Emergence of Collective Behavior Induced by Interlayer Coupling in Ferromagnetic Multilayers

Abstract

Magnetic multilayer systems have attracted considerable interest due to their relevance to emerging technologies such as spintronics and nanotechnology, as well as their theoretical interest. In films with strong perpendicular magnetic anisotropy, frustration arising from the competition between exchange and dipolar interactions leads to alternating stripe-like domains. In multilayer systems, coupling between magnetic layers separated by a nonmagnetic spacer provides an opportunity to investigate the effects of interlayer interactions on thermal behavior, magnetic ordering, and collective phenomena. In this work, we investigate a ferromagnetic bilayer system separated by a nonmagnetic spacer, in the absence of an external magnetic field. The system is modeled using the ferromagnetic Ising model, including dipolar interactions and ferromagnetic interlayer coupling. The exchange and dipolar intralayer coupling constants are fixed at J1=1 and J2=1.5, respectively, while the interlayer coupling J3 is varied within the interval [0.3, 0.5]. These values are selected from the J1- J3 phase diagram at T=0, along a line where the ground state is degenerate between two phases characterized by alternating stripe domains of equal width, h=1 and h=2, in both layers. This allows us to study the competition between the magnetic ordering patterns associated with the minimum-energy configurations. Monte Carlo simulations were performed using Metropolis and Wang–Landau algorithms. Equilibrium configurations obtained with Metropolis sampling were obtained. Short-time dynamics was employed to determine the transition temperatures between stripe-ordered and the high-temperatures disordered tetragonal liquid (TL) phases through the study of the orientational order parameter and its moments. The Wang–Landau method was employed to calculate the density of states, entropy, and specific heat, providing complementary information about the transitions. The results show that increasing the interlayer coupling enhances collective behavior between the layers, with the corresponding transition temperatures progressively converging toward a common value.

Keywords: magnetic bilayer anisotropic systems; phase transitions; collective behavior