The current saturation associated with the acoustoelectric effect has been investigated in photoconducting crystals of cadmium sulphide. Under normal band gap irradiation the current-voltage characteristic measured under pulsed conditions shows a departure from Ohm's law at a critical field, E(_c), of the order of l0(^3)v cm(^-1). The current saturation is associated with the internal generation of acoustic flux which occurs when the electron drift velocity, v(_d), exceeds the velocity of sound, v(_s), in the crystal. In consequence the critical field for saturation is determined by the drift mobility µ(_d) = v(_s) /E(_c). The work described in this thesis concerns the measurement of the drift mobility and its relationship to the experimentally determined Hall mobility, fd^, as a function of conductivity and temperature. In uniform samples the critical field at room temperature was independent of conductivity over the range 10(^-4) to 10(^-2) ohm(^-1)cm(^-1). The corresponding values of µ(_d)were all about 300 cm(^2)v(^-1)sec(^-1) in most of the samples studied and agreed well with the measured values of the Hall mobility. With non-uniform samples, however, the critical field varied strongly with the intensity of incident illumination. Under saturated conditions potential probe measurements on uniform samples revealed the presence of a stationary high field domain near the positive electrode.

Grazing incidence scattering of synchrotron x-rays has been used to characterize the structure of magnetic thin films and periodic nanostructures. The combined metal and metal oxide films have been chosen to clarify the effects of growth processing techniques in technologically important magnetic and magnetoresistive thin film materials, and have particular relevance to the magnetic tunnel junction (MTJ) class of magnetic sensor. Co/Al2O3 thin films and Co/MgO multilayer thin films have been characterized using x-ray reflectivity and diffuse scatter analysis to explain how preparatory oxidation of the lower ferromagnet in an MTJ can reduce Néel interlayer coupling and improve the consistency of magnetoresistance. Measurements reveal differing effects for Al2O3 and MgO tunnel barrier materials. In Co/Al2O3 systems, preoxidation is found to reduce significantly chemical interdiffusion at the interface between the two layers, implying a more uniform oxidation of the barrier layer. In Co/MgO multilayers, an increased in-plane correlation length of the inherited interface roughness is seen after preoxidation. This implies that preoxidation suppresses the short wavelength undulations on both sides of the tunnel barrier that cause Néel coupling. Grazing incidence in-plane diffraction measurements on epitaxial Fe/MgO/Fe [001] and Fe/Au/MgO/Fe [001] films during annealing to 600 K have shown that, in both cases, the MgO lattice is initially strained towards being commensurate with the iron and gold layers, but relaxes after annealing towards a typical bulk MgO lattice. The iron and gold layers display linear thermal expansion at rates consistent with the bulk material. These in-plane lattice measurements demonstrate how the strain and strain dispersion in an epitaxial MgO barrier layer can be relieved under controlled annealing conditions. Finally, patterned thin film surfaces with submicron periodic symmetries have been studied by grazing incidence x-ray scattering. A novel semi-kinematical theory has been developed into a numerical algorithm capable of simulating the scatter from a wide range of arbitrary and disordered nanoscale arrays. This has allowed key structural parameters including array periodicity, symmetry and array coherence lengths to be extracted from experimental data.

The ability to characterise dynamic processes in the solid state is crucial to our understanding of many important materials. Nuclear magnetic resonance (NMR) and molecular dynamics (MD) simulations are two, highly complementary techniques that can be used in this pursuit, with NMR providing robust measurements of kinetic parameters across a large range of time-scales while MD can give insight into the form that the motion takes. The aim of the work presented in this thesis has been to demonstrate the extent of this complementarity by combining both techniques to investigate interesting systems, and also to expand upon it by implementing extended time scale MD methods that allow slower dynamic processes to be accurately simulated. Atomistic simulations of a urea inclusion compound and of octafluoronaphthalene (OFN), a molecular solid, illustrate the dynamic range and complicated nature of motions that can be present in solid phases of matter, and the inherent difficulty of modeling them without explicit knowledge of their form. Whilst the MD simulation can provide this information, there are computational limits to the range of time-scales it can conventionally access. The OFN system provides an example of this limit, as slow molecular motions observed by NMR experiments are shown to be inaccessible to long, ambient temperature simulations. To combat this deficiency, the metadynamics extended time scale technique has been implemented, allowing rare dynamic events to be observed in very short simulations, and the effects of complex correlated motions to be explored.

Nickel: An optical grid method has been used to check the anomaly in the λl00, T curve (Comer and Hunt 1955) in a nickel crystal. The results appeared to confirm the anomaly but as the method could not be calibrated independently there was considerable doubt about their accuracy. A solenoid capable of giving fields of 10,500 oersteds has been constructed, and magnetostriction measurements carried out using the capacitance bridge method of Corner and Hunt, on the [100] specimen in the temperature range 20 K to 630 K, and on the [111] specimen in the temperature range 78 K to 630 K, The results show that the λl00,T anomaly does not exist. Reasons are given for the error in the optical, grid method, and the results obtained by the capacitance bridge method are discussed in relation to those of Corner and Hunt, and to the theoretical equation of Vonsovsky (194-0). Extrapolation of the X,T curves to 0 K gives λlll = -28 x 10(^-6), λ100 = -57 x 10(^-6). Gadolinium: A zone melting apparatus has been constructed for the purpose of segregating the impurities in a piece of polycrystalline gadolinium in order to grow a single crystal. The grain size has been increased but no single crystal has been produced. Magnetostriction and intensity of magnetization measurements have been made on an ellipsoid of polycrystalline gadolinium in the temperature range 78 K to 350 K. The results show that gadolinium has a large volume effect, and the volume magnetostriction is proportional to the square of the paramagnetic magnetization above the Curie point. The saturation magnetization shows an anomaly at 150 K, and becomes zero at 233 K. The contribution of the volume magnetostriction to the thermal expansion of gadolinium is shown to be too small to account for the thermal expansion anomaly except in the immediate neighbourhood of the Curie point.

The hyperfine field at the cobalt nucleus in GdCo(_2), Gd(_1-x)Y(_x)co(_2), Gd(_1-x)Dy(_x)C0(_2), and Gd(Co(_1-x)Ni(_x))(_2) were studied using the technique of "Spin-Echo" N.M.R. At 4.2ºK cobalt has a negative hyperfine field of 60.8 Koe with a line width of 1.94 Koe. This field is composed of contributions from two main sources; a) from conduction electron (C,E.) polarization and core polarization of the cobalt electron configuration by the cobalt sub- lattice; b) from C.E. polarization of the cobalt electron configuration by the gadolinium sub-lattice. The spin of the gadolinium sub-lattice is decreased in the yttrium substituted series. This is accompanied by a slight decrease in the hyperfine field and a simultaneous increase in the line width of the resonance. An analysis of the variation in H(_c0), taking into account the induced change in cobalt sublattice magnetization, indicates that the contribution from the gadolinium sublattice is approximately 10 Koe per unpaired 4f electron. In addition, line width measurements show that the R.E. - T.M. interaction is long range in character. Magnetization measurements on the nickel substituted series show that the ToM, moment remains constant initially; indicating that the cobalt-nickel sublattice has an itinerant electron configuration. The variation in H(_CO) along this series is ascribed to a decrease in C.E, concentration with nickel substitution, A T,M, - T,M, interaction of short-range is indicated by the line width variation.

A technique has been developed for growing large, strain-free, single crystals of the rhombohedral (ɑ) semimetallic form of arsenic from vapour phase. Previously, only small crystals have been obtained: large crystals can only be grown when the deposition temperature is above a critical value of 700ºc. Temperature fluctuations must be kept within 0.2ºC to prevent the development of severe mosaic structure. Dislocations have been examined by etching techniques. Due to the 'puckered layer' structure of arsenic, either trigonal or hexagonal pits may be produced at the same site of emergence of dislocations on the (lll) cleavage plane: the etch determines the pit shape. Galvanomagnetic effects in arsenic have been studied for the low-field condition µH<<1 by systematically measuring the twelve coefficients that define the isothermal magnetoresistivity tensor to second order in magnetic field, at selected temperatures between 77ºK and 305ºK. Because the effects are small, their measurement requires certain experimental refinements: the measuring system has a resolution of 10 (^-9)v: electronic systems have been designed to stabilize .a sample current of 5 amps, to 1 in 10^ and the sample temperature to 0.001 K.A two carrier, multivalley ellipsoidal model of the energy bands of arsenic is invoked to interpret the galvanomagnetic measurements and so determine carrier densities and mobilities and tilt angles of the Fermi ellipsoids. To solve the consequent twelve equations in nine unknowns, a new method of computation, incorporating a least-mean-squares criterion, has been devised. The electrons are sited in pockets tilted at +82 to the trigonal axis and holes in pockets tilted at +40º ; equal carrier densities are essentially temperature independent, ranging from 1.9x10(^20)cm.(^-3)at 77 ºK to 2.1x10(^20) cm.(^-3) at 305ºK. These findings are in close agreement to recent theoretical calculations and measurements of the de Haas-van Alphen effect. Carrier mobility~1 7temperature dependences are close to T(^-1.7) , considerably greater than the expected T(^-1.0)probably owing to intervalley scattering. A measurement of the two components of the thermoelectric-power tensor has shown that S(_33) is negative, not positive, as reported previously.

The study of organic based materials is a flourishing area of interest as the physical/chemical properties of the compound can be tuned through functionalisation or simple chemical changes to the organic component. This thesis will focus on the magnetic behaviour of metal-organic magnetic materials where a variety of techniques will be used to study the magnetism such as bulk magnetometry and muon spin relaxation. As well as the magnetic properties, some comments will be made on the chemical properties such as molecular structure. The thesis begins with an overview of the theory of magnetism and details regarding experimental techniques. Ni(TCNQ)$_2$ is a recently discovered non-solvated metal organic magnet that was reported to show ferromagnetic behaviour below 20 K where there was evidence of a glassy magnetic component. This thesis reports the synthesis of both a protio and deutero form of the material where upon deuteration of the TCNQ molecule, a shift in critical temperature ($T_{\rm C}$) was observed to a higher temperature by approximately 15%. Diffraction experiments were conducted to attempt to provide information on the atomic structure however this proved unsuccessful. Magnetometry experiments showed a ferromagnetic transition at approximately 20 K in the deuterated and 17 K in the protonated materials where at low temperatures the sample appeared to be a three-dimensional magnetically order material. Muon spin relaxation studies were conducted on the deuterated sample which showed two peaks within the dynamical relaxation in zero-field; one associated with the transition and a low temperature (5 K) spin freezing effect where it is believed there are interactions between magnetic clusters that enter a quasi-static regime. It may be possible that the glassy component and the ferromagnetic behaviour of the material are not due to the same exchange mechanism or magnetic interactions. KTCNQ is a compound that undergoes a spin-Peierls transition, $T_{\rm SP}$, at approximately 400 K where below this temperature there is a dimerisation of the TCNQ radical spins that couple antiferromagnetically and the system goes from a conductive to insulating state. In an attempt to tune the TCNQ-TCNQ interactions different materials were synthesised where the protons on the TCNQ ring were substituted for fluorine and bromine atoms. On substitution of the protons with other elements a dramatic shift in Tsp was observed where for the fluorine based compounds $T_{\rm SP}$ = 150 K and the KTCNQ-Br$_2$ compound showed no evidence of a transition. Both KTCNQ-H$_4$ and KTCNQ-F$_4$ were studied further using muon spin relaxation where the transition is clearly modelled using a stretched exponential where an increase in electronic fluctuation rate is shown by a gradual move from an exponential to Gaussian relaxation. At low temperatures the relaxation again changes and within the KTCNQ-F$_4$ sample 2 F-$\mu^+$-F states are observed. Another controversial organic based magnet is Ni$_2$TCNQ which was first synthesised in 2007. Here a study of a similar material is reported where the TCNQ has been swapped for TCNQF$_4$ and the magnetic properties are shown to be a result of nickel nanoparticles trapped within a metal-organic or purely organic based matrix. The room temperature ferromagnetism is not strictly due to only the bulk Ni particles as this would result in a blocking temperature below 20 K and so the matrix is shown to play an important role. The size of the Ni nanoparticles was shown to be tuneable when using different solvents within the reaction, generally use of chlori- nated solvents lead to rapid decomposition of the starting material, Ni(COD)$_2$ and lead to larger nanoparticles, however use of a nitrile based solvent led to Ni clusters that were approximately 1 nm in size and dispersed within the matrix. A novel scaling of the magnetisation curves as a function of field showed that once the ferromagnetic component had been subtracted the matrix or Ni based clusters show an antiferromagnetic ground state at low temperature. The final chapter describes an investigation of the starting material, Ni(COD)$_2$ which was studied using a SQUID magnetometer where it was shown that there was a high level of magnetic impurities, which was attributed to small Ni clusters that showed a similar scaling relationship of the magnetisation as for the Ni2TCNQF$_4$ based material. This demonstrates the inappropriate nature of Ni(COD)$_2$ as a starting material for metal-organic based magnetic compounds.

Measurements have been made of the lattice parameters, density and magnetic characteristics of a series of alloys between Terbium and Scandium. Both single crystal and poly-crystalline materials have been used in this investigation. It is found that the lattice parameters show a slight deviation from Vegard's Law. The Neel temperatures follow a T(_N)OCG(^1.33) dependence unlike the rare earth metals and other alloy systems, where a T(_N)OCG(^0.66) law is obeyed.(G=effective de Gennes function). The paramagnetic Curie points show an approximate dependence on G(^-2) unlike the θ(_ρ)ϬĞ variation observed for the rare earth metals and the alloys with Yttrium. Thermal expansion and magnetostriction measurements have been made along the principal directions of single crystals of alloys of composition T(^b)0.89(^sc)0.11 and Tb(_0.825)(^sc)0.175. An estimate of the exchange constants and their derivatives has been made for the alloy T(^b)0.89(^sc)0.11 with the help of the turn angle data provided by Dr. H,R, Child. The contribution of magnetostrictive energy has been estimated in the two alloys following Cooper's analysis. It is observed that the magnetostrictive energy drops rapidly with composition and this is assumed to be the cause of the absence of ferromagnetism even in materials containing fairly high proportion of Terbium.

Thin film devices have found many applications in recently developed technology. With the need to increase data storage capacity and performance there are ever more demanding requirements of these devices. Gaining an understanding at the atomic scale of the growth and subsequent manufacturing treatments is fundamental to improving the device design. Grazing incidence x-ray scattering techniques have been used to study the interfaces in a sequence of samples, starting with repeated bi layers of single element material and sequentially working up to a realistic Magnetic Tunnnel junction (MTJ) structure. The width of the diffuse Bragg sheet from repeated bi-layers of Co/Pd and Co/Ru shows that the correlation length of the out-of-plane toughness is shorter for higher frequency roughness components than longer wavelength features. Scaling behaviour in the intensity profile demonstrates that the interfaces become more two-dimensional as more layers are deposited Reflectivity measurements with in-situ annealing reveal that the interfaces in CoFe/Ru repeated bi-layers are stable with temperature. The interfaces of amorphous CoFeB with ruthenium are also stable until the CoFeB crystallises. Similar measurements on repeated bi-layers of CoFeB/AlO, show sharpening of the interface during annealing. The diffuse scatter shows this to be a reduction in the intetdiffusion of the interface and not a change in topological roughness. The scatter from a single CoFeB/A10, interface on a realistic MTJ sub-structure also shows changes with annealing which are consistent with interface sharpening. This sharpening is matched to enhancements in the tunnel magneto- resistance of the MTJ. The changes occurring cannot be explained solely by sharpening of this particular interface and more sophisticated modelling has been attempted to identify the changes. Simulations show that changes in the manganese profile from an IrMn pinning layer in the MTJ should result in a significant change in the variable energy reflectivity recorded at a constant scattering vector.

In this thesis we investigate the magnetism of a selection of crystalline mate- rials that display fascinating macroscopic properties, largely due to the complex behaviour of rare-earth ions. The technique of choice is diffraction; shown to pro- vide the most lucid of results in this field of research. Neutron powder diffraction, a well established tool for determining magnetic structures, has been employed in order to determine two magnetic structures of the intermetallic R2CoGa8 series. This was the first such study of these materials, which are of great interest as the magnetic exchange interactions of the rare-earth ions compete with the crystal elec- tric field, giving rise to a wealth of magnetic properties tuneable by the choice of rare-earth ion. We then move on to study two of the most extreme multiferroic mate- rials, TbMn2O5 and TmMn2O5. We have developed a new technique for determin- ing electronic state specific magnetic structures through resonant x-ray diffraction, which we have successfully employed in an investigation of the terbium magnetic sub-lattice in TbMn2O5. The outstanding question in many multiferroics regards the exact microscopic mechanisms at play. Due to the huge potential in technology, this has been the subject of intense debate over that past decade. We have shown, through ab-initio computation and the simultaneous measurement of electric polar- isation and magneto-striction, that the exchange-striction model is dominant in the main ferroelectric phase of TbMn2O5 and TmMn2O5. Through ion specific resonant x-ray diffraction measurements, we have clarified the behaviour of the terbium sub- lattice upon the magnetic field induced electric polarisation reversal in TbMn2O5. Furthermore, we have made the discovery of additional incommensurate magnetic diffraction signals, believed to be indicative of the response of magnetic domains to applied magnetic fields.