Sun Ieee Composition Ally Controlled Fept Nano Particle Materials

IEEE TRANSACTIONS ON MAGNETICS, VOL. 37, NO. 4, JULY 2001

1239

Compositionally Controlled FePt Nanoparticle Materials Shouheng Sun, Eric E. Fullerton, Dieter Weller, and C. B. Murray

Abstract—High temperature solution phase decomposition of Fe(CO)5 and reduction of Pt(acac)2 in the presence of stabilizers oleic acid and oleyl amine are employed to produce 4 nm diameter FePt nanoparticles. The Fe and Pt composition of the nanoparticle materials can be tuned by adjusting the molar ratio of Fe(CO)5 to Pt(acac)2 , and the compositions ranging from Fe30 Pt70 to Fe80 Pt20 are obtained. The nanoparticle materials are easily dispersed into alkane solvent, facilitating their self-organization into nanoparticle superlattices. As synthesized FePt nanoparticles possess disordered fcc structure and show superparamagnetic behavior. Thermal annealing induces the change of internal particle structure and thus the magnetic properties of the particles. Composition dependent structure analysis shows that an annealed FePt nanoparticle assembly with a composition around Fe55 Pt45 will lead to the highly ordered fct phase. This Fe55 Pt45 nanoparticle assembly yields high coercivity, and will be a candidate for future ultra-high density magnetic recording media applications. Index Terms—FePt nanoparticle, high u FePt, magnetic recording, nanoparticle synthesis, self-assembly, thermal annealing.

I. INTRODUCTION

T

HE REQUIREMENTS for ultra-high density magnetic recording have driven the development of new magnetic thin film media with smaller grains, higher coercivity, low magnetization and minimal magnetic exchange coupling between neighboring grains. The FePt-based nanostructured materials are excellent candidates for this approach because of their good chemical stability and high magnetocrystalline anisotropy erg cm observed in the ordered intermetallic phase [1], [2]. This large crystalline anisotropy allows for thermally stable grain diameters down to 2.8 nm. A well-organized magnetic array of such particles will contribute to an effort to design a magnetic medium capable of recording densities beyond 1 Tb/in [3]. The challenges to realize this Tb/in goal are to make uniform high-coercivity FePt nanoparticles and nanoparticle assemblies with controlled assembly thickness, surface roughness, and mechanical robustness. Various vacuum deposition techniques have been developed to fabricate high coercive FePt nanocrystalline films [1], [2], [4]–[6]. Post deposition Manuscript received October 13, 2000. S. Sun and C. B. Murray are with the IBM T. J. Watson Research Center, Yorktown Heights, NY 10598 USA (e-mail: [email protected]). E. E. Fullerton is with the IBM Almaden Research Center, San Jose, CA 95120 USA (e-mail: eef@almaden. ibm.com). D. Weller was with the IBM Almaden Research Center, San Jose, CA 95120 USA. He is now with Seagate Research, Pittsburgh, PA 15203 USA (e-mail: [email protected]). Publisher Item Identifier S 0018-9464(01)06007-1.

annealing has proven essential to transform the as-deposited chemically disordered fcc structure into the chemically ordered fct phase that has high magnetocrystalline anisotropy. Random nucleation in the initial stages of the growth, however, typically results in broad distributions of particle sizes, which may be further aggravated by agglomeration during annealing. Research efforts on preventing this uncontrolled agglomeration have recently been focused on embedding the FePt particles [7] [8], into a variety of insulator matrixes, such as [10] and [11]. We recently reported that solu[9], tion phase chemistry could be used to prepare monodispersed FePt nanoparticle materials [12]. The procedure involved and polyol reduction of decomposition of in solution with oleic acid and oleyl amine. Controlled evaporation of the solvent from dispersed particles followed by thermal treatment led to self-organized ferromagnetic nanocrystalline assemblies that can support high-density magnetic recording. In this paper, we report on the generalized synthesis and characterization of 4 nm FePt nanoparticle materials. The Fe and Pt compositions are easily tuned by controlling the molar ratio and . As-synthesized FePt nanoparticles of have the disordered fcc structure and are superparamagnetic at room temperature. Thermal annealing induces the change of internal particle structure and thus the magnetic properties of the particles. Both structure and magnetic properties of the FePt nanoparticle materials are very sensitive to the particle composition. Composition dependent structural and magnetic is the optimum composition for FePt data reveal that nanoparticle materials with high quality fct phase formation and . high coercivity II. EXPERIMENTAL PROCEDURES A. Synthesis of FePt Nanoparticles The synthetic experiments were carried out using standard airless procedures and commercially available reagents. The compositionally controlled synthesis is as follows: platinum acetylacetonate (197 mg, 0.5 mmol), 1,2-hexadecanediol (390 mg, 1.5 mmol), and dioctylether (20 mL) were mixed and heated to 100 C. Oleic acid (0.16 mL, 0.5 mmol), oleyl (see Fig. 1 for the amine (0.17 mL, 0.5 mmol) and amount added to control the composition of the final product) were added and the mixture was heated to reflux (298 C) for 30 minutes. The heat source was then removed and the reaction mixture was allowed to cool to room temperature. The inert gas protected system could then be opened to ambient environment. The black product was precipitated by adding mL and separated by centrifugation. The ethanol

0018–9464/01$10.00 © 2001 IEEE

1240

IEEE TRANSACTIONS ON MAGNETICS, VOL. 37, NO. 4, JULY 2001

assembly were performed on a Hitachi HF-2000 TEM equipped with the parallel-detection electron energy-loss spectroscopy (PEELS) and energy dispersive x-ray spectroscopy (EDS) [13]. Particle size was determined using a Philips EM 430 TEM (300 KV). A thin FePt nanoparticle assembly was made by spreading a dilute particle dispersion onto a grid. Leo 1560 HRSEM was used to study the annealed FePt nanoparticle assemblies on naturally oxidized silicon substrates. The nanoparticle structure was determined by X-ray powder diffracradiation tion on a Siemens D-500 diffractometer under Co . The sample was m thick on a micro cm . slide substrate Magnetic studies were carried out using a MPMS2 Quantum Design SQUID magnetometer with fields up to 5 T and temperatures from 5 K to 350 K. Measurements were done on thick nm on a micro slide or MgO nanoparticle assemblies (100). The substrate was loaded parallel to the magnetic field. III. RESULTS AND DISCUSSION Fig. 1. Compositional relation between Fe(CO) and x in Fe Pt based on 0.5 mmol of Pt(acac) .

yellow-brown supernatant was discarded. The black precipitate mL in the presence of oleic was dispersed in hexane mL and oleyl amine mL . Then, acid mL was added to the dispersion. The resulting ethanol dispersion was centrifuged to remove any remaining unsolved precipitation. The product was precipitated out by adding more mL to the dispersion and centrifuging. Further ethanol purification of the product was performed by dispersing the product into hexane, precipitating it out with ethanol, and centrifuging. The materials were re-dispersed in hexane, and . stored under B. Synthesis of Nanoparticle Assemblies To form a self-organized assembly, 0.05 mL of hexane dismg/ml were deposited on persed FePt nanoparticles cm . The solvent was allowed a solid substrate to evaporate slowly (5–10 minutes) at room temperature. The glove box as-deposited thin film was then transferred into a concentration ppm) and annealed in a Thermolyne ( 1300 furnace. The temperature was raised to 580 C from room temperature over a period of 14 minutes and maintained at this temperature for 30 minutes. The sample was taken out of the box. The furnace and cooled to room temperature in the cooled FePt nanoparticle assembly is smooth, reflective, and can be handled and stored under ambient condition. C. Characterization of FePt Nanoparticles and Nanoparticle Assemblies Fe and Pt elemental analyses of the as-synthesized FePt nanoparticle materials were performed on an ICP-Atomic Emission Spectrometer at Galbreith Lab, Tennessee. Composition and thickness of the FePt nanoparticle assembly were determined by Rutherford backscattering spectrometry. Microanalyzes of both single FePt nanoparticle and FePt nanoparticle

A. Synthesis and Stabilization of the FePt Nanoparticles Solution phase metal salt reduction and organometallic precursor decomposition are well-known techniques for the preparation of monodisperse magnetic metal particles [14]–[17]. However, the technique cannot easily be applied to synthesize binary metal nanoparticles. The reduction or decomposition of two metal-containing precursors often does not proceed in a way that facilitates the formation of binary metal nuclei. In most cases, the product contains two phase-separated elemental nanoparticles. For example, the and reduction of in decomposition of the same reaction condition as that in the FePt synthesis did not yield CoPt nanoparticle materials. Instead, Co particle aggregation and very small separate Pt particles are formed. Therefore, forming the binary nuclei in solution phase is key for the synthesis of binary metal nanoparticles. It is well known that chemical modification of metal precursor can either change its reduction potential or thermal stability. To make binary metal nanoparticles, care should be taken in choosing the stabilizer, solvent, reducing agent, and reaction temperature. Polymers have been widely used to control particle growth, stabilize metal dispersions and limit oxidation of the particles. However, in our reaction, they would not yield monodispersed nanoparticles. This may be because a polymer usually contains multiple uneven coordination sites within its chain; these uneven coordination sites would lead to nonhomogeneous nucleation. Oleic acid and oleyl amine are proven to be a good ligand combination for FePt formation and stabilization. Oleic acid has long been used to stabilize varieties of colloids including Fe nanoparticles [14]. Alkyl amines, on the other hand, are good stabilizing ligand for noble metal such as Pt. The structural similarity between oleic acid and oleyl amine provides a smooth ligand shell around each FePt nanoparticle, facilitating superlattice formation. The FePt nanoparticles are prepared by the combination of polyol reduction of and thermal decomposition of in the presence of oleic acid and oleyl amine and can be easily dispersed into alkane solvent [12].

SUN et al.: COMPOSITIONALLY CONTROLLED FePt NANOPARTICLE MATERIALS

Fig. 2.

TEM image of a 4 nm Fe

Pt

1241

nanoparticle assembly.

B. FePt Compositional Control The composition of FePt nanoparticle materials is tuned via and . Based on 0.5 mmol the molar ratio of , oleic acid and oleyl amine, and 20 mL each of the and the resulting of dioctylether, the molar amount of are shown in Fig. 1. It can be seen that not all contributes to the FePt formation. has a the low boiling point (103 C). At reaction temperature of 298 C, is actually in the vapor phase. The formation of this vapor phase results in the slow decomposition of at a rate that matches with the reduction rate of . The FePt nanoparticles are formed in a shorter period of time. cannot be completed Therefore the consumption of on this synthetic time scale. As a result, 0.5 mmol of and 0.5 mmol of yield , while 1.1 mmol and 0.5 mmol of lead to of nanoparticle materials. The elemental analysis from both EELS on a group of isolated FePt particles and EDX on the FePt nanoparticle assembly shows that the composition of either a single FePt nanoparticle or a group of FePt nanoparticles matches well with that from ICP analysis of bulk FePt nanoparticle materials. This indicates that the composition of the FePt nanoparticle materials is relatively uniform. TEM analysis on monolayer assemblies of the FePt nanoparticles shows that the image contrast from particle to particle is about the same. As the electron density of Fe differs from that of Pt, the uniform contrast image of the FePt nanoparticles also suggests that the Fe and Pt form uniform composition in each of the FePt nanoparticles. C. Formation of Nanoparticle Assemblies When the FePt nanoparticle dispersions are spread on a substrate, and the carrier solvent is allowed to slowly evaporate, FePt nanoparticle superlattices are produced. For TEM L of a dilute FePt dispersion observation, a drop mg/mL was deposited on a -coated copper grid. Fig. 2 is a TEM image of a cubic assembly of 4 nm particles. Although the cubic assembly symmetry will be ideal for future ultra-high density recording media, in reality, we do not have, as yet, control of the symmetry of the

Fig. 3. XRD patterns of (a) Fe Pt , (b) Fe Pt , (c) Fe Pt , and (d) Fe Pt nanoparticle assemblies annealed at 580 C for 30 minutes.

self-organization process. Extensive studies on nanoparticle assembly have shown that the symmetry depends on many factors, such as particle size, shape and relative dimensions of the particle core and the organic capping [18], [19]. These FePt particle assemblies show no obvious aggregation upon atmosannealing at temperatures up to 600 C under static phere (1 atm). Rutherford backscattering measurements on these annealed 4 nm FePt particle assemblies indicate 40–50% (at.%) carbon content. This shows that annealing at high temperature does not result in the loss of stabilizing ligands; rather, they are converted to a carbonatious coating around each particle, effectively protecting particles from agglomeration. D. FePt Nanoparticle Structure X-ray diffraction of the as-synthesized FePt particles reveals a typical chemically disordered fcc structure. Annealing induces the Fe and Pt atoms to rearrange into the long range chemically ordered fct structure. The change of the internal particle structure upon annealing depends on annealing temperature, as well as the Fe, Pt composition. XRD [12] and Kerr effect measureparticle assemblies show that ments [20] on annealed the onset of this phase change occurs at about 500 C. While annealing at higher temperatures or for longer time can increase C, the chemical ordering, too high a temperature, e.g., or too long an annealing time, will result in either particle’s partial aggregation or phase separation. XRD combined with TEM and HRSEM studies show that the optimum temperature for the phase transformation is 580 C. It is worth mentioning, however, that the exact structure after thermal annealing depends strongly on particle composition. Fig. 3 shows a series of XRD patterns of differently composed FePt assemblies annealed at 580 C for 30 minutes. It shows that among all these 580 C annealed FePt assembly yields a nanoparticle assemblies, only the high quality fct phase.

1242

Fig. 4. Hysteresis loops of the as-synthesized Fe assembly at (a) 5 K, (b) 15 K, (c) 35 K, and (d) 85 K.

IEEE TRANSACTIONS ON MAGNETICS, VOL. 37, NO. 4, JULY 2001

Pt

nanoparticle

E. Magnetic Properties of the FePt Nanoparticle Assembly The role of the annealing temperature on the chemical ordering and magnetic properties was probed by measuring the temperature dependent magnetic properties. SQUID magnetometry measurements of the as-synthesized FePt particles show that they are superparamagnetic at room temperature. At low temperature, however, they all show ferromagnetic variations among properties, even though there exist large differently composed FePt nanoparticle materials. The temperare shown in ature dependent hysteresis behavior of Fig. 4. The coercivity drops sharply as temperature is raised from 5 K to 15 K, indicating these small nanoparticles are thermally unstable. This is consistent with the low magnetocrystalline anisotropy of the fcc structure of the particles. Shown in Fig. 5 are the room temperature hysteresis loops for particles that were annealed at 500, 550 and 4 nm values increase dramatically with annealing 580 C. The temperature showing the transition from superparamagnetic to ferromagnetic behavior. The sample annealed at 500 C appears nearly superparamagnetic at room temperature. An expanded view of the low-field part of the loop shows an value of 330 Oe and the loop is hysteretic up to 6 kOe fields. This likely results from a minority fraction of the particles having sufficient anisotropy to be ferromagnetically ordered at room temperature. Increasing the annealing temperature to 550 C increases the ferromagnetic fraction of particles with value increasing to 3200 Oe. However, there is still the , indicating an inflection in the magnetization near particles. Annealing at C results some low value just over 9000 Oe with a in a room temperature loop shape characteristic of an isotropic distribution of high anisotropy particles. Hysteresis data of 580 C annealed FePt nanoparticle assemblies show that, like the as-synthesized FePt

Fig. 5. Room temperature hysteresis loops of Fe Pt assemblies annealed at (a) 500 C, (b) 550 C, and (c) 580 C.

Fig. 6. Composition dependent coercivity nanoparticle assemblies.

Hc

nanoparticle

of 580 C annealed FePt

nanoparticle material, the coercivities of these annealed FePt particles depend strongly on FePt composition, as shown in Fig. 6. The extrapolation of the Gaussian type fit yields the best , a conclusion that matches composition of FePt to be that obtained from the as-synthesized FePt materials and from FePt thin films [21].

SUN et al.: COMPOSITIONALLY CONTROLLED FePt NANOPARTICLE MATERIALS

1243

the magnetization should be stable for more than ten years and makes this material suitable for magnetic storage application. IV. CONCLUSION REMARKS The present synthesis provides general procedures for the preparation of 4 nm differently composed FePt nanoparticle materials and nanoparticle assemblies. Detailed structure and magnetic analyses of these nanoparticles show that the FePt composition control in FePt nanoparticle materials is the key to control the FePt particle structure transformation and magnetic hardness, and the optimized composition for the highest coercive . The synthetic search FePt nanoparticle materials is for 2D arrays of these ferromagnetic dots is underway.

H

Fig. 7. Temperature dependent c of (a) 500 C, (b) 550 C, and (c) 580 C annealed Fe Pt nanoparticle assemblies. The solid symbols are the measured values and the lines are the fits.

Shown in Fig. 7 is the temperature dependence of for the three annealing temperatures. Each sample shows a monotonic with decreasing temperature with the 500 C increase in below annealed particles showing a strong increase in K. The behavior can be quantitatively understood by relating the measured coercivity to the low temperature coerand the stability factor [3]: civity

where is the magnetic anisotropy energy, is the magnet switching volume, is Boltzman’s constant, is the time the applied magnetic field is applied sec , and is the thermal attempt frequency Hz . The fits of the data in Fig. 7 to above equation, where the is assumed to be constant with temperature, yield values of values , 6.7, 12.8, and 18.7 kOe and , and ergs. Assuming that is determined by the individual grain volume, we get an estimate of , , and ergs/cm for the 500, of 550 and 580 C annealing temperatures respectively. The ergs/cm of 580 C annealed sample agrees with that – ergs/cm . The expected for fully ordered FePt fit for the 500 C sample give a blocking temperature of 210 K for the average particle which agree with zero-field cooled measurements of the magnetization which shows a peak near this temperature. The room temperature stability parameter for the which indicates that 580 C annealed film is

ACKNOWLEDGMENT The authors thank A. Kellock of IBM Research for RBS experiments, Z. L. Wang of Georgia Institute of Technology for elemental microanalysis of single FePt nanoparticles, and L. Folks and A. Moser of IBM Research for fruitful discussions. REFERENCES [1] A. Cebollada, D. Weller, J. Sticht, G. R. Harp, R. F. C. Farrow, R. F. Marks, R. Savoy, and J. C. Scott, Phys. Rev. B., vol. 50, p. 3419, 1994. [2] R. F. C. Farrow, D. Weller, R. F. Marks, M. F. Toney, A. Cebollada, and G. R. Harp, J. Appl. Phys., vol. 79, p. 5967, 1996. [3] D. Weller and A. Moser, IEEE Trans. Magn., vol. 35, p. 4423, 1999. [4] Y. Ide, T. Goto, K. Kikuchi, K. Watanabe, J. Onagawa, H. Yoshida, and J. M. Cadogan, J. Magn. Magn. Mater., vol. 245, p. 177, 1998. [5] N. Li and B. M. Lairson, IEEE Trans. Magn., vol. 35, p. 1077, 1999. [6] R. A. Ristau, K. Barmak, L. H. Lewis, K. R. Coffey, and J. K. Howard, J. Appl. Phys., vol. 86, p. 4527, 1999. [7] C. P. Luo, S. H. Liou, and D. J. Sellmyer, J. Appl. Phys., vol. 87, p. 6941, 2000. [8] B. Bian, K. Sato, Y. Hirotsu, and A. Makino, Appl. Phys. Lett., vol. 75, p. 3686, 1999. [9] B. Bian, D. E. Laughlin, K. Sato, and Y. Hirotsu, J. Appl. Phys., vol. 87, p. 6962, 2000. [10] C. P. Luo, S. H. Liou, L. Gao, Y. Liu, and D. J. Sellmyer, Appl. Phys. Lett., vol. 77, p. 2225, 2000. [11] C.-M. Kuo and P. C. Kuo, J. Appl. Phys., vol. 87, p. 419, 2000. [12] S. Sun, C. B. Murray, D. Weller, L. Folks, and A. Moser, Science, vol. 287, p. 1989, 2000. [13] Z. L. Wang and Z. C. Kang, Functional and Smart Materials—Structural Evolution and Structure Analysis: Plenum Press, 1998, ch. 6–8. [14] K. S. Suslick, M. Fang, and T. Hyeon, J. Am. Chem. Soc., vol. 118, p. 11 960, 1996. [15] S. Sun and C. B. Murray, J. Appl. Phys., vol. 85, p. 4325, 1999. [16] S. Sun, C. B. Murray, and H. Doyle, Mat. Res. Soc. Symp. Proc., vol. 577, 1999, p. 385. [17] C. Petit, T. Cren, D. Roditchev, W. Sacks, J. Klein, and M. P. Pileni, Adv. Mater., vol. 11, p. 1198, 1999. [18] R. L. Whetten, M. N. Shafigullin, J. T. Khoury, T. G. Schaaff, I. Vezmar, M. M. Alvarez, and A. Wilkinson, Accounts of Chemical Research, vol. 32, p. 397, 1999. [19] Z. L. Wang, Z. R. Dai, and S. Sun, Adv. Mater., vol. 12, p. 1944, 2000. [20] D. Weller, S. Sun, C. B. Murray, L. Folks, and A. Moser, IEEE Trans. Magn., vol. 37, no. 4, July 2001. [21] M. Watanabe and M. Homma, Jpn. J. Appl. Phys., Part 1, vol. 35, p. 1264, 1996.