Dynamic and Magnetotransport Properties of Perpendicularly Magnetized CoFeB Magnetic Tunnel Junctions

Abstract

We present the study of the dynamic properties of CoFeB (CFB) based magnetic tunnel junctions (MTJ) by Brillouin light scattering spectroscopy and micro-strip ferromagnetic resonance technique as well as magnetoresistance measurements. Two different MTJs: single and double barrier systems have been investigated. The dynamic studies revealed mostly similar perpendicular anisotropy field for both CoFeB layers for the single barrier MTJ with the thickest CFB layers, while two sensibly different perpendicular anisotropy fields were revealed in the case of the double barrier MTJ with the thickest CFB soft and hard layers. Modeling the spin wave frequencies behavior allows us the analysis of the experimental data and the derivation of the pertinent magnetic parameters. Our results suggest an interfacial origin of the anisotropy. Therefore, by changing the thickness of the perpendicularly magnetized CFB layers, it is possible to obtain a free and pinned layer usually ensured by the use of synthetic antiferromagnetic material. The tunnel magnetoresistance has been measured in elliptical elements of double barrier MTJ of different sizes. Strong size-dependent switching fields and antiparallel state being more unstable in the smallest device are observed. Patterning effect, lowering the anisotropies around the edge areas and increasing the dipolar coupling that favors the parallel state in the case of stacked layers, are probably the origin.