Collective behavior in arrays of coupled ferromagnetic magneto-dipole

Abstract

The analytical model of ellipsoid chains (ECh) has been used in several works to describe magnetic (static and dynamic) properties. In the present work the model validity was tested, applied to hexagonal arrays of Ni ECh by means of Prolate ellipsoids with transversal diameter of 30 nm and longitudinal dimensions of 60 nm. Two different chains were tested, one formed by 1 ellipsoid and the other formed by 6 ellipsoids. Two inter-chains distances were tested to analyze the strong dipole interaction effects in the arrays. Hexagonal arrays of 100 chains (constituted by 1 and 6 ellipsoids) were analyzed with the inter-chain distance of 45 nm and 60 nm. The dimensions were chosen motivated by the experimental results reported in literature in ferromagnetic nanowires deposited in porous alumina membranes. The ferromagnetic resonance results were generated by means of micromagnetic simulation through the Nmag package. A standard problem was executed, in which the magnetization deviated from the equilibrium direction and, through the temporal dependence, the main frequency was determined by means of a Fast Fourier Transformation. The simulation results were analyzed and fitted by using the analytical model based on ellipsoids (with uniaxial anisotropy). The effective field of magnetic anisotropy was obtained from the fitting of the above analytical model with the calculated data. From the results, even with ellipsoids in such small sizes (close to the one domain configuration for Ni particles), the model in which the magnetization is assumed to be homogeneous is not completely valid. The best correspondence was in the limiting case of longer inter-chain distances. This shows the limitations of the analytical model, often used in systems with very close particles.