Linear Chains of Nanomagnets: Engineering the Effective Magnetic Anisotropy

Abstract

We have investigated the control of effective magnetic anisotropy in Permalloy (Ni80Fe20) linear chain arrays by tuning the symmetry arrangements of the ellipsoidal nanomagnets and the film thickness. The coupled ellipsoidal nanomagnets are arranged in two distinct configurations (Fig. 1 (a) and (b)), namely: type-I, where elements are arranged along the major axis of the ellipsoid, and type-II, in which the neighboring elements are coupled along the minor axis of the ellipsoid [1, 2]. Hence, the shape anisotropy of each nanomagnet and the configurational anisotropy due to the lattice arrangements favor the same direction of magnetization for type-I arrays while competition exists between the shape and configurational anisotropy for type-II arrays. The hysteresis loops depicted in Fig. 2 (a) and (c) clearly show the variation of the effective anisotropy with thickness which is also supported by the ferromagnetic resonance (FMR) spectra, shown in Fig. 2 (b) and (d) for type-I and type-II arrays respectively. A clear transition from a single domain states to a combination of complex flux closure states such as vortex, double vortices are observed at different applied field angles (φ) when the film thickness is varied in the range from 20 nm to 100 nm. Tunable microwave absorption spectra established the complex interplay between the shape anisotropy and magnetostatic interactions at different film thicknesses and applied field angles. Micromagnetic simulations are in good agreement with the experimental results. Our results demonstrate the possible ways of manipulating the effective magnetic anisotropy in the arrays of nanomagnets for magnonics and microwave applications.