Tunneling spectra of sputter-deposited CoFeB/MgO/CoFeB magnetic tunnel junctions showing giant tunneling magnetoresistance effect

We measured differential tunneling conductance (dI/dV, d2I/dV2) spectra of spin-valve-type magnetic tunnel junctions (MTJs) with a MgO(001) tunnel barrier layer and amorphous CoFeB ferromagnetic electrodes that show 315% magnetoresistance (MR) ratio at 4.3 K. The dI/dV spectra showed clear reduction in the conductance at around ±400 mV for a parallel magnetic configuration. Such anomalous spectra have never been observed for MTJs with an amorphous Al-O barrier. The d2I/dV2 spectra showed several distinct peaks between 5 and 100 mV. Magnon excitations are assigned to an origin of those peaks and thought to be a dominant process to reduce MR at finite bias voltage.