| Summary: | 碩士 === 國立中興大學 === 材料科學與工程學系 === 96 === The structural and magnetic properties of NiCo(18 nm)/(Ni,Co)O(45 nm) bilayers were investigated. X-ray diffractometry results have shown that the top NiCo layer consisted of a fcc NiCo phase (a= 3.53 Å). The bilayer bottom was either a pure rock-salt (Ni,Co)O (a= 4.26 ) or a composite [NiCo+(Ni,Co)O] phase, depending on the percent of O2/Ar ratio used during deposition. The grain sizes of these polycrystalline NiCo/(Ni,Co)O bilayers range from 5 nm to 15 nm, as revealed by Transmission electron microscopy (TEM).
A double-shifted hysteresis loop exhibiting components that were from positive or negative coupling was observed at 5 K under a field-cooling process (FC at 20 kOe) in the NiCo/(Ni,Co)O (8%O2/Ar) bilayers.
Further, an unusual exchange bias was found in NiCo/(Ni,Co)O bilayers that results from the surface of the (Ni,Co)O layer being bombarded with different Ar-ion energies using End-Hall deposition voltages (VEH) from 0 to 150 V. The largest exchange bias field, Hex~ -400 Oe at 5 K, was observed in NiCo/(Ni,Co)O bilayers (VEH= 150 V) whereas no significant Hex was found in unbombarded NiCo/(Ni,Co)O bilayers (VEH= 0 V). In addition, the coercivity, Hc, was found to change with VEH in an opposite fashion to Hex. The behavior of Hex and Hc with VEH is attributed to surface reorientation that is due to moderate ion-beam bombardment effects on the surface of the (Ni,Co)O layer.
Angular dependence of NiCo/(Ni,Co)O bilayers on Hc and Hex have shown that both positive and negative Hex was observed in zero-field-cooling process (ZFC). However, the typical negative Hex was found in the FC process indicating that the role of the cooling field is used to maintain a single FM domain state.
Direct evidence of exchange coupling between the AF (Ni,Co)O and the FM NiCo layer is provided by Photoemission electron microscopy (PEEM). The NiCo/(Ni,Co)O (8%O2/Ar) bilayer displays stripe domains which are characteristic of the exchange bias. However, a sudden change in PEEM images was observed in NiCo/(Ni,Co)O (30%O2/Ar) bilayers. This is likely due to the AF (Ni,Co)O spin reorientation from in-plane toward out-of-plane direction resulting from ion-beam bombardment.
The magnetotransport studies have shown that these NiCo/(Ni,Co)O bilayers exhibit the anisotropic magnetoresistance (AMR) behavior. The total MR ratio is larger at 77 K, ascribed to the reduced interfacial scattering between FM and AF layer.
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