3 Nm, Annealing Conditions

INSTITUTE OF PHYSICS PUBLISHING

NANOTECHNOLOGY

Nanotechnology 16 (2005) 1603–1607

doi:10.1088/0957-4484/16/9/033

Optimized synthesis and annealing conditions of L10 FePt nanoparticles I Zafiropoulou1 , V Tzitzios1 , D Petridis1 , E Devlin1 , J Fidler2 , S Hoefinger2 and D Niarchos1 1 2

Institute of Materials Science, NCSR ‘Demokritos’ Agia Paraskevi, 15310 Athens, Greece Vienna University of Technology, Wiedner Hauptstrasse 8-10/138, A-1040 Vienna, Austria

E-mail: [email protected] and [email protected]

Received 5 January 2005, in final form 2 March 2005 Published 1 July 2005 Online at stacks.iop.org/Nano/16/1603 Abstract The fabrication of ∼3 nm FePt nanoparticles and detailed studies of the effect of the annealing conditions (temperature and time) on the formation of the face-centred tetragonal phase and on the magnetic properties are described. Additionally, the effect of the precursor molar ratio on the final product and the composition of the as-prepared samples were studied. The main criteria for the determination of the best alloy are the properties desired for applications in the field of magnetic recording media.

1. Introduction Iron–platinum alloy nanoparticles exhibit remarkable magnetic properties according to their size, form, and crystal structure. In particular, the high magnetocrystalline anisotropy (6.6 × 107 ergs cm−3 ) [1] of the face-centred tetragonal (fct) phase permits a reduction in the size of nanoparticles below 10 nm while retaining the stability of their magnetization against thermal fluctuations and demagnetizing effects [2]. These features make the fct phase of FePt very desirable in the field of ultra-high-density information storage [3]. However, there are particular values of the coercivity and magnetization, as well as of the size and size distribution of the alloy nanoparticles, that are necessary for the successful application in recording media. The coercivity should range between 4 and 8 kOe, so as to be high enough for stable storage, but also low enough for overwriting to be possible. The magnetization should be of significant magnitude so as to produce an intense signal for reading. The size of the nanoparticles should be as small as possible (preferably 10 nm maximum) in order to obtain the best storage density (bits in−2 ). Finally, the size reduction should be accompanied by a narrow size distribution and weak interparticle interactions in order to keep the signal to noise ratio within acceptable levels. A successful way to address the above requirements is to employ chemical routes for the fabrication of monodispersed FePt nanoparticles. A general method applied successfully for the synthesis of these nanoparticles involves the reduction of Pt(acac)2 by a diol, e.g. 1,2-hexadecanediol, with the concurrent thermal decomposition of an organometallic iron 0957-4484/05/091603+05$30.00 © 2005 IOP Publishing Ltd

source in a high boiling point organic solvent, e.g. dioctyl ether [4]. The reaction is conducted in the presence of oleic acid and oleylamine. These surface-active compounds coat the surface of each particle with organic matter that prevents undesired effects such as agglomeration and oxidation. This modification simultaneously provides the necessary solubility in organic non-polar solvents for spontaneous self-assembly into three-dimensional superlattices. Understanding the process of the fct phase formation is the key point for the production of FePt nanoparticles with the largest possible percentage of the fct phase under mild annealing conditions. Accordingly, the object of the present work is to study the synthesis conditions, i.e. molar ratio and annealing conditions (temperature and time), on the size and magnetic properties of the nanoparticles. The synthetic route followed was that of Sun et al [4], which leads to very small nanoparticles (as low as 2 nm) with a narrow size distribution. In previous work of our research group the commercially available and inexpensive polyethyleneglycol (PEG) has been successfully used as the reducing agent for the preparation of CoPt nanoparticles, instead of the 1,2-hexanodiol [5]. M¨ossbauer spectroscopy was found to be a very useful tool for this study. This technique enables the identification and quantification of the magnetic and other iron-containing phases and their evolution in the course of the process. In the present case the fcc and fct phases have spectra with distinct M¨ossbauer parameters. The isomer shift (IS) values for both phases are very similar because of the very similar electronic state of the iron. The different environments of the iron atoms in each phase, however, induce different quadrupole splittings (QSs)

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Figure 1. XRD patterns (a), hysteresis loops (b) and M¨ossbauer spectra ((c), (d)) for the equiatomic FePt. I: as-prepared, II: annealed at 700 ◦ C for 1 h and III: annealed at 700 ◦ C for 3 h.

and magnetic hyperfine fields (HFs) in the two phases. The cubic phase has a high hyperfine field of 340–350 kG and a zero quadrupole splitting, while the tetragonal phase has a lower field, 310–315 kG, and a quadrupole splitting in the range 0.09– 0.15 mm s−1 . Thus the application of M¨ossbauer spectroscopy enabled the identification and estimation of the iron phases formed during the fcc to fct transformation. Iron oxide phases were also identified from their characteristic values.

2. Experimental section The experimental procedure for the synthesis of iron–platinum nanoparticles was as follows. About 20 ml of diphenylether were heated to 100 ◦ C under N2 atmosphere in a round flask. Pt(acac)2 was then added, followed by PEG 600, oleic acid and oleylamine. Finally the Fe(CO)5 was added, after which the system was fitted with a condenser. The temperature was raised to 250 ◦ C and the system was refluxed for at least 45 min. Nanoparticle formation was indicated by the solution becoming a dense black suspension. At the end of the reaction, the suspension was left to cool at room temperature and ethanol (about 50 ml) was added to precipitate the nanoparticles. The product was separated by centrifugation, washed repeatedly with ethanol and exposed to dry in air on a flat glass surface. The amount of the Fe precursor was varied in order to synthesize alloys with different compositions. The FePt nanoparticles were annealed under a reducing atmosphere (95% Ar + 5% H2 ) for various times and at different temperatures. 1604

The structure of the as-prepared and annealed nanoparticles was determined using x-ray powder diffraction (XRD) measurements with a D 500 Siemens diffractometer. The samples were measured in the range 2θ = 20◦ –90◦ , with Cu Kα radiation (λ = 1.5418 cm−1 ). The XRD data were also used for the estimation of nanoparticle mean size via the Scherrer formula [6]. Magnetic properties were measured at room temperature using a VSM PAR Model 155. M¨ossbauer spectrometry was carried out using a constant acceleration spectrometer with a 57 Co(Rh) source calibrated at room temperature using an iron metal foil. The low-temperature measurements were carried out using an Oxford Variox cryostat. For the TEM pictures an HRTEM JEOL JEM 2000 FX microscope was used. The samples were dispersed in chloroform and the solution was dropped on a carbon-coated Cu grid.

3. Results and discussion 3.1. Effect of molar ratio on FePt properties Initially a series of alloys were synthesized with a molar ratio Fe/Pt ranging between 4.5/1 and 1/3. The x-ray diffraction patterns indicated that all the as-prepared samples crystallized in the fcc phase, but after annealing only the equiatomic sample showed the desired transformation to the fct phase (figure 1(a)). Hysteresis loops led to the same conclusion, since only the equiatomic alloy showed a significant improvement in the magnetic properties (coercivity and magnetization) after annealing (figure 1(b)).

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Figure 2. HRTEM images and electron diffraction pattern from the as-prepared equiatomic FePt.

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The mean nanoparticle sizes, extracted from the XRD patterns (using the Scherrer equation), range between 3 and 4 nm for the as-prepared samples and after annealing rise to 18 nm (1 h) and 29 nm (3 h). Grain growth is through agglomeration and sintering [7, 8]. A small decrease in the coercivity values is seen in all samples after 3 h annealing compared to 1 h. This is likely due to the continuing growth of the nanoparticles above a critical value where non-coherent modes of magnetization reversal are allowed [9, 10]. For the equiatomic composition, figure 1(b) shows the hysteresis curves, which show an initial increase in coercivity on annealing from 0.1 kOe for the as-prepared sample to 7 kOe for the sample annealed for 1 h, and then a slight decrease to 6.5 kOe on annealing for 3 h. Figures 1(c) and (d) show the M¨ossbauer spectra from the as-prepared and the 3 h annealed equiatomic FePt alloy. The majority of the as-prepared sample (figure 1(c)) consists of iron oxide (96%), and only 4% of the sample forms the FePt alloy. Nevertheless, after 3 h annealing, this sample gave a M¨ossbauer spectrum (figure 1(d)), showing complete reduction and phase 100% transformation to the fct phase. The as-prepared equiatomic FePt alloy was also studied with an HRTEM (figure 2). The nanoparticles are spherical in shape with a size between 3 and 5 nm, in good agreement with that evaluated from the XRD patterns using the Scherrer formula. A key issue refers to the formation of iron oxides during the synthesis of the fcc FePt phase from the thermal decomposition of Fe(CO)5 . The XRD results of the as-prepared sample, as shown in figure 1(a), do not indicate the formation of the fcc phase. In fact, when the XRD pattern was confined to the region of 2θ values up to 38◦ the characteristic reflections at 2θ 29.9◦ and 36.3◦ of the ferrite phase [9] (Fe3 O4 , γ -Fe2 O3 ) are quite clear (figure 3). The M¨ossbauer spectrum, on the other hand (figure 1(c)), provides not only qualitative but also quantitative evidence for the formation of the iron oxide phase; the analysis of the spectrum from the as-prepared sample showed 96% iron oxide in this sample. The oxide present in the sample is cubic, with parameters suggesting either γ -Fe2 O3 or Fe3 O4 . Because of the small size of the nanoparticles and their non-stoichiometry, these two oxides cannot be distinguished, but haematite can be excluded due to the absence of any significant quadruple splitting. The presence of iron oxide in the as-prepared samples disagrees with many literature reports which claim that the synthesis leads directly to the cubic phase

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(fcc) of the FePt alloy. According to our M¨ossbauer results, the fcc phase represents only 4% of the sample, while the rest of the sample consists of iron oxides and probably metallic platinum. The same conclusion is reached from the XRD patterns. Here the (111) peak of the fcc alloy would be expected to appear at 2θ = 40.986◦ . A strong reflection appearing at 2θ ∼ 39.96◦ is wrongly taken to indicate the formation of the fcc phase. This peak should be attributed to the metallic Pt (in accordance to the data base [11] where the (111) peak of Pt is at 2θ = 39.763◦ ). The peak is very intense and obscures that of the iron oxides due to the high atomic number of Pt [12]. Our conclusion is that there is no need to use Fe(CO)5 as an iron precursor, since it does not prevent the formation of iron oxides and in addition possesses numerous disadvantages such as its chemical instability and toxicity. 3.2. Effect of annealing conditions on FePt properties Based on the above results, the equiatomic sample was chosen for the study of the annealing conditions required to yield the fct phase. In the first stage the effect of annealing temperature (Tan ) was examined. The samples were annealed for 2 h at 400, 450, 500, 600 and 700 ◦ C. The XRD measurements show the gradual formation of the fct phase (figure 4(a)). This is accompanied, as expected, with a corresponding increase in the coercivity values, with a maximum value of 7 kOe at 600 ◦ C 1605

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Figure 5. XRD patterns (a) and hysteresis loops (b) for samples annealed at 450 ◦ C for various times.

(figure 4(b)). We note that the sample annealed at 700 ◦ C has a lower coercivity than that annealed at 600 ◦ C, because the 700 ◦ C annealing is accompanied by a significant increase in the nanoparticle size from 20 to 28 nm. It is also important to point out that the coercivity values achieved after annealing at 400 ◦ C (1.4 kOe, size 6 nm), and especially at 450 ◦ C (4.5 kOe, size 9.5 nm), are satisfactory for many applications and, accordingly, there is no need for annealing at the more elevated temperatures mentioned in the literature. For these reasons the temperature of 450 ◦ C was selected to examine the effect of the annealing time (tan ) on the coercivity. In this second stage the equiatomic alloy was annealed at 450 ◦ C for 10, 30 60 and 120 min. In the XRD patterns, figure 5(a), there is no clear evidence for the formation of the fct phase. There is, however, a gradual development of the characteristic doublet at 2θ values, indicating the partial formation of the fct phase. Because of the low Tan the size of nanoparticles remains low, reaching a maximum value of 9.5 nm for the 120 min annealing time. The coercivity values increase with increasing the annealing time, as shown by the hysteresis loops (figure 5(b)). This increase happens abruptly at first but slows down as tan increases. For example, after 30 min annealing the coercivity is 3.5 kOe, and after 60 min it increases only to 3.7 kOe. This conclusion was further supported by the M¨ossbauer spectra obtained from samples annealed for 10 and 120 min (figures 6(a) and (b)). The sample annealed for 10 min contains 42% of the fct phase, a surprisingly large amount considering 1606

the Tan and tan . In this spectrum (figure 6(a)) iron oxides are also present, presumably since 10 min is too short a time for their reduction. For the 120 min annealed samples the fct percentage increases to 48%. Consequently, the evolution of the fct phase is a process which slows down with increasing the Tan or tan . For this reason, there is no need to apply strong annealing conditions. Annealing at 450 ◦ C for 30 min leads to the formation of 42% fct phase accompanied by a 3.5 kOe coercivity, while the nanoparticle size remains at 8.5 nm. These values are satisfactory enough for future technological applications of this system.

4. Conclusions In conclusion, the fabrication of hard magnetic L10 FePt nanoparticles (with a size of 3–5 nm) is reported, along with a study of the effect of molar ratio of the metallic precursors on the composition and magnetic properties of the alloy. Although Fe(CO)5 was used as the source of metallic iron, the as-prepared nanoparticles consist mainly of iron oxides and metallic platinum. This leads to the conclusion that for the synthesis of FePt iron precursors other than Fe(CO)5 , which is toxic, explosive and chemically unstable, can be used. The study of the annealing conditions (temperature and time) on the magnetic properties of the equiatomic alloy showed that a temperature of 450 ◦ C is sufficient for a adequate coercivity value to develop (3.5 kOe), while the size is maintained below 10 nm. As far as the annealing time is concerned, 30 min was

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Figure 6. M¨ossbauer spectra for two samples annealed at 450 ◦ C for 10 min (a) and 120 min (b).

adequate for the desired properties, since the formation of the fct phase seems to be a process that starts rapidly but slows down with both temperature and the annealing time.

Acknowledgment Work supported by E C Growth Program HIDEMAR.

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