Ferromagnetic resonance of perpendicular magnetic anisotropy MgO/CoFeB based tri-layers

Magnetic tunnel junctions with perpendicular magnetic anisotropy (PMA) are good candidates for next generation spin-transfer-torque magnetic random access memory (STT-MRAM) [1]. Besides a comparatively higher thermal stability than in-plane MA, the recently reported appearance of PMA in MgO/Co40Fe40B20/Ta structures allows for lower switching currents while maintaining high quality MgO barrier growth.
In this report we use the ferromagnetic resonance technique to investigate the magnetic properties of a CoFeB layer sandwiched by Ta and MgO layers. Samples were grown by RF or DC sputtering using a TIMARIS cluster deposition tool (Singulus Technologies AG) [2] and annealed in N2 environment for 30 minutes at temperatures between 150 and 250°C in steps of 50°C. FMR measurements are performed at room temperature using a microwave cavity [3]. Thin films are also investigated by standard magnetometry measurements in a SQUID/VSM and also by the extraordinary Hall effect to extract the saturation magnetization and effective anisotropy.
Through FMR we are able to simultaneously extract the out-of-plane anisotropy, the effective magnetization and the damping co-efficient, as a function of annealing temperature. Our results show that post-annealing systematically shifts the magnetic easy axis from in-plane to out-of-plane direction. As the annealing temperature is increased the effective magnetization also increases.
[1] S. Ikeda et al., Nature Materials 9,721–724 (2010).
[2] B. Ocker, Singulus Technologies AG, Hanauer Landstrasse 103, 63796 Kahl am Main, Germany.
[3] J. Lindner and M. Farle: Magnetic Anisotropy of Heterostructures, STMP 227, 45–96 (2007).