Extending spintronics to the third dimension is regarded as one of the promising alternatives to meet the ever increasing demands for new functionalities and more energy-efficient computing technologies. In comparison to 2D computing devices, 3D structures can not only offer higher density and better device connectivity, leading to proposals such as using 3D racetracks for high density memory, but also provide access to 3D geometrical effects such as chirality and curvature, which may lead to new physics. However, significant advances in both the fabrication of 3D spintronic devices and our fundamental understanding of the influence of 3D geometries on magnetotransport which is known as the first generation of spintronics, are required before we can fully realise the promise of 3D spintronics.

In this thesis, we present new fabrication methods for the realisation of two types of 3D nanomagnetic circuits. The first one directly integrates a complicated 3D ferromagnetic cobalt nanostructure into a circuit by employing recent developments in a 3D nanoprinting technique (Focused electron beam induced deposition), allowing exploration of complex 3D geometrical effects on magnetotransport signals. The second one incorporates multi-layered magnetic thin film materials on top of 3D non-magnetic nanostructures via physical vapour deposition, paving the path for the use of high-quality spintronic materials in 3D devices.

After these key advances in nanofabrication, we experimentally studied the magnetotrans- port properties of these systems under external fields applied along multiple directions, in order to understand the underlying spin states present in these systems as well as the magnetotransport signals they generate. These were complemented with several computational tools to interpret the complex magnetotransport signals measured from 3D structures. We discovered that the three dimensionality directly affects the magnetotransport signals in several ways, including deviations from the usual angular dependence of well-known effects. Specifically, we observed an angular dependent magnon magnetoresistance which had not been reported so far in planar systems. We also observed key features in magnetoelectrical signals during magnetisation reversal that originated from curling magnetic configurations that are characteristic of 3D structures. The fabrication and characterisation methodologies developed are easily adaptable to other geometries and materials, and these findings mark the first step towards exploring new spintronic effects emerging in three dimensions and in the long run, the realisation of 3D devices.