Magnetic Order in Multilayer Films with antiferromagnetic interlayer coupling

J. J. MELO QUINTERO; G. P. SARACCO; M. A. BAB

Workshop; SPIN ARGENTINA International workshop on Spintronics 2022; 2022

Advances in magnetic material fabrication techniques have made it possible to obtain multilayer filmswith nanometric thicknesses, made up of ferromagnetic layers and separated by non-magnetic layers.These exhibit interlayer exchange constants ranging from ferromagnetic to antiferromagnetic with thethickness of the intermediate non-magnetic layer. The case of multilayers with antiferromagneticexchange constants between layers was crucial for the discovery of phenomena of great interest innanotechnology and spintronics, such as giant magnetoresistance and tunnel magnetoresistance. Thesematerials are commonly referred as synthetic antiferromagnets. In multilayers with a strongperpendicular anisotropy, alternate stripes domain structures have been experimentally observed, wherethe magnetization is perpendicular to the film, presenting different interlayer coupling patterns. Theseconfigurations can be related to the competition between exchange and dipolar interactions, andconsequently 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 thelatter in order to consider the increase of the thickness of the non-magnetic layer. In this way, theHamiltonian 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 usingMonte Carlo simulations for the values of the constants J1 = 1, J2 = 2, J3 = -0.5, -1 and -1.5, and keepingg1 = g2 = 1. In all cases, at low temperatures, the equilibrium configurations consist of alternating spinstripes 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 intersectperpendicularly, and finally reach the paramagnetic phase. It should be noted that the described phasesare present in the phase diagram of a monolayer with J = 2, which also includes a nematic phase asintermediate between h = 2 and TL. The analysis of the non-equilibrium dynamic behavior of theorientational order parameter and its moments allows us to characterize the h = 2-TL transition ascontinuous, obtaining critical temperatures that increase with J3, as well as determining the absence ofthe nematic phase.