Exchange Stiffness in pMTJ Free Layers by Magnetometry and Spin-Torque Ferromagnetic Resonance

Exchange interactions play a critical role in setting the scale of micromagnetic structure and the spin-wave excitation spectrum in nanometer scale magnetic elements. Specifically, in perpendicularly magnetized free layers of magnetic tunnel junction (pMTJ) nanopillars, exchange, magnetic anisotropy and dipolar interactions determine the spin-wave modes and magnetization reversal pathways. Here we report studies of the exchange stiffness A in composite CoFeB free layers in pMTJ (extended) thin films. Vibrating sample magnetometry (VSM) was used to determine the free layer magnetization versus temperature, which is fit to Bloch’s law to determine A. We further perform field modulated spin-transfer FMR (ST-FMR) measurements on 30 to 80nm diameter nanopillars to infer the confined spin-wave modes, providing, in principle, an independent means of determining A. We discuss the correspondence between these methods and the nature of the spin-transfer excited spin-wave modes in such pMTJ nanopillars.