Micromagnetic behavior of permalloy (Ni80Fe20) nanodots as a function of aspect ratio

Abstract

We present the results of computational micromagnetic simulations at zero temperature under free boundary conditions for Permalloy nanodots. The nanodot’s diameter ($D$) was varied from 20 to 120 nm, and the thickness ($t$) ranged from 4 to 120 nm, which allows to obtain different aspect ratios $t/D$. Simulations were conducted using the Ubermag platform and the Object Oriented Micromagnetic Framework (OOMMF). The hysteresis loops exhibited a strong dependence on the aspect ratio ($t/D$), which was evident in the narrowing of the hysteresis curves as this ratio approached unity. This phenomenon led to the formation of nucleation and annihilation fields, resulting in the formation of vortex-type magnetic textures with a central core capable of moving within the basal plane. Furthermore, the time dynamics at each step of the magnetic field were addressed by solving the time-dependent Landau–Lifshitz–Gilbert differential equation, where the system’s Hamiltonian is defined in terms of magnetocrystalline anisotropy, demagnetization, exchange, and Zeeman contributions. Energy diagrams illustrate the competition among these energies, attempting to attain their equilibrium state, thereby creating a complex energy landscape. Moreover, they operate on different orders of magnitude, whence their relative importance is discussed. Final results are summarized in a proposal of phase diagrams.