Magnetic configurations of isolated and assemblies of iron 30 nm nanocubes studied by electron holography

The magnetic remanent configurations of single and assemblies of crystalline 30 nm Fe nanocubes were studied using off axis electron holography (EH). Experiments were performed on the SACTEM-Toulouse (Tecnai-F20 microscope (FEI)) fitted with an aberration corrector (CEOS) in which we use the first transfer lens of the Cs corrector as a pseudo Lorentz lens to perform EH [1]. The magnetic contribution to the phase shift was obtained by recording two holograms between which the Fe cubes were magnetized in two opposite directions. This is achieved by tilting the sample by +45° and switching the microscope objective lens to apply a large field of about 2 Tesla to the sample. The objective lens was then switched off and the sample is tilt back to zero to record a first hologram. This procedure is done a second time but tilting the sample by −45°. This procedure relies on the ability to saturate the magnetization in the sample in two opposite directions before decreasing the magnetic field to zero. EH experiments performed on an isolated cube (Figure 1) evidence a vortex state which fulfilled the largest cylinder, fitting perfectly the central part of the cube. The magnetization continuously curls around the [001] axis with components in the (001) plane and turns progressively in the [001] axis with components in the (001) plane and turns progressively in the [001] direction when approaching its core. Additional features are visible along the [001] edges of the nanocube. In the four corners close to the border of the cube the magnetic flux cannot circulate around the vortex. The resulting magnetostatic energy is however reduced creating a flux closure around the [001] edges of the cube (see zoom in Figure 1d). This magnetic configuration was simulated by micromagnetic calculations carried on using the three-dimensional version of the micromagnetic code OOMMF [2].