  To develop a full understanding of interactions in nanomagnet arrays is a
persistent challenge, critically impacting their technological acceptance. This
paper reports the experimental, numerical and analytical investigation of
interactions in arrays of Co nanoellipses using the first-order reversal curve
(FORC) technique. A mean-field analysis has revealed the physical mechanisms
giving rise to all of the observed features: a shift of the non-interacting
FORC-ridge at the low-H$_c$ end off the local coercivity H$_c$ axis; a stretch
of the FORC-ridge at the high-H$_c$ end without shifting it off the H$_c$ axis;
and a formation of a tilted edge connected to the ridge at the low-H$_c$ end.
Changing from flat to Gaussian coercivity distribution produces a negative
feature, bends the ridge, and broadens the edge. Finally, nearest neighbor
interactions segment the FORC-ridge. These results demonstrate that the FORC
approach provides a comprehensive framework to qualitatively and quantitatively
decode interactions in nanomagnet arrays.
