Effect of CoFe Dusting Layer and Annealing on the Magnetic Properties of Sputtered Ta/W/CoFeB/CoFe/MgO Layer Structures*

Ultrathin CoFeB films are an essential building block of most emerging spintronic applications due to the tunable perpendicular magnetic anisotropy (PMA) energy, low Gilbert damping and high annealing stability. Improvements in these magnetic properties are being explored through interface engineering. We explored the effect of a CoFe wedge inserted as a dusting layer (0.1 nm – 0.3 nm thick) at the CoFeB/MgO interface of a sputtered Ta(2 nm)/W(3 nm)/CoFeB(0.9 nm)/MgO(3 nm) film – a typical structure for spin-orbit torque devices. Films were annealed in a Rapid Thermal Annealer (RTA) at temperatures varying between 300 °C and 400 °C in an Ar environment. Ferromagnetic resonance studies were carried out to estimate the effective PMA field and the Gilbert damping as a function of the CoFe thickness and across several annealing temperatures. While as-deposited films present only easy-plane anisotropy, a transition along the wedge from easy-plane to out-of-plane was observed across several annealing temperatures. Interlaying a CoFe dusting film between CoFeB and MgO provides an alternative approach for PMA in ultrathin films.