Exchange energies in CoFeB/Ru/CoFeB synthetic antiferromagnets

The interlayer exchange coupling confers specific properties to synthetic antiferromagnets that make them suitable for several applications of spintronics. The efficient use of this magnetic configuration requires an in-depth understanding of the magnetic properties and their correlation with the material structure. Here we establish a reliable procedure to quantify the interlayer exchange coupling and the intralayer exchange stiffness in synthetic antiferromagnets; we apply it to the ultrasmooth and amorphous Co40Fe40B20 (5-40 nm)/Ru/ Co40Fe40B20 material platform. The complex interplay between the two exchange interactions results in a gradient of the magnetization orientation across the thickness of the stack which alters the hysteresis and the spin wave eigenmodes of the stack in a nontrivial way. We measured the field dependence of the frequencies of the first four spin waves confined within the thickness of the stack. We modeled these frequencies and the corresponding thickness profiles of these spin waves using micromagnetic simulations. The comparison with the experimental results allows us to deduce the magnetic parameters that best account for the sample behavior. The exchange stiffness is established to be 16 +/- 2 pJ/m, independent of the Co40Fe40B20 thickness. The interlayer exchange coupling starts from -1.7 mJ/m(2) for the thinnest layers, and it can be maintained above -1.3 mJ/m(2) for CoFeB layers as thick as 40 nm. The comparison of our method with earlier characterizations using the sole saturation fields argues for a need to revisit the tabulated values of interlayer exchange coupling in thick synthetic antiferromagnets.

Automated summary

We developed a reliable method to quantify the interlayer exchange coupling and intralayer exchange stiffness in synthetic antiferromagnets, and applied it to a specific material platform. We found that the interplay between the two exchange interactions leads to a gradient of magnetization orientation across the stack, which nontrivially affects its hysteresis and spin wave eigenmodes. Using micromagnetic simulations and experimental measurements, we determined the magnetic parameters that best describe the sample behavior, revealing the independence of exchange stiffness on thickness and the variation of interlayer exchange coupling with thickness.