006) 995–998www.elsevier.com/locate/matlet

Materials Letters 60 (2

Oriented self-assembly of cubic FePt nanoparticles

Nisha Shukla a,⁎, Chao Liu a, A.G. Roy b

a Seagate Research, Pittsburgh, PA 15222, USAb Department of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, PA 15213, USA

Received 6 July 2005; accepted 17 October 2005Available online 10 November 2005

Abstract

Chemically synthesized FePt nanoparticles have potential for application as magnetic media for data storage. In order to be used as data storagemedia, however, they must be deposited on substrates in such a way that their magnetic axes are aligned with respect to one another and withrespect to the substrate, a problem that has yet to be overcome with spherical FePt nanoparticles. In this work we report the initial results of effortsto synthesize single crystalline FePt nanoparticles with cubic shapes. Our synthesis yields particles with shapes that are close to cubic orrectangular. When deposited on surfaces these particles assemble with local order and a square lattice. More importantly, they orient themselveswith their [100] axes perpendicular to the surface and with local interparticle alignment of the [010] and [001] axes oriented parallel to the surface.These films of FePt nanocubes begin to overcome several of the obstacles associated with the development of magnetic data storage media basedon self-oriented magnetic arrays of FePt nanoparticles.© 2005 Elsevier B.V. All rights reserved.

Keywords: Magnetic materials; Nanomaterials; Nanoparticles; FePt; Self-assembly

1. Introduction

Synthesis and self-assembly of FePt nanoparticles have beenof great interest to the data storage industry as a possible meansfor making nanoparticle based magnetic media which canenable recording densities as high as 1 Tbit/in.2. The magneticbits which record data in thin film magnetic media arecomprised of many grains. In order to achieve high recordingdensities with bits having well defined shapes and boundaries itis necessary to decrease the size of the grains such that theyremain significantly smaller than the bit size. If the grainsbecome too small, however, one reaches the superparamagneticlimit in which the magnetic moment of the individual bitsbecomes thermally unstable. Self-assembled arrays of nano-particles can, in principle, reduce transition jitter by havinguniformly sized magnetic grains separated by a well-definedcenter-to-center grain distance. Jitter is the noise in the magneticsignal from the media that arises from the poorly definedboundary of the magnetic bit formed by randomly shaped andrandomly sized magnetic grains. Thus the ideal magnetic media

⁎ Corresponding author.E-mail address: nisha@andrew.cmu.edu (N. Shukla).

0167-577X/$ - see front matter © 2005 Elsevier B.V. All rights reserved.doi:10.1016/j.matlet.2005.10.059

would be formed of nanoparticles which are of uniform, smallsize and packed into arrays with the highest possible density.

Chemical syntheses have been developed that producespherical FePt nanoparticles that are highly monodispersedwith uniform diameters of 3–4 nm. The chemical synthesis ofFePt nanoparticles was first reported by Sun et al. [1] and sincethen several groups [2–6] have reported synthesis of FePtnanoparticles using similar or modified syntheses. Theseproduce spherical nanoparticles coated with films of oleicacid and oleylamine surfactants that are∼2 nm thick. One of thedisadvantages of these small spherical nanoparticles is that theyhave low magnetic moment per unit area. In part, this arisesfrom the low packing fraction of spherical nanoparticles whichcan occupy only ∼70% of the area in a two dimensional array,at best. In reality, the fractional coverage of a surface by thespherical FePt cores is significantly less than 70% because thecores are coated with surfactant films which are needed tostabilize the nanoparticles. A more critical problem which isprobably generic to spherical particles is the fact that themagnetic axes of the particles cannot be aligned once depositedon a surface. Sato and Hirotsu [7] have reported the fabricationof oriented FePt nanoparticles with high coercivity but thesenanoparticles were fabricated by electron beam evaporation and

Fig. 1. Bright-field TEM micrographs of unannealed FePt nanocubes depositedfrom a non-polar solvent on a carbon-coated copper TEM grid. The cubes areroughly 5 nm on a side. The particles in image A form a monolayer. Theparticles in image B that are marked in circles have formed a bilayer. Theparticles in the first and second layer have their sides aligned and one canobserve an interference pattern between the two.

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grown epitaxially on MgO(001) and NaCl(001) surfaces.Increasing the packing fraction and obtaining alignment of themagnetic axis are both crucial for utilization of chemicallysynthesized FePt nanoparticles in high density recording media.

Synthesis of cubic nanoparticles has been reported in theliterature for various metals and alloys. Yu and Yam [8] havereported phase aqueous synthesis of monodispersed silvernanocubes with edge length of 55 nm. Lu et al. [9] havereported synthesis of cubic PbTe nanoparticles. Pietryga et al.[10] have reported synthesis of cubic PbSe nanoparticles for useas quantum dots. Chen et al. [11] have reported the formation ofcore shell cubic FePt/Fe3O4 nanoparticles but showed no netorientation or long range self-assembly of these nanoparticleswhen deposited on substrates.

In this work, we have demonstrated the chemical synthesis ofcrystalline FePt nanoparticles with shapes that are close torectangular and cubic. When deposited onto surfaces fromsolution their shape anisotropy causes these FePt nanocubes toassemble with oriented crystallographic axes. The FePtnanocubes are 4–10 nm in size and assemble into arrays thatare locally square and thus have higher packing densities thanspherical FePt nanoparticles. Thus, this work demonstratesprogress in two of the directions critical to the successfuldevelopment of magnetic recording media based on FePtnanoparticles.

2. Experimental

The synthesis of FePt nanoparticles was performed undernitrogen in a glove box using iron pentacarbonyl and platinumacetylacetonate as sources of Fe and Pt, respectively. In thissynthesis Fe generated by decomposition of Fe(CO)5 is used asa reducing agent for platinum (II) acetylacetonate. A solution ofplatinum (II) acetylacetonate (0.5 mmol), Fe(CO)5 (0.5 mmol),oleic acid (4 mmol), and oleylamine (4 mmol) in dichloroben-zene was heated at 4 °C per minute until the reflux temperatureof 170 °C was achieved. The heating rate and the reactiontemperature are both significantly lower than those used in thestandard synthesis with dioctyl ether [1]. The reaction mixturewas then refluxed at 170 °C for 24 h. This synthesis generates acolloidal solution of FePt nanoparticles coated with a layer ofmixed oleic acid and oleyl amine. The colloidal solution waswashed using a standard process described previously byShukla et al. [12] to remove the smallest particles. The finalsolution of FePt nanoparticles was a 50:50 mixture of hexaneand octane with only oleic acid as an excess surfactant in thesolution. This solution of nanoparticles was then used to depositthe particles on carbon-coated TEM grids for subsequentimaging and analysis.

The FePt nanoparticles were self-assembled by placing a dropof solution onto carbon coated copper TEM grids and allowingthe solvent to dry. The concentration of FePt nanoparticle wasapproximately 2 mg/ml. TEM imaging and selected areaelectron diffraction of FePt self-assembled films was performedusing a Philip EM420T microscope operating at 120 keV.

All chemicals used during the process of synthesis andwashing of FePt nanoparticles were bought from Aldrich and

Strem chemicals. Platinum acetylacetonate (99.99%), oleic acid(90%), oleylamine (70%), Fe(CO)5 (99%), and dichlorobenzene(99%) were used as obtained without further purification.Anhydrous ethyl alcohol and hexanes were also used withoutfurther purification.

3. Results

The goal of this work has been to prepare FePt nanoparticles thatwill assemble on surfaces into textured arrays in which thecrystallographic axes of the particles are oriented with respect to thesurface. This has not been achieved using the spherical FePtnanoparticles produced by the standard synthesis [1,2]. We havemodified the synthesis by using dichlorobenzene as the solvent, heatingthe solution much more slowly and allowing the reaction to proceed atmuch lower temperatures and for longer times with the goal ofpreparing nanoparticles that have cubic shapes that will self-orient theircrystallographic axes during deposition on surfaces. As revealed in theTEM images of Fig. 1 our synthesis has successfully producednanoparticles that are roughly cubic in shape with sides that are about 5nm in length. Although the shapes and sizes of these particles are notyet uniform, their shapes clearly approximate those of cubes andrectangles more closely than those of spheres. Furthermore, the profilesof the nanoparticles on the TEM grid indicate that they have oriented

Fig. 3. Bright-field TEM micrograph of unannealed FePt nanocubes self-assembled with local square arrays on a carbon-coated copper TEM grid.Monolayer and bilayer self-assembly is observed.

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themselves such that one face of the nanoparticle is in contact with thesurface. In other words, if these particles are in fact crystalline andhave, as one would expect, the fcc atomic structure of bulk FePt, thenthey are oriented with their [100] axes perpendicular to the surface.

The crystallinity and crystallographic orientation of the FePtnanocubes are revealed by the SAD patterns shown in Fig. 2. Thediffraction rings clearly arise from the fcc crystal structure of the FePtlattice. It is important to point out that for magnetic recordingapplications, one ultimately wants to produce the fct phase rather thanthe fcc phase. The important feature of the SAD patterns is that theintensities of the rings suggest that the particles have a net orientationwith the [100] axes aligned along the surface normal. At 0° tilt this oughtto yield intense diffraction from the [0kl] planes perpendicular to the[100] plane. Theweak diffraction features from the [111] planes indicatethat the order is imperfect. The electron diffraction patterns obtained at asample tilt angle of 20° reveal anisotropy in intensity around thediffraction rings, also indicative of texture in the nanoparticle films[13,14]. Similar diffraction features have been observed by Ristau et al.[13] from [111] textured films of sputter deposited FePt and CoPt films.

Close examination of Fig. 1B also reveals several features of theoriented FePt nanocubes. The features highlighted with circles appearto be two nanocubes stacked on top of one another. Close examinationreveals interference patterns that arise from the superposition of the twolattices. These interference patterns appear in number of such caseswhere two particles are stack on top of one another. The orientation ofthese interference features indicates that the [100] planes are orientedparallel to the faces of the nanocubes. Another interesting feature of theTEM image of Fig. 1 is that locally the nanocubes themselves assembleinto a square array. From the point of view of magnetic recordingapplications this has the advantage of providing higher packing fractionthan the assembly of spheres into close packed hexagonal lattices. Thenanocubes assemble into a square array with a spacing betweennanocubes of roughly 3–4 nm. The FePt nanocubes are coated withsurfactant films formed by a mixture of oleic acid and oleylamine. Theoleyl chain of these surfactants is roughly 2 nm in length and so thespacing between particles is consistent with expectations based on thedimensions of the surfactant chains. Future attempts to further increasethe packing density of these cubic nanoparticles will have to explorethe use of surfactants with shorter chain lengths. Finally, the nanocubefilm imaged in Fig. 1B has a mixture of both monolayer and bilayer.The interesting feature of the film is that in the bilayer the nanocubesare stacked on top of one another. Furthermore, the sides of the

Fig. 2. Selected area diffraction patterns of unannealed FePt nanocubes obtained withfrom the (200) rings and weak diffraction from the (111) rings indicates a net [100]patterns taken at a tilt of 20° also reveals texture in the nanoparticle orientation.

nanocubes are aligned with one another indicating that even in thesecond layer, the [010] and [001] axes retain the same orientation as inthe first layer. This suggests that if allowed to assemble into a three-dimensional structure, these nanocubes could form a cubic lattice witha high degree of interparticle crystallographic alignment.

In order to be useful in magnetic recording applications, FePtnanocubes must be prepared as thin films with a fairly uniformdistribution across large areas. Such films could be one or more layersin thickness. Fig. 3 shows a large area TEM image of a nanocube filmformed by dip coating onto a TEM grid. As revealed by the image thesenanocubes can be deposited with fairly uniform areal density acrosslengths on the order of 1 μm. Although the film does not display longrange order, within the film, the nanocubes are ordered into a lattice thathas domains with local square symmetry.

The work presented addresses some of the challenges to thedevelopment of magnetic recording media based on FePt nanoparticles.Our initial success at synthesizing fePt nanoparticles with roughlycubic shapes has enable us to deposit them on surfaces such that theircrystallographic axes have net alignment with respect to the surfacenormal. Challenges that remain include the synthesis of particles withthe fct rather than fcc structure and the development of methods todeposit nanoparticles into films with long range order and high packingdensities.

the TEM sample holder tilt angle set at (a) 0° and (b) 20°. The intense diffractiontexture. The anisotropy of the intensity in the rings observed in the diffraction

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4. Conclusions

A new synthesis has been developed that produces magneticFePt nanocubes coated with surfactant. These nanocubes self-assemble on surfaces to form arrays with locally squaresymmetry. Within the array the nanocubes are oriented withtheir [100] axes aligned along the surface normal. The [100]axes lying parallel to the surface are also aligned with oneanother. These properties have potential for applications in highdensity magnetic recording and patterned media.

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