Monodisperse 3 d Transition-Metal (Co, Ni, Fe) Nanoparticles and Their Assembly into Nanoparticle Superlattices
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Monodisperse 3d
Transition-Metal (Co, Ni, Fe) Nanoparticles and Their Assembly into Nanoparticle Superlattices
spins aligned in a single direction.3 Further reducing the size of a single-domain particle decreases the number of spins exchange-coupled to resist spontaneous reorientation of its magnetization at a given temperature. A double-well potential (Figure 1f) is used to conceptualize the rotation of the magnetization direction for a uniaxial magnetic particle.8 The energy barrier E between the orientations is proportional to the particle volume V and the material’s anisotropy constant K, which describes the preference for spins to align in a particular direction within the particle due to the influence of crystal symmetry, shape, and surface effects. As the particle size decreases, E becomes comparable to thermal energy (kBT), and the energy barrier no longer pins the magnetization on the time scale of observation. The particle is said to be superparamagnetic. Mapping the scaling limits of magnetic storage technology8,9 and understanding
C.B. Murray, Shouheng Sun, Hugh Doyle, and T. Betley Introduction Magnetic colloids, or ferrofluids, have been studied to probe the fundamental size-dependent properties of magnetic particles and have been harnessed in a variety of applications.1–4 The magnetorheological properties of magnetic colloids have been exploited in high-performance bearings and seals.5 The deposition of magnetic dispersions on platters and tapes marked the earliest embodiments of magnet information storage.6 Magnetic particles enhance contrast in magnetic resonance imaging and promise future diagnostic and drug delivery applications.7 The need to explore the scaling limits of magnetic storage technology has motivated the preparation of size-tunable monodisperse magnetic nanoparticles8–10 with controlled internal structures. The study of these nanoparticles is critical to efforts to separate the role of defects from intrinsic, finite size effects. We report the preparation of colloidal magnetic nanoparticle samples with controlled size, surface coordination, and crystallinity that are monodisperse to 1 atomic shell. Figures 1a–d depict the stages of nanoparticle synthesis, size-selective pre-
MRS BULLETIN/DECEMBER 2001
cipitation, self-assembly, and nanoparticle superlattice formation, respectively. Figure 1e shows a schematic diagram of a model nanoparticle with its crystalline metallic core, oxidized surface, and a monolayer coat of organic stabilizers (surfactants). In this article, crystalline particles with low concentrations of defects are referred to as nanocrystals (NCs), while the more general term nanoparticle (NP) will refer to particles containing gross internal grain boundaries, fractures, or internal disorder. Comparing the size evolution of the magnetic properties in these systems reveals the importance of internal crystal structure. The magnetic properties of nanometersized particles arise from the competition between strong, short-range exchange interactions and long-range, di
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