Abstract
Three-dimensional (3D) nanomagnetism, where spin configurations extend into the vertical direction of a substrate plane allow for more complex, hierarchical systems and the design of novel magnetic effects. As an important step towards this goal, we have recently demonstrated the direct-write fabrication of freestanding ferromagnetic 3D nano-architectures of ferromagnetic CoFe in shapes of nano-tree and nano-cube structures by means of focused electron beam induced deposition. Here, we present a comprehensive characterization of the magnetic properties of these structures by local stray-field measurements using a high-resolution micro-Hall magnetometer. Measurements in a wide range of temperatures and different angles of the externally applied magnetic field with respect to the surface plane of the sensor are supported by corresponding micromagnetic simulations, which explain the overall switching behavior of in part rather complex magnetization configurations remarkably well. In particular, the simulations yield coercive and switching fields that are in good quantitative correspondence with the measured coercive and switching fields assuming a bulk metal content of 100 at % consisting of bcc CoFe. We show that thermally-unstable magnetization states can be repetitively prepared and their lifetime controlled at will, a prerequisite to realizing dynamic and thermally-active magnetic configurations if the building blocks are to be used in lattice structures.
Highlights
Three-dimensional (3D) nano-magnetism is an increasingly popular research topic that radiates into several fields of applied and fundamental research, as well as technological application [1,2,3,4].At the same time, the fabrication and magnetic characterization of 3D nano-magnetic structures faces several methodical and technological challenges
The focus of this paper is on the magnetic properties of nano-cube and nano-tree type magnetic building blocks
As discussed in [19], a simple macro-spin approach—where stem and edges of the nano-cubes are represented by a single, macroscopic spin moment with uniaxial anisotropy based on the assumption that all microscopic magnetic moments within the macrospin point in the same direction and rotate collectively—reveals that an almost vertical decrease or increase of h Bz i of the simulated up- and down-sweep curves, respectively, is associated with the rotation of the four stems, whereas smaller steps are caused by the almost simultaneous flipping and canting of the edge macro-spins. In comparison to these macro-spin simulations, the more rounded and sheared hysteresis loops with smaller area and coercive field observed in the experiments shown in Figure 2a point to a non-uniform magnetization reversal of the stems dominated by multi-domain switching events, which is confirmed by micromagnetic simulations
Summary
Three-dimensional (3D) nano-magnetism is an increasingly popular research topic that radiates into several fields of applied and fundamental research, as well as technological application [1,2,3,4]. We have presented the FEBID nano-fabrication of magnetic 3D geometries in nano-tree or nano-cube shape, as well as their arrangement in small array structures [19]. By writing these structures, consisting of metallic Co3 Fe, on custom-made micro-Hall sensor devices, we were able to measure their magnetic stray field component perpendicular to the sensor layer. The first micromagnetic simulation analysis presented in [19] already indicated that the magnetization reversal on the different edges of the nano-tree and nano-cube structures does not proceed via coherent rotation but via complex vortex-like magnetization configurations. We discuss the implications of our findings with a view to using the nano-cube and nano-tree building block in artificial lattices
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