| Summary: | 碩士 === 國立中興大學 === 材料科學與工程學系所 === 97 === The structural and magnetic properties of NiFe(10 nm)/Mn-oxide(20 nm) bilayers were investigated. Transmission electron microscopy results have shown that the top NiFe layer consisted of a f.c.c. NiFe phase (a~ 3.55 Å). The bilayer bottom consisted of either b.c.c. α-Mn, rocksalt MnO, tetragonal Mn3O4, or a composite phase, depending on the percent of O2/Ar ratio used during dual ion-beam deposition. The grain sizes of these polycrystalline NiFe/Mn-oxide bilayers range from 5 nm to 15 nm.
Magnetometry results at 5K indicate that the exchange bias field (Hex~ −300 Oe) is largest in a NiFe/Mn bilayer. The smallest Hex(~ −75 Oe) was found in NiFe/Mn-oxide (21% O2 /Ar ) bilayers. This is ascribed to the MnO (low magnetocrystalline anisotropy) formation by in situ Mn oxidation. In contrast, a further increase in the O2 /Ar ratio during deposition results in larger Hex(~ −150 Oe) and Hc. This is attributed to the oxidation of MnO into a harder ferrimagnet Mn3O4 (high magnetocrystalline anisotropy). The variations of the Hex are attributed to different bottom layer structures (Mn, MnO, and Mn3O4).
However, Moderate ion-beam bombardment on the surface of the Mn layer may create uncompensated Mn spins that results in an enhanced Hex in the NiFe/Mn (VEH= 70V) bilayer. A misalignment of spins at the FM/AF interface from the effects of a higher ion-beam bombardment energy resulted in a drop in Hex.
The magnetotransport studies have shown that these NiFe/Mn-oxide bilayers exhibit the anisotropic magnetoresistance (AMR) behavior. The total MR ratio measured at 77 K is larger than at room temperature, ascribed to the reduced interfacial scattering between FM and AF layer. The NiFe/Mn-oxide(41% O2/Ar) bilayer has the largest total MR ratio(~3.26%) among all samples at 77K. It is due to the strong anisotropic scattering at the interface.
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