Structure and magnetic properties of ni nio, co coo composite films

noise and the mismatch noise is relatively small. Figure 101. Measured SNDR, SNR and SFDR Figure 102. Measured output spectrum with PDWA off and input signal. 117 Figure 103. Measured output spectrum with PDWA on and input signal. The measured DR is 78dB, and the peak SNR and SNDR are 77dB and 69dB respectively. The differential FS (Full Scale) input is 5Vp-p. The power consumption is 62mW under a 3.3V supply voltage. The performance is summarized in Table and the chip microphotograph is shown in Figure 104. The core chip area is 1.5x1.3mm2. 118 Gain Stage 5th Integrator 4th Integrator 3rd Integrator 2nd Integrator 1st Integrator Diferentiator Figure 104. Die microphotograph 119 Table 8. Performance Summary PARAMETER VALUE Signal Bandwidth 2.2MHz Clock Frequency 35.2MHz Oversampling Ratio SFDR 94dB Dynamic Range 78dB Peak SNR 77dB Peak SNDR 69dB Input Range 5Vpp (differential) Core Area 1.5x1.3mm2 Total Power Consumption 62mW Power Supply Voltage 3.3V Technology 0.35μm CMOS 120 Chapter Conclusions and Future Work 6.1 Conclusion In this thesis, an NS-DEM technique has been proposed to improve the linearity of the multi-bit ΔΣM. NS-DEM works with most existing DEM techniques and provides further spectral noise-shaping to the DAC errors. The proposed technique eliminates the trade off between SNR and SFDR in most existing DEM techniques, The NS-DEM is analyzed and evaluated in the behavior simulation and has shown an improvement in both SFDR and the SNR. The NS-DEM is more effective at low input signal level or signal with less dc component, while less effective at large input amplitude. . This is because of the overflow of the accumulator, which impair the noise shaping performance of the NS-DEM and result in the SNR degradation at high input signal level and low frequency signal. The noise shaping concept of the NS-DEM is first experimentally demonstrated in a dithered noise shaping DAC in a 0.35-µm CMOS technology. The first-order noise shaping has been observed in the measurement. The NS-DEM has also been employed in a lowpass 5th-order 4-bit quantization ΔΣM emplying NS-PDWA, which is aimed for ADSL2+ application. The fabricated ΔΣM chip is fully operational and achieves 94dB for SFDR and 78dB for DR in 2.2MHz BW, which meets the specification intended for ADSL2+ application. 6.2 Original Contribution z A new noise shaping dynamic element matching (NS-DEM) technique for improving the DAC linearity. Unlike most of the existing DEM techniques that trade SNR for SFDR, the NS-DEM improves both SFDR and SNR of the 121 DAC. z NS-DEM technique is validated with existing widely used DEM algorithms. In principle, it can work with any DEM algorithms. z A 5-bit DAC with the NS-DEM (dither) in 0.35 μm CMOS technology. z A lowpass 5th-order 4-bit quantization ΔΣM employing NS-DEM (PDWA) in 0.35 μm CMOS technology. z The concept of a bandpass ΔΣM employing NS-DEM. z The incorporation of the differentiator into the first-stage loop filter in a lowpass ΔΣM without incurring any additional analog circuit. z An accumulator realized by a register and a proposed new adder based on the thermalmeter code. 6.3 Future Work In this design, it has been noted that tones come out when a large input signal is distorted by the input switch. Therefore, some solutions, such as bootstrapping, can be employed to remove this limitation, making the ΔΣM more linear over the whole input level. One of the common solutions is to use the bootstrapped circuit to ensure the constant Vgs. Due to the reduced supply voltage and increased signal bandwidth, the continuous-time ΔΣM becomes more favorable nowadays. .The proposed NS-DEM may be further employed in continuous-time ΔΣMs where the DAC nonliearity is also a threat. . Since the overflow is a major concern in NS-DEMS, the input-referred offset and 1/f noise of the whole ΔΣM should be minimized. Therefore, some techniques, such as auto-zero and chopper, may be employed to minimize the in-band offset and 1/f noise. 122 Re Chow. B.H. Liu, and J.H. Yin. “Evolution of structural and magnetism of NiO from amorphous, clusters to nanocrystalline” Phys. Rev. B 76, 224402 (2007). 14. H. Pan, J.B. Yi, R.Q. Wu, S. Lei, J.H. Yang, J.Y. Lin, Y.P. Feng, J. Ding, L.H. Van, and J.H. Yin “Room temperature ferromagnetism in carbon-doped ZnO” Phys. Rev. Lett. 99, 127201 (2007). iii Table of Contents Acknowlegements . i Publications during PhD study .ii Table of contents………… . iv Summary…………….…. x List of tables ……… xii List of figures and illustration…… xiii Chapter Introduction . 1.1 Brief review of magnetic properteis of materails 1.1.1 Origin of magetism . 1.1.2 Type of magnetism . 1.1.2.1 Diamagnetism 1.1.2.2 Paramagnetism 1.1.2.3 Ferromagnetism . 1.1.2.4 Antiferromagnetism . 1.1.2.5 Ferrimagnetism 1.2 Hysteresis loops . 1.3 Exchange coupling between ferromagnet and antiferromagnet . 11 1.3.1 Theories . 11 1.3.2 High coercivity in exchange coupling system . 15 1.3.3 △ M curves 16 1.3.4 Antiferromagnet in exchange coupling . 17 1.3.5 NiO and CoO in the exchange coupling 17 1.4 The magnetization of NiO particles . 19 1.5 Motivations . 20 iv 1.6 Summary………. 22 References… . 23 Chapter Experimental Procedures 28 2.1 Film deposition: Sputtering 29 2.2 X-ray diffraction (XRD) 31 2.3 Scanning electron microscopy (SEM) 33 2.4 Energy disperse x-ray spectrometer (EDS) 34 2.5 Atomic force microscopy (AFM) . 34 2.6 Raman spectroscopy 35 2.7 X-ray photonelectron spectroscopy (XPS) . 36 2.8 Tansmission electron microscopy (TEM) 37 2.9 Vibrating sample magnetometer (VSM) 39 2.10 Superconducting quantum interference device (SQUID) 40 2.10 Extending absorption X-ray fine structure (EAXFS) 41 2.10 X-ray magnetic circular dichroism (XMCD) 45 2.11 Summary . 48 References… . 49 Chapter An investigation of structure, and magnetic properties of Ni film prepared by sputtering ………… . 50 3.1 Introduction . 51 3.2 Experimental procedure . 51 3.3 Characterization and microstructure analysis . 52 3.3.1 Calibration for the thickness and deposition rate…………… 52 3.3.2 XRD analysis. 53 3.3.3 TEM analysis. 54 v 3.3.4 Coordination number examined by EXAFS 58 3.3.5. SEM analysis of the annealed Ni films…………… . 59 3.4 Magnetic properties . 62 3.5 Resistivity . 63 3.6 Discussion . 64 3.6.1 The effect of substrate and sputtering power on the saturation magnetization of Ni films. . 65 3.6.2 XMCD analysis. 68 3.7 Summary 68 References… . 69 Chapter Magnetism evoution of NiO from amorphous, cluster to nanocrystalline structures…………………. . 71 4.1 Introduction . 72 4.2 Experimental procedure . 72 4.3 Synthesis and analysis of amorphous, cluster, and nanocrystalline specimens ………………………………… . 73 4.3.1 Synthesis of fully amorphous NiO by sputtering. 73 4.3.2 Synthesis of NiO in the cluster and nanocrystalline structure (Coprecipitation and subsequent annealing) . 75 4.3.2.1 Thermal gravitation analysis (TGA) . 75 4.3.2.2 XRD analysis . 77 4.3.2.3 Structure analysis by TEM . 78 4.4 Maxmium magnetization of NiO with different structures (Amorphous, cluster and nanocrystalline) . 81 4.5 Antiferromagnetism in fully amorphous NiO . 83 vi 4.6 Ferromagnetism in NiO clusters 85 4.6.1 Estimation of the Curie temperature of the ferromagnetism . 85 4.6.2 Spin glass behavior . 87 4.7 First principle calculation 90 4.8 Core/shell interactions in nanocrystals . 92 4.8.1 The description of surface spins and antiferromagnetic core. 93 4.8.2 Annealing temperature dependence of exchange bias and coercivity of NiO powders . 93 4.8.3 Temperature dependence of exchange bias and coercivity of NiO powders annealed at different temperatures. 96 4.9 Superparamgnetism in NiO powders 98 4.9.1 Superparamagnetism according to Néel model 98 4.9.2 Anisotropy of NiO powders. . 99 4.10 Summary . 102 References… . 104 Chapter Magnetic properties of Ni/NiO nanocomposites prepared by reactive sputtering………. 106 5.1 Introduction . 107 5.2 Experimental procedure . 107 5.3 Structure and characterization 100 5.3.1 Cross-section TEM analysis 108 5.3.2 XRD analysis 101 5.3.3 In-plane TEM analysis 109 5.4 Magnetization of Ni/NiO composite films. 113 vii 5.4.1 Saturation magnetization of Ni/NiO composites after annealing at different temperatures…………… . 113 5.4.2 In-plane and out-of-plane hysteresis loops of Ni/NiO composites annealed at 200 oC. . 115 5.4.3 Temperature dependence of magnetization of Ni/NiO composites annealed at 200 oC. . 116 5.4.4 Cure Temperature of Ni/NiO composites annealed at 200 oC. 117 5.5 Exchange bias phenomenon in Ni/NiO composites 119 5.5.1 Hysteresis loops taken at different temperatures 119 5.5.2 Temperature dependence of exchange bias and coercivity of the Ni/NiO annealed at 350 oC. . 122 5.5.3 ZFC and FC curves of Ni/NiO composites anealed at 350 oC 123 5.6 Composition effect 124 5.7 Summary . 125 References… . 127 Chapter Coercivity and exchange bias of Ni/NiO nanocomposite films prepared by oxidation during magnetic annealing 129 6.1 Introduction . 130 6.2 Experimental procedure . 130 6.3 Effect of composition on the magnetic properties of Ni/NiO composites . 131 6.4 Effect of magnetic field on the magnetic properteis of Ni/NiO composites. . 132 6.5 Phase and microstructure analysis 133 6.5.1 XRD analysis . 133 6.5.2 TEM analysis . 133 6.6 Magnetic properties of Ni/NiO composites prepared by magnetic annealing 136 viii 6.6.1 Room temperature hysteresis loops . 1136 6.6.2 Temperature dependence of coercivity and exchange bias . 137 6.6.3 Blocking temperature . 138 6.6.4 ∆M curve analysis 139 6.7 Summary… . 140 References… . 141 Chapter Coercvity and exchange bias of Co/CoO nanocomposite films prepared by oxidation during magnetic annealing………. 143 7.1 Introduction . 144 7.2 Experimental procedure . 144 7.3 The results of Co and CoO prepared by sputtering . 144 7.4 Characterization and microstructure analysis . 146 7.4.1 XRD analysis . 146 7.4.2 Raman spectroscopy analysis 147 7.4.3 TEM analysis . 148 7.5 Composition study and its effect on the magnetic properties 150 7.6 Temperature dependence of magnetic properties 152 7.7 Comparison of the hysteresis loops of Co/CoO composite films by magnetic annealing under an oxygen partial pressure and by sputtering with an oxygen partial pressure… . 154 7.8 Summary…… . 155 References… . 156 Chapter Conclusions and future work 157 8.1 Conclusions . 150 8.2 Future works… 160 ix [...]... exchange bias and coercivity as a function of the oxygen ratio during magnetic annealing at 200 oC for 1 h (all the values were taken at 80 K) Fig 7.6 The coercivity and exchange bias of a Co/ CoO composite prepared by the annealing of a Co film under 3 % O2 at 200 oC for 1 h Fig 7.7 ZFC and FC curves of a Co/ CoO composite prepared by magnetic annealing Fig 7.8 Hysteresis loops (80 K) of a Co/ CoO composite. .. focused on understanding of the magnetic properties of Ni and NiO in the amorphous or disordered, cluster, nanocrystalline and well crystalline states and the understanding of the exchange coupling and magnetic behaviors of ferromagnetic/antiferromagnetic Ni/ NiO nanocomposites In this study, nanostructured samples were fabricated by co- precipitation and magnetron sputtering It was found that NiO exhibited... xvii Fig 7.3 HRTEM micrographs of a) a pure Co film; b) a Co film after magnetic annealing at 200 oC for 1 h under an oxygen ratio of 3 %; c) a Co film after magnetic annealing at 200 oC for 1 h under an oxygen ratio of 10 % Fig 7.4 a) The saturation magnetization of a Co/ CoO composite annealed under different oxygen ratios and b) the composition of the CoO in the Co/ CoO composites annealed under different... micrograph of a Ni/ NiO composite The inset is the selected area electron diffraction (SAED) pattern, indicating a Ni phase Fig 5.2 XRD spectra of Ni/ NiO composites annealed at different temperatures Fig 5.3 The average grain size of the Ni and NiO phase in Ni/ NiO composites calculated from Scherrer equation The line is a guide to eyes The two triangle points in the graph are the grain sizes of the NiO films. .. magnetization of Ni/ NiO composites annealed at different temperatures under an argon atmosphere Fig 5.6 In-plane and out -of- plane hysteresis loops of a Ni/ NiO composite after annealing at 200 oC (the loops were taken at 300 K) Fig 5.7 Temperature dependence of saturation magnetization of a Ni/ NiO composite after annealing at 200 °C Fig 5.8 Curie temperature of the Ni foil and Ni/ NiO composite annealed at... large range of applied field (10 kOe) of the corresponded plot Fig 5.11 Temperature dependence of the exchange bias (HE) and coercivity (HC) of a Ni/ NiO composite annealed at 350 oC Fig 5.12 ZFC and FC curves of a Ni/ NiO composite annealed at 350 oC Fig 6.1 The hysteresis loops of sample N0 films with a subsequently magnetic annealing under a field of 0 kOe (N0e, dash), 0.5 kOe (N0f, solid), and 10 kOe... hysteresis loops of a) the as-deposited Ni film (N0) and b) the Ni film performed magnetic annealing at 380 oC under an oxygen partial pressure of 0.001 torr (N0b) Fig 6.6 Coercivity and exchange bias of N0b as a function of the temperature Fig 6.7 Zero-field-cooling (ZFC) and field-cooling (FC) curves of a Ni/ NiO composite prepared by magnetic annealing at 380 oC under an oxygen partial pressure of 0.001... has a low coercivity The study of the mechanisms indicated that the high coercivity of the composite was due to the exchange coupling between Ni and NiO The small grain size of NiO was attributed to the high coercivity Similarly, high coercivity was achieved in a Co/ CoO composite prepared by magnetic annealing The coercivity of the composite is evidently higher than that prepared by sputtering under an... enhanced xi List of Tables Table 3.1 The dependence of saturation magnetization Ms of Ni films with a thickness of 100 nm on the substrate and sputtering power Table 4.1 The calculated anisotropy energy constant of NiO powders annealed at different temperatures Table 5.1 Composition effect of Ni and NiO on the coercivity and exchange bias The exchange bias and coercivity were measured from the composite after... loops of Ni/ NiO composites a) in the as-deposited state; b) annealed at 200 oC with an applied magnetic field of 10 kOe The insets are the hysteresis loops in a large range of the applied field (10 kOe) of the corresponded plot xvi Fig 5.10 Hysteresis loops of Ni/ NiO composites a) annealed at 350 oC with an applied magnetic field of 10 kOe and b) annealed at 500 oC under an applied magnetic field of 10 . STRUCTURE AND MAGNETIC PROPERTIES OF Ni/ NiO, Co/ CoO COMPOSITE FILMS YI JIABAO (B. ENG. TIANJIN UNIV. CHINA) A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY. an oxygen ratio of 10 %. Fig. 7.4. a) The saturation magnetization of a Co/ CoO composite annealed under different oxygen ratios and b) the composition of the CoO in the Co/ CoO composites annealed. Composition study and its effect on the magnetic properties 150 7.6 Temperature dependence of magnetic properties 152 7.7 Comparison of the hysteresis loops of Co/ CoO composite films by magnetic annealing