Evidence for magnon excitation contribution to the magnetoresistance behavior during thermal annealing in CoFeB/MgO/CoFeB magnetic tunnel junctions

Abstract

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.