Ga Implantation in a MgO-based Magnetic Tunnel Junction With $\hbox {Co}_{60}\hbox {Fe}_{20}\hbox {B}_{20}$ Layers

A Co₆₀Fe₂₀B₂₀-based tunneling magnetoresistance multilayer stack with an MgO barrier has been exposed to 30 keV Ga ions at doses corresponding to ion etching and metal deposition in a focused ion beam (FIB) instrument, to study the applicability of these processes to magnetic tunnel junction (MTJ) fabrication. MTJs were fabricated and irradiated to investigate how the exposures affected their coercivity and magnetoresistance. Elemental depth profiles, acquired using electron spectroscopy for chemical analysis, showed that Ga gathered in and around the two Co₆₀Fe₂₀B₂₀ layers. Correlated with the results of the magnetic measurements, this Ga presence was found to cause a reduction of magnetoresistance and an increase in coercivity. Quantitatively, a dose of 10¹⁴Ga⁺·cm^-2 reduced the magnetoresistance by 60%, whereas a dose of 10¹⁵Ga⁺·cm^-2 reduced the magnetoresistance by 67% and also increased the coercivity by 2 mT and changed the dipole coupling between the sensing and the pinning layers by 1.6 mT. The latter was attributed to an imbalance in the synthetic antiferromagnetic structure, where the stack´s Ru spacer served as an implantation barrier. The magnetoresistance was lost at a dose of 10¹⁶Ga⁺·cm^-2. Annealing reduced the content of Ga around the magnetic layers but also caused diffusion of Cu from one of the layers in the stack. Apart from the observation and explanation of implantation damages in the multilayer, this work concludes on the applicability of FIB processes for prototyping of MTJs.