Quantitative imaging of the magnetic field distribution in an artificial spin ice studied by off-axis electron holography

The magnetic state, including the stray fields, of a chiral pattern of interacting permalloy nanomagnets is studied using off-axis electron holography in the transmission electron microscope. The projected in-plane magnetisation of the nanomagnets is reconstructed from the experimental magnetic phase shift using model-based iterative reconstruction. The thickness and chemical composition of the nanomagnets are characterised in cross-sectional geometry. The average value of the magnetic polarisation of the permalloy through the thickness of the sample is measured to be 0.73 T. This value is lower than the bulk value of 1 T, likely as a result of a combination of the microstructure, composition and possible oxidation of the nanomagnets. The experimental results are compared to micromagnetic simulations to confirm the magnetic states and to understand the switching processes in the magnetic nanoislands.