臺灣博碩士論文加值系統

本研究在探討鎳鐵合金與鉻多層磁性薄膜與鐵鉻多層膜結構近似，但不具巨磁阻之原因。本樣品系統由磁光柯爾效應儀與極化中子反射率量測，來瞭解樣品之磁性特質。在鉻隔離層厚度約2.4奈米時，鐵磁層間存在有類似反鐵磁性耦合，但磁阻特性卻類似異向性磁阻。在低溫下此磁性耦合會轉變成鐵磁耦合或喪失耦合關係，其原因可能與鉻之尼爾溫度有關。
藉由X光散射與吸收光譜，可研究樣品之結構與界面形貌。隨隔絕層鉻之厚度變化，鉻的結構可能存在一相變厚度，由面心立方結構轉變成體心立方結構。在界面間局部的互融會影響巨磁阻效應。而不完整之隔絕層，也可能是造成巨磁阻在本系統降低之原因之一。

The magnetic coupling between two permalloy layers with Cr separating layer was studied using longitude magneto-optical Kerr effect measurement (LMOKE), polarized neutron and X-ray diffraction XRD. The coupling between two permalloy layers is antiferromagnetic-like when the Cr thickness is near 2.4 nm along the easy axis. A strong biquadratic coupling term has been found from the polarized neutron reflectivity study. The strong biquadratic coupling may be caused by the rough interface between the permalloy and Cr layers. The coupling strength is getting stronger as lower temperature but higher than a phase change temperature TN. As the temperature is lower than TN, the coupling strength drops sharply. It's believed that the TN is short-range antiferromagnetic order and long-range antiferromagnetic order of the type of an incommensurate spin density wave transition. The missing GMR effect of this system might be due to this strong biquadratic coupling of the permalloy layers.
X-ray diffraction, X-ray anomalous scattering and X-ray absorption near edge spectroscopy are used to study the structure of epitaxial films. The hysterisis loop with two-fold symmetry was discussed with the crystal structure. Usual X-ray diffraction was hard to determine the structure of Cr because the film was so thin. The anomalous X-ray diffraction and reflectivity is helpful to analyze the structure and interface roughness of Cr layer with permalloy neighbors. X-ray absorption near edge spectroscopy was also done to study the Cr structure. As the Cr layer is thinner, the absorption spectrum of Cr is more fcc-like as permalloy. On the opposite, as the Cr layer is thicker, the spectrum of Cr is more similar to bcc bulk Cr. The results imply that the Cr layer with permalloy neighbors could have slight of interdiffusion. Another possibility is the thin Cr layer could be strained by permalloy neighbors and formed psudomorphic fcc structure.

Chapter 1. Introduction 01
Chapter 2. Theory & experimental methods
2.1 Magnetic coupling and magnetoresistance 04
2.2 Spin density wave (SDW) phenomena using Cr spacer layer 11
2.3. X-ray and polarized neutron reflectivity 18

Chapter 3. Sample preparation 27
Chapter 4. Magnetic properties
4.1 Longitude magneto-optical Kerr effect (MOKE) measurement 31
4.2 Magnetoresistance calculation 37
4.3 SQUID Measurement 41
Chapter 5. X-ray diffraction measurements on growth mode 45
Chapter 6. X-ray analyze on diffusion at interface
6.1 X-ray absorption near edge fine structure 51
6.2 X-ray Anomalous scattering 59
Chapter 7 .Polarized neutron reflectivity and X-ray reflectivity studies on magnetic Interface
7.1 X-ray reflectivity and PNR measurement 64
7.2 PNR measurement at low temperature 70

Chapter 8. Conclusion 72
References 73
Appendix A
A.1 The procedure of sample preparation
A.2 The sample list and the LMOKE measurements of Ni80Fe20/Cr/Ni80Fe20 system
A.3 The results of the magnetoresistance measurement
A.4 The simulation spectra of polarized neutron reflectivity
Appendix B
B.1 Curriculum Vitae
B.2 Major publications copies in first author

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