Takeharu Kuroiwa

Suppression of Switching-Field Variation by Surface Oxidation Depending on the Shape of the CoFeB Free Layer

We investigated the suppression of the variation of switching field Hsw by reducing magnetization M in submicron-sized magnetic tunnel junctions (MTJs) by oxidizing the surface of the CoFeB layer. Examination confirmed a reduction in the M of oxidized CoFeB film. The results also confirmed that oxidization enlarges anisotropy field Hk of CoFeB films. We applied this film to the free layer of submicron-sized MTJs of various aspect ratios. The results revealed that Hsw variation depending on the aspect ratio is reduced more with oxidation than without. We confirmed that Hsw variation in submicron-sized MTJs originating in the CoFeB free layers shape decreased due to the oxidation, which reduced the switching field distribution (SFD) for the MTJs

Read-cycle endurance of magnetic random access memory elements

The read-cycle endurance of a magnetic tunneling junction (MTJ) has been investigated, focusing on the spin-dependent tunneling current at high temperatures. The MTJ structure used in this study consisted of Ta-NiFe-CoFe-AlOx-CoFe-IrMn-NiFe-Ta prepared on a thermally oxidized Si wafer. Both the tunneling current and tunneling magnetoresistance ratio showed no significant degradation during the 1E9 read-cycle test using unipolar voltage pulses of 0.5 V at room temperature. Temperature dependence of the MTJs resistance calculated from the measured tunneling current has also been examined in a temperature range from room temperature to 175/spl deg/C. Furthermore, in the case of read-cycle tests for thermally stressed MTJs, the variation of the tunneling current was less than 2% after 1E9 cycles at a stress temperature of 175/spl deg/C. This indicates that MTJs have sufficient read-cycle endurance under high-temperature operation.