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 (K_eff ~ 2.9×10⁵J/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.