also like to thank S. Bae of MINT Center, University of Alabama.

## Footnotes

Corresponding author: Y.-K. Hong (ykhong@eng.ua.edu).

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This paper appears in:
IEEE Magnetics Letters
Issue Date:
DECEMBER
On page(s):
4500204 - 4500204
ISBN:
1949-307X
Print ISBN:
N/A
INSPEC Accession Number:
N/A
Digital Object Identifier:
10.1109/LMAG.2010.2087315
Date of Original Publication:
17 Nov, 2010

Umemoto, M.