Magnetic Properties of FePt Nanoparticles Prepared by a Micellar Method
© to the authors 2009
Received: 30 July 2009
Accepted: 1 September 2009
Published: 16 September 2009
FePt nanoparticles with average size of 9 nm were synthesized using a diblock polymer micellar method combined with plasma treatment. To prevent from oxidation under ambient conditions, immediately after plasma treatment, the FePt nanoparticle arrays were in situ transferred into the film-growth chamber where they were covered by an SiO2 overlayer. A nearly complete transformation of L 10 FePt was achieved for samples annealed at temperatures above 700 °C. The well control on the FePt stoichiometry and avoidance from surface oxidation largely enhanced the coercivity, and a value as high as 10 kOe was obtained in this study. An evaluation of magnetic interactions was made using the so-called isothermal remanence (IRM) and dc-demagnetization (DCD) remanence curves and Kelly–Henkel plots (ΔM measurement). The ΔM measurement reveals that the resultant FePt nanoparticles exhibit a rather weak interparticle dipolar coupling, and the absence of interparticle exchange interaction suggests no significant particle agglomeration occurred during the post-annealing. Additionally, a slight parallel magnetic anisotropy was also observed. The results indicate the micellar method has a high potential in preparing FePt nanoparticle arrays used for ultrahigh density recording media.
KeywordsFePt nanoparticles Reverse micelles Self-assembly Interparticle exchange coupling Magnetic recording
Patterned single-domain magnetic nanoparticles, with each one carrying one bit of information, have been considered as one of the best candidates for future ultrahigh density recording media . In such a scheme, enhancing the storage density needs a reduction in nanoparticle diameter and spacing, hence materials with high magnetocrystalline anisotropy constant K u are required to enhance the thermal stability. Face-centered-tetragonal (FCT) L 10 phase FePt has been receiving considerable attention due to its high K u (7 × 107 ergs/cc) , a value that is of the largest among the known hard magnetic materials. Theoretical calculations indicate that FePt particles as small as 3 nm are sufficiently stable and satisfy the requirements of permanent data storage . Great efforts have been devoted to search a convenient way for synthesizing L 10 phase FePt nanoparticle arrays. Traditional physical deposition techniques have difficulties in controlling the particle size and morphology due to the random nucleation during the initial growth stage. A colloidal-chemical route, since the seminal work of Sun et al. , has proven to be a powerful way to synthesize FePt nanoparticles with uniform size and well-defined interparticle spacing. The as-prepared nanoparticles by colloidal technique possess atomically disordered face-centered-cubic (FCC) structure and are superparamagnetic in nature. Thermal annealing with temperature higher than 500 °C is usually needed to induce the FCC–FCT transition. During the post-annealing process, however, the agglomeration of particles due to the rather small interparticle spacing can result in a drastic increase of particle size, as well as a very strong interparticle exchange coupling [5, 6]. In addition, the ligand shell is expected to influence the magnetic properties and can be converted to a carbonaceous coating around particles during the post-annealing process .
A micellar method has been adopted by Ethirajan et al. and Qu et al.  to overcome these obstacles. The preparation of metal nanoparticle arrays by this method is based on the self-assembly of suitable diblock copolymers . When the diblock copolymers are dissolved into an apolar solvent, they form spherical reverse micelles. The cores of such reverse micelles can be loaded with metal salts. The reverse micelles self-organize into hexagonally ordered arrays when a monolayer of them covers a smooth substrate. Properly removing the polymer matrix, the resultant metal nanoparticles hold the initial pattern of the reverse micelles. This method has a high flexibility in controlling the particle size, interparticle spacing, and composition. The size distributions of the obtained nanoparticles are comparable to or even better than those of the colloidal nanoparticles . Although the micellar method has been demonstrated successful in synthesizing FePt nanoparticle arrays [8, 9], few works are available on their magnetic properties. The reported coercivity of FePt nanoparticles synthesized by the micellar method is merely 1500 Oe , which is far from being ideal for a recording medium.
Herein, we present a detailed investigation on the structural and magnetic properties of FePt nanoparticles fabricated by the micellar method. Based on previous works [9, 12], the ordered FePt nanoparticle arrays with ideal 1:1 stoichiometry were successfully synthesized. Post-annealing treatments were employed to induce the formation of L 10 phase FePt nanoparticles. A high value (10 kOe) of the coercivity (H C) has been achieved for the sample annealed at temperatures above 700 °C. Moreover, the absence of interparticle exchange coupling suggests no significant particle agglomeration occurred during the post-annealing, which leads to lower media noise.
The preparation of FePt nanoparticles was based on the self-assembly of poly(styrene)–poly(4-vinylpyridine), i.e., PS–P4VP, into reverse micelles in toluene. H2PtCl6 and FeCl3 were added into the micellar solution and then micelles loaded with metal salts were deposited on silicon substrates by a dip-coating process. Details on the process have been published elsewhere . In order to remove the polymer matrix and to finally reduce the nanoparticles into metallic states, the as-coated samples were subsequently subjected to oxygen and hydrogen plasma treatment for 30 min (150 Pa, 100 W) within a home-made radio frequency (RF) plasma system. Immediately after the plasma treatments, the samples were in situ transferred into the film-deposition chamber where they were covered by a 10-nm-thick SiO2 layer using RF magnetron sputtering. Coating the SiO2 layer is to protect the FePt nanoparticles from oxidation in ambient, as well as to prevent them from agglomeration during annealing. The procedure from dip-coating to SiO2 deposition was repeated for 10 times. Finally, vacuum annealing treatments were performed to drive the FCC–FCT transition.
Surface morphologies of the monolayer samples were characterized by a NT-MDT Solver P47 atomic force microscopy (AFM) in a semi-contact mode. For the X-ray diffraction (XRD) measurements in a θ–2θ mode, a Panalytical X’Pert-MPD Pro diffractometer with a CuK α X-ray source was used. Transmission electron microscopy (TEM) analysis was carried out by a JEM-2010 microscope. Room-temperature magnetic hysteresis loops and ΔM curves were measured by a model MicroMag-2900 alternating gradient magnetometer (AGM).
Results and Discussion
It is very important to insure the complete removal of the PS–P4VP matrix by plasma treatment, otherwise the residuals would form carbonaceous contamination during post-annealing, influencing the magnetic behaviors of FePt nanoparticles. The X-ray electron spectroscopy (XPS) signals from C 1s and N 1s were used to determine whether the matrix is absent . The results show that the polymer matrix can be completely removed through a 30 min oxygen plasma treatment. After the oxygen plasma treatment, Fe and Pt atoms are in highly oxidized state . Undergone a subsequent hydrogen plasma treatment (30 min, 100 W), the Fe and Pt atoms were reduced to a metallic state. Rutherford backscattering spectroscopy (RBS) measurements reveal that the plasma treatment adopted here produces scarcely any loss of the metal atoms . The synthesis of carbon-contamination free, metallic FePt nanoparticles favors their applications in ultrahigh density data recording.
A suitable high H C is technologically important for high-density recording media. The coercivity of ~10 kOe we obtained is much higher than 1.5 kOe reported in Ref.  and 2.2 kOe in Ref. . However, it is still less than the theoretical coercivity value of 9 nm FCT FePt nanoparticles. For assembly of randomly distributed single-domain particles with uniaxial anisotropy, by taking the theoretically saturation magnetization (M S) and magnetocrystalline anisotropy values, a coercivity of 30 kOe is calculated for fully ordered 9 nm Fe50Pt50 particles . Generally, stoichiometry deviation from 1:1 [16, 18], surface oxidization , the multidomain effect, and the formation of a silicide structure  may be possible explanations for the smaller H C as compared with the calculated one. In order to obtain L 10 phase, the alloy must have a composition between 33 and 55 at.%, as determined from the Fe–Pt phase diagram . It is difficult for nanoparticle with Fe/Pt ratio significantly deviating from 1:1 to transform into the hard magnetic phase upon annealing . In a recent work , we have investigated the composition deviation of FePt nanoparticles from the nominal Fe:Pt molar ratio of the precursors. Based on the research, a better control over the particle stoichiometry has been achieved. Furthermore, in this work, to avoid the surface oxidization of FePt nanoparticles, the FePt samples were in situ transferred into the film-deposition chamber and covered by an SiO2 overlayer after hydrogen plasma treatment. Therefore, in the present work, the possibilities of stoichiometry deviation and surface oxidization can be excluded, and the multidomain effect and the formation of a silicide structure may be responsible for the smaller H C.
In conclusion, we have prepared FePt nanoparticles on Si substrate by a micellar method, with small particle size (9.0 nm) and a very narrow size distribution. The as-prepared samples by dip-coating were exposed to plasma to completely remove the polymer matrix and reduce the atoms into metallic state. Post-annealing was performed to induce the formation of chemically ordered FCT L 10 phase. An ordering parameter higher than 0.9 was achieved for the samples annealed at 700 °C or above for 60 min. By better control on the FePt stoichiometry and avoidance from surface oxidation, a large in-plane coercivity of ~10 kOe was obtained in this study. A slight parallel magnetic anisotropy was also observed. In addition, no evident interparticle exchange coupling was observed for the FePt particles, suggesting no significant agglomeration occurred during the post-annealing. The FePt nanoparticles prepared by micellar method have a high potential for use in magnetic data recording.
This work was financially supported by the National High-tech R&D Programme of China under Grant No 2006AA03Z306, and the National Natural Science Foundation of China under Grant No 50601025 and 60806044.
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