Size and shape effects on the order-disorder phase transition in CoPt nanoparticles

• Published in: Nature materials Vol. 8; no. 12; pp. 940 - 946
• Main Authors: Alloyeau, D, Ricolleau, C, Mottet, C, Oikawa, T, Langlois, C, Le Bouar, Y, Braidy, N, Loiseau, A
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 01.12.2009; Nature Publishing Group
• Subjects: Biomaterials; Chemical engineering; Chemistry and Materials Science; Condensed Matter Physics; Magnetism; Materials Science; Monte Carlo simulation; Nanoparticles; Nanotechnology; Optical and Electronic Materials; Physical properties; Transition temperatures
• ISSN: 1476-1122; 1476-4660
• DOI: 10.1038/nmat2574

Chemically ordered bimetallic nanoparticles are promising candidates for magnetic-storage applications. However, the use of sub-10 nm nanomagnets requires further study of possible size effects on their physical properties. Here, the effects of size and morphology on the order–disorder phase transition temperature of CoPt nanoparticles ( T C NP ) have been investigated experimentally, using transmission electron microscopy, and theoretically, with canonical Monte Carlo simulations. For 2.4–3-nm particles, T C NP is found to be 325–175  ∘ C lower than the bulk material transition temperature, consistent with our Monte Carlo simulations. Furthermore, we establish that T C NP is also sensitive to the shape of the nanoparticles, because only one dimension of the particle (that is, in-plane size or thickness) smaller than 3 nm is sufficient to induce a considerable depression of T C NP . This work emphasizes the necessity of taking into account the three-dimensional morphology of nano-objects to understand and control their structural properties. The structure of magnetic nanoparticles has a strong influence on the properties of these materials at present being considered for magnetic-storage applications. It is now shown that size and shape of magnetic nanoparticles such as CoPt affect the transition from an ordered to a disordered phase, highlighting the need to take morphology into account to understand the structural properties.