Magnetic Order in Multilayer Films with antiferromagnetic interlayer coupling

J.J. MELO QUINTERO; BAB, MARISA ALEJANDRA; SARACCO, GUSTAVO PABLO

Bariloche

Workshop; The International Workshop on Spintronics ?Spin Argentina 2022?; 2022

Advances in magnetic material fabrication techniques have made it possible to obtain multilayer films with nanometric thicknesses, made up of ferromagnetic layers and separated by non-magnetic layers. These exhibit interlayer exchange constants ranging from ferromagnetic to antiferromagnetic with the thickness of the intermediate non-magnetic layer. The case of multilayers with antiferromagnetic exchange constants between layers was crucial for the discovery of phenomena of great interest in nanotechnology and spintronics, such as giant magnetoresistance and tunnel magnetoresistance. These materials are commonly referred as synthetic antiferromagnets. In multilayers with a strong perpendicular anisotropy, alternate stripes domain structures have been experimentally observed, where the magnetization is perpendicular to the film, presenting different interlayer coupling patterns. These configurations can be related to the competition between exchange and dipolar interactions, and consequently their thermal behavior can be studied by using the Ising model with dipolar interactions. In this work, we model the multilayer film with a system consisting of two bidimensional ferromagneticlattices coupled by means of an antiferromagnetic exchange constant, studying different values of the latter in order to consider the increase of the thickness of the non-magnetic layer. In this way, the  Hamiltonian of the model depends on the exchange (J1, J2) and dipole (g1, g2) constants of each layer, and on the coupling constant between layers, J3. The thermal behavior of the model is studied by using Monte Carlo simulations for the values of the constants J1 = 1, J2 = 2, J3 = -0.5, -1 and -1.5, and keeping g1 = g2 = 1. In all cases, at low temperatures, the equilibrium configurations consist of alternating spin stripes of width h = 2 in both layers, antiferromagnetically coupled. When the temperature is increased, both layers become the liquid tetragonal phase (TL), which is characterized by stripes that intersect perpendicularly, and finally reach the paramagnetic phase. It should be noted that the described phases are present in the phase diagram of a monolayer with J = 2, which also includes a nematic phase as intermediate between h = 2 and TL. The analysis of the non-equilibrium dynamic behavior of the orientational order parameter and its moments allows us to characterize the h = 2-TL transition as continuous, obtaining critical temperatures that increase with J3, as well as determining the absence ofthe nematic phase