Bimagnetic Core/Shell FePt/Fe 3 O 4 Nanoparticles Hao Zeng, †,§ Jing Li, ‡ Z. L. Wang, ‡ J. P. Liu, § and Shouheng Sun* ,† IBM T. J. Watson Research Center, Yorktown Heights, New York 10598, School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, and Department of Physics, UniVersity of Texas at Arlington, Arlington, Texas 76019 Received November 10, 2003; Revised Manuscript Received November 13, 2003 ABSTRACT Bimagnetic core/shell Fe 58 Pt 42 /Fe 3 O 4 nanoparticles are synthesized from high-temperature solution phase coating of 4 nm Fe 58 Pt 42 core with Fe 3 O 4 shell. The shell is tunable from 0.5 to 3 nm. Magnetic properties of the as-synthesized core/shell particles are dependent on shell thickness due to the exchange coupling between core and shell. Upon reductive annealing, an assembly of the core/shell nanoparticles is transformed into a hard magnetic nanocomposite with enhanced energy product. Surface modification of nanometer sized inorganic core with different inorganic shell to form core/shell type nanostruc- tures has become an important route to functional nanoma- terials. Such modification has brought about interesting physical and chemical properties of the nanostructured materials that have shown important technological applica- tions. The growth of a wide band gap (e.g., ZnS) semicon- ducting shell around a narrow band gap (e.g., CdSe) core has resulted in core/shell materials with higher luminescence quantum yields than single component semiconductor nanoc- rystal materials. 1-9 A core/shell type Ag/Au particle pos- sesses not only a strong surface plasma band of Ag between 390 and 420 nm but also a Au surface that allows site- specific binding to various biomolecules and may be used as optic labels for highly sensitive and selective diagnostic detection. 10 Creation of core/shell nanostructures containing metal oxide, such as MgO/Fe 2 O 3 , CaO/Fe 2 O 3 , has greatly enhanced the efficiencies of these structures over pure MgO and CaO particles as destructive adsorbents for environmental toxins, such as SO 2 and H 2 S. 11,12 Procedures leading to novel inorganic core/shell structures with controlled dimensions on both core and shell and new functionality have also been reported. 13-23 We report synthesis and characterization of bimagnetic core/shell FePt/Fe 3 O 4 nanoparticles with FePt core being 4 nm and shell Fe 3 O 4 tunable from 0.5 to 3 nm. We recently demonstrated that using self-assembly of two different magnetic nanoparticles, FePt and Fe 3 O 4 , followed by reduc- tive annealing to remove organic surfactant around each nanoparticle, a magnetic nanocomposite FePt-Fe 3 Pt could be fabricated. 24 This nanocomposite contains the modulated FePt and Fe 3 Pt phases, with the latter having dimension of about 5 nm, leading to effective exchange coupling between FePt hard phase and Fe 3 Pt soft phase, and enhanced energy product. However, this binary self-assembly approach re- quires precise control over the mass ratio and diameters of both the hard and soft magnetic nanoparticles, and the assembly conditions have to be controlled carefully to achieve the intermixing of different phases. A more conve- nient alternative to controlling the dimension of different phases and achieving the needed intermixing and exchange coupling is using self-assembly of core/shell structured FePt/ Fe 3 O 4 or Fe 3 O 4 /FePt nanoparticles. In such system, the dimensions of the hard and soft phases are tuned by the core diameter and shell thickness, and the direct contact between the core and shell ensures strong exchange coupling. Here, we demonstrate that the core/shell structured FePt/Fe 3 O 4 nanoparticles, with core and shell being exchange coupled, can be readily synthesized and used as building blocks to form hard magnetic nanocomposites. Magnetic properties of each core/shell nanoparticle can be tuned by varying the thickness of the Fe 3 O 4 shell. Such bimagnetic core/shell nanoparticles represent a novel class of nanostructured magnetic materials that will allow precise engineering of magnetic properties by selectively tuning anisotropy, mag- netization, and the dimensions of both core and shell, and can further be used to fabricate devices for novel nanomag- netic applications. The 4 nm Fe 58 Pt 42 nanoparticles were made by the combination of reduction of Pt(acac) 2 and decomposition of Fe(CO) 5 in octyl ether solvent. 25 These 4 nm FePt nanopar- † IBM T. J. Watson Research Center. ‡ Georgia Institute of Technology. § University of Texas at Arlington. NANO LETTERS 2004 Vol. 4, No. 1 187-190 10.1021/nl035004r CCC: $27.50 © 2004 American Chemical Society Published on Web 12/05/2003