3D Nanomagnetic Architectures Investigated by micro-Hall Sensors

Abstract

Three-dimensional (3D) magnetic nanostructures open a path for more complex, functional, and novel magnetic properties by allowing the magnetization to orientate itself out of a plane in comparison to two-dimensional (2D) systems. Taking full advantage of the remarkable properties, the possibility of creating complex 3D networks of nanomagnets will very likely trigger the development and emergence of novel spintronics devices such as ultrahigh density memories that store information in three-dimensions. However, 3D nanostructuring is highly challenging and constrained by existing capabilities in standard fabrication and characterization techniques. Here, an advanced combination of focused electron beam-induced deposition (FEBID) and ultra-sensitive micro-Hall magnetometry based on a home-built GaAs/AlGaAs micro-scaled sensor with a two-dimensional electron gas (2DEG) as an active layer is employed to directly engineer individual CoFe 3D nano-architectures and probe the 3D spin distributions. Their magnetization reversal was investigated by comprehensive measurements in a wide range of temperatures and angles of the applied magnetic field supported by corresponding macrospin and micromagnetic simulations. Firstly, for 2x2 arrays of nano-trees and nano-cubes, applied field and temperature protocols are employed to probe the effective thermal dynamics, which reveals thermally activated processes originating from their intricate three-dimensional structure. Such findings enhance the basic knowledge about the thermally-activated magnetization dynamics and may open new avenues for research. Moreover, by investigating the angular dependence of magnetic hysteresis loops supported by macrospin and micromagnetic simulations, it becomes possible to elucidate their overall switching behavior. A highly complex switching process with a vortex-like magnetization distribution across the magnetic elements is observed. Also, advanced hysteresis loop measurements represented by first-order reversal curves (FORCs) display signatures of nonuniform vortex magnetization states. Secondly, for 2x2 arrays of nano-tetrapods (single-units of intricate diamond lattice) grown in plus and cross arrangements, which was motivated by achieving different dipolar coupling, fine-truing of the applied field angle reveals that the resulting hysteresis loops show a complex step-like switching and development of abrupt jumps around the remanence at higher angles of the applied magnetic field, i.e., close to parallel to the sensor plane. The micromagnetic simulations are in favourable agreement with the experiment and unveil that the switching process is mostly sequential from one shape anisotropy-dominated magnetic element to another and generally proceeds by vortex states nucleation and annihilation scenarios promoting a circular path being initiated at the borders of the elements. Also, some elements reverse their magnetization via the nucleation and propagation of vortex domain wall-like structures. Noticeable differences in the shape of the hysteresis loops are observed for both arrays. This suggests that the source of the variance is likely linked to the dipole-dipole coupling formed between the nearest-neighbour units. With an extensive analysis of FORC diagrams, in particular, characteristic butterfly-like signatures are observed providing strong support for the interpretation of the reversal process by vortex states nucleation and annihilation in nonuniformly magnetized elements. The work presented in this thesis describes a comprehensive investigation of individual 3D magnetic nanostructures. The insights gained from this study may be of assistance to facilitate the advanced study and exploitation of the rapidly expanding field of 3D nanomagnetism which is imperative for the development of modern computing, sensing, and biological applications.