| Summary: | 碩士 === 國立中正大學 === 物理所 === 98 === The study of ferromagnetic layers with perpendicular anisotropy has attracted great attention due to the potential application on the high density magneto-resistance random access memory (MRAM). Among these materials, Co/Pt multilayered structure, which has high magnetic anisotropic constant and simple fabrication process, becomes one of the most promising candidates. In this thesis, a series of Co/Pt multilayers are fabricated by dc and rf sputtering and they are designed for the studies of the ferromagnetic/antiferromagnetic pinning effect and the thickness dependence of the Pt layer. The characterizations of thee films include: magnetoresistance (MR), extraordinary hall effect (EHE), vibrating sample magnetometer (VSM), and magnetic force microscope (MFM). Four different configurations are fabricated as follows.
(1)SiO2/Co(4Å)/Pt(28Å)/Co(4Å)/Pt(xÅ)/Co(4Å)/Pt(20Å)
(2)SiO2/Co(4Å)/Pt(28Å)/Co(4Å)/Pt(6Å)/Co(4Å)/Pt(tÅ)/IrMn(xÅ)
/Pt(20Å)
SiO2/Co(3Å)/Pt(28Å)/Co(3Å)/Pt(6Å)/Co(3Å)/Pt(2Å)/IrMn(xÅ)
/Pt(20Å)
(3)SiO2/MgO(15Å)/Co(2.5Å)/Pt(28Å)/Co(2.5Å)/Pt(6Å)/Co(2.5Å)
/Pt(20Å)
(4)SiO2/MgO(15Å)/Co(2.5Å)/Pt(28Å)/Co(2.5Å)/Pt(6Å)/Co(2.5Å)
/Pt(2Å)/IrMn(60Å)/Pt(20Å)
In configuration (1): All Co layers have fixed thickness 4 Å and the top two Co layers are separated by a Pt layer with fixed thickness of 28Å. The second and the third Co layer are separated by a Pt layser with either 6 Å or 28 Å. This configuration allows us to test whether the Pt thickness may affect the magnetic coupling in this multilayer configuration. A single magnetization hysteresis loop is observed for the configuration with the Pt thickness of 6 A while a double M-H loops are observed for the configuration with the Pt thickness of 28 A.
In configuration (2): This configuration is similar to the first configuration except an IrMn(x) layer is deposited as the top pinning layer and a extra Pt(t) is also deposited between IrMn(x) layer and top Co layer to prevent the inter-diffusion. The pinning effect is observed from the structure of SiO2/Co(4)/Pt(28)/Co(4)/Pt(6)/Co(4)/Pt(t)/IrMn(x)/Pt(20). An asymmetrical triple M-H hysteresis loop shows interesting magnetization reversal process. Further, the increase of the Pt(t) thickness, the triple loop reduces to a double loop indicating the reduction of the antiferromagnetic pinning effect.
In configuration (3):MgO(15) layer was deposited as the buffer layer to improve the crystalline quality of configuration (1). Each Co layer’s thickness was also changed to 2.5 Å. The results of EHE show similar hysteresis loops as configuration (1). The MR measurement showed the consistency of the change of MR with parallel and antiparallel magnetic states observed from the EHE. Highest MR ratio was about 0.05%. In addition, MR oscillations were observed during the variation of the magnetic states from parallel to antiparallel, which may indicate intermediate states exist in the perpendicular coupled system.
In configuration (4): A MgO (15 Å) layer was deposited as the buffer layer in configuration (2) to improve the film quality and each Co layer’s thickness was 2.5Å. The results of EHE show similar hysteresis loops as configuration (2). The MR measurement showed the consistency of the change of MR with parallel and antiparallel magnetic states observed from the EHE. Highest MR ratio was about 0.02%. MR oscillations were also observed during the variation of the MR regions which is similar to the films of configuration (3).
MFM measurements showed domain structure similar to bubble-like domain and these domains can be generally interpreted by the competition of four origins: (a)exchange coupling in each Co layers, (b) antiferromagnetic coupling between two Co layers through Pt(28Å) layer, (c) the pinning effect from IrMn layer, and (d) anisotropic energies.
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