Interfacial effects on magnetoresistance in nano-devices

다국어 초록 (Multilingual Abstract)

The lifestyles of people living in the high-speed and cutting-edge era represented by the 4th industrial revolution are changing from off-line to on-line. As on-line replaces off-line, people experience and need more digital technology. Due to this demand, the amount of information and data to be processed is increasing exponentially. The development of high-level storage and processing devices is required for rapid information processing. In particular, there is an urgent need to develop materials and process technologies to realize ultra-small size, ultra-low power, and high integration of the device. Among the various alternatives, spintronic device technology using the spin of electrons is receiving great attention. Spintronics technology is a technology that implements a new concept of device by using two physical properties of electrons, charge and spin, together, rather than improving or miniaturizing an existing semiconductor device using the charge of electrons. It controls electrons by distinguishing not only the charge of electrons but also spin information, that is, the spin-up and spin-down states. Compared to other types of information processing technology, it has high-speed operation, low power consumption and strong non-volatile properties. Many previous studies have made important advances in the industry with products using giant magnetoresistance, tunneling magnetoresistance, and spin transfer torque. It forms one axis of nanotechnology, which is a hot topic in recent years, and with the miniaturization of the nano scale, a new quantum mechanical phenomenon that has not been seen before is realized, and the technology is being developed in the field of nano-spintronics.
This paper deals with three studies applied to spintronic devices using nanomaterials. The key keyword of these studies is the interfacial effect. The effect of each unique nanomaterial on the device surface was confirmed using various electrical measurement methods.
First, the effect of the Ca-doped Bi2Se3 surface properties on the ferromagnet was confirmed through anisotropic magnetoresistance. Bi2Se3, a kind of three-dimensional topological insulator, is an insulator on the inside but a metallic material on the surface. It has a unique band structure in which the spin direction changes according to the electron motion direction. Simply put, when a current is applied in x-axis direction to the topological insulator channel, the spins are aligned in a fixed y-axis direction on the channel surface. This property is called spin-momentum locking. Anisotropic magnetoresistance determines the resistance of the channel according to the angle between the current and magnetization. When the current and magnetization are perpendicular, the scattering probability of electrons is low, which lowers the resistance, and when the current and magnetization are parallel, the scattering probability increases and the resistance increases. In the TI/FM hybrid structure, the magnetization reversal process was quantified through the effect of this surface characteristic on the magnetization of a ferromagnet having an easy-axis in the in-plane direction.
Second, the intrinsic properties and local modulation of Fe3GeTe2 were confirmed through anomalous Hall effect. Two-dimensional magnetic material has the following advantages. It is possible to separate the layers in the atomic scale and maintains the magnetic properties in the monolayer. Layers can be stacked easily without considering properties such as lattice miss-match. And it reacts sensitively to various external factors that can change the magnetic properties. (i.e. magnetic/electric field, strain, spin-torque, etc.) Fe3GeTe2 has a high TC of 200 K, a metal characteristic that is easy to apply to a spin device, and a large perpendicular magnetic anisotropy. In a ferromagnetic, spin has an easy-axis that is stable and a hard-axis that is unstable depending on magnetic anisotropy. Perpendicular magnetic anisotropy refers to a state where it is comfortable for the spin to exist in the vertical direction of the channel. In general, as the thickness decreases, the in-plane state corresponds to the easy-axis. Fe3GeTe2 has a large perpendicular magnetic anisotropy despite being a very thin two-dimensional material. Anomalous Hall effect, a kind of Hall effect, refers to spin polarization by magnetization inside a ferromagnetic without an external magnetic field. When the channel has magnetization in z-axis direction, the spin moves along the y-axis of the channel, and the spin in the same direction as the magnetization becomes the majority and accumulates more. This accumulation difference is measured as Hall voltage. Through this, the magnetic anisotropy field, one of the magnetic properties, was derived from two-dimensional ferromagnet for the first time. The correlation between bulk effect and interfacial effect of Fe3GeTe2 was confirmed, and the surface was partially controlled through a heterostructure with other two-dimensional materials.
Finally, the effect of the stray field of Fe3O4 nanoparticles located on the surface on the semiconductor channel was confirmed by ordinary magnetoresistance. A bio-magnetoresistive sensor for detecting liver cancer cells sensitive to magnetoresistance changes based on indium antimonide semiconductor material with a narrow bandgap and high mobility has been developed. It is a good means to give efficiency to the complicated and time-consuming liver cancer diagnosis and to detect liver cancer biomarkers present in very small amounts in the blood. Ordinary magnetoresistance increases the resistance by affecting the channel current path by Lorentz force. When an external magnetic field perpendicular to the channel is applied through the antibody-antigen-antibody-NP complex attached to the sensor surface, a stray field is strongly generated and the magnetoresistance is changed. Compared to the area where liver cancer tumors exist in the blood (about 20 ng/ml), it reacts sensitively to very low concentrations (< 1 pg/ml), and has the advantage of being selective for specific biomarker and reusable.
In this research reveals several potential applications for nanomaterials and new technologies in demand today. Moving from the era of bulk to the era of interfaces, it presents the possibility as a new device, not just downsizing in thickness and size. Research on intrinsic properties of new materials is a valuable work that can help establish this new paradigm and contribute to real industrial development as well as academic research. Therefore, spintronics research using nanomaterials is expected to provide a new path for next-generation material and device research.

목차 (Table of Contents)

• Abstract i
• Contents v
• List of Figures ix
• List of Table xviii
• Introduction 1
• 1 Background 2
• 1.1 Nanomaterials 2
• 1.1.1 Size effect 2
• 1.1.2 Surface effect 3
• 1.1.3 Superparamagnetic 4
• 1.2 Spin−orbit coupling 6
• 1.3 Magneto-transport 8
• 1.3.1 Ordinary magnetoresistance 8
• 1.3.2 Anisotropic magnetoresistance 9
• 1.3.3 Anomalous Hall effect 10
• 2 Anisotropic magnetoresistance in a Ni81Fe19/SiO2/Ca-Bi2Se3 hybrid structure 12
• 2.1 Introduction 13
• 2.2 Samples and methods 13
• 2.3 Results and discussion 16
• 2.3.1 Anisotropic magnetoresistance of TI/FM structure 16
• 2.3.2 Magnetization switching process of TI/FM structure 19
• 2.3.3 Anisotropic magnetoresistance of with a rotational magnetic field 22
• 2.4 Summary 27
• 3 Interface engineering of magnetic anisotropy in van der Waals ferromagnet-based heterostructures 28
• 3.1 Introduction 29
• 3.2 Samples and methods 30
• 3.3 Results and discussion 34
• 3.3.1 Magneto-transport properties of FGT-based heterostructures 34
• 3.3.2 Analysis of Hk using anomalous Hall measurements 40
• 3.3.3 Thickness Dependences of Hk 46
• 3.3.4 Local modulation of PMA in FGT-based heterostructures 52
• 3.4 Summary 58
• 4 An InSb-based magnetoresistive biosensor using Fe3O4 nanoparticles 59
• 4.1 Introduction 60
• 4.2 Samples and methods 61
• 4.3 Results and discussion 64
• 4.3.1 Sensing mechanism 64
• 4.3.2 Characterization of Fe3O4 nanoparticles 64
• 4.3.3 Magnetoresistance of sensors 66
• 4.3.4 Selectivity of sensors 72
• 4.3.5 Reusability of sensors 74
• 4.3.6 Comparison with other sensors and further considerations 76
• 4.4 Summary 79
• Bibliography 80
• Curriculum Vitae 89
• Korean abstracts 93
• Acknowledgements 97

참고문헌 (Reference)

1 R. C. O ’ Handley, "Modern Magnetic Materials : Principles and Applications", 2000

2 S. Gene, "Structural , Optical and Magnetic Characterization of Spinel Zinc Chromite ( ZnCr2O4 ) Nanocrystals Synthesized by Thermal Treatment Method.", 2014