Stripe-like domain nucleation in synthetic antiferromagnetic multilayers revealed by soft spin-wave modes

Abstract

Magnetism is one of the most active and dynamic areas of physics, attracting great interest from scientists in recent decades. This interest relates not only to its relevance in the fundamental understanding of matter but also to its role in the development of new technologies. At the microscopic level, magnetism originates primarily in the intrinsic magnetic moments of atoms, associated with the spin, and the interaction between these moments. These fundamental properties give rise to a variety of collective phenomena, including magnetic ordering in ferromagnetic material and magnetic moment excitations, known as spin waves, which can carry information through the material without generating a net flow of electric charge. This thesis focuses on the investigation of magnetization instability processes in ferromagnetic multilayer systems composed of thin and thick layers. These processes are explored by studying spin wave dynamics in the framework of micromagnetism, particularly through the soft modes, which are specific spin wave modes whose frequencies are close to zero, indicating that the system is approaching a phase transition or magnetic instability. The results indicate that in thin systems, the instability is localized at the surface or within the volume, depending on whether the number of layers is even or odd. Furthermore, when perpendicular anisotropy is considered, instability can occur under certain conditions through the formation of stripe domain textures. As the system becomes thicker, the formation of stripe domains becomes more favored. A phase diagram is obtained by analyzing the soft modes, which enables the prediction of conditions for stripe domain formation. The periodicity of the stripe domains is also determined by the soft mode properties. When the ferromagnetic state is close to destabilization, the stripe domains exhibit chiral properties. In contrast, when the antiferromagnetic state is at the onset of reversion processes, the domains are non-chiral, preferring only one rotation sense. These results offer new insights into predicting the onset of the reversal processes without calculating the complex magnetic textures, but just analyzing the soft spin-wave modes.