Thesis advisor: Fazel Tafti / Since the idea of topology was realized in real materials, the hunt is on for new candidates of topological semimetals with novel electromagnetic responses. For example, topological states can be highly conductive due to a topological protection, which can be destroyed in a magnetic field and lead to an extremely high magnetoresistance. In Weyl semimetals, a transverse current that would usually require a magnetic field to emerge, can be generated by intrinsic Berry curvature without a magnetic field -- the celebrated anomalous Hall effect. In this dissertation, both phenomena mentioned above are studied in rare-earth monopnictides and RAlX material family (R=rare-earths, X=Ge/Si), respectively. The monopnictides are ideal for the study of extreme magnetoresistance because of their topological transitions and abundant magnetic phases. In LaAs, we untied the connection between topological states and the extreme magnetoresistance, the origin of which is clarified. In HoBi, we found an unusual onset of extreme magnetoresistance controlled by a magnetic phase dome. On the other hand, RAlX material family is a new class of Weyl semimetals breaking both inversion and time-reversal symmetries. In particular, in PrAlGeₓSi₁₋ₓ (x=0-1), we unveiled the first transition from intrinsic to extrinsic anomalous Hall effect in ferromagnetic Weyl semimetals, and the role of topology is discussed. In CeAlSi, we found that the Fermi level can be tuned as close as 1 meV away from the Weyl nodes; moreover, a novel anomalous Hall response appears only when the Fermi level is tuned to be near the Weyl nodes. Thus, we established a new transport response solely induced by Weyl nodes. / Thesis (PhD) — Boston College, 2021. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Physics.

The past few decades have witnessed a race towards developing smaller, faster, cheaper and ultra high capacity data storage technologies. In particular, this race has been accelerated due to the emergence of the internet, consumer electronics, big data, cloud based storage and computing technologies. The enormous increase in data is paving the path to a data capacity gap wherein more data than can be stored is generated and existing storage technologies would be unable to bridge this data gap. A novel approach could be to shift away from current two dimensional architectures and onto three dimensional architectures wherein data can be stored vertically aligned on a substrate, thereby decreasing the device footprint. This thesis explores a data storage concept based on vertically aligned cylindrical magnetic nanowires which are promising candidates due to their low fabrication cost, lack of moving parts as well as predicted high operational speed. In the proposed concept, data is stored in magnetic nanowires in the form of magnetic domains or bits which can be moved along the nanowire to write/read heads situated at the bottom/top of the nanowire using spin polarized current. Cylindrical nanowires generally exhibit a single magnetic domain state i.e. a single bit, thus for these cylindrical nanowire to exhibit high density data storage, it is crucial to pack multiple domains within a nanowire. This dissertation demonstrates that by introducing compositional variation i.e. multiple segments along the nanowire, using materials with differing values of magnetization such as cobalt and nickel, it is possible to incorporate multiple domains in a nanowire. Since the fabrication of cylindrical nanowires is a batch process, examining the properties of a single nanowire is a challenging task. This dissertation deals with the fabrication, characterization and manipulation of magnetic domains in individual nanowires. The various properties of are investigated using electrical measurements, magnetic microscopy techniques and micromagnetic simulations. In addition to packing multiple domains in a cylindrical nanowire, this dissertation reports the current assisted motion of domain walls along multisegmented Co/Ni nanowires, which is a fundamental step towards achieving a high density cylindrical nanowire-based data storage device.

Les tests « in vitro » permettent d’établir près de 70% des diagnostics et leur développement pour une utilisation au plus près du patient apparaît donc comme un enjeu majeur de santé publique. Dans ce contexte, les critères ASSURED (« Affordable, Sensitive, Specific, User-friendly, Rapid and robust, Equipment-free and Deliverable to end-users ») a été défini par l’organisation mondiale de la santé pour que les chercheurs développent des outils de diagnostic dits « Point of Care » utilisables par le plus grand nombre. Avec l’essor de la microfluidique, la gamme des dispositifs possibles s'est élargie et des biocapteurs intégrés ont été développés, transformant le signal biologique d’une reconnaissance d’un biomarqueur par une sonde biologique en un signal optique, électrochimique, mécanique ou encore magnétique. Comme les milieux biologiques sont en grande majorité amagnétiques, les capteurs magnétiques ne sont pas affectés par l’utilisation de matrices biologiques complexes comme peuvent l’être les mesures optiques ou électrochimiques. De plus ces capteurs sont faciles à produire et intégrables dans les puces microfluidiques. Cette thèse a pour objectifs de concevoir un outil de diagnostic in vitro basé sur des capteurs à magnétorésistance géante et de tester ses performances. Cette étude a été réalisée en utilisant une lignée cellulaire de myélome murin. Les cellules sont marquées spécifiquement par des particules magnétiques fonctionnalisées par des anticorps dirigés contre un de leurs antigènes et sont passées dans le canal microfluidique au-dessus des capteurs. Cette méthode de détection dynamique permet de compter les objets magnétiques un par un. La difficulté réside dans la distinction des signaux spécifiques provenant des cellules marquées des signaux faux positifs induits par les billes restant en solution. Deux types de dispositifs ont été conçus dans cette thèse pour lever ce verrou. Le premier possède une couche inerte de séparation de quelques micromètres entre les capteurs GMR et le canal qui permet de supprimer les signaux des billes isolées. Le second dispositif, qui a des capteurs à la fois au-dessus et au-dessous du canal microfluidique, permet une double détection simultanée de chaque objet magnétique. Il est ainsi possible de connaître le nombre de billes qui les marquent et de déterminer s’il s’agit d’un agrégat de billes ou d’un objet biologique. / The « in vitro » tests are requested for the establishment of nearly 70% of diagnoses and their development for on-site detection therefore appears to be a major public health issue. In this context, the ASSURED criterion (« Affordable, Sensitive, Specific, User-friendly, User-friendly, Rapid and robust, Equipment-free and Deliverable to end-users ») has been defined by the World Health Organization to encourage researchers to develop diagnostic tools called « Point of Care » that can be widely used.With the rise of microfluidics, the range of possible devices has broadened and integrated biosensors have been developed, transforming the biological signal from a biomarker recognition by a biological probe into an optical, electrochemical, mechanical or magnetic signal. As biological environments are largely non-magnetic, magnetic sensors are not affected by the use of complex biological matrices as are optical or electrochemical measurements. In addition, these sensors are easy to produce and can be integrated into microfluidic chips. The objectives of this thesis are to design a diagnostic tool in vitro based on giant magnetoresistance sensors and to test its performance. Its development was carried out using a murine myeloma cell line. The cells are specifically labeled by magnetic particles functionalized by antibodies directed against one of their antigens and flown in the microfluidic channel above the sensors. This dynamic detection method allows magnetic objects to be counted one by one. The challenge is to distinguish the signals coming from the labeled cells from those of the beads remaining in solution. In order to address this problem, two labs on chips are developed in this thesis. In a first device, an inner layer of a few micrometers separates the sensors from the channel which allows to suppress the signals of the isolated beads. The second device has sensors both above and below the microfluidic channel and can measure the number of beads corresponding to each doubly detected object which can thus be identified (aggregates or biological objects).

La polyvalence de la technique de track-etching a permis d’étudier plus avant l’effet piezoélectrique direct et indirect d’un film polarisé en poly(fluorure de vinylidène) PVDF en créant des membranes nanostructurées hybrides de nanofils de nickel (Ni NWs)/PVDF. Les propriétés magnétiques du nanofil de nickel, telle que la magnétorésistance anisotrope (AMR), ont été exploitées afin d’étudier la réponse de l’aimantation à la déformation mécanique de la matrice PVDF. En particulier, les déformations ont été induites soit par contrainte thermo-mécanique, soit par contrainte électromécanique (effet piezoélectrique indirect). La sensibilité d’un nanofil unique a permis de déterminer l’amplitude et la direction de la contrainte mécanique exercée à l’échelle nanométrique par la matrice PVDF. La résistance exceptionnelle de la réponse piezoélectrique directe du film PVDF polarisé à l’irradiation, telle que l’irradiation aux ions-lourds accélérés et aux électrons (domaine de doses < 100kGy) a été observée. Mis à part la conservation de la réponse piezoélectrique, les défauts engendrés par l’irradiation dans ce domaine de dose (scissions de chaines, augmentation de phase crystalline, réticulations) ont eu un impact significatif sur la structure du matériau polymère. L’ensemble de ces défauts, les uns prépondérants en-dessous de la dose-gel ( 10kGy), les autres au-dessus, forme une compensation d’effets antagonistes qui mènent à une réponse piezoélectrique globalement inchangée. Stimulé par la grande résistance du PVDF à l’irradiation en termes de réponse piezoélectrique, l’idée a été d’exploiter, en vue d’une application dans la récupération d’énergie, le réseau de nanofils de nickel inclus dans la membrane en PVDF polarisé pour étudier l’influence des nanofils de nickel sur la l’efficacité piezoélectrique. La présence du réseau de nanofils de nickel mène à un accroissement non négligeable de l’efficacité piezoélectrique. Reliée à la présence des nanofils, une augmentation de la permittivité diélectrique dans le PVDF nanostructuré a également été enregistrée. Une polarisation interfaciale entre les nanofils de nickel et la matrice PVDF pourrait expliquer cette valeur accrue par rapport au PVDF nanoporeux sans nanofils. / The versatility of the track-etching technique has allowed to investigate deeper the direct and inverse piezoelectric effect of a polarized Poly(vinylidene fluoride) (PVDF) film in building nanostructured hybrid Nickel nanowires (Ni NWs)/PVDF membrane. The magnetic properties of the Ni NW, such as anisotropic magneto resistance (AMR), are exploited to investigate the response of the magnetization to a mechanical deformation of the PVDF matrix. In particular, the deformations were induced either by thermo-mechanical or an electro-mechanical (inverse piezoelectric effect) stress. The sensitivity of the single NW has allowed to determine the amplitude and direction of a mechanical stress exerted at the nano-scale by the PVDF matrix. The outstanding resistance of the direct piezoelectric response of polarized PVDF film to radiation, such as SHI and e-beam, (doses range < 100kGy) was reported. Beyond the conservation of the piezoelectric response, in this dose range, irradiation defects (chain scissions, increase of the crystalline -phase, crosslinking) had a significative impact on the polymer material. All these defects, ones predominant above the gel dose (herein 10 kGy), and the other ones below, compensate their antagonistic effects towards the globally unchanged piezoelectric responses. Motivated by the high radiation resistance of the PVDF in terms of piezoelectric response, the idea was to exploit Ni NWs array embedded in the polarized PVDF membrane to study the influence of the Ni NWs on the piezoelectric response in view of harvesting energy application. The presence of the Ni NWs array leads a non-negligible increase of the piezoelectric efficiency. Related to the presence of the NWs, an increase of the dielectric permittivity in the nanostructured PVDF was also reported. An interfacial polarization between the Ni NWs and the PVDF matrix could explain the higher efficiency value respect to nanoporous PVDF, without NWs.

Spin-torque nano-oscillators (STNOs) are prospective successors of transistor-based emitters and receivers of radio-frequency signals in commonly used remote communication systems. In comparison to the conventional electronic oscillators, STNOs offer the advantage of being tunable over a wide range of frequencies simply by adjusting the applied current, the smaller lateral size (up to 50 times) and the lower power consumption as the lateral size of the device is reduced. It has already been demonstrated that the output signal characteristics of STNOs are compatible with the requirements for applications: they can provide output powers in the µW range, frequencies of the order of GHz, quality factors Q (equal to df/f, where f is the frequency, and df is the linewidth) up to several thousands (e.g., 3 200), and can be integrated into Phase-Locked Loop (PLL) circuits. The most promising type of spin-torque oscillators is the hybrid geometry STNOs utilizing an in-plane magnetized fixed layer, an out-of-plane magnetized free layer and the MgO tunnel barrier as a spacer. This geometry maximizes the output power, since the full parallel-to-antiparallel resistance variation can be exploited in the limit of large magnetization precession angle (i.e., when the magnetization oscillates fully within the plane of the STNO stack). Moreover, the considered hybrid geometry allows for the reduction of the critical currents, enables functionality regardless of the applied magnetic or current history and requires a simplified fabrication process in comparison to the opposite hybrid geometry, consisting of an in-plane magnetized free layer and an out-of-plane reference layer, which requires an additional read-out layer. Simultaneously, the choice of the spacer material in considered STNOs is motivated by the increase of both the output power (via large magnetoresistance ratios) and the power conversion rate ('output power to input power' ratio), compared to their fully metallic counterparts. Despite the many advantages of MgO-based hybrid geometry STNOs, unexplained issues related to the physics behind their principle of operation remained. In this thesis, the main focus is put on the two key aspects related to the out-of-plane steady-state precession in hybrid STNOs: the precession mechanism (combined with the analysis of the influence of the bias dependence of the tunnel magnetoresistance) and the zero-field oscillations stabilized by an in-plane shape anisotropy. State-of-the-art theoretical studies demonstrated that stable precession in hybrid geometry STNOs can only be sustained if the in-plane component of the spin-transfer torque (STT) exhibits an asymmetric dependence on the angle between the free and the polarizing layer (which is true for fully metallic devices, but not for the MgO-based magnetic tunnel junctions (MTJs)). Nevertheless, recent experimental reports showed that spin-transfer driven dynamics can also be sustained in MgO-based STNOs with this particular configuration. In this thesis, a phenomenological and straightforward mechanism responsible for sustaining the dynamics in considered system is suggested. The mechanism is based on the fact that, in MgO-based MTJs, the strong cosine-type angular dependence of the tunnel magnetoresistance, at constant applied current, translates into an angle-dependent voltage component, which results in an angle-dependent spin-transfer torque giving a rise to the angular asymmetry of the in-plane STT and, thus, enabling steady-state precession to be sustained. Subsequently, the bias dependence of the tunnel magnetoresistance (TMR), which has been so far neglected in similar calculations, is taken into account. According to the results of analytical and numerical studies, the TMR bias dependence brings about a gradual quenching of the dynamics at large applied currents. The theoretical model yields trends confirming our experimental results. The most important conclusion regarding to this part of the thesis is that, while the angular dependence of the tunnel magnetoresistance introduces an angular asymmetry for the in-plane spin transfer torque parameter (which helps maintain steady-state precession), the bias dependence of the resistance works to reduce this asymmetry. Thus, these two mechanisms allow us to tune the asymmetry of the in-plane STT as function of current and to control the dynamical response of the actual device. Except for the precession mechanism, the thesis is also focused on the issue of zero-field oscillations, which would be especially desirable from the point of view of potential applications. According to the state-of-the-art theoretical studies, for hybrid geometry devices with circular cross-section (i.e., exhibiting no other anisotropy terms), current-driven dynamics cannot be excited at zero applied field. Indeed, zero-field oscillations have only been experimentally observed for systems having the free layer magnetization slightly tilted from the normal to the plane, which has usually been achieved by introducing an in-plane shape anisotropy. In the thesis, the influence of the in-plane shape anisotropy of the MTJ on zero-field dynamics in the hybrid geometry MgO-based STNOs is analytically and numerically investigated. In agreement with the previous reports, no zero-field dynamics for circular nano-pillars is observed; however, according to the numerical data, an additional in-plane anisotropy smaller than the effective out-of-plane anisotropy of the free layer enables zero-field steady-state precession. Accordingly, the lack of an in-plane anisotropy component (e.g., for circular cross-section nano-pillars), or the presence of an in-plane shape anisotropy equal or greater than the out-of-plane effective anisotropy, inhibits the stabilization of dynamics in the free layer at zero field. The results of analytical and numerical studies and the general trends identified in the corresponding experimental data are found to be in excellent qualitative agreement.:1. Introduction 1.1. Short history of magnetotransport applications 1.2. Spin-transfer torque induced effects and devices 1.3. Goals of the thesis 2. Fundamentals 2.1. Electronic transport in single transition metal layers 2.2. Tunnel magnetoresistance (TMR) 2.2.1. Electronic transport in magnetic tunnel junctions 2.2.2. Tunnel magnetoresistance versus structural properties of the multilayer 2.2.3. Bias voltage and temperature dependence of tunnel magnetoresistance 2.2.4. Angular dependence of tunnel magnetoresistance 2.3. Spin-transfer torque in GMR/TMR structures 2.3.1. Spin-transfer torque 2.3.2. Landau-Lifshitz-Gilbert (LLG) equation 2.3.3. LLG equation and spin-transfer torques 2.3.4. Bias voltage dependence of spin-transfer torques in MTJs 2.3.5. Angular dependence of spin-transfer torque 2.4. Spin-torque-based phenomena 2.4.1. Current-induced switching 2.4.2. Current-induced dynamics 3. Experimental 3.1. General characteristics of MgO-based magnetic tunnel junctions 3.2. STNO samples 3.2.1. Samples by AIST (Tsukuba, Japan) 3.2.2. Samples by HZDR / SINGULUS (Dresden / Kahl am Main, Germany) 3.3. Magnetotransport measurements 3.3.1. Experimental setup and data analysis 3.3.2. Experimental results 3.4. Aspects to be explained 4 Numerical and analytical calculations 4.1 Out-of-plane steady-state precession in hybrid geometry STNO 4.1.1 Angular dependence of tunnel magnetoresistance as a mechanism of stable precession 4.1.2. Influence of the bias dependence of tunnel magnetoresistance 4.1.3. Comparison with the experimental data 4.1.4. Comparison with the GMR-type counterpart 4.1.5. Summary 4.2. Zero-field dynamics in hybrid geometry STNO stabilized by in-plane shape anisotropy 4.2.1. Effect of the in-plane shape anisotropy 4.2.2. Zero-field dynamics 4.2.3. Summary 5. Conclusions 6. Outlook Appendix Bibliography

This thesis includes a study of a touch-free sensor systems and different principles for measuring a magnetic field. An in-depth study was conducted to describe two important principles in these contexts, i.e. Magnetoresistance principle and the Hall Effect principle. A market evaluation of different sensor chips developed on these principles was conducted. Furthermore, the study should be linked to existing principles to determine the position of a rotating axis. Function tests show that the performance requirement for each component was met. The results based on verification tests show that it is important to have a better mechanical connection between drive and device. Temperature tests show that the system meets the project specification at room temperature, but large angular deviations occur in temperature change, especially when driving with short steps. This thesis can be continued by reprogramming the source code and developing new software that can control more parameters and make it more accurate at positioning. Another recommendation would be to investigate and compare other sensor chips because there are several sensors that can apply to the system. Further improvements can be made by performing more tests on the system. / Examensarbetet innefattar en studie om beröringsfritt givarsystem och olika principer för att mäta ett magnetfält. En fördjupad litteraturstudie utfördes för att förklara två viktiga principer i dessa sammanhang d.v.s. magnetoresistiva principen och hallgivarprincipen. En marknadsutvärdering av olika sensor-chip som utvecklats enligt dessa principer genomfördes. Vidare ska studien kopplas till kända principer för att bestämma positionen för en roterande axel. Funktionstester visar att prestandakravet för varje komponent uppfylldes. Resultaten baserade på verifieringstester visar att det är viktigt att ha en bättre mekanisk koppling mellan drivdon och enhet. Temperaturtester visar att systemet uppfyller projektets kravspecifikation vid rumstemperaturen men det uppkommer stora vinkelavvikelser vid temperaturförändring framför allt vid körning med korta steg. Detta arbete kan fortsättas genom omprogrammering av källkoden och utveckling av ny programvara som kan styra fler parametrar och göra det mer exakt vid positionsbestämning. En annan rekommendation skulle vara att undersöka och jämföra andra sensorchip eftersom det finns flera sensorer som kan tillämpa i systemet. Ytterligare förbättringar kan göras genom att utföra fler tester på systemet.

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We present studies of edge states induced by the Aharonov-Casher vector potential or Rashba-type spin-orbit interaction using quantum transport in InGaAs/InAlAs herterostructures. The Aharonov-Casher effect is electromagnetically dual to the Aharonov-Bohm effect and is predicted to lead to edge states in a parabolic confinement at two-dimensional sample edges. As a narrow gap material, InGaAs has a low effective mass, high mobility, and strong spin-orbit interaction, which indicate that it can be used as a good material to detect the Aharonov-Casher effect or SOI interaction. Using InGaAs, we measured the magnetoresistance in a quantum antidot in narrow short channels in a tilted magnetic field. The fine structure (mT spacing) observed in the magnetoresistance indicate a probable energy spacing between AC edge states. We also fabricated side-gate channel structures in InGaAs/InAlAs quantum wells and investigated the values of the Rashba spin-orbit coupling constant α using the weak antilocalization analysis as a function of the side-gate voltage. We take the effect of the finite width into account and find the corrected values. With the simulation of electric fields in the wide channel and narrow channel, we found that the electric field components can be changed using side-gate voltages. While our results do not indicate which electric field component is responsible, the data indicate that the deduced spin-orbit strength values in a narrow channel are tunable by the side-gate voltage. / Ph. D.