Magnetotransport measurements of connected and unconnected NiFe artificial spin ice lattices

Artificial spin ice analogues formed from hexagonal arrays of individual magnetic nanoislands or connected nanowires have been studied via magnetotransport measurements. Asymmetry in the magnetoresistance data at temperatures below 50K has previously been attributed to long-range order and chiral loop states forming at the edges of the lattice. After capping the magnetic lattice with a nonmagnetic non-conducting layer to avoid oxidation there was no evidence for this asymmetry below 25K. A complex combination of effects due to exchange bias and the onset of a modified magnetisation reversal mechanism are responsible for the observed asymmetry in uncapped samples. Electrical transport measurements of an unconnected hexagonal lattice of artificial spin ice show an increase in the magnitude of the anisotropic magneto-resistance compared with connected lattices. The switching signal size in an unconnected lattice is 0.1% of the overall measurement resistance, compared to just 0.01% in a connected lattice. This results from magnetic domain walls no longer being free to propagate through the lattice, in comparison to the case of the connected lattice, forcing a domain wall nucleation in each individual nanowire. Connected artificial spin ice lattices can be made to mimic their unconnected counterparts by reducing the magnetic nanowire width at the vertices. This restricting lattice impedes magnetic domain wall propagation in a manner similar to the unconnected lattice. A lattice with 300nm-wide wires requires 25% restriction to completely prevent the domain wall propagation, compared with just 7% in a 150nm wire. Finally a two-layer square lattice has been created, in order to produce an artificial spin ice with equivalent magnetostatic interactions between all nanowires across a vertex. This new variant of artificial spin ice has been achieved by vertically offsetting two magnetic sub-lattices via a non-magnetic polymeric spacer layer