Superstructure magnetic anisotropy in Fe3O4 nanoparticle chains.

Magnetic anisotropy is essential for many applications of ferromagnetic/ferrimagnetic materials, including permanent magnets and magnetic recording media. Attempts have been made recently to build up 3-D nanoparticle and quantum dot assemblies, however, it is not understood yet if a nanoparticle assembly can possess high magnetic anisotropy with low anisotropic materials. In this article, we report our discovery of high magnetic anisotropy resulted from Fe3O4 nanoparticle chains. We started with closely-packed nanoparticle assemblies of spherical Fe3O4 nanoparticles that exhibit low magnetocrystalline anisotropy and shape anisotropy, and corresponding negligible coercivity. When the nanoparticle assemblies are compressed under pressure, they form bundles or arrays that consist of Fe3O4 chains with a length scale of several hundred nanometers. Magnetic measurements show that these Fe3O4 chain arrays possess a high uniaxial magnetic anisotropy (Keff ~ 2.9×105 J/m³) and significant magnetic coercivity. Our simulations reveal that interparticle magnetic dipolar interactions contribute to this type of superstructure magnetic anisotropy. This study demonstrates the feasibility and approaches to create "patterned" high magnetic anisotropy in nanoparticle superstructures/assemblies.