Superstructure anisotropy in Fe₃O₄ magnetic nanoparticle chains

Magnetic anisotropy is essential for many applications of ferromagnetic/ferrimagnetic materials, including permanent magnets and magnetic recording media. Conventionally, significant uniaxial magnetic anisotropy originates from either magnetocrystalline anisotropy or shape anisotropy. The former requires magnetic compounds containing heavy elements, like rare earths. The latter can only be realized in low-dimensional magnetic structures. Attempts have been made recently to build up 3-D nanoparticle and quantum dot assemblies with novel collective properties, including magnetic performance. However, it is not understood yet if a nanoparticle assembly can possess high magnetic anisotropy with low anisotropic particles and how the collective anisotropy is related to the assembly geometry. In this talk, 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 neglectable magnetocrystalline anisotropy and shape anisotropy. However, 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 ~ 2.9×10⁵ J/m³) and significant magnetic coercivity, compared with the low anisotropy (K_eff ~ 9.4 ×10⁴ J/m³), and zero coercivity of the uncompressed nanoparticle assemblies. Simulations reveal that interparticle magnetic dipolar interactions contribute to this type of superstructure magnetic anisotropy which is related to.the geometric parameters of the nanoparticle chains, including the length of the chains, the inter-chain distance and the inter-particle spacing. This study demonstrates the feasibility and approaches to create “patterned” high magnetic anisotropy in nanoparticle superstructures/assemblies.