Tuning shape, exchange and surface anisotropy of core/shell NPs

Nanoscale Magnetism Control via Surface and Exchange Anisotropy for Optimized Ferrimagnetic Hysteresis.
Seung-hyun Noh, Wonjun Na, Jung-tak Jang, Jae-Hyun Lee, Eun Jung Lee, Seung Ho Moon,
Yongjun Lim, Jeon-Soo Shin, and Jinwoo Cheon
Nano Letters 12, 3716 (2012)

Morphological and structural evolution of magnetic nanoparticle and correlated tunability of nanomagnetism. (a) Magnetic NPs with various structural motifs exhibiting differences in size, surface anisotropy, and exchange anisotropy. (b) Magnetism tuning by the systematicchanges of magnetic nanoparticles. Graphs i−iv correspond to the nanoparticles shown in part a where modulation of structural motifs is needed to control parameters such as K, Hc, Ms, or Mr.

Images and magnetization behaviors of cube and sphere nanoparticles. (a) TEM images of cube (18 nm (σ ≈ 5%) in edge length) and (b) sphere nanoparticle (22 nm (σ ≈ 7%) in diameter). Nanoparticles have identical composition (Zn_0.4Fe_2.6O₄) and magnetic volume (5.8 × 10^–24 m³). (c) High resolution TEM image of cube exhibiting well-defined lattice fringes of {100} faces. (d) M-H curves of cube and sphere measured at 300 K using SQUID. M_s of cube is 165 emu/g_(Fe+Zn), and that of sphere is 145 emu/g_(Fe+Zn). Simulated magnetic spin states of (e) cube and (f) sphere by using OOMMF program. The color map indicates the degree of spin canting against external magnetic field (B₀) where red indicates nondeviated spins and blue indicates highly canted spins. Local spin states on the surfaces of nanoparticles are depicted on the right corners of parts e and f. Cube exhibits lower spin disorder rate of 4% than sphere of 8%.