Bilayer magnetic nanostructures are currently of interest in a variety of applications due to the ability to combine complementary properties of each layer. One key area is data storage where extending hard disk drive (HDD) storage density may be achieved by storing each bit of data in an individual magnetic nanostructure. Whilst several magnetometry techniques are capable of measuring the properties of arrays of magnetic nanostructures, very few are sensitive enough to measure an individual magnetic nanostructure. An electrical technique termed anomalous Hall effect (AHE) magnetometry is used in this work due to its high sensitivity. In this technique the structure is fabricated on a Hall cross, and a current is applied and the transverse voltage measured whilst sweeping an external magnetic field. Bilayer magnetic nanostructures formed from separated Co/Pd and Co/Ni multilayers were measured for the first time with this technique, which showed that an asymmetry is seen in their hysteresis loops when the two layers are magnetically decoupled. It is demonstrated that this is due to a combination of a very small giant magnetoresistance (GMR) effect present in the magnetic nanostructure, and the Hall cross offset voltage which results from small imperfections in the shape of the cross. A finite element simulation is used to provide quantitative evidence for this model. These results indicate that asymmetry will be seen in Hall measurements of other materials which exhibit magnetoresistance. Bilayers of FePt and FeRh were also investigated as these materials are of interest for a future HDD system where the structure is heated, and the FeRh undergoes a ferromagnetic transition causing it to assist the switching of the FePt. These thin films are difficult to deposit whilst achieving chemical ordering in both layers. FeRh/FePt films are deposited, before greater ordering is demonstrated with an FePt/FeRh film. It is demonstrated that AHE magnetometry can provide a similar level of measurement information of bilayer nanostructures of these materials as bulk magnetometry techniques can provide of the thin film.

The electrical conductivity of InN, group III-V semiconductor, is measured by four point measurement at low temperatures and high magnetic fields. From Resistance Vs temperature measurements (done in the absence of magnetic field) there is a transition from semiconducting state to superconducting state at 2.5K. This superconducting state disappears when the measurements are repeated but at a magnetic field of 0.1 Tesla. Mover the Hall voltage is not proportional to the magnetic field.

The anomalous Hall effect (AHE) has been discovered for over a century, but its origin is still highly controversial theoretically and experimentally. In this study, we investigated the scattering effect on the AHE for both exploring the underlying physics and technical applications. We prepared Cox(MgO)100-x granular thin films with different Co volume fraction (34≤x≤100) and studied the interfacial scattering effect on the AHE. The STEM HAADF images confirmed the inhomogeneous granular structure of the samples. As x decreases from 100 to 34, the values of longitudinal resistivity (pxx) and anomalous Hall resistivity (pAHE) respectively increase by about four and three orders in magnitude. The linear scaling relation between the anomalous Hall coefficient (Rs) and the pxx measured at 5 K holds in both the as-prepared and annealed samples, which suggests a skew scattering dominated mechanism in Cox(MgO)100-x granular thin films. We prepared (Fe36/n/Au12/n)n, (Ni36/n/Au12/n)n and (Ta12/n/Fe36/n)n multilayers to study the interfacial scattering effect on the AHE. The multilayer structures were characterized by the XRR spectra and TEM images of cross-sections. For the three serials of multilayers, both the pxx and pAHE increase with n, which clearly shows interfacial scattering effect. The intrinsic contribution decreases with n increases in the three serials of samples, which may be due to the crystallinity decaying or the finite size effect. In the (Fe36/n/Au12/n)n samples, the side-jump contribution increases with nn, which suggests an interfacial scattering-enhanced side jump. In the (Ni36/n/Au12/n)n samples, the side-jump contribution decreases with n increases, which could be explained by the opposite sign of the interfacial scattering and grain boundary scattering contributed side jump. In the (Ta12/n/Fe36/n)n multilayers, the side-jump contribution changed from negative to positive, which is also because of the opposite sign of the interfacial scattering and grain boundary scattering contributed side jump. The interfacial scattering effect on the AHE is much more complicated than surface scattering in thin films or scattering by delta-impurities in bulk-like samples.

This dissertation focuses on the study of spin-dependent transport in systems with strong spin-orbit coupling within their band structure. In particular we focus on the anomalous Hall effect, the spin Hall effect, and the Aharonov-Casher effect whose origins, are linked to the presence of spin-orbit coupling. Given the theoretical controversy surrounding these effects we further simplify our studies to semiconductor systems where the band structure is much simpler than in metallic systems with heavy elements. To obtain finite analytical results we focus on reduced dimensions (two and one dimensions) which can be explored experimentally. To set the stage, we discuss the origins of the strong spin-orbit coupling in semiconductors deriving the effective interaction from the Dirac equation. We discuss in detail the skew scattering contribution to the anomalous Hall effect in two-dimensional systems, which is dominant for systems with low impurity concentrations, and find that it is reduced when the two chiral subbands are partially occupied in an electron gas and vanishes for a hole gas, regardless of the band filling. We also present calculations for all contributing mechanisms. We propose a device to test this prediction and study the crossover from the intrinsic to the extrinsic anomalous Hall effect. We calculate all contributions to the anomalous Hall effect in electron systems using the Kubo-Streda formalism. We find that all contributions vanish when both subbands are occupied and that the skew scattering contribution dominates when only the majority subband is occupied. We calculate the interference effects due to spin-orbit interaction in mesoscopic ring structures patterned from HgTe quantum wells related to the Aharonov-Casher effect and the spin Hall effect. We find that the transport properties are affected by the carrier density as well as the spin orbit interaction. We find that the conductivity is larger in hole gas systems. We also show that devices with inhomogenous spin orbit interaction exhibit an electrically controlled spin-flipping mechanism.

For the past 50 years, the magnetic storage industry has been the ultimate provider, covering the ever-expanding needs of digital mass storage. Up to now, scaling to smaller dimensions has been the core approach for achieving higher areal densities in conventional media. However, recent studies have shown that further miniaturisation is physically bound by superparamagnetic effects. Bit patterned media (BPM) is one of the most promising solutions for extending the areal densities beyond 1Tbit/in². The ability to magnetically characterise novel BPM is a key requirement for its future commercialisation. The work presented in this thesis describes the investigation of the Magneto-Optic Kerr Effect (MOKE) and the Anomalous Hall Effect (AHE) techniques, used in the characterisation of BPM samples. These samples were produced as part of this work using a customised subtractive fabrication process. Continuous thin magnetic films with Ptseed(10nm)/[Co(0.4nm)/Pt(1nm)]x15 composition and perpendicular anisotropy, were deposited using electron-beam (e-beam) evaporation, and subsequently patterned into magnetic nano-islands through means of e-beam lithography and Ar⁺ ion milling. In terms of the MOKE technique, a fully customised polar MOKE system has been developed and successfully used to observe the hysteretic behaviour of magnetic nano-island arrays with varying dimensions. An expected reduction in the MOKE signal due to pattering has been observed, as well as an additional signal reduction due to the scattering effect arising from the non-vertical sidewalls of the dome-shaped islands. In the case of islands with improved shape profile, the magnetic reversal of BPM arrays with islands sizes down to 35nm has been successfully detected, demonstrating a sensitivity of approximately 6x10⁻¹³emu for the MOKE system. In terms of the AHE technique, Au Hall crosses were deposited on top of the BPM arrays that had been previously characterised by MOKE, allowing the direct comparison and cross-verification of the results obtained by the two techniques. A revised Hall cross design, where the Hall crosses were generated by direct patterning of the Pt seed layer, has provided a significant improvement in AHE signal. This has allowed the detection of the magnetic reversal of individual sub-50nm magnetic islands, demonstrating a sensitivity of approximately 6.6x10⁻¹⁵emu for the AHE technique. The spatial sensitivity of these Hall cross structures has been experimentally determined, indicating a decrease in sensitivity with increasing distance from the cross centre and with decreasing island diameter. In conclusion, the sensitivity profile of Hall cross structures must be taken into account for the accurate interpretation of BPM characterisation results obtained by the AHE technique. Finally, some suggestions are outlined with regard to the future improvement of the developed characterisation techniques and the further continuation of the work presented in this thesis.

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 concept of Berry phase, since its proposition in 1984, has found numerous applications and appears in almost every branch of physics today. In this work, we study several physical effects in ferromagnetic metal materials which are manifestations of the Berry phase. We first show that when a domain wall in a ferromagnetic nanowire is undergoing precessional motion, it pumps an electromotive force which follows a universal Josephson-type relation. We discover that the integral of the electromotive force over one pumping cycle is a quantized topological invariant equal to integer multiples of h/e, which does not depend on the domain wall geometry nor its detailed dynamic evolution. In particular, when a domain wall in a nanowire is driven by a constant magnetic field, we predict that the generated electromotive force is proportional to the applied field with a simple coefficient consisting of only fundamental constants. Our theoretical prediction has been successfully confirmed by experiments. Similar effect known as spin pumping occurs in magnetic multilayer heterostructures, where a precessing free magnetic layer pumps a spin current into its adjacent normal metal layers. Based on this effect, we propose two magnetic nanodevices that can be useful in future spintronics applications: the magnetic Josephson junction and the magneto-dynamic battery. The magnetic Josephson junction has a drastic increase in resistance when the applied current exceeds a critical value determined by the magnetic anisotropy. The magneto-dynamic battery acts as a conventional charge battery in a circuit with well-defined electromotive force and internal resistance. We investigate the condition under which the power output and efficiency of the battery can be optimized. Finally we study the side jump contribution in the anomalous Hall effect of a uniformly magnetized ferromagnetic metal. The side jump contribution, although arises from disorder scattering, was believed to be independent of both the scattering strength and the disorder density. Nevertheless, we find that it has a sensitive dependence on the spin structure of the disorder potential. We therefore propose a classification scheme of disorder scattering according to their spin structures. When two or more classes of disorders are present, the value of side jump is no longer fixed but depends on the relative disorder strength between classes. Due to this competition, the side jump contribution could flow from one class dominated limit to another class dominated limit when certain system control parameter changes. Our result indicates that the magnon scattering plays a role distinct from the normal impurity scattering and the phonon scattering in the anomalous Hall effect, because they belong to different scattering classes. / text

Heusler compounds form a large class of intermetallic materials, which attracted a lot of interest in recent years. The reason is their enormous flexibility, which makes it possible to observe almost every physical effect in one of the 1000 members known nowadays. Especially many magnetic Heusler compounds display promising properties, which offer potential application in fields like rare-earth free permanent magnets, magnetocalorics, spin transfer torque devices and tunnel junctions. Apart from that, magnetic Heusler systems are also interesting for fundamental research since some members host skyrmion lattices and other magnetically complex orders. The search for new Heusler compounds is therefore fruitful in many ways. Accordingly, the present thesis followed the approach of synthesizing and characterizing such Heusler compounds that were either entirely new or had unexplored magnetic properties. Exactly this second approach was demonstrated in Chapter 3, namely for IrMnGa. With help of combined neutron and x-ray diffraction experiments it was possible to correct the structural model from literature and show that the compound crystallizes indeed within the half-Heusler space group but with a substantial degree of Y -disorder. In contrast to older suggestions, the subsequent magnetic characterization revealed a robust canonical spin glass state instead of antiferromagnetic order. The magnetic phase diagram was found to be similar to Au1−xFex and thus hinted on a Heisenberg-like spin glass with considerable anisotropy. Contrary to synthesis route and heat treatment, changing the composition allowed to tune the spin glass state extensively. Increasing the Mn content caused a transition from spin to cluster glass behavior and for Mn contents above 40 at%, it was even possible to introduce ferrimagnetic order. Notably, the composition dependence of spin glasses was only studied for binary systems before. It turned out that many trends are quite similar for the Ir-Mn-Ga ternary scenario with the exception of magnetic behavior near the percolation limit being more complex. Generally, spin glass order is rather rare in Heusler compounds and especially for half-Heusler systems a report remained elusive up to IrMnGa. Chapter 4 then summarized investigations on the Heusler series Fe3−xMnxSi, which features a spin reorientation transition at low temperatures. Despite being one of the most studied Heusler systems, the magnetotransport properties were not yet covered systematically in literature. The presented investigations unveiled that the mechanisms of longitudinal as well as Hall resistivity change upon cooling through the spin reorientation transition. For the Hall effect, skew scattering dominates above TR whereas it is the intrinsic mechanism below. The finding emphasizes the dependency of the intrinsic Hall contribution on the magnetic structure and it should be possible to generalize this change of the Hall mechanism to all magnetic transitions, where the intrinsic contribution is affected. The subsequent Chapter 5 provided a reevaluation of the Fe-Mn-Si phase diagram. The approach seemed well justified since the obtained phase boundaries agreed better with theory than the old experimental studies. Furthermore, it was found that those compounds, which were previously identified as β-Mn, actually crystallize in a superstructure. The ordered version has a Mn3IrSi as prototype and derives from β-Mn by splitting of the 8c site into two 4a sites. Due to the close relation of both structures, this phase was named β’-Mn. Moreover, it turned out that the ’mysterious’ secondary phase, which was mentioned for Mn-rich Fe3−xMnxSi Heusler compounds but never specified, is given exactly by β’-Mn. The investigations of its magnetic properties indicated a transition to a canonical spin glass state at low temperatures. β’-Mn thus adds a further type of magnetic ordering to the Fe-Mn-Si system. Indeed, the latter comprised all kinds of solid state magnetism but no spin glass order was reported before. Finally, the spin glass state was demonstrated to exhibit a similar composition dependence as in Ir-Mn-Ga, which illustrated nicely the universal character of the spin glass concept. The last chapter dealt with the difficult search for entirely new Heusler compounds. It was explained that high-throughput studies struggle to predict phase stabilities, which is why they have to be treated with care. To overcome these issues, some design rules were suggested to evaluate whether a Heusler compound is likely to be experimentally stable or not. Usually, there are no reports for systems, which do not form as single phase. Since this is a highly inefficient habit, 26 multi-phase ’Heusler compounds’ were listed. In the end of the chapter, the successful synthesis of three new compounds was presented, namely Ru2CrAl, Ru2CrGa and Ru2CrSb. Ru2CrGa was identified as Pauli paramagnet whereas Ru2CrSb exhibited an antiferromagnetic transition around 100 K. A second transition at 40 K was accompanied by a small increase of magnetization, which hinted on some more complex magnetic structure at low temperatures.

Transition metal and related compounds have been extensively studied over the past several decades. These investigations revealed a wide range of behavior, encompassing colossal magnetoresistance (CMR), high-TC superconductivity, and magnetic semiconductivity, all of which continue to present fundamental challenges to the understanding of such phenomena. There is, however, a close correlation between such characteristics and the appearance of magnetic order. This correlation underlies the present study, which focuses on the magnetic and transport behavior of various Manganese (Mn), Iron (Fe) and Cobalt (Co) containing materials, with particular emphasis on the nature of the magnetic order they display and the critical exponents that characterize the accompanying phase transition. The magnetic and transport properties of two specific systems will be covered: first various doped manganites from the series (La,Pr)1-x(Ca,Ba)xMnO3, and second the magnetic semiconductors Fe0.8Co0.2Si and Ga0.98Mn0.02As. In the manganites, the influence of doping on; (i) the evolution of the metal-insulator transition (MIT) with composition; (ii) the universality class of the magnetic critical behavior associated with the paramagnetic to ferromagnetic transition, which occurs in the vicinity of a MIT with which CMR is associated; (iii) the mechanisms underlying ferromagnetism across the MIT; (iv) the correlation between the appearance of a Griffiths-like phase and CMR, and (v) the origin of Griffiths-like phase have been investigated. Four different systems have been studied: La1-xCaxMnO3 (0.18 ≤ x ≤ 0.27), La1-xBaxMnO3 (x ≤ 0.33), (La1-yPry)0.7Ca0.3Mn16/18O3 (y ≤ 0.85), and Pr1-xCaxMnO3 (x = 0.27, 0.29). In Fe0.8Co0.2Si and Ga0.98Mn0.02As, the scaling between magnetization and conductivity has been the subject of ongoing debate. In bulk Fe0.8Co0.2Si, a novel scaling between the anomalous Hall effect (AHE) and the magnetization enables the anomalous Hall coefficient to be accurately determined. In turn, this enables the universality class for the transition to ferromagnetism to be established independently from the anomalous Hall conductivity. In an epitaxial (metallic) Ga0.98Mn0.02As microstructure, the magnetization has been indirectly determined from the AHE. Subsequent analysis yields magnetic critical exponents consistent with the Mean-Field model, direct support for which had previously been lacking.

Transition metal and related compounds have been extensively studied over the past several decades. These investigations revealed a wide range of behavior, encompassing colossal magnetoresistance (CMR), high-TC superconductivity, and magnetic semiconductivity, all of which continue to present fundamental challenges to the understanding of such phenomena. There is, however, a close correlation between such characteristics and the appearance of magnetic order. This correlation underlies the present study, which focuses on the magnetic and transport behavior of various Manganese (Mn), Iron (Fe) and Cobalt (Co) containing materials, with particular emphasis on the nature of the magnetic order they display and the critical exponents that characterize the accompanying phase transition. The magnetic and transport properties of two specific systems will be covered: first various doped manganites from the series (La,Pr)1-x(Ca,Ba)xMnO3, and second the magnetic semiconductors Fe0.8Co0.2Si and Ga0.98Mn0.02As. In the manganites, the influence of doping on; (i) the evolution of the metal-insulator transition (MIT) with composition; (ii) the universality class of the magnetic critical behavior associated with the paramagnetic to ferromagnetic transition, which occurs in the vicinity of a MIT with which CMR is associated; (iii) the mechanisms underlying ferromagnetism across the MIT; (iv) the correlation between the appearance of a Griffiths-like phase and CMR, and (v) the origin of Griffiths-like phase have been investigated. Four different systems have been studied: La1-xCaxMnO3 (0.18 ≤ x ≤ 0.27), La1-xBaxMnO3 (x ≤ 0.33), (La1-yPry)0.7Ca0.3Mn16/18O3 (y ≤ 0.85), and Pr1-xCaxMnO3 (x = 0.27, 0.29). In Fe0.8Co0.2Si and Ga0.98Mn0.02As, the scaling between magnetization and conductivity has been the subject of ongoing debate. In bulk Fe0.8Co0.2Si, a novel scaling between the anomalous Hall effect (AHE) and the magnetization enables the anomalous Hall coefficient to be accurately determined. In turn, this enables the universality class for the transition to ferromagnetism to be established independently from the anomalous Hall conductivity. In an epitaxial (metallic) Ga0.98Mn0.02As microstructure, the magnetization has been indirectly determined from the AHE. Subsequent analysis yields magnetic critical exponents consistent with the Mean-Field model, direct support for which had previously been lacking.