Junction size dependence of the spin-waves modes in perpendicular magnetic tunnel junction nanopillars

Perpendicularly magnetized tunnel junctions (pMTJs) nanopillars with CoFeB free and reference layers are being intensely explored for applications as spin-torque switched magnetic memories. They also provide interesting structures to study spin-waves in confined geometries. Here we study spin waves modes as a function of nanopillar diameter (50 to 100nm) and applied in-plane field by measuring thermal ferromagnetic resonance (FMR) spectra at room temperature. For 50 and 60 nm diameters only one mode is observed. While for larger diameters, 2 modes with a spacing of about 2GHz is observed. At field large enough to saturate free layer, the 2 modes have almost the same variation in frequencies with applied field, which suggested they are both associated with the free layer. Micromagnetic simulation show that for small devices only the fundamental spin-wave mode is accessible in our experiments, while for larger devices higher order modes with radial nodes can be excited, but with a very small amplitude.