Magnetization reversal in finite size dot arrays: Global Configurational Anisotropy

We analyzed with MFM and MOKE finite squared (rectangular) arrays of circular (elliptical) magnetic dots and performed simulations with MuMax, a GPU-based software. We showed as for limited size of the periodic arrays the transition of the magnetization during the reversal starts at the edges and corners of the array and propagates inside the pattern, so that in a restricted field range the magnetization results to be not uniformly distributed. While the shape of the dots (circular, elliptical, etc.) introduces a Configurational Anisotropy, we find that the finite array dimensions introduce an additional Global Configurational Anisotropy. Both effects originate at the demagnetizing interactions playing at different space scales: the dot and total array space scale, respectively. Simulations of dot arrays are often restricted to one dot assuming isolated non-interacting magnetization processes. Periodic boundary conditions are often used to incorporate interdot interactions, still limiting computations to a restricted number of dots and assuming infinite lattice periodicity. Then, configurational anisotropy is accounted for, but global configurational anisotropy is not. We show that mutual dot interactions together with finite array dimensions have a nonnegligible impact on the magnetization reversal of a dot array. We numerically and experimentally study the hysteresis properties of Permalloy (Py) arrays of 16x16 circular and elliptical dots, with thickness ranging between 10 and 25 nm and lateral size between 300 and 500 nm. In magnetooptical Kerr effect (MOKE) measurements, in-field magnetic force microscope (MFM) measurements and simulations, we find that global shape anisotropy steers the magnetization reversal of the array: the dots run through different magnetization states depending on the dot location and collective magnetization processes occur, leading to transition avalanches and formation of magnetization chains. Moreover, we find that imperfections as edge roughness and external perturbations, as the MFM measurement itself, anticipate the dots reversal path set by the global configurational anisotropy and promote field induced magnetization state changes. These findings are important in the development of applications that rely on a robust control of dot magnetization states in dot arrays