Synthesis and Characterization of Bimagnetic Bricklike Nanoparticles Girija S. Chaubey, Vikas Nandwana, Narayan Poudyal, Chuan-bing Rong, and J. Ping Liu* Department of Physics, UniVersity of Texas at Arlington, Arlington, Texas 76019 ReceiVed September 28, 2007. ReVised Manuscript ReceiVed NoVember 8, 2007 Bimagnetic FePt/CoFe 2 O 4 nanoparticles having a bricklike morphology were synthesized by growing a soft magnetic CoFe 2 O 4 phase on FePt cubic nanoparticle seeds. The size of the soft phase could be controlled by tuning the material ratio of the FePt seeds to the CoFe 2 O 4 component. To obtain magnetic hardening, the as-synthesized bricklike nanoparticles were annealed at elevated temperatures under a reductive atmosphere to convert the disordered face-centered cubic FePt phase into the ordered L1 0 phase having high magnetic anisotropy. When the particles were annealed, a gradual change was observed in morphology from bricklike particles to spherical polycrystalline nanocomposite particles because of diffusion. Meanwhile, the magnetic energy density was enhanced as a result of the exchange coupling between the hard and soft phases. The enhancement was dependent on the ratio of the volumes of the soft phase and the hard phase. Introduction The major interest in recent materials research lies not only in synthesizing new materials but also in controlling the morphology of materials to produce desired nanostructures, for instance, nanoparticles with controllable sizes and shapes. The developments in nanoparticle synthesis from single- component nanoparticles to nanoparticles with two or more components are attractive for advanced applications. For instance, core/shell-structured semiconductor nanoparticles with wide-band-gap shells over narrow-band-gap cores have higher luminescence quantum yields than single-component semiconductor nanocrystalline materials. 1–3 For biological applications, nanoparticles with a magnetic material encap- sulated in a nonmagnetic material or a nonmagnetic material encapsulated in a magnetic material are interesting. 4–6 Bimagnetic nanoparticles consisting of a hard magnetic phase and a soft magnetic phase have high potential applications in magnetic recording media 7–9 and permanent magnetic materials 10–12 because the intimate contact between the hard and soft magnetic phases in the nanoparticles enhances interphase exchange coupling. Most of the bimagnetic nanoparticle systems reported to date have core/shell struc- tures in which the core components are covered completely by the shells. In this paper, we report the synthesis and characterization of bimagnetic nanoparticles having a novel morphology: bricklike nanoparticles, which consist of two components, FePt and CoFe 2 O 4 (with the oxide phase attached to the FePt phase), forming shadow images of each other. Unlike a magnetic hard-core/soft-shell structure, the thickness of both components can be tuned to a large extent. This makes it easy to control the ratio of the two components and thus to adjust the magnetic properties. Experimental Procedures All of the reagents used in this synthesis were commercially available and used as received without further puriﬁcation. Iron(III) acetylacetonate [Fe(acac) 3 ], cobalt(II) acetylacetonate [Co(acac) 2 ], iron pentacarbonyl [Fe(CO) 5 ], 1,2-hexadecanediol, oleylamine, and oleic acid were purchased from Sigma-Aldrich. Platinum(II) acetylacetonate [Pt(acac) 2 ] was obtained from Strem Chemical. All of the reactions were carried out using standard Schlenk-line technique. 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Mater. 2008, 20, 475–478 10.1021/cm7028068 CCC: $40.75  2008 American Chemical Society Published on Web 12/22/2007