For sputtered CoFeB/MgO/CoFeB magnetic tunnel junctions, it is well known that the tunnel magnetoresistance (TMR) ratio increases with increasing annealing temperature ($T_a$) up to a critical value ($T_p$), and then decreases with further increasing $T_a$, resulting in a peak around $T_p$. The improved crystallinity of the MgO barrier and CoFeB electrodes due to annealing has been considered as the main reason for the enhancement of the TMR ratio, especially for $T_a<T_p$. In this work, the evidence is provided that the magnon excitation plays a great contribution to the magnetoresistance (MR) behavior in annealed samples based on the measurement of dynamic conductance and inelastic electron tunneling (IET) spectra. The magnon activation energy ($E_c$) obtained from the fits for IET spectra exhibits a similar temperature dependence with that of the TMR ratio. A detailed analysis shows that the magnon excitation, together with improved crystallinity of the MgO barrier and CoFeB layers, is the main contribution to the annealing-temperature-dependent MR behavior.

• Received 17 January 2011

DOI:https://doi.org/10.1103/PhysRevB.83.224430