Synthesis and magnetic properties of gold coated iron oxide nanoparticles J. Appl. Phys. 105, 07B504 (2009); 10.1063/1.3059607 Synthesis of self-assembled monodisperse 3 nm FePd nanoparticles: Phase transition, magnetic study, and surface effectThe monodisperse Fe 3 O 4 nanocubes with controllable sizes from 6.5 to 30.0 nm have been synthesized in one pot. The shape-induced texture of the self-assembled nanocube superlattices has been deposited onto substrates by a convective assembly technique. The cubelike shapes affect the crystalline orientation of individual particles within the self-assembled superlattices compared with the nanoparticles with an isotropic spherical shape. The nanocubes with a size less than 25 nm show a typical superparamagnetic behavior at room temperature. Comparison of the hysteresis loop at 5 K of cubes with different sizes reveals a size-dependent behavior of saturation magnetization and coercivity.

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Monodisperse magnetite (Fe3O4) nanoparticles (NPs) were synthesized and coated using a SiO2 shell with controlled thickness ranging from 3.0 to 20.0 nm. The temperature-dependent zero-field-cooled (ZFC) and field-cooled (FC) magnetizations of the 7.5 nm Fe3O4 NPs with systematically increasing interparticle spacing were studied using the continuous and intermittent cooling protocol. The experimental evidence from dc magnetization and simulated ZFC/FC curves reveal that the increasing interparticle spacing modulated the collective magnetic behavior by effectively lowering the interparticle dipolar coupling, and for 7.5 nm Fe3O4 NPs a noninteracting particle system formed with interparticle spacing above 31.5 nm.

Equiatomic FePt and CoPt nanoparticles with the ordered L1 0 structure are attractive as ultrahigh density magnetic recording media. In a recent work, chemically synthesized fcc-FePt nanoparticles with narrow size distribution and their self-assembled array with close-packed microstructure has been achieved successfully. However, the particles coalesced during the subsequent annealing step necessary to obtain L1 0 FePt nanoparticles. In the present study, we have successfully demonstrated the direct synthesis of L1 0 FePt nanoparticles at low temperature of 553 K using the "modified polyol method" without subsequent annealing, whose diameter is 5 -10 nm and intrinsic magnetocrystalline anisotropy field ͑H k ͒ is 31 kOe. This indicates that precisely controlling the reaction kinetics, especially low reduction rate through optimizing the polyol/Pt mole ratio and type of polyol are very important for directly synthesizing the L1 0 FePt nanoparticles. Furthermore, we investigated the size, morphology and composition dependence of the magnetic properties of FePt nanoparticles in order to clarify the L1 0 ordering mechanism. As a result, clear evidence of the existence of the critical diameter for the thermodynamical L1 0 ordering is not observed in the size range above 2 nm. Furthermore, the recrystallization and sintering process can be a driving force for promoting the L1 0 ordering, and hence, Fe and Pt atom diffusion at the grain boundary plays an important role to the L1 0 ordering of FePt nanoparticles.

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