Domain formation in rectangular magnetic nanoparticle assemblies

Abstract

Rectangular assemblies with different aspect ratios were prepared with spherical magnetite nanoparticles (diameter $d=20\phantom{\rule{0.16em}{0ex}}\mathrm{nm}$) on lithographically patterned substrates using a variant of the meniscus force deposition method. The aspect ratio (width:length) of the rectangular assemblies was varied from very low (1:1) to very high (1:1000) values. Angle-dependent ferromagnetic resonance measurements were performed to study the influence of the shape anisotropy on the magnetic properties. Using an analytical model based on the Smit-Suhl formalism for single-domain magnets, the demagnetizing factors were determined. The analysis of the resonance signals shows that, for small aspect ratios, the ratio of the in-plane demagnetization factors is inversely proportional to the corresponding ratio of width:length; that is, the assemblies behave like a single ferromagnet due to dipolar magnetic coupling between the particles. At larger aspect ratios a more complicated behavior is observed which indicates the formation of a multidomain-like structure inside the assemblies caused by geometrical inhomogeneities in the filling of the assemblies. Micromagnetic simulations of the magnetic properties of the assemblies support this assumption qualitatively and suggest that the formation of these inhomogeneities can be controlled by the fabrication process. The results provide insights into the collective magnetic behavior of nanoparticle assemblies, highlighting the high degree of tunability of the magnetic properties of such assemblies, which makes them promising building blocks for future magnetic devices.