Enhanced Magnetic Transition of Core-Shell Cobalt-Platinum Nanoalloys
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Enhanced Magnetic Transition of Core-Shell Cobalt-Platinum Nanoalloys Jong-Il Park, Nam-Jung Kang, Sang-Min Lee, Sehun Kim, S. J. Oh1, H. -C. Ri1, Jinwoo Cheon2* Department of Chemistry and School of Molecular Science (BK21), Korea Advanced Institute of science and Technology (KAIST), Taejon, 305-701, Korea. 1 Korea Basic Science Institute (KBSI) Taejon, 305-333, Korea. 2 Department of Chemistry, Yonsei University, Seoul, 120-749, Korea. ABSTRACT Synthesis of ‘solid solution’ and ‘core-shell’ types of well defined Co-Pt based nanoalloys smaller than 10nm have been achieved by redox transmetalation reactions. This redox transmetalation are selectively observed only if the redox potential between two metals is favorable. The composition of the magnetic alloys can also be tuned by adjusting the ratio of reactants. Annealed core-shell nanoparticles transformed into mixed nanoalloys with face centered tetragonal (fct) structures, which show large coercivity and ferromagnetism at room temperature. These nanoparticles can potentially be used as an independent single magnetic bit of tera-bit information storage. Also, this kind of redox transmetalation reaction can be utilized as a general process to synthesize various types of nanoalloys with controlled composition in a selective fashion. INTRODUCTION Controlled synthesis of nanomaterials have drawn interest due to the novel opto-electronic, magnetic, and catalytic properties that arise from the quantum size effects and large surface areas that are characteristic of these species [1]. In particular, recent efforts have concentrated on the development of magnetic nanoalloys due to the potential use of each nanoparticle as an independent magnetic bit for future information storage systems [2-5]. The magnetic and chemical properties of the monometallic elements are known to be significantly improved by the formation of alloys with additional metals and they provide many advantages such as high magnetic anisotropy, enhanced magnetic susceptibility, and large coercivities [4-7]. Specifically, the CoPt alloy is one of the candidates for ultrahigh-density magnetic recording media because of its high magnetic anisotropy, good chemical stability and resistance to corrosion [4, 6, 7]. Most of the magnetic alloy syntheses to date have been focused in the area of thin films by using vacuum deposition techniques [7]. However, random nanoparticle nucleation and growth, relatively large crystallite sizes, and broad size distributions adversely affect their magnetic performance. Recent reports suggest solution based chemical routes are an excellent alternative method, allowing better control of magnetic nanomaterials growth [5, 6]. In this paper, we would like to explore the redox transmetalation reactions for the synthesis of two different types, solid solution and core-shell, of CoPt nanoalloys under 10 nm. Stoichiometry tuned solid solution type CoPt nanoalloys are synthesized by the reaction between Co2(CO)8 and Pt(hfac)2. Core-shell type nanoalloys in which Pt resides as a shell arou
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