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Understanding the relationship between skyrmions and the behaviour of the Hall effect is essential to realise skyrmionic data storage. In this thesis, possible contributions to the Hall effect, anomalous and topological, in [CoB/Ir/Pt]xn multilayers are determined and the temperature and repeat layer, n, dependent behaviour characterised. The excess Hall resistivity remaining after the ordinary and anomalous Hall effects are subtracted from the Hall resistivity is measured in CoB/Ir/Pt multilayers and shown to depend on the temperature and n. The number, and variation in number, of peak and trough-like features in ρxy,excess as the magnetic field is varied is inversely proportional to the temperature and n. Correlation between the temperature dependence of maximum ρxy,excess signal and anomalous contribution, ρA, is observed possibly suggesting shared physical origins between ρxy,excess and ρA. The observations reported suggest that ρxy,excess is, at least partially, topological in nature. This is supported by the dependence of maximum ρxy,excess on n in agreement with previous reports of skyrmion presence in similar multilayer samples with 2 ≤ n ≤ 10. The temperature dependence of the anomalous Hall effect in [CoB|Ir|Pt]xn is accounted for by the temperature dependence of the saturation magnetisation, MS(T). Extrinsic (skew scattering, α, and side-jump, β) and intrinsic (b) contributions to the anomalous Hall effect are determined by scaling with longitudinal resistivity, ρxx. The AHE is described well in these samples by summing linear and quadratic terms in ρxx0 from impurity scattering, and a quadratic term in ρxx from phonon-lattice interactions. α, β, and b are of similar magnitude to those found in literature for single-layered ferromagnetic materials. Side-jump scattering is found to be the largest contribution to the anomalous Hall resistivity in [CoB|Ir|Pt]×n multilayers suggesting that the large side-jump coefficient observed could be contributing to the temperature dependence of ρxy,excess.

We present ways to characterise and optimise multilayers for skyrmion-based devices using Co and CoB with Pt, Ir and Ta. Magnetic proximity effects at the interface of a ferromagnet and a non-magnet were studied to find agreements between literature - a topic which is often disputed. It was confirmed that Pt and Ir experience induced moments at a magnetic interface, particularly Pt, and Ta causes a magnetic dead layer. More moment was measured for CoB/Pt interfaces than Co/Pt, and an inhomogeneity in the layer was seen below 8 Å of ferromagnet. The Pt spin polarisation depth was shown to be ∼10 Å, and the top interface to contribute more moment than the bottom. These effects must be considered in the saturation magnetisation of a multilayer thin film. Attention was also given to the Dzyaloshinskii-Moriya interaction and its strength, a key parameter of a skyrmion. It was shown, via collaborative work, that this value could vary, even when the same sample was measured in different laboratories using both different and the same method. Proximity effects, and using an approximation of Bloch's law specific for thin films could also change the strength of the Dzyaloshinskii-Moriya interaction. Synthetic antiferromagnets consisting of CoB, Ir and Pt were also designed to produce a device with capabilities more suited for skyrmion racetrack memories. Multiple parameters were characterised and trends were established as a function of repetitions, material thicknesses and temperature. An understanding of the hysteresis mechanisms was established via electrical measurements. A synthetic antiferromagnet was built with the ability to host magnetic field-nucleated skyrmions, however, the skyrmions were not coupled together antiferromagnetically across the two ferromagnetic layers. These samples were also probed with THz waves by depositing the material on a nanofabricated coplanar waveguide. Adaptations of the waveguide and the measurement set-up were successful, however, further work is necessary to confirm the observed trends.

Two sets of lateral spin valves (LSVs) were fabricated - Permalloy (Py)/ Silver (Ag) and Py/ Copper (Cu). Both sets of devices were made with 99.9999 % (six 9s) pure metal, evaporated at a rate that would optimise their crystal structure. However, the deposition of the non-magnetic material in both sets was paused midway, for 25 minutes, before the growth was completed. This was done in order to purposefully introduce ambient impurities from the deposition chamber (such as N2) into the central channel of the devices and, as such, act to disrupt the quality of the crystal structure. Both 'local' and 'non-local' direct current (DC) reversal sweeps (± 500 µA) were performed at a range of temperatures (3 K up to 290 K) and applied magnetic field strengths (± 100 mT), for devices with a variety of injector-detector separations (550 nm to 2650 nm) in order to extract their thermal, electrical and spintronic properties. A model recently presented by Stefanou et al. [1] was applied to the thermal voltages measured in both sets of devices resulting in a detailed insight to the thermal differences between Cu and Ag nanowires. It was found that Cu's relatively larger thermal conductivity resulted in a completely different temperature dependence for thermal voltages measured at temperatures below 50 K. The length dependence of the spin signals in both sets of devices were extracted and fitted with a Valet-Fert model for spin diffusion. Careful analysis showed no correlation between the presence of a characteristic Kondo 'upturn' in the resistivity measurements and the magnitude of a low temperature 'downturn' in the spin diffusion lengths for both the Ag and Cu. The introduction of non-magnetic impurities during deposition and hence the disruption of the crystal structure, appears to have significantly reduced the applicability of the Elliott-Yafet (EY) model for spin-flip scattering when compared to similar devices [2-4]. Additionally, a low temperature deviation from the usual quadratic, nonlocal, current-voltage relationship (NLIV), was shown to be very strongly correlated to the magnitude of thermal voltages, caused by Joule heating in the devices' injectors, measured at their detectors [4-6].

This research consideres the effect of molecular overlayers on the spin Hall magnetoresistance (SHMR) and anisotropic magnetoresistance (AMR) of heavy metals (Pt, Ta, and PtMn). The objective is to investigate the effect of molecular overlayers on the effective spin orbit coupling (SOC), i.e. the charge-spin conversion of heavy metals. We have pursued this objective by measuring the SHMR & the AMR in YIG/Metal and YIG/Metal/C60 structures. This effect has generated a large interest over the last few years, and they are critical in the study of the spin Hall angle and its applications, e.g. for devices such as spin torque MRAM. "In this study" when C60 is grown on a metal, the electronic structure is altered due to hybridisation and charge transfer. A conducting layer with high SOC is formed at the interface between a heavy metal and molecules. The SHMR for Pt/C60 and Ta/C60 at room temperature are up to a factor 6 higher than for the pristine metals, with the spin Hall angle increased by 20-60 %. At low fields of 1-30 mT, there is an AMR which increases up to 700 % at room temperature by C60. Given the dielectric properties of C60, this opens the possibility of gating the effective SOC of metals, with applications for spin transfer torque memories and pure spin current dynamic circuits. Annealing sample plays a crucial part in our transport properties of (Pt, PtMn) and (Pt, PtMn)/C60 because of changes to the crystallinity and metallo-molecular coupling at the interface. As a result, the SHMR/MR can increase by a further factor 4 when annealing at a temperature of 300 ◦C.

Both gadolinium gallium garnet (GGG) and yttrium aluminium garnet (YAG) substrates are used to grow thin YIG films. It is known that substrates of GGG are associated with reduced lattice strain (ε = 0.016%) relative to YAG substrates (ε = 1.12%) for YIG lattices. In addition, it was found that the structural and magnetic properties of thin YIG films are influenced by oxygen contents. For instance, we found in this work that the lattice constant for YIG/GGG films with 6.2% of the Oxygen content is similar to bulk value of YIG lattice constant ( 12.376 A) and the magnetic properties are similar to those of bulk YIG that have a coercive field (H c ) < 1 Oe and saturation magnetisation (M s= 143emu/cm 3 ). The current study concerns all of the features of good quality nm-thin sputtered YIG films with the identification of interfacial diffusion and the impact it has in terms of suppressing magnetisation. The structural and magnetic properties of nm-thin YIG films deposited on GGG and YAG substrates by RF magnetron sputtering. A combination of X-ray diffraction (XRD), X-ray reflectivity (XRR) and atomic force microscopy (AFM) were employed to assess the films' morphology and structural characterisation. Meanwhile, both a VSM and SQUID magnetometer were employed to determine the associated magnetic qualities. The YIG films have a 111 crystalline orientation and a 1-3 A surface roughness. The YIG films have a coercive field of 0.30 ± 0.05 Oe, saturation magnetisation of 140 ± 6 emu/cc. Furthermore, the saturation magnetic moment's thickness dependence has a dead layer. The dead layer thickness can be changed by changing the Oxygen contents during sputtering. The thinnest dead layer was 2.35nm in YIG film with 6.2% of Oxygen and the thickest dead layer was 6 nm in YIG film with 5% of Oxygen. When a normal metal (NM) makes contact with a magnetic material, this can result in spin Hall magnetoresistance (SHMR). Insulating magnetic materials help to avoid magnetic effects because they are able to effectively restrict electrons from leaving the NM. To help analyse this, an innovative means of producing YIG has been devised that utilises sputtering methods, thereby enabling applications to be created in less time and at lower cost relative to the liquid phase epitaxy (LPE) approach. The magnetic qualities of the YIG material have been specified at varying thicknesses. For thin samples, it is known that magnetisation is lower than the bulk value and this is understood to be attributable to the substrate interface having a layer that is interdiffused. Owing to the fact that the SHMR is an interface effect, it is necessary to employ AFM and XRR to examine how rough the films are. The first measurements of SHMR were made using platinum and the results achieved broadly matched those reported in the empirical literature. A cryostat is used to measure angular dependence with four probe resistance measures and a split pair magnet which enables the sample to be positioned in all conceivable directions relative to the applied field. This concurs with the established theory and research was conducted across various temperatures from 2K to 300K. A range of models were then applied to fit the data for the spin diffusion length and it was found that the temperature dependence is best explained by the Elliot-Yafet mechanism (EY). The research is conducted into magneto-transport with bilayers of platinum and YIG using different interfaces (2 nm of Pt on 40 nm of YIG with different Oxygen contents) . Measuring resistivity as a function of temperature, conventional magnetoresistance and angle-dependent magnetoresistance reveals that there is considerable reliance on the interface. Consequently, it is necessary to develop the methods for preparing interfaces so as to observe a pure SHMR from enhanced spin mixing conductance.

This thesis deals with three interconnected subjects all relevant to the study of spin transport in lateral spin valves (LSV): spin injection and transport; the thermal voltages that accompany spin transport; and, measurements of the spin diffusion length of a ferromagnetic nanowire. Spin injection and diffusion were examined in shadow evaporated CoFe/Cu LSVs with different CoFe electrode thicknesses, namely, 15 and 30 nm. Compared to previous studies of CoFe based LSVs, fabricated with two-step lithography, extracted spin polarisations were higher. It was concluded that this was due to the high quality transparent interfaces achieved with a shadow deposition technique and/or the slightly more Co rich composition being favourable for high spin polarisations. Thus, the potential combination of both factors is promising for achieving high spin signals. However, this work also highlighted one important caveat; a thin CoFe electrode must be used as the spin diffusion length of the Cu was strongly reduced as the amount of CoFe evaporated increased. As suggested in the literature, this was due to magnetic impurities from the electrode material being incorporated into the Cu channel during shadow deposition. Additionally, the spin injection efficiency was lower in LSVs with 30 nm thick CoFe electrodes as a consequence of the increased CoFe/Cu interface area. The thermal background voltages that arise from Peltier and Joule heating during electrical spin injection were reported and analysed as a function of temperature and electrode separation in CoFe/Cu LSVs. The contributions from the amount of heat generated, efficiency of detection, heat flow into the substrate and along the transport channel were discussed. The dramatic reduction of heat flow into the substrate was shown to dominate the device response at low temperatures. Finally, a spin absorption method was employed to determine the temperature dependence of the spin diffusion length of CoFe. Values obtained were comparable to the literature and scaled linearly with the CoFe resistivity; demonstrating that spin relaxation in CoFe occurs through the Elliott-Yafet mechanism.

Extrinsic spin-orbit scattering is a mechanism which can give rise to, and enhance, both the anomalous and spin Hall effects. In this thesis, it is shown that spin-dependent scattering from extrinsic impurities, localised as a discontinuous δ-layer, significantly modifies the ordinary Hall coefficient of thin films of Cu. The position of the impurity δ-layer profoundly impacts the magnitude of the Hall coefficient, giving rise to maximum values for positions between the edges and centre of the film consistently for Ir and Ta impurities. For an impurity δ-layer of Fe situated between ultrathin Pt and the substrate, a paramagnetic spin Hall magnetoresistance (PSHMR) is discovered which strongly depends upon the concentration of the impurity, the temperature and the magnitude of the applied magnetic field. This has implications for measurements of the spin Hall magnetoresistance (SHMR) in bilayers of the ferrimagnetic insulator yttrium iron garnet (Y₃Fe₅O₁₂, YIG) and Pt, where the magnetic proximity effect (MPE) may play a role. To investigate this, YIG/Pt bilayers with various interfaces are prepared and magnetotransport phenomena studied. Through measurements of the resistivity as a function of temperature, the conventional magnetoresistance, the angle-dependent magnetoresistance and the Hall effect, it is shown that a strong dependence upon the interface exists. Etching the YIG surface with piranha solution is found to enhance the spin mixing conductance, giving rise to the spin Hall-anomalous Hall effect and a significant increase in the SHMR in comparison to a film without this treatment. An enhanced MPE is also found after the etching process, giving rise to a PSHMR and a Hall effect reflecting a larger concentration of magnetic impurities. Interface preparation methods hence need to be improved to observe a pure SHMR from an enhanced spin mixing conductance.

This thesis describes the development of on-chip terahertz (THz) waveguides for use in ultrafast magnetic measurements. An epitaxial transfer method is used to bond photoconductive switch material to quartz substrates. This, combined with optical lithography methods, allows on-chip generation and detection of THz pulses. Normally, the metal in a waveguide has a high conductivity (eg. gold or copper) in order to reduce ohmic loss. However, if this metal is instead a magnetic multilayer, then the THz pulse interaction with the metal can be used to measure ultrafast magnetic effects. This was performed using Cobalt/Copper multilayers, and a THz GMR was seen in a transmitted pulse. Additionally, an ’inverse’ GMR was noted in input pulses and in the pulse tail of transmitted pulses. It is believed that this is the first time such behaviour has been documented. Lateral spin valves offer the ability to separate charge and spin effects; however, the dimensions of these devices is in the hundreds of nanometers range. Coplanar waveguides were used to confine and concentrate THz pulses into this spatial regime. Curved waveguides were fabricated to prove confinement, and magnetoresistance properties were used to demonstrate field concentration. Devices were then designed to allow picosecond current injection into a lateral spin valve geometry. Initial results are obscured due to interference from the excitation pulse, but improvements to the design and fabrication could pave the way for this technology in the future.

The current study is focused on the investigation of the spin and heat properties of a Permalloy/silver/Permalloy, Permalloy/silver (doped with Iron)/Permalloy and Vanadium/silver/Vanadium spin valves in a lateral geometry. The presence of a downturn in the spin-signal of these lateral spin valves at low temperatures, below 30 K, can be avoided by depositing the Ag at a faster rate. This will promote a better microstructure that seems to have a crucial role in systems with very low or no magnetic impurities. A small part of the report is dedicated to test the two different behaviours of the spin signal that have been suggested in the literature, in the case of spin transport through an oxidised normal metal channel. According to literature, it would either remove the downturn of the spin signal at low temperatures, or form one. Our devices exhibited no downturn below 30 K, but by oxidising them, did not lead to the creation of it. By doping the normal metal channel with a dust layer of magnetic impurities (MIL) one would expect that by increasing scattering centers the spin signal and spin diffusion length would decrease no matter where the MIL would be. Interestingly that was not confirmed, as not only the spin signal and diffusion length vary depending on the position of the MIL in the NM channel, but also in the occasion of the device where the MIL was place right in the middle of the NM channel. At temperatures above 70 K the spin signal presented was bigger than that of the reference device. Moreover, the spin diffusion length presented in the particular set of devices, exceeded that of the reference set of devices for temperatures higher than 140 K. Finally, thermal effects were also expected to get picked up during the measurements as in order to create the spin accumulation, a charge current of 500 μ A is driven through the resistive Permalloy injector. That would give rise to Joule heating and Peltier effect once it reaches the Permalloy/silver junction due to the mismatch of the Peltier coefficients. Heat would then diffuse along the silver channel and through the substrate and would be detected in the second silver/Permalloy junction. The latter acted as a thermocouple and due to the difference in their Seebeck coefficients the temperature difference that was picked up, was converted to potential difference. The Scanning Thermal Microscopy technique was employed to image these effects. Based on the results of the technique, an analytical model that can predict the injector and detector junction temperature increase from that of the substrate was developed. In addition to that, since the voltage output is proportional to the product of the effective Seebeck coefficient with the temperature difference, the detector voltage could be predicted. To test the model a simple system of V/Ag/V device was used to deconvolute the thermal effects avoiding having a spin accumulation in the detector signal. After that, the thermal effects arising in a Py/Ag/Py lateral spin valve were investigated. Finally, since the Peltier effect was present in the measurements, the non-local resistance, spin signal and baseline resistance were analysed as a function of current direction to find a so far not studied effect. There was a splitting in the non-local and baseline resistance across all different devices that had temperature and current dependence. To confirm that the effect was not an artefact, three cryostats, three transport sticks, two sample holders and two more systems (Py/Cu/Py and CoFe/Cu/CoFe) were studied.

This thesis outlines the construction of an ultra-high-vacuum angle-deposition system, developed specifically for the fabrication of lateral spin valves (LSVs). The thesis then proceeds to answer two important questions: what causes the loss of spin accumulation at low temperatures seen in LSVs? and how do spin currents interact in multi-terminal circuits? Through a double-dose electron beam lithography and angle-deposition technique, Cu/Py LSVs are fabricated and shown to have transparent contacts. By means of a DC injection method, the nonlocal voltage is measured as a function of injection current, magnetic field and temperature, enabling contributions from heat and spin currents to be isolated. The spin diffusion length is obtained from LSVs fabricated with Cu containing magnetic impurities $ < $1 ppm and $\sim$4 ppm. Temperature dependent charge and spin transport measurements provide evidence linking the presence of the Kondo effect in Cu to the suppression of the spin diffusion length below 30 K. The spin-flip probability for magnetic impurities is found to be 34\%, orders of magnitude larger than other scattering mechanisms. This is extended to explain similar observations in the spin accumulation. These measurements establish the dominant role of Kondo scattering in spin-relaxation, even in low concentrations of order 1 ppm. Finally, a new multi-terminal LSV (MTLSV) is fabricated and the interaction between two spin currents is investigated. Fan-out and fan-in measurements are performed, demonstrating that spin currents separate and combine at junctions in a circuit with magnitudes dictated by the spin resistance of the conduction channels. It is also shown that two spin currents of opposite polarity will cancel out. Whether Kirchhoff's law holds for spin currents is discussed and this chapter helps lay the ground work for spin current based circuits and computation.

C60 has been included in hybrid magneto-molecular devices as an active layer where the C60 is thermally sublimated under high vacuum and the metals are sputter deposited in-situ. Multilayer C60/Co samples were measured by x-ray reflectivity and transmission electron microscopy to show that there is little interdiffusion of layers suggesting C60 is robust to the deposition of metals by sputtering. We have observed 25% magnetoresistance through 60 nm of C60 at 10 K as well as 8% magnetoresistance at room temperature through 20 nm of C60. Unexpected asymmetric effects in external magnetic field were observed in some C60 spin valves as a result of interfacial hybridisation between C60 and ferromagnetic materials. It has also been shown that when C60 is in contact with a ferromagnetic material, spin polarised electrons from the ferromagnet are transferred into the C60 as a result of differences in chemical potential. This has been measured in C60/Co and C60/Py bilayer samples and observed via ferromagnetic resonance and point contact Andreev reflection. The molecular properties of C60 can be manipulated by an adjacent ferromagnetic material at ferromagnetic resonance dissipating energy into the C60. This changes the frequency/energy of molecular vibrations, as well as an enhanced photoemission from the C60. The changes can be correlated to spin pumping and phonon injection. The conductivity through a C60 layer is also increased during ferromagnetic resonance of the adjacent Py by up to 8% owing to increased phonon-assisted hopping steps.

Two epitaxial ferromagnets FePd and its isoelectronic partner FePt, which order in the well known face-centered tetragonal L10-phase, have been fabricated to (a) evaluate the current spin polarization in the ballistic and in the diffusive electron transport regime, (b) to elucidate whether there is a discrepancy in the current spin polarizations depending on the transport regime, how high this discrepancy is, and finally (c) how the spin-orbit scattering strength changes by replacing Pd by Pt. These are questions of fundamental importance in the field of electron transport studies in magnetic materials, in the recent years known as spintronics. The study began with experimental investigations on the structural, magnetic, and electron transport properties of a L10-ordered epitaxial iron-platinum (FePt) alloy layer fabricated by magnetron-sputtering on a single-crystal MgO(001) substrate. At all temperatures ranging from 2 K to 258 K, a diffusive spin current polarization of > 80% was found. To study the ballistic transport regime, point-contact Andreev-reflection measurements at 4.2 K were performed and a value for the ballistic current spin polarization of ∼ 42% was obtained (which compares very well with that of a polycrystalline thin film of elemental Fe). This discrepancy is attributed to a difference in the characteristic scattering times for oppositely spin-polarized electrons, such scattering times influencing the diffusive but not the ballistic current spin polarization. One of the main parameters in the L10-phase ferromagnets is the degree of crystallographic long range ordering. To investigate the influence of the long range ordering on the current spin-polarization, epitaxial FePd magnetic thin films of varying degrees of L10-phase chemical order were used. By quantitatively accounting for changes in micromagnetism through simulation and magnetic force microscopy, it is shown within the Levy-Zhang spin-mistracking theory of domain wall resistance, that this is connected to a change in the average degree of spin-dependence at the scattering centers that control the diffusive transport in this system, suggesting a possible means of producing highly spin-polarized currents in the diffusive electron transport regime. Finally, both materials L10-FePd and -FePt were investigated with respect to the anomalous Hall effect as a consequence of spin-orbit scattering. A linear dependence between transverse and longitudinal resistivity in epitaxial L10- ordered FePd and FePt films due to the anomalous Hall effect (AHE) has been observed. In ordered ferromagnets such a linear relation may be anticipated due to the skew-scattering mechanism; the absence of an additional quadratic term suggests that side-jump scattering, which is dominant in less well ordered material, has been effectively suppressed. One finds that on exchanging Pd for the heavier, though isoelectronic Pt, the skew-scattering deflection angle in an individual electronic scattering event dramatically increases from 17 to 49 mrad. This increase corresponds well to a Z3/2 dependence of the atomic spin-orbit interaction energy, contrary to commonly assumed Z4 dependence.

This thesis describes the design, construction, and use of a sputter deposition facility at the University of Leeds for the growth of magnetic multilayer samples. Now completed, the machine is a UHV sputtering facility, capable of growing up to 15 samples of arbitrary complexity under computer control in a single vacuum cycle. The system currently has five sputter targets. The system was used to grow Co/Cu multilayers. It was found that the level of residual gas in the chamber is of primary importance in determining the final quality of the samples. A good vacuum is required to achieve good antiferromagnetic coupling between adjacent Co layers in the multilayer stack. Consequentially the giant magnetoresistance of such samples is very high. A poor vacuum leads to poor coupling between Co layers, and the magnetoresistance of such samples is minimal. When grown under clean conditions the giant magnetoresistance of the Co/Cu multilayers was very high - as much as 75% at room temperature, rising to 130% at 4.2K. Coupling oscillations as the Cu spacer thickness was varied were strong, whilst none were detected as a function of Co thickness. It was found that the >< Cu spacer layer was the part of the multilayer where the effects of damage by residual gases were most severe, and that damage to this area could cause biquadratic coupling. This form of the coupling arranges spins in adjacent layers at 90° to one another, rather than antiparallel, reducing the giant magnetoresistance response of the sample. The saturation magnetisation, bilinear and biquadratic coupling energies of 180° and 90° coupled samples were measured. These three quantities were found to scale as a function of temperature.