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

Abstract

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 degrees 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.

Charge distribution and Fermi level in bimetallic nanoparticles

“…In recent years, bimetallic nanoparticles have received increasing attention due to their promising electrocatalytic, [1][2][3] catalytic, [4][5][6][7][8] magnetic, 4,9,10 and optical properties. 4,11 The interaction of nanoparticles with their environment can, sometimes drastically, shift the Fermi level of electrons in the nanoparticles, influencing their chemical and electrochemical properties, as highlighted in a recent review.…”

Prospects for nanoparticle-based permanent magnets

“…First, synthesis of magnetic nanoparticles with precise control over the size, size distribution and phase purity is challenging, owing to the requirement of hightemperature thermal annealing >500 °C under thermalequilibrium conditions [13][14][15]. Second, the particles must be assembled with their easy axes aligned to obtain a large remanent magnetization (M r ).…”

Surface modification-induced phase transformation of hexagonal close-packed gold square sheets

“…Conventionally, the phase transformation of inorganic nanocrystals is realized under high pressure 7,[10][11][12] or at an elevated temperature 6,8,13 . In addition, the electron beam-induced phase transformation has also been observed, such as the transformation of Cu 2 S nanorods from low-chalcocite to highchalcocite structures 14 .…”