| Summary: | 碩士 === 國立中正大學 === 物理所 === 94 === High quality Fe(12nm)/Gd(6nm)/Fe(12nm)/MgO(1.6nm)/FeCo (30nm)/MgO(001) film is fabricated by molecular beam epitaxy in the absence of any external magnetic fields. After structural and magnetic characterizations using XRD and VSM, these films are patterned into oval-shaped device-array with cell size 40 to 100 square micrometers. Magnetoresistance (MR) of 20 cells is measured using a four-probe ac bridge technique and most of the MR ratios showed ~0.1%. The free layer switching field extracted from MR measurements is ~ 32 Oe which is similar to the coercivity (Hc) of the non-patterned MTJ film. Strikingly, about 1/3 of the cells show clearly negative MR ratios . In addition, the switching filed decreases down to 8 Oe for these cells. This result can be attributed to be similar to the previous observation on rare earth-transition metal GMR effect, where it is claimed that the Gd layer forces the adjacent Fe layer to be antiparallel in the high field state and parallel in a low-field state [1]. As a result, since 4f electron does not contribute to MR it shows an inverse MR effect for the present configuration. Although the present MTJ configuration shows relatively low MR ratio, the interplay between 4f and 3d spins is identified in the present study. Furthermore, the I-V curve of the sample presents non-linear effects, which are consistent with Simmon's theory. In this paper, we also discuss the influence in adding the Gd into MTJ and the way to improve MR ratio in the future.
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