Kojiro Komagaki

Inverse tunnel magnetoresistance in magnetic tunnel junctions with an Fe4N electrode

The magnetotransport properties of Fe4N∕MgO∕CoFeB and Fe/MgO/CoFeB magnetic tunnel junctions (MTJs) were investigated at room temperature. In the Fe/MgO/CoFeB-MTJ, normal tunnel magnetoresistance (TMR) effect and roughly symmetric bias voltage (VB) dependence were observed, similar to the MTJs exhibiting coherent tunneling such as Fe/MgO/Fe. On the other hand, the inverse TMR effect, showing higher tunnel resistance for parallel magnetization configuration than for antiparallel configuration, and strong asymmetric VB dependence of TMR ratio were observed in the Fe4N∕MgO∕CoFeB-MTJ. The maximum TMR magnitude of 18.5% was obtained at VB=−200 mV, where the current flows from Fe4N to CoFeB. The enhancement of the inverse TMR ratio around VB=−200 mV is due to the broad peak of tunnel conductance in antiparallel configuration of Fe4N and CoFeB magnetizations. A large peak of the density of state at +300 meV from the Fermi level for minority spin electrons of bulk Fe4N might be an origin of this phenomenon.

In situ heat treatment of ultrathin MgO layer for giant magnetoresistance ratio with low resistance area product in CoFeB/MgO/CoFeB magnetic tunnel junctions

In order to promote the grain growth of ultrathin MgO barrier deposited on a CoFeB layer, in situ infrared (IR) heat treatment just after the deposition of MgO barrier was examined. In case that IR heat treatment was not applied, tunneling magnetoresistance (TMR) ratio of CoFeB/MgO/CoFeB magnetic tunnel junction (MTJ) was significantly decreased with decreasing resistance area (RA) product to less than 10 Ω μm2. On the other hand, TMR ratio of 206% was achieved at the RA product of 2.1 Ω μm2 when the IR heat treatment was applied. According to the cross sectional transmission electron microscope images for the samples with 0.76-nm-thick (∼4 ML) MgO barrier, the (001) oriented well crystallized structure with smooth interface was observed for the IR heated sample. Moreover, it revealed that the lateral grain size of MgO was significantly enlarged compared to that for the sample without IR heating. The improvement of TMR properties at low RA product region by the heat treatment might be due to the decrease ...