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Three-dimensional fabrication of nanostructures opens unique possibilities to understand not only the fundamental effects of the shape of and interactions between the nanostructures but also technological applications. This is of particular importance for magnetic nanostructures, where curvilinear geometry can lead to the emergence of topological effects and spin excitations. In this work, we used the focused electron beam ion deposition (FEBID) method to fabricate three-dimensional magnetic cobalt nanohelices with controlled geometry such as chirality and curvature. Using a combination of off-axis electron holography and electron tomography, we are able to determine the way in which the quantitative nanoscale magnetization distribution is related to the 3D morphology and curvature of the nanohelices. These results pave a way forward for development of future 3D magnetic nanostructures to explore novel physics as well as for applications based on domain walls such as magnetic memory and magnetic field sensors.

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The effect of curvilinear geometry and resulting boundary conditions in confined magnetic nanostructures can lead to novel exchange-driven interactions such as effective anisotropy and an antisymmetric vector exchange anisotropy [1]. These novel effects are dependent on the curvature and the curvature-gradient of the nanostructure, and result in magnetochiral and topologically-induced spin textures that have led to theoretical predictions of unlimited domain wall velocities and spin chirality symmetry breaking [2]. In order to understand the interplay between geometric topology and magnetic spin topology, it is critical to understand the magnetic domain configuration and the domain wall behavior in such curved nanostructures at high-resolution and in three-dimensions. Lorentz transmission electron microscopy (LTEM) provides a unique combination to characterize not only the microstructure but also the magnetic domain structure of such nanofibers at a high spatial resolution [3].In this work, we have fabricated curved magnetic nanohelices using direct focused electron beam ion deposition (FEBID) from Co precursor. This was achieved using a Co precursor gas in the gas injection system on a FEI Nova FIB. We were able to control the growth rate and susbequently the three-dimensional structure of these nanohelices using various electron beam currents and electron accelerating voltage. We will discuss in detail the growth of such nanohelices as shown in Figure 1. The three-dimensional morphology of the nanohelices was studied using electron tomography. We calculated the variation of mean curvature along the helix, which can then be correlated with the resultant magnetic domain structure. The magnetic domain structure was studied using combination of off-axis electron holography and transport-of-intensity formalism. Figure 2 shows the magnetic phase shift, and resultant magnetic induction map from one such helix. We will further discuss the results of correlating the magnetic domain structure with the geometric curvature of the 3D nanohelices, as well as present the three-dimensional magnetization mapping.

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