Damping constant in a free layer in nanoscale CoFeB/MgO magnetic tunnel junctions investigated by homodyne-detected ferromagnetic resonance

We investigate the damping constant of a free layer with a perpendicular magnetic easy axis in nanoscale CoFeB/MgO magnetic tunnel junctions (MTJs) with a reference layer with an in-plane easy direction. The built-in noncollinear magnetization configuration in the MTJs allows us to measure homodyne-detected ferromagnetic resonance without tilting the magnetization direction of the free layer from the device normal. The damping constants determined from the spectral linewidths after the subtraction of the inhomogeneous broadening are nearly independent of the device diameter ranging from 70 to 100 nm, and take values similar to those reported for blanket CoFeB films.