林敏聰臺灣大學：物理研究所顏宏聿Yen, Hong-YuHong-YuYen2007-11-262018-06-282007-11-262018-06-282007http://ntur.lib.ntu.edu.tw//handle/246246/54513Interest in magnetic nanoparticles has increased in the recent years due to the industrial applications such as the ultra-high density storage device and the fundamental interest in finite size effect. By naturally grown stripe structures with ~ 4 nm interdistance on the Al2O3 layer, Fe and Mn nanoparticles were prepared by self assembling. The surface morphology and magnetic properties were characterized by STM and MOKE, respectively. The 9~33 ML Fe and 0.1~16.9 ML Mn nanoparticles both reveal that the separation of particles decreases with increasing coverage. [1 ML is defined as the surface atom density: 1.54 × 10^15 at./cm2 on Cu(100).] The Fe nanoparticles are magnetic isotropic until 23 ML, and the twostep hysteresis loops of 23, 33 ML are ascribed to the uniaxial anisotropies with the higher order term along the stripe directions, which is supported by the Stoner-Wohlfarth simulation. Mn nanoparticles are proven to be non-ferromagnetic at 0.1~8.5 ML and there is no exchange bias with Fe capped on 3.4~16.9 ML Mn at our lowest accessible temperature ~ 130 K. For 17.6 ML Fe capped n ML Mn (n= 3.4~16.9), the drastic reduction of magnetic moments and the enhancement of coercivity were found at RT (room temperature), while at LT (150 K), the coercivity decreases unusually. The increasing roughness with increasing coverage of Mn and Fe-Mn interdiffusion account for the RT observations, and the temperature dependent reversed domain nucleation and domain wall pinning may be responsible for the LT behaviors.1 Introduction 1 2 Basic Concept 4 2.1 Growth of thin film and islands . . . . . . . . . . . . . . . . . . . . . 5 2.2 Energy of a magnetic system . . . . . . . . . . . . . . . . . . . . . . . 7 2.3 Magnetic domain and magnetization reversal . . . . . . . . . . . . . . 9 2.4 Magnetic hysteresis loop . . . . . . . . . . . . . . . . . . . . . . . . . 9 3 Experiment Apparatus 12 3.1 Multi-functional UHV system . . . . . . . . . . . . . . . . . . . . . . 13 3.2 Pumping system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 3.3 Sputtering and Annealing . . . . . . . . . . . . . . . . . . . . . . . . 16 3.4 Auger Energy Spectrum (AES) . . . . . . . . . . . . . . . . . . . . . 17 3.5 Molecular Beam Epitaxy (MBE) . . . . . . . . . . . . . . . . . . . . 20 3.6 Medium Energy Electron Diffraction (MEED) . . . . . . . . . . . . . 21 3.7 Low Energy Electron Diffraction (LEED) . . . . . . . . . . . . . . . . 22 3.8 Scanning Tunneling Microscopy (STM) . . . . . . . . . . . . . . . . 24 3.9 Magneto-Optical Kerr Effect (MOKE) . . . . . . . . . . . . . . . . . 27 4 Magnetic Properties of Self-Aligned Fe and Mn Nanoparticle Assembly on Al2O3/NiAl(100) 31 4.1 Preparation of Fe and Mn nanoparticles on Al2O3/NiAl(100) . . . . . 32 4.1.1 Experiment . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 4.1.2 Preparation of single-crystalline Al2O3/NiAl(100) . . . . . . . 33 4.1.3 Calibration of deposition rate . . . . . . . . . . . . . . . . . . 33 4.1.4 The STM image of Fe and Mn nanoparticles . . . . . . . . . . 37 4.2 Magnetic properties of Fe nanoparticles on Al2O3/NiAl(100) . . . . . 42 4.3 Magnetic properties of Mn nanoparticles on Al2O3/NiAl(100) . . . . 48 4.4 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49 5 Magnetic Properties of Fe Capped Mn Nanoparticle Assembly on Al2O3/NiAl(100) 53 5.1 Growth of Fe capped Mn nanoparticle assembly on Al2O3/NiAl(100) 54 5.2 Magnetic properties of Fe capped Mn nanoparticle assembly on Al2O3/NiAl(100) 55 5.2.1 The room temperature magnetic behavior . . . . . . . . . . . 55 5.2.2 The low temperature (150 K) magnetic behavior . . . . . . . 60 5.3 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64 6 Conclusion 65 Bibliography 67en-US鐵奈米顆粒錳奈米顆粒磁性鐵磁覆蓋層臺階狀磁滯曲線Fe nanoparticleMn nanoparticlemagnetismFe capping layertwo-step hysteresis loopthesis