Appl Phys A (2013) 113:61–66 DOI 10.1007/s00339-013-7856-3 RAPID COMMUNICATION Decoration of carbon nanotube with size-controlled L1 0 -FePt nanoparticles for storage media Reza Moradi · Seyed Ali Sebt · Hadi Arabi · Majid Mojtahedzadeh Larijani Received: 12 April 2013 / Accepted: 10 July 2013 / Published online: 20 July 2013 © Springer-Verlag Berlin Heidelberg 2013 Abstract In this work, first multi-wall carbon nanotubes (MWCNTs) with outer diameter about 20–30 nm are syn- thesized by a CVD method; they have been purified and functionalized with a two-step process. The approach con- sists of thermal oxidation and subsequent chemical oxida- tion. Then, monosize FePt nanoparticles along carbon nan- otubes surface are synthesized by a Polyol process. The syn- thesized FePt nanoparticles are about 2.5 nm in size and they have superparamagnetic behavior with fcc structure. The CNTs surfaces as a substrate prevent the coalescence of particles during thermal annealing. Annealing at the tem- perature higher than 600 ◦ C for 2 h under a reducing atmo- sphere (90 % Ar + 10 % H 2 ) leads to phase transition from fcc to fct-L1 0 structure. So, the magnetic behavior changes from the superparamagnetic to the ferromagnetic. Further- more, after the phase transition, the FePt nanoparticles have finite size with an average of about 3.5 nm and the coercivity of particles reaches 5.1 kOe. R. Moradi (B ) · S.A. Sebt Department of Physics, Science and Research Branch, Islamic Azad University, P.O. Box 14665-678, Tehran, Iran e-mail: reza.moradi.58@gmail.com Fax: +98-151-2277733 H. Arabi Magnetism and Superconducting Research Lab., Physics Department, University of Birjand, Birjand, Iran H. Arabi Department of Physics, Faculty of Science, Ferdowsi University of Mashhad, Mashhad, Iran M.M. Larijani Agricultural, Medical and Industrial Research School, NSTRI, Karaj, 31485/498, Iran 1 Introduction In recent years, magnetic nanoparticles have attracted exten- sive attention because of their extremely diverse applications in magnetic recording media, permanent magnets, magnetic sensors and drug delivery in biomedical applications [1– 4]. L1 0 FePt nanoparticles with face-centered tetragonal (fct) structure have very high magnetocrystalline anisotropy (k u = 7 × 10 7 erg/cm 3 ) and also they have very high co- ercivity which makes them suitable candidates for using in ultrahigh density magnetic recording media [5]. Chemical methods are adequate to produce nanoparticles with uniform size distribution. But, the nanoparticles synthesized using the chemical method have disordered A1 phase with face- centered cubic (fcc) structure, they have small anisotropy and show superparamagnetic behavior at room temperature [6]. Therefore, an additional thermal treatment process is re- quired to transform the nanoparticles from the disordered A1 phase to the ordered L1 0 phase. With respect to the ag- gregation of nanoparticles at high temperature, nanoparti- cle size increases and makes them inappropriate for using at data storage media [7]. Various methods have been suggested for preventing the aggregation of nanoparticles, such as decreasing of phase transition temperature with using the additive metals like Au, Ag and B 2 O 3 . The salt method (NaCl) able to prevent the sintering of nanoparticles, the direct synthesis method, rapid thermal annealing and core/shell structure are all in- teresting approaches to prevent coalescence of magnetic nanoparticles [8–15]. But the results after annealing show that in these methods the coercivity decreases significantly. To solve this problem, an appropriate substrate is required during the annealing process. Carbon nanotubes (CNTs) have received great attention because of their electrical, optical and mechanical proper- ties and high thermal stability [16]. In this paper, we suggest