Magnetic properties of hybrid nanostructures 1

MAGNETIC PROPERTIES OF HYBRID NANOSTRUCTURES DENG SUZI NATIONAL UNIVERSITY OF SINGAPORE 2010 MAGNETIC PROPERTIES OF HYBRID NANOSTRUCTURES DENG SUZI B. Appl. Sc. (Hons) National University of Singapore A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY DEPARTMENT OF CHEMISTRY NATIONAL UNIVERSITY OF SINGAPORE 2010 Acknowledgements First of all, I would like to express my sincere gratitude to my supervisor Associate Professor Loh Kian Ping. Throughout my PhD candidature, Prof Loh provided me with lots of ideas, good teaching and encouragement, at the same time allowing me the research freedom in my doctoral projects. I would like to extend my gratitude to my co-supervisor, associate Professor Sow Chorng Haur. Prof Sow is always more than willing to help me with any questions I faced for my research. I have benefited and learnt a lot from his kind and modest nature, his passion in pursuing science, and nurturing students. I would like to express my gratitude to S. N Piranayaganam, and Mr Aung KyawOo from Data Storage Institute, for their support and assistance in patterned media studies. My gratefulness also goes to Dr Foo Yong-Lim, Dr Lin Ming and Tan Huiru from IMRE for the assistance in the nanocluster beam deposition project and TEM characterization. I am grateful to Dr Yi Jiabao who had patiently helped me with SQUID experiments and his optimism, generosity and efficiency had left a deep impression on me. My sincere appreciation is dedicated to Dr Fan Haiming. He freely shared his ideas with me and explained scientific phenomena previously unknown to me. I will always remember the enlightening discussions with him as well as encouragement and critiques by him. i Last but not least, I would like to thank past and current group members for their help, assistance and friendship. Without their daily help and support, this thesis would not be possible. ii Publications 1. Room temperature ferromagnetism at self-assembled monolayer modified Ag nanocluster-ZnO nanowire interface Deng, S.; Loh, K. P.; Ding, J.; Yi, J. B.; Tan, H. R.; Lin, M; Foo, Y. L.; Zheng, M.; Sow, C. H. Applied Physics Letters, 2008, 93, 193111. 2. An effective surface-enhanced Raman scattering template based on a Ag nanocluster–ZnO nanowire array Deng, S.; Fan, H. M.; Zhang, X. J.; Loh, K. P.; Cheng, C. L.; Sow, C. H.; Foo, Y. L. Nanotechnology, 2009, 20, 175705. 3. Antiferromagnetically coupled patterned media Piramanayagam, S. N.; Aung, K. O.; Deng, S. and Sbiaa, R. Journal of Applied Physics, 2009, 105, 1. 4. Magnetostatic interactions in Antiferromagnetically Coupled Patterned Media Deng, S.; Aung, K. O.; Piramanayagam, S. N. and Sbiaa, R. Journal of Nanoscience and Nanotechnology (Manuscript accepted for publication in Special Issue, 2010) 5. Room-Temperature Synthesis of Soluble Carbon Nanotubes by the Sonication of Graphene Oxide Nanosheets Wang, S.; Tang, Lena A. L.; Bao, Q.; Lin, M.; Deng, S.; Goh, B. M. and Loh, K. P. Journal of American Chemical Society. 2009, 131, 16832 iii 6. Thiol-capped ZnO Nanowires/Nanotubes Arrays with Tunable Magnetic Properties at Room Temperature Deng, S. Z.; Fan, H. M.; Wang, M.; Zheng, M. R.; Yi, J. B.; Wu, R. Q.; Tan, H. R.; Sow, C. H.; Ding, J.; Feng, Y. P. and Loh, K. P. ACS Nano, 2010, 4, 495 7. Antiferromagnetically Coupled Patterned Media and Switching Field Distribution Ranjbar, M.; Piramanayagam, S. N.; Deng, S.; Aung, K. O.; Sbiaa, R.; Chong, T. C. IEEE Transactions in Magnetics, 2010, 46, 1787 iv Chapter Introduction 1.1 Magnetism in nanostructures 1.2 Fundamentals of Magnetism 1.2.1 Magnetic Anisotropy in low dimensions . 10 1.2.2 Single Domain Theory and Superparamagnetism . 12 1.2.3 Exchange interactions and magnetostatic interactions between magnetic dipoles 14 1.3 Magnetic Gradient Nanomaterial . 16 1.4 Antiferromagnetically exchange-coupled (AFC) patterned media . 21 1.5 Interfacial Magnetism . 24 1.6 Scope of work . 29 Chapter Experimentals . 33 2.1 Introduction . 33 2.2 Magnetic Measurements . 34 2.2.1 Alternating Gradient Magnetometry 34 2.2.2 Vibrating Sample Magnetometry . 37 2.2.3 SQUID Magnetometer . 38 2.2.4 Magnetic force microscopy . 42 2.2.5 Magneto-optical Kerr Effect 45 2.3 Surface Analysis . 48 2.3.1 X-Ray Photoelectron Spectroscopy (XPS) 48 2.3.2 Scanning Electron Microscopy (SEM) 49 v Chapter Fabrication of Magnetic Gradient Nanomaterial by Combined Nanocluster Beam Deposition and Colloidal Crystals Patterning . 53 3.1 Introduction . 54 3.2 Experimental Section 58 3.2.1 Formation of colloidal crystal assembles and Reactive-ion etching using CF4/O2 . 58 3.2.2 Fabrication of colloidal crystal patterned Ni and FePt nanostructures by RF sputtering and FePt nanocluster beam deposition . 58 3.2.3 Characterization of magnetic nanostructures by VSM and MFM . 61 3.3 Results and Discussions 62 3.3.1 Reactive Ion Etched Colloidal Patterns Consisting of Magnetic Metal Ni . 62 3.3.2 Fabrication and Characterization of Hollow FePt Hemispherical Caps Arrays 66 3.3.3 Multidomain flux closure magnetic domains observed for FePt-Fe/Co coated microspheres) 72 3.3.4 Quasi–single-domain or bidomain remanent states observed for FePt-Ag coated microspheres) 74 3.4 Conclusions . 77 Chapter Magnetostatic interactions in Antiferromagnetically Coupled Patterned Media 83 4.1 Introduction . 84 4.2 Experimental Section 87 4.2.1 Varying stabilizing layer thickness of AFC structure 87 4.2.2 Varying deposition pressure of 3nm thick stabilizing layer 87 4.2.3 Characterization of magnetic nanostructures by MOKE and MFM 88 vi 4.2.3.1 Measurement of remanence curves . 88 4.2.3.2 Determination of SFD . 89 4.3 Results and Discussions 91 4.3.1 Schematic of AFC structure . 91 4.3.2 Hysteresis loops of unpatterned thin film . 91 4.3.3 Reduction in SFD with AFC 92 4.3.4 Improvement in stability and coercivity with AFC . 96 4.3.5 Effect of stabilizing layer coercivity on AFC 97 4.3.6 AFC for different stabilizing layer coercivities . 98 4.4 Conclusions . 102 Chapter Room Temperature Ferromagnetism at Self-Assembled Monolayer Modified Ag Nanocluster-ZnO Nanowire Interface 105 5.1 Introduction . 106 5.2 Experimental Section 109 5.2.1 Fabrication of thiol-capped Ag NCs-ZnO NWs 109 5.2.2 Magnetic characterization . 110 5.2.3 Electronic Structure Characterization . 111 5.3 Results and Discussions 112 5.3.1 Characterization of Ag NC-ZnO NWs by SEM, TEM and XPS . 112 5.3.2 Effect of Ag NC size on Magnetic Properties of Thiol-capped Ag NC-ZnO NWs . 114 5.3.3 Anomalous Temperature dependence of Ms for Thiol-capped nm Ag NC-ZnO NWs 118 vii 5.3.4 Charge redistribution in thiol-capped Ag NCs-ZnO NWs – UPS and NEXAFS study 120 5.4 Conclusions . 123 Chapter Thiol-capped ZnO Nanowires/Nanotubes Arrays with Tunable Magnetic Properties at Room Temperature 127 6.1. Introduction 128 6.2 Experimental Section 131 6.2.1 Fabrication of single-crystalline ZnO NWs by Vapor Phase Transport 131 6.2.2 Fabrication of single–crystalline ZnO NWs and NTs by hydrothermal method . 131 6.2.3 Fabrication of polycrystalline ZnO NTs by ALD 132 6.2.4 Self-assembly of thiol on ZnO NWs/NTs 133 6.2.5 Characterizations of bare and thiol-capped ZnO NWs/NTs 133 6.3 Results and Discussions 135 6.3.1 Characterizations of bare and thiol-capped ZnO NWs/NTs 135 6.3.2 Mechanism of ZnO nanotube formation 139 6.3.3 Characterization of ZnO NTs/NWs by XRD, Raman, FTIR, PL and XPS . 141 6.3.4 Magnetic moment tunable by height of NW arrays s 145 6.3.5 Enhancement of magnetic moment of thiol-capped ZnO NT relative to NWs . 147 6.3.6 Temperature dependence of magnetization of thiol-capped ZnO NWs/NTs 149 6.3.7 DFT calculations of magnetic moment of thiol-capped ZnO 151 6.3.8 Effect of crystallinity on magnetic anisotropy . 153 6.4 Conclusions . 158 viii F = (B/I)µ (2.2) where B is the magnetic field which would be produced by a fictitious current I circulating in the detection coils. The induced voltage is given by the relation: E=− d ( B / I ) dz dF = −μ ⋅ dt dz dt (2.3) The magnetic moment signal may lag the field as the magnetic moment is continuously measured during the sweeping of the magnetic field is swept. This results in a field error (or coercivity error) that is approximately equal to the product of the time constant and the sweep rate, which also depends on the slope of the hysteresis loop. A significant error in coercivity can cause the roundoff of steep curves, leading to errors in the determination of the measured slopes, the squareness S, the coercive squareness S*, and the remanent magnetization Mr. 2.2.3 SQUID Magnetometer Superconducting QUantum Interference Devices (SQUID) are the most sensitive devices for measuring magnetic flux.4 , , 6, SQUID have being widely used in a variety of measurements including current, voltage, magnetic field, gravitational field, and magnetic susceptibility. The SQUID consists of two superconductors separated by thin insulating layers to form two parallel Josephson junctions (figure 2.2). The great sensitivity of the SQUID is associated with measuring changes in magnetic field 37 associated with one flux quantum. One of the discoveries associated with Josephson junctions was that flux is quantized in units Φ0 = 2= ≅ 2.0678 × 10−15 Tesla ⋅ m 2e (2.4) Figure 2.2 The schematic diagram of superconducting ring with two Josephson junctions in DC SQUIDs. In order to operate the DC SQUID, a constant bias current is fed through the superconducting ring. If a constant biasing current is maintained in the SQUID device, the measured voltage oscillates with the changes in phase at the two junctions, which depends upon the change in the magnetic flux. When a variation in magnetic flux caused 38 by a sample occurs, an additional supercurrent starts to run through the ring in an attempt to compensate the change in flux, giving rise to a higher voltage across the SQUID. Since the magnetic flux is quantized as it passes through a superconducting ring, the average voltage across SQUID becomes a periodic function of the change in magnetic flux. Counting the oscillations allows the evaluation of the flux change which has occurred. Thus, the DC SQUID is a magnetic flux-to-voltage converter. Similarly, the DC SQUID may be operated by applying a constant voltage and measuring the current as a function of magnetic flux. The detection of the variation in magnetic flux in the SQUIDs is coupled to a superconducting wire wound detection coil through induction. For the DC SQUID, the detection coil consists of a set of three superconducting wire wound coils configured as a second-derivative gradiometer, as shown in Figure 2.3. Such gradiometer configuration not only reduces the noise caused by the fluctuations in the large magnetic field of the superconducting magnet, but it can also minimize the background noise and fluctuation due to the relaxation of the magnetic field of superconducting magnet. However, the magnetic moment of a sample is still measurable because the gradiometer coil set measures the local variations in magnetic flux density induced by the dipole field of the sample. Conventionally, a measurement procedure in DC SQUID is performed by moving a sample through superconducting gradiometer at a certain temperature and magnetic field. Any differentiation in the magnetic flux in gradiometer coils generates a proportional variation in the persistent current. Since the SQUID is a highly linear current and voltage converter, any change in the current produces the corresponding variation in voltage, which is essentially proportional to the magnetic moment of the 39 sample. Therefore, the magnitude of magnetic moment of the sample can be determined by the measurement of the variations in voltage in SQUID. Figure 2.3 The schematic configuration of second-derivative gradiometer superconducting detection coils. The routine measurements in SQUID magnetometers can be performed in two ways. The first measurement is the field dependence of magnetization at constant temperature M (H), and the second is the temperature dependence of the magnetic materials under constant magnetic field M (T). According to the cooling process, the temperature dependent measurements can be further performed by zero-field-cooling (ZFC) and field-cooling (FC), respectively. In ZFC measurements, the magnetic sample is cooled from room temperature without a magnetic field. On the other hand, for the FC 40 measurements, the sample is cooled from room temperature in the presence of a magnetic field. In this research, the magnetic measurements are carried out on a Quantum Design SQUID magnetometer. The temperature ranges from K to 400 K for temperature dependent magnetic studies, and the magnetic field is from T to -5 T in the field dependent measurements. 2.2.4 Magnetic force microscopy The macroscopic properties of a magnetic material are determined by its magnetic microstructure. Therefore, domain observation is essential for the development of magnetic materials and understanding of magnetization processes. Magnetic force microscope (MFM) can be used to image the spatial variation of magnetic forces on a sample surface (Figure 2.4). The MFM is based on the simple idea of measuring the force (or the force gradient) between a magnetized tip and a sample that has magnetic or diamagnetic properties. In MFM, a tapping cantilever equipped with a MFM tip that is coated with a ferromagnetic thin film is first scanned over the surface of the sample to obtain topographic information (1&2 in Figure 2.5). Using LiftMode (3-5 in Figure 2.5), 41 Figure 2.4 MFM maps the magnetic domains of the sample surface.8 Figure 2.5 Principles for MFM LiftMode. Dotted line represents route taken by MFM cantilever. 1&2: Cantilever traces surface topography on first trace and retrace. 3: Cantilever ascends to Lift scan Height. 4&5: Lifted cantilever profiles topography while responding to magnetic influences on second trace and retrace.9 the tip is then raised just above the sample surface. The surface topography is scanned while being monitored for the influence of magnetic forces. These influences are measured using the principle of force gradient detection. The resonant frequency of the cantilever f is shifted by an amount Δf proportional to vertical gradients in the magnetic 42 forces on the tip dFz/dz, induced by magnetic interaction between tip and sample (Figure 2.4). Δf dFz =− f 2k dz (2.5) f being the resonant frequency and k the cantilever spring constant. Due to the minus sign in (2.5), a positive force gradient, which is characteristic of an attractive magnetic interaction between tip and sample, produces a negative frequency shift, whilst a negative force gradient, typical of a repulsive interaction, induces a positive frequency shift. The shifts in resonant frequency tend to be very small, typically in the range 1-50 Hz for cantilevers having a resonant frequency f ~100 kHz. An image taken with a magnetic tip contains information about both the topography and the magnetic properties of a surface. Which effect dominates depends upon the distance of the tip from the surface, because the inter-atomic magnetic force persists for greater tip-to-sample separations than the van der Waals force. If the tip is close to the surface, in the region where standard non-contact AFM is operated, the image will be predominantly topographic. With an increase in the separation between the tip and the sample, magnetic effects become apparent. Collecting a series of images at different tip heights is one way to separate magnetic from topographic effects. 2.2.5 Magneto-Optical Kerr Effect 43 Figure 2.6. The orientations are defined in terms of the direction of the magnetisation vector M with respect to the surface of the material and the plane of incidence of an incident optical beam. (a) In the longitudinal case the magnetisation vector is in the plane of the surface and parallel to the plane of incidence. (b) In the Polar case the magnetisation vector is perpendicular to the plane of the surface. 10 When a beam of polarised light reflects off a magnetic surface, the plane of polarisation of the light is rotated. This phenomenon is known as the magneto-optic Kerr effect, named after Reverend Kerr who discovered the effect in the 19th Century. The technique is sometimes referred to as SMOKE, where the S stands for surface. However, the light is known to penetrate about 20 nm into the surface for most metals which means that MOKE is not particularly surface sensitive. Figure 2.6 illustrates the magneto-optical effects for longitudinal and polar orientations. In the longitudinal case the magnetisation vector is in the plane of the surface and parallel to the plane of incidence. 11 The effect is simple and occurs for 44 radiation incident in the P-plane (E-vector parallel to the plane of incidence) or the SPlane (E-vector perpendicular to the plane of incidence). The effect is that radiation incident in either of these linearly polarised states is, on reflection, converted to elliptically polarised light. The major axis of the ellipse is often rotated slightly with respect to the principal plane and this is referred to as the Kerr rotation. There is an associated ellipticity and this is called the Kerr ellipticity. The polar case is similar to the longitudinal case, except that the magnetisation vector is perpendicular to the plane of the surface. Figure 2.7. POLAR MOKE setup configuration 12 The experimental setup normally involves passing laser light through a polarising filter and then reflecting the light off the sample. The light then passes through another cross-polarising filter (Figure 2.7). Slight changes in the plane of polarisation will thus cause variations in the detected light intensity after the second filter. MOKE is frequently 45 used to measure the hysteresis loops of thin magnetic films, by studying the light intensity as a function of applied magnetic field. 46 2.3 Surface Analysis 2.3.1 X-Ray Photoelectron Spectroscopy (XPS) X-ray photoelectron spectroscopy is one of the most widely used technique in the area of surface analysis as it can measure the elemental composition, empirical formula, chemical state and electronic state of the elements that exists within a material.13 XPS uses the highly focused monochromatised soft x-ray to irradiate the sample surface under ultrahigh vacuum condition. The commonly used x-ray sources for XPS are Al Kα (1486.6 eV) and Mg Kα (1253.6 eV) as these photons are relatively “clean” with few satellites peaks, resulting in relatively narrow line widths. X-ray photon is absorbed by an atom near the surface, leading to the photoionization and the emission of a core inner shell electron to the vacuum 14 as illustrated in Figure 2.8. The kinetic energy of the emitted electron can be measured by using electron energy analyzer. The binding energy (EB) is calculated as EB = hv - Ekin – Φ (2.6) where Φ is the work function of the spectrometer, EB is the binding energy with respect to Fermi level, and Ekin is the kinetic energy of the emitted electron. Each photoexcited atom will exhibit characteristic binding energy of the core level electron and it varies with different chemical environment (oxidation state, lattice site and molecular environment etc.) of the atom by chemical shift up to a few eVs. This provides a very useful information for the investigation of surface modification. 47 Figure 2.8 Schematic diagram showing photoionization and electron emission by incident x-ray. 2.3.2 Scanning Electron Microscopy (SEM) Figure 2.9 Primary electrons interaction with sample and generated signals. 48 In order to gain better understanding on surface morphology, maximum resolution on the surface image must be obtained. However, the maximum resolution mainly depends on the wavelength of the radiation selected for the image. For the normal light optical microscopes, the maximum resolution of the sample image is limited by the visible light wavelength of between 400 nm to 700 nm and the degree of magnification beyond 1000 would be impossible. To achieve higher magnification, electrons are used instead of optical waves as the latter provide smaller wavelength. In Scanning Electron Microscope (SEM), images of the samples are obtained by scanning the surface with a high energy beam of electrons in a raster scan pattern. An electron beam is thermionically emitted from an electron gun fitted with a tungsten filament cathode. Tungsten is normally used in thermionic electron guns as it has the highest melting point and lowest vapour pressure of all metals, thereby allowing it to be heated for electron emission and because of its low cost. The electron beam, which typically has an energy ranging from a few hundred eV to 40 keV, is focused by one or two condenser lenses to a spot about 0.4 nm to nm in diameter. The beam passes through a pairs of scanning coils in the objective lens, which deflect the beam in a raster fashion over a rectangular area on the sample surface. When the primary electron beam interacts with the sample, the electrons lose energy by repeated random scattering and absorption within a teardrop-shaped volume of the specimen known as the interactive volume, which extends from less than 100 nm to around μm into the surface. Depending on the sample, the interaction can generate secondary electrons from the primary electrons, backscattered electrons, x-rays, light, heat and even transmitted 49 electrons that pass through the sample. The interaction and generated signals can be schematically illustrated as Figure 2.9. The generated signals will be detected by the scintillator-photomultiplier device and the digital image will be generated. The resolution of the SEM is within nm region and the magnification up to 200000 times can be obtained. 50 References Frey, T., Jantz, W. and Stibal, R., J. Appl. Phys. 1988, 64, 6002. Reeves, R., J. Phys. E 1972, 5, 547. O'Grady, K., Lewis, V. G. and Dickson, D. P. E., J. Appl. Phys 1993, 73, 5608. Gallop, J. C. SQUIDs, the Josephson Effects and Superconducting Electronics; Adam Hilger: Bristol, 1991. Weinstock, H.; Editor SQUID Sensors: Fundamentals, Fabrication and Applications.; Kluwer Academic: Dordrecht, 1996. Rondinone, A. J.; Zhang, Z. J. Handbook of Nanophase and Nanostructured Materials 2003, 2, 252. McElfresh, M. Fundamentals of Magnetism and Magnetic Measurements; Quantum Design: San Diego, 1994. "A Practical Guide to Scanning Probe Microscopy" http://mechmat.caltech.edu/~kaushik/park/1-3-0.htm Digital Instruments Magnetic Force Microscopy (MFM) Support Note No. 229, Rev. B http://www.nuance.northwestern.edu/media/nifti%20pdf/MFM_Manual.pdf 10 http://qub.ac.uk 11 Wrona, J.; Stobieckia, T.; Czapkiewicza, M.; Raka, R.; zakb, T.; Koreckib, J.; Kimc, C. G.; Journal of Magnetism and Magnetic Materials, 2004, 3, 2294-2295. 12 http://korek.uci.agh.edu.pl/mokenew.html 51 13 Moulder J. F.; Stickle, W. F.; Sobol, P. E.; Bomber, K. D. Handbook of X-ray Photoelectron Spectroscopy, Physical Electronics Division, Perkin-Elmer Corporation, Minnesota, 1991. 14 Nordling, C.; Sokolowski, E.; Siegbahn, K. Phys. Rev. 1957, 105, 1676. 52 [...]... varieties of magnetic orderings are schematically depicted in Figure 1. 2 Figure 1. 2 Varieties of magnetic orderings (a) paramagnetic, (b), ferromagnetic, (c) ferrimagnetic, (d) antiferromagnetic, and (e) superparamagnetic Diamagnetic material has a negative susceptibility with typical values on the -5 -6 order of 10 to 10 If a magnetic field is applied to a diamagnetic material, the induced magnetic. .. (AT) and 1H,1H,2H,2H-perfluoroalkanethiol (FT) Ag NC-ZnO NW 11 8 Table 6 .1 Summary of geometric parameters of ZnO NWs and NTs, synthesized by VPT, hydrothermal growth and ALD 13 8 Table 6.2 Summary of Mr/Ms, Ms and Hc for the thiol-capped ZnO NWs and NTs 15 5 List of Schemes Scheme 4 .1 Schematic explanation of effect of grain size on coercivity and AFC 10 0 Scheme 5 .1 Schematic... ~ 1/ D In the Stoner model (right), ferromagnetism this means D(EF) > 1/ I The dotted line describes the onset of ferromagnetism, D(E) = 1/ I 2 Fig .1. 2 Varieties of magnetic orderings (a) paramagnetic, (b), ferromagnetic, (c) ferrimagnetic, (d) antiferromagnetic, and (e) superparamagnetic 8 Fig .1. 3 The inverse susceptibility varies with T for (a) paramagnetic, (b) ferromagnetic, (c) ferrimagnetic,... is illustrated in Figure 1. 4 Figure 1. 4 A spin-reorientation from in-plane to out -of- plane occurs when the magnetocrystalline anisotropy is large enough to overcome the shape anisotropy to favors an outof-plane magnetization 1. 2.2 Single Domain Theory and Superparamagnetism 10 Figure 1. 5 Schematic representation of a 18 0º domain wall .11 A bulk magnetic material is comprised of magnetic domains, which... characterized by the magnetic induction or the flux density (B) These three magnetic quantities, magnetic field, magnetic induction and magnetization are related by B = μ (H + M) (1. 1) B=μH (1. 2) 0 where μ is a universal constant of permeability in a free space and μ is the permeability 0 of a material The magnetic susceptibility (χ) is defined as the ratio of magnetization to magnetic field χ=M/H (1. 3) Therefore,... the above equations, the permeability and susceptibility of a material are correlated by μ = μ (1+ χ) 0 (1. 4) The susceptibility is a major parameter in characterization of magnetic properties of a material The magnitude of the susceptibility and its temperature and field dependencies 6 provide a measure of magnetic behavior of different types of magnetic materials, which can be classified into diamagnetism,... signal Temperature dependence of magnetization (Ms) and coercivity (Hc) measured for thiol-capped (c) NWs and (d) NTs arrays 14 9 Fig 6 .11 Thermal dependence of magnetization of thiol-capped ZnO NWs array measured under ZFC and FC conditions under a field of 10 00 Oe (magnetic moment not normalized to area) 15 0 Fig 6 .12 Schematics of the distribution of dipole moments on the lateral... 6 .1 Ordered alignment of magnetic moments in thiol-capped single crystalline ZnO NW/NT vs disordered alignment of magnetic moment in polycrystalline ZnO NT 13 0 xviii Chapter 1 Introduction 1. 1 Magnetism in nanostructures Magnetism in reduced dimensions has been an active topic in the last two decades 1 Although much of the physical phenomena can be understood by quantum descriptions of. .. property of spin.6, 7 The magnetic properties of a matter are fundamentally the result of the electrons of the atom, which creates a magnetic moment as a result of the electron motion At the atomic level, both types of electron motion, spin and orbital, are associated with a magnetic moment When a material is placed in a magnetic field H, its magnetic induction consists of the magnetic induction generated... antiferromagnetic materials TN and TC are Néel temperature and Curie temperature, respectively 9 Fig .1. 4 A spin-reorientation from in-plane to out -of- plane occurs when the magnetocrystalline anisotropy favors an out -of- plane magnetization and is large enough to overcome the shape anisotropy 11 Fig 1. 5 Schematic representation of a 18 0º domain wall 12 Fig 1. 6 Schematic of Stoner-Wohlfarth . in Magnetics, 2 010 , 46, 17 87 v Chapter 1 Introduction 1 1. 1 Magnetism in nanostructures 1 1. 2 Fundamentals of Magnetism 7 1. 2 .1 Magnetic Anisotropy in low dimensions 10 1. 2.2. MAGNETIC PROPERTIES OF HYBRID NANOSTRUCTURES DENG SUZI NATIONAL UNIVERSITY OF SINGAPORE 2 010 MAGNETIC PROPERTIES OF HYBRID NANOSTRUCTURES. Fabrication of thiol-capped Ag NCs-ZnO NWs 10 9 5.2.2 Magnetic characterization 11 0 5.2.3 Electronic Structure Characterization 11 1 5.3 Results and Discussions 11 2 5.3 .1 Characterization of Ag NC-ZnO