The utilization of ferromagnetic shape memory alloys (FSMAs) in small scale devices has attracted considerable attention within the last decade. However, the lack of sufficient studies on their reversible shape change mechanisms, i.e, superelasticity, magnetic field-induced martensite variant reorientation and martensitic phase transformation, at the micron and submicron length scales prevent the further development and the use of FSMAs in small scale devices. Therefore, investigating the size effects in these mechanisms has both scientific and technological relevance. Superelastic behavior of Ni54Fe19Ga27 shape memory alloy single crystalline pillars was studied under compression as a function of pillar diameter. Multiple pillars with diameters ranging between 200 nm and 10 µm were cut on a single crystalline bulk sample oriented along the [110] direction in the compression axis and with fully reversible two-stage martensitic transformation. The results revealed size dependent two-stage martensitic transformation which was suppressed for pillar sizes of 1 µm and below. We also demonstrated that the reduction in pillar diameter decreases the transformation temperature due to the difficulty of martensite nucleation in small scales. Size effects in the magnetic field-induced martensite variant reorientation were investigated in the Ni50Mn28.3Ga21.7 single crystals oriented along the [100] direction of the austenite phase. Single crystalline compression pillars were fabricated on the martensite twins between the sizes of 630 nm and 20 µm. It was found that the stress-induced and magnetic field-induced martensite variant reorientation are size dependent and became more difficult with the reduction in sample size. Surprisingly, it was still possible to magnetically activate the shape change in the micropillars which indicates the fact that magnetocrystalline anisotropy energy increases with the reduction in sample dimensions. Ni45Mn36.6Co5In13.4 pillars between the 600 nm and 10 µm diameters were investigated along the [100] direction of the austenite to study the size effects in the magnetic field-induced phase transformation (MFIPT). MFIPT was obtained down to 5 µm size in these pillars with reasonable magnetic field levels similar to their bulk counterparts.

In the present thesis, we have explored multiple aspects concerning the stochasticity of magnetic domain wall motion during magnetization reversal, all of which originated from our initial study of magnetic field sensing using planar Hall effect. Magnetic field sensors occupy a very important and indispensable position in modern technology. They can be found everywhere, from cellphones to automobiles, electric motors to computer hard disks. At present there are several emerging areas of technology, including biotechnology, which require magnetic field sensors which are at the same time simple to use, highly sensitive, robust under environmental conditions and sufficiently low cost to be deployed on a large scale. Magnetic field sensing using planar Hall effect is one such feasible technology, which we have explored in the course of the thesis. The work was subsequently expanded to cover some fundamental aspects of the stochasticity of domain wall motion, studied with planar Hall effect, which forms the main body of work in the present study. In Chapter 1, we give an introduction to the phenomenology of planar Hall effect, which is the most important measurement technique used for all the subsequent studies. Some early calculations, which had first led to the understanding of anisotropic magnetoresistance and planar Hall effect as being caused by spin-orbit interaction are discussed. In Chapter 2, we discuss briefly the experimental techniques used in the present study for sample growth and fabrication, structural and magnetic characterization, and measurement. We discuss pulsed laser ablation, which is the main technique used for our sample growth. Particular emphasis is given to the instrumentation that was carried out in-house for MOKE and low field magnetotransport (AMR and PHE) measurement. This includes an attempt at domain wall imaging through MOKE microscopy. Some of the standard equipments used for this work, such as the SQUID magnetometer and the acsusceptometer are also discussed in detail. In Chapter 3 we discuss our work on planar Hall sensors that led to the fabrication of a device with a very simple architecture, having transfer characteristics of 650V/A.T in a range of _2Oe. The sensing material was permalloy (Ni81Fe19), and the value had been obtained without using an exchange biased pinning layer. Field trials showed that the devices were capable of geomagnetic field sensing, as well as vehicle detection by sensing the anomaly in Earth's magnetic field caused by their motion. Its estimated detection threshold of 2.5nT made it well suited for several other applications needing high sensitivity in a small area, the most prominent of them being the detection of macromolecules of bio-medical significance. Chapter 4: The work on Barkhausen noise was prompted by reproducibility problems faced during the sensor construction, both between devices as well as within the same device. Study of the stochastic properties led us to the conclusion that the devices could be grouped into two classes: one where the magnetization reversal occurred in a single step, and the other where it took a 0staircase0 like path with multiple steps. This led us to simulations of Barkhausen noise using nucleation models like the RFIM whence it became apparent that the two different groups of samples could be mapped into two regimes of the RFIM distinguished by their magnetization reversal mode. In the RFIM, the nature of the hysteresis loop depends on the degree of disorder, with a crossover happening from single-step switching to multi-step switching at a critical disorder level. Appropriate changes also appear in the Barkhausen noise statistics due to this disorder-induced crossover. By studying the Barkhausen noise statistics for our permalloy samples and comparing them with simulations of the RFIM, we found nearly exact correspondence between the two experimental groups with the two classes resulting from crossing the critical disorder. What remained was to quantify the 0disorder0 level of our samples, which was done through XRD, residual resistivity and a study of electron-electron interaction effects in the resistivity. All these studies led to the conclusion that the samples reversing in multiple steps were more 0defective0 than the other group, at par with the model predictions. This completed the picture with respect to the modeling of the noise. In experiments, it was found that a high rate of film deposition yielded less 0defective0 samples, which severed as an important input for the sensor construction. These results can be viewed from a somewhat broader perspective if we consider the present scenario in the experimental study of Barkhausen noise, or crackling noise in general. Two classes of models exist for such phenomena: front propagation models and nucleation models. Both appear to be very successful when it comes to experiments with bulk materials, while the comparison with experiments on thin films is rather disappointing. It is still not clear whether the models are at fault or the experiments themselves. Through our study, we could demonstrate that there can be considerable variation in the Barkhausen noise character of the same material deposited in the same way, and what was important was the degree of order at the microscopic level. This may be a relevant factor when experimental papers report non-universality of Barkhausen noise in thin films, which can now be interpreted as either insufficient defects or a sample area too small for the study. Chapter 5: Defects in a sample are not the only cause for stochastic behavior during magnetization. In most cases, random thermal 0events0 are also an important factor determining the path to magnetization reversal, which was also true for our permalloy samples. We studied the distribution of the external fields at which magnetization reversal took place in our samples, and tried to explain it in terms of the popular Neel-Brown model of thermal excitation over the anisotropy barrier. The analysis showed that even though the coercivity behaved 0correctly0 in terms of the model predictions, the behavior of the distribution width was anomalous. Such anomalies were common in the literature on switching field distributions, but there seemed to be no unified explanation, with different authors coming up with their own 0exotic0 explanations. We decided to investigate the simplest situations that could result in such a behavior, and through some model-based calculations, came to the conclusion that one of the causes of the anomalies could be the different magnitudes of barrier heights/anisotropy fields experienced by the magnetic domain wall when the reversal occurs along different paths. Though an exact match for the behavior of the distribution width could not be obtained, the extended Neel-Brown model was able to produce qualitative agreement. Chapter 6 contains a study of some interesting 0geometrical0 effects on Barkhausen noise of iron thin films. By rotating the applied magnetic field out-of plane, we could observe the same single-step to multi-step crossover in hysteresis loop nature that was brought about by varying disorder in Chapter 4. We could explain this through simulations of a random anisotropy Ising model, which, apart from exhibiting the usual disorder induced crossover, showed a transition from sub-critical to critical hysteresis loops when the external field direction was rotated away form the average anisotropy direction. Once again, simulation and experiment showed very good agreement in terms of the qualitative behavior. In the second part of this chapter, a study of exchange biased Fe-FeMn system was carried out, where it was observed that the reversal mode has been changed from domain wall motion to coherent rotation. Barkhausen noise was also suppressed. Though many single-domain models existed for this type of reversal, our system was not found to be strictly compatible with them. The disagreement was with regard to the nature of the hysteresis, which, if present, had to be a single step process for a single domain model. The disagreement was naturally attributed to interaction with the nearby magnetic moments, to verify which, simulations were done with a simplified micromagnetic code, which produced excellent agreement with experiment. In Chapter 7, we have studied the temporal properties of Barkhausen avalanches, to compare the duration distributions with simulation. We had used a permalloy sample that was sub-critical according to avalanche size distributions, and our measurement was based on magneto-optic Kerr effect. We measured duration distributions which showed a similar manifestation of finite-size effects as were shown by the size distributions. The power law exponent was calculated, which was deemed 0reasonable0 upon comparison simulations of the sub-critical RFIM. Appendix A contains a study of high-field magnetoresistance of permalloy, which shows that the dominant contribution to magnetoresistance is the suppression of electron-magnon scattering. An interesting correlation is observed between the magnetization of samples and an exchange stiffness parameter d1, that was extracted from magnetoresistance measurements. Here we also re-visit our earlier observation of permalloy thin films possessing a resistance minimum at low temperature. The origin of this minimum is attributed to electron-electron interaction. Appendix B contains the source codes for most of the important programs used for simulation and data analysis. The programs are written in MATLAB and FORTRAN 95. LabView programs used for data acquisition and analysis are not included due to space requirements to display their graphical source codes. Appendix C discusses the studies on a disordered rare-earth oxide LaMnO3. The re-entrant glassy phase is characterized with ac susceptibility and magnetization measurements to extract information about the nature of interactions between the magnetic 0macrospins0 in the system. Appendix D deals with electron scattering experiments performed with spinpolarized electrons (SPLEED) from clean metal surfaces in UHV. A study of the scattering cross sections as a function of energy and scattering angle provides information about spin-orbit and exchange interactions of the electrons with the surface atoms, and can answer important questions pertaining to the electronic and magnetic structure of surfaces. In the course of this study, planar Hall effect is seen to emerge as a powerful tool to study the magnetic state of a thin film, so that it is interesting to apply it to thin films of other materials such as oxides, where magnetization noise studies are next to nonexistent. What also emerged is that there is still a lot of richness present in the details of supposedly well-understood magnetization phenomena, some of which we have explored in this thesis in the context of stochastic magnetization processes.

Interest into the effects of weak static magnetic fields on chemical reactions involving spin correlated radical pairs has increased over the last few decades, particularly as scientists have become more curious about the mechanisms by which animals can sense and respond to small variations in the Earth's weak (50 µT) magnetic field. The magnetosensitivity of radical pairs, as dictated by the radical pair mechanism, lies at the heart of the most heavily supported hypothesis of this magnetoreception phenomenon. This thesis is concerned with the spectroscopic investigations of isotropic and anisotropic magnetic field effects in fluorescent systems. First of all, an introduction to spin chemistry and magnetoreception is presented. In chapter 3, the effects of weak radiofrequency oscillating fields when applied in combination with weak static fields are explored in isotropic solutions. The validity of the high-field model, typically used to describe spin dynamics in magnetic resonance, is tested and the effects of orientation and field strength on magnetic field effects are discussed in detail. In Chapter 4, a range of exciplex systems are studied by fluorescence methods and their energetics are explored. The factors which determine the formation of an exciplex, i.e. the complex equilibrium between the exciplex and the spin-correlated radical pair,are considered and used to assess the existence and magnitude of MFEs. Radical pair systems investigated, using MARY spectroscopy, with respect to their potential to act as model chemical compasses are introduced in chapter 5. Solid-state media are used to align the exciplex systems to detect any magnetic field direction dependence. Finally, in chapter 6, AMELIA, an experiment which can directly measure the anisotropic magnetic field response of a system, is presented and applied successfully to systems to detect directly the anisotropic field response of a photoexcited anthracene crystal.

Magnetic fields are present throughout the universe and are very important for many astrophysical processes. Magnetic field influences a star throughout its life and affects nearby objects such as planets. Stellar magnetic field can be detected by measuring the Zeeman splitting of spectral lines in the intensity spectra (Stokes I) if the field is strong, or by analyzing polarization spectra if the field is weak. Magnetic fields in stars similar to the Sun are ubiquitous but, in general, relatively weak. Until recently these fields were detected through circular polarization (Stokes V) only since linear polarization (Stokes QU) is significantly weaker. The information embedded in different Stokes spectra is used for reconstruction of the surface magnetic field topology with Zeeman Doppler imaging (ZDI) technique. However, cool stars often have complex field geometries and this, combined with a low field strength, partial Stokes parameter observations and the presence of cool spots, makes accurate magnetic mapping difficult. We have performed numerical tests of ZDI to investigate some of the problems of magnetic inversions and ways to overcome them. The most reliable results were found when magnetic field and temperature inhomogeneities were modelled simultaneously and all four Stokes parameters were included in the reconstruction process. We carried out observations of active cool stars in all four Stokes parameters trying to find an object with linear polarization signatures suitable for ZDI. The RS CVn star II Peg was identified as a promising target, showing exceptionally strong linear polarization signatures. We reconstructed the magnetic field in II Peg using full Stokes vector observations for the first time in a cool star. Compared to the magnetic maps recovered from the Stokes IV spectra, the four Stokes parameter results reveal a significantly stronger and more complex surface magnetic field and a more compact stellar magnetosphere. Spectropolarimetric observations and magnetic inversions can also be used to investigate magnetic activity of the young Sun and its implications for the solar system past. To this end, we studied a sample of six stars with parameters very similar to the present Sun, but with ages of only 100-650 Myr. Magnetic field maps of these young solar analogues suggest a significant decrease of the field strength in the age interval 100-250 Myr and a possible change in the magnetic field topology for stars older than about 600 Myr.

Optical trapping is a method which uses focused laser light to manipulate small objects. This optical manipulation can be scaled below the diffraction limit by using interactions between light and apertures in a metal film to localize electric fields. This method can trap objects as small as several nanometers. The ability to determine the properties of a trapped nanoparticle is among the most pressing issues to the utilization of this method to a broader range of research and industrial applications. Presented here are two methods which demonstrate the ability to determine the properties of a trapped nanoparticle. The first method incorporates Raman spectroscopy into a trapping setup to obtain single particle identification. Raman spectroscopy provides a way to uniquely identify an object based on the light it scatters. Because Raman scattering is an intrinsically weak process, it has been difficult to obtain single particle sensitivity. Using localized electric fields at the trapping aperture, the Raman integrated trapping setup greatly enhances the optical interaction with the trapped particle enabling the required sensitivity. In this work, the trapping and identification of 20 nm titania and polystyrene nanoparticles is demonstrated. The second method uses an aperture assisted optical trap to detect the response of a magnetite nanoparticle to a varying applied magnetic field. This information is then used to determine the magnetic susceptibility, remanence, refractive index, and size distribution of the trapped particle. / Graduate / 0544 / 0752 / stevenjones3.14@gmail.com

Studies on strongly correlated electron systems over decades have allowed physicists to discover unusual properties such as spin density waves, ferromagnetic and antiferromagnetic states with unusual ordering of spins and orbitals, and Mott insulating states, to name a few. In this thesis, the focus will be on the specific heat property of these materials exhibiting novel electronic ground states in the presence and absence of a field. The purpose of these measurements is to characterize the phase transitions into these states and the low energy excitations in these states. From measurements at the phase transitions, one can learn about the amount of order involved [i.e. entropy: ΔS = ∫Δc p/T dT], while measurements at low temperatures illuminate the excitation spectrum. In order to study the thermodynamic properties of the materials at their phase transitions, a high sensitive technique, ac-calorimetry was used. The ac-calorimeter, workhorse of our low dimensional materials lab, is based on modulating the power that heats the sample and measuring the temperature oscillations of the sample around its mean value. The in-house ac-calorimetry set up in our lab has the capability to produce a quasi-continuous readout of heat capacity as a function of temperature. A variety of single crystals were investigated using this technique and a few among them are discussed in my thesis. Since many of the crystals that are studied by our group are magnetically active, it becomes useful for us to also study them in the presence of a moderate to high magnetic field. This motivated me to design, develop, and build a heat capacity probe that would enable us to study the crystals in the presence of non-zero magnetic fields and at low temperatures. The probe helped us not only to revisit some of the studied materials and to draw firm conclusions on the previous results but also is vital in exploring the untouched territory of novel materials at high magnetic fields (~ 14 T).

Experiments in high energy density (HED) plasma physics have become more accessible with the increasing availability of high-intensity pulsed lasers. Extending the experiment parameters to include magnetized HED plasmas requires a field source that can generate fields of order 100 tesla. This dissertation discusses the design and implementation of a pulsed field driver with a designed maximum of 2.2 MA from a 160 kJ capacitor bank. Faraday rotation measurement of 63 T for a 1.0 MA discharge supported Biot-Savart estimates for a single-turn coil with a 1 cm bore. After modification, the field driver generated up to 15 T to magnetize supernova-like spherical blast waves driven by the Texas Petawatt Laser. The presence of the high field suppressed blast wave expansion, and had the additional effect of revealing a cylindrical plasma along the laser axis. / text

The existence of a Fisk-type heliospheric magnetic field (HMF) is one of the most debated questions in cosmic-ray modulation. Recently, Burger and Hitge [2004] developed a divergence-free Fisk-Parker hybrid magnetic field model to demonstrate the behaviour of cosmic rays in the heliosphere due to such a field. This approach has been refined and the properties of the consequent field are investigated. It is found that randomly directed magnetic field diffusion in and above the photosphere significantly influences the solar magnetic field both at the solar poles and near the polar coronal hole boundary. The solar cycle dependence of this field is investigated, a study which is of particular importance for studies of the long-term behaviour of cosmic rays, such as those undertaken at the SANAE base in Antarctica. The amplitudes of the 26-day recurrent cosmic-ray variations are modelled as function of both latitudinal gradient and heliolatitude and are found to agree qualitatively and in some cases quantitatively with the observational results reported by Zhang 119971 and Paizis et al. 119991. Although magnetic field data do not clearly indicate the existence of the Fisk field [see, e.g., Fursyth et al., 20021, this study supports the existence of a Fisk-type HMF. / Thesis (M.Sc. (Physics))--North-West University, Potchefstroom Campus, 2006.

Micromirrors and diffraction gratings were developed for spectroscopy and magnetic field sensor in this thesis. MEMS blazed gratings were successfully fabricated in different grating periodicities to cover a wide infrared wavelength range. Lorentz force actuated micromirrors were investigated, and two types of mirrors were fabricated: rotating and pop-up micromirrors. The deflection angle of the mirrors was controllable by altering the driving current on the mirror. Deflection angle vs. driving current was studied for different mirror types and different spring dimensions. A Lorentz force based magnetic field sensor is also demonstrated. The sensor employs the rotating micromirror as a resonator. With an AC current flowing around the micromirror, a periodic Lorentz force is generated which drives the resonator. The rotational amplitude of the micromirror is measured with an optical positioning system and external circuits. The highest resolution of the magnetic field sensor is 0.4 nT at 50 mArms, and 53 mHz filter bandwidth. With appropriate current level, this sensor can measure a wide range of magnetic field, from nT to T. / October 2016

Storage of large amounts of grain post-harvest is common during drying, distribution and preservation of crops. During storage, where grain is usually held in a large metallic container or bin, changes in temperature, moisture, and insect infestation can cause grain to spoil annual post-harvest crop losses are estimated up to 30% in some countries while Canadian losses of 2% exceed a billion dollars. Currently, locally-sensitive temperature and moisture sensors are a common way to monitor grain bins. Sensors are generally strung on heavy duty cables that can withstand the forces generated when unloading grain. This monitoring method provides a coarse sampling of the storage environment due to system cost and the fact that using many sensor cables would require significantly reinforcing the bin. Further, these cables are not suitable for monitoring stored crops that are dried by a combination of stirring and aeration. Over the past four years, I have developed multiple electromagnetic imaging based grain-monitoring systems with the goals of overcoming the deficiencies of existing sensor technology and allowing farmers and distributors a robust way to preserve our food stores and increase revenue. The proposed technology aims to produce global, quantitative images of grain properties throughout the bin from measurements taken by a few side-mounted antennas used to interrogate the bin contents. To develop this technology intensive research was put into the design of low profile, robust antennas as well as numerical analysis of the effects of different field distributions within conducting boundaries. Both electric and magnetic field sensitive antennas were built and tested in small lab-scale as well as full-scale grain bins to experimentally evaluate the performance of such imaging system. This thesis provides details on different system designs and analysis and describes the advantages and challenges associated with the techniques described. / October 2016