Off-axis electron holography was used to investigate the barrier profile of the $\mathrm{Py}∕\mathrm{Al}{\mathrm{O}}_x∕\mathrm{Zr}{\mathrm{O}}_y∕\mathrm{Py}$ magnetic tunnel junctions with different $\mathrm{Zr}{\mathrm{O}}_y$ thicknesses. The tunneling magnetoresistance (TMR) has a strong dependence on bias voltage and the bias voltage for maximum TMR is shifted from zero. This shift increases with $\mathrm{Zr}{\mathrm{O}}_y$ barrier thickness due to the increasing barrier asymmetry in the junctions. The evolution of barrier asymmetry was directly observed by the phase change of the off-axis electron holography, which unambiguously shows the barrier profile changes from triangular to trapezoidal shape as increasing of $\mathrm{Zr}{\mathrm{O}}_y$ thickness.

• Received 21 November 2006

DOI:https://doi.org/10.1103/PhysRevB.75.134420