Magnon-assisted inelastic excitation spectra of a ferromagnetic tunnel junction

Inelastic-electron-tunneling spectroscopy (IETS) has been applied to investigate the spin dependent tunneling process for a $\mathrm{Ta}\text{(50\hspace{0.05em}Å)/}{\mathrm{Fe}}_{\mathrm{20}}{\mathrm{Ni}}_{\mathrm{80}}\text{(60\hspace{0.05em}Å)/}\mathrm{IrMn}\text{(300\hspace{0.05em}Å)/}\mathrm{Co}\text{(60\hspace{0.05em}Å)/}\mathrm{Al}\text{(13\hspace{0.05em}Å)-}\mathrm{oxide/}$ $\mathrm{Co}\text{(40\hspace{0.05em}Å)/}{\mathrm{Fe}}_{\mathrm{20}}{\mathrm{Ni}}_{\mathrm{80}}\text{(200\hspace{0.05em}Å)}$ spin-valve-type tunnel junction. IET spectra for both parallel and antiparallel magnetization configurations of ferromagnetic electrodes showed a strong peak at 12 mV. The subtraction spectrum defined by the difference between the spectra of both the configurations was calculated. Spin-independent inelastic excitation processes are not affected by an external magnetic field, and thus, the subtraction spectrum indicates the inelastic modes induced only by the magnetic origin. It showed a strong peak at 12 and 20 mV for the positively and negatively biased direction of the bottom electrode, respectively. The tendency of the tunneling magnetoresistance ratio to decrease with bias voltage agreed with the shape of the subtraction spectrum. By assuming the surface magnon excitation, we obtained the distributions of the correlation length and the Curie temperature for both ferromagnetic electrode surfaces faced on the insulator.