Journal of Magnetism and Magnetic Materials 231 (2001) L9–L12 Letter to the Editor Magnetic properties of Fe nanoparticles trapped at the tips of the aligned carbon nanotubes X.X. Zhang a, *, G.H Wen a , Shaoming Huang b , Liming Dai b , Ruiping Gao c , Zhong L. Wang c a DepartmentofPhysics,UniveristyofScienceandTechnologyHong Kong,ClearWaterBay,Kawloon,Hong Kong,China b CSIROMolecularScience,Bag 10,ClaytonSouth,Victoria3169,Australia c SchoolofMaterialsScienceandEngineering,GeorgiaInstituteofTechnology,AtlantaGA30332-0245,USA Received 23 November 2000 Abstract The magnetic properties of iron nanoparticles partially encapsulated at the tips of aligned carbon nanotubes have been studied. The carbon nanotube wall not only protects the metallic particles from oxidization, but also reduces the inter-particledipolarinteractionbynon-magneticseparation.Magneticcharacterizationsperformedinthetemperature range of 5–350K with magnetic field up to 3T show that these carbon-nanotube-supported iron particles are good candidates for high-density magnetic recording media. # 2001 Elsevier Science B.V. All rights reserved. PACS: 36.40.Cg; 61.48.+C; 75.50.Tt; 61.46.+W Keywords: Fe nanoparticles; Aligned carbon nanotube; Recording media Magnetic nanoparticles have been fabricated using a variety of techniques [1–3], and many interestingphenomenahavebeenobserved[1,3–5]. Their novel properties are the focus of many fundamental research and practical applications [1,2,4–7]. Most of the current studies are on oxide nanoparticles, because it is difficult to prevent metallic magnetic (Fe,Co,Ni) nanoparticles from oxidization at conventional experimental condi- tions. Several techniques, such as carbon encapsu- lation [8–11], reagent stabilizing [12], and passivation of nanoparticles [13], have been devel- oped to protect the metallic particles. Magnetic properties of the carbon-encapsulated nanoparti- clesandnanowireshavebeeninvestigatedrecently [14,15]. The metallic particles in the carbon cages and films are believed to be a potential candidate for the high-density magnetic recording media [8,9]. It is well known that transition metals, such as Fe and Co, are mandatory for the nucleation and growth of carbon nanotubes by pyrolysis [16]. In preparation of aligned carbon nanotubes by pyrolyzing iron (II) phthalocyanine, FePc, it has been demonstrated that the nanotube growth involves two iron nanoparticles [17]. A small particle serves as the nuclei of the nanotube and a large particle locates at the growth front to be responsible for the growth of the nanotubes [17]. *Corresponding author. Fax:+852-2358-1652. E-mailaddress: phxxz@ust.hk (X.X. Zhang). 0304-8853/01/$-see front matter # 2001 Elsevier Science B.V. All rights reserved. PII:S0304-8853(01)00134-2 LETTER TO THE EDITOR