Colloids and Surfaces A: Physicochem. Eng. Aspects 301 (2007) 419–424 Fabrication of CoPt nanoparticles with high coercivity on a polymer film Jung Hoon Kim, Jeon Kim, Kwang H. Baek, Dong Hyun Im, Chang Kyung Kim, Chong Seung Yoon ∗ Division of Materials Science and Engineering, Hanyang University, Seoul 133-791, Republic of Korea Received 18 October 2006; received in revised form 22 December 2006; accepted 2 January 2007 Available online 12 January 2007 Abstract L1 0 structured CoPt nanoparticles were fabricated on a polyimide film by depositing a 1.7-nm-thick Co–Pt alloy film, followed by annealing at 650–800 ◦ C during which the Co–Pt alloy film was converted into a monolayer of 4 nm sized L1 0 phase CoPt particles. In order to produce well- separated larger particles, multiple depositions of 1.7-nm-thick Co–Pt alloy film were carried out. Heterogeneous nucleation on the pre-existing CoPt nanoparticles allowed the film deposition and annealing to be repeated up to four times to increase the average particle size from 4 nm to 7 nm as needed. The monolayer of 7-nm-szied CoPt nanoparticles exhibited a high coercivity of 9.8 kOe with a remanence ratio of 0.72. The method can be easily extended to a large area as well as other material systems to create a monolayer of metal nanoisland structures on a polymer film. © 2007 Elsevier B.V. All rights reserved. Keywords: CoPt; Magnetic nanoparticle; Monolayer; Polyimide 1. Introduction L1 0 structure is a derivative variant of fcc structure in which Co and Pt atoms occupy lattice sites in a ordered manner instead of randomly occupying the lattice sites as in a solid solution [1]. The ordering converts the fcc structure to a tetragonal struc- ture and provides extremely large magnetocrystalline uniaxial anisotropy (hence, large coercivity) to L1 0 structured CoPt and FePt. Because of the large magnetocrystalline anisotropy, CoPt and FePt have become an important class of materials for future ultra-high density magnetic recording applications [2] as Co- based conventional granular magnetic recording materials are rapidly approaching the physical limit imposed by the super- paramagnetic effect. FePt and CoPt, expected to overcome the effects of thermal fluctuation and demagnetizing field, have been prepared in the form of a thin film [3–6] and nanoparticles [7–16] by a number of means. To produce a bit pattern, electron-beam lithography can be used to produce an array of sub-100 nanome- ter bits by pattering the continuous thin film [17]; however, this technique is not practical due to low throughput. Alternatively, ∗ Corresponding author. E-mail address: csyoon@hanyang.ac.kr (C.S. Yoon). CoPt and FePt magnetic nanoparticles can be arranged into a highly regular lattice using surfactant chemistry [10,11]. Such self-organized arrays of CoPt nanoparticles have been produced using various techniques, based largely on chemical methods [7–11] and physical deposition methods [12–15] or, recently, using protein engineering [16]. In general, compared to physi- cal deposition techniques, chemical methods produce superior quality nanoparticles in terms of size distribution and compo- sition. Chemical methods, however, typically require organic surfactants to drive the self-assembly of pre-fabricated nanopar- ticles so that monolayer particle array configuration on a large area substrate cannot be easily achieved. Compared to chemical methods, physical deposition methods can attain the monolayer configuration relatively easily. How- ever, physical deposition methods rely on thermal annealing at temperatures exceeding 650 ◦ C to induce ordering of the L1 0 structure, which, therefore, requires thermally robust substrates and excludes use of an organic substrate. The aim of the present study is to demonstrate that it is possible to prepare a monolayer of L1 0 CoPt nanoparticles on a polyimide (PI) film for possi- ble applications in a flexible high density data storage device. An additional rationale for using a polymer substrate for thin film deposition is the higher probability of obtaining discrete nanoparticle configuration rather than a continuous film CoPt, 0927-7757/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2007.01.005