Paramagnetic FexTa1-x alloys for engineering of perpendicularly magnetized tunnel junctions

Abstract

Exchange coupling between two magnetic layers through an interlayer is of broad interest for numerous recent applications of nano-magnetic systems. In this Letter we study ferromagnetic exchange coupling through amorphous paramagnetic Fe-Ta alloys. We show that the exchange coupling depends exponentially on spacer thickness and scales with the Fe-Ta susceptibility, which can be tuned via the alloy composition and/or temperature. Such materials are of high interest for the engineering of perpendicularly magnetized CoFeB-MgO based tunnel junctions as it enables ferromagnetic coupling of magnetic layers with differing crystalline lattices, suppresses dead layers and can act as an inter-diffusion barrier during annealing.

Optimization of Tunneling Magnetoresistance in Perpendicular Magnetic Tunnel Junctions With Co|Pd Reference Layers

“…On blanket film level, the CoFeB interfacial layer was strongly coupled to the Co|Pd multilayer with up to 0.4 nm of the Ta-spacer layer. This is consistent with the recent work by Gottwald and coworkers [Gottwald 2013] However, in patterned devices, the CoFeB layer was found to decouple from the Co|Pd multilayers with Ta thicker than 0.2 nm. PMA stacks with 2 Ta|0.8 CoFeB|1 MgO|0.5 Fe|0.8 CoFeB|y Ta|[0.25 Co|0.8 Pd]x4|0.95 Ru|[0.3 Co|0.8 Pd]x8|Ru were grown with y = 0, 0.2 nm.…”

Enhanced tunneling magnetoresistance and perpendicular magnetic anisotropy in Mo/CoFeB/MgO magnetic tunnel junctions

“…The dead-layer thickness for each sample is calculated with a linear fit from the CoFeB thickness dependence of the areal magnetization (not shown), and the corresponding values for the Ta and Mo samples are 0.44 and 0.16 nm, respectively. Since the alloy layers are likely to be paramagnetic, 29 the large magnetic dead layer with Ta is in good agreement with the formation-energy difference between FeTa and FeMo alloys. As discussed previously, 20 the large negative formation energy of tantalum oxide and the crystalline nature of the Mo buffer layer might also contribute to the differences observed in the two types of samples.…”

Evolution of perpendicular magnetized tunnel junctions upon annealing

“…The ferromagnetic resonance eigenmodes indicate however substantial evolutions within the stack, with significant lowering of the anisotropies of both the Co/Pt-based parts and of the CoFeB-based parts of the tunnel junction. The exchange coupling through Ta is not robust to annealing at 400 • C, which argues for advanced texture breaking layers enriched with Co and Fe that seem to promote stronger ferromagnetic coupling 9,25 . The antiferromagnetic exchange coupling through Ru is also decaying regularly.…”