Specimens of the pseudobinary series, Dy(CO,Ni)(_2), were made in order to investigate the variation in the shape of the hysteresis loop across the series. A pulsed-field magnetometer was used to obtain oscillo grams of the loops and also to derive the ordering temperature and molecular moment for each specimen. Using this data, the variation of critical field with composition was explained and demonstrated by assuming the domain walls in those materials to be very narrow (of the order of ten atomic spacings) and to be intrinsically pinned. The applicability of the models of thermal activation and, at lower temperatures, tunnelling of the domain wall through the barrier, due to Taylor and also those proposed by Egami were tested by studying the variation of the initial part of the magnetisation curve (i.e., below the critical field at which the magnetisation increases abruptly) with field sweep rate, composition, and temperature. Measurements of the magnetisation rate, (^dM)/(^dt), were also taken to understand the variation of domain wall mobility with applied field and temperature and hence examine the applicability of Taylor's and Egami's models further.

A brief introduction to the theory of susceptibility in antiferromagnets Is given and the experimental studies on antlferromagnetic domains are reviewed. X-ray topographic techniques are discussed. Low temperature domain studies in moderate fields have not been performed previously by X-ray topography due to the geometrical constraints. A new topographic technique using synchrotron radiation has been developed for domain wall motion studies in magnetic fields up to 14 KOe at temperatures down to 4.2K. Details of four successful cryostat designs are presented. The perfection of crystals KNiP(_3), KCoF(_3) and KFeF(_3) grown from the flux has been studied by X-ray topography. Very low dislocation densities and dynamical diffraction effects were observed. Dislocations directions in the flux, grown crystals of KNIF(_3) and KCoF(_3) prefer to run in such directions as to minimise their elastic energy per unit growth length in a similar manner to dislocations in crystals grown from aqueous solution. The topographic experiments on highly perfect crystals of KNIF(_3), and KCoF(_3), have revealed three types of domains with spin orientations parallel to each of the cube edges. Controlled domain wall motion in KNIF(_3) and KCoF(_3) has been studied using synchrotron radiation. Wall movement occurs to align spins perpendicular to the applied field and walls between domains already so oriented are found not to move. The general behaviour of antiferro magnetlc domains in KNIF^ and KCoF(_3) is in excellent agreement with the theoretical model proposed by Neel (1954). No catastrophic spin flop is observed, and some wall movements are reversible. Both direct observations of domain wall motion and the susceptibility measurements on single crystals of KNIF(_3) show that the spin flop occurs via domain wall motion. The effective anlsotropy is measured from the critical and threshold field data for both KCoF(_3) and KNIF(_3).

A study of the elastic properties of the scheelite structure crystals CaW0(_4), CaMo0(_4), ArMo0(_4) and PbMo0(_4) is presented. The seven independent components of the elastic stiffness constant tensor of CaW0(_4) have been determined between 1.5 K and 300 K by pulse superposition measurements of ultrasound velocities. A computer fitting procedure has been used to obtain the elastic constants from the velocity data; this has allowed the elastic constants to be determined even though large energy flux deviations prevented the measurements of the velocities of the quasi-shear modes propagated in the [100] and [110] directions. Experiments on SrMo0(_4) single crystals have shown that similar difficulties were the cause of inconsistencies in recently published velocity data; a new, correct set of elastic constants for SrMo0(_4) has been determined. Scheelites belong to the TII Laue group. The signs of C(_16) and S(_16) depend on the definition of the sense of the +z axis with recpect to the atomic arrangement. A convention previously used for CaMo0(_4) has bee adopted, the Z axis sensehas been found in SrMo0(+4) by an x-ray method: in each material C(_16) has been shown to be negative. Elastic behaviour and wave propagation characteristic, and their orientation dependence in the four materials, have been compared and contrasted. The work has provided a new in-sight into the acoustic properties of TI and TII Laue group materials; in the latter the “axes of acoustic symmetry” in the (001) plane do not coincide with the <100> and <110> crystallographic directions (as in TI's) but deviate from them by angles which are between 15 and 23 in the scheelites studied. Knowledge of the positions of these axes should facilitate further ultrasonic studies.

The Keyes model has been used to formulate the free electron contribution to the elastic constants of bismuth and its alloys with antimony (0-10 at.% Sb) The electronic contribution has been shown theoretically to be negligible in the intrinsic materials but to be observable in highly donor doped bismuth. The elastic stiffness constant tensor components of single crystals of bismuth and its alloys (3 at.% Sb, 5 at % Sb, 7 at.% Sb end 10 at.% Sb) with antimony have been obtained between 4.2 K and room temperature from measurements of ultra sonic wave velocities made by the pulse echo overlap technique. In contradiction with previous work, on polycrystalline alloys the ultrasonic velocities are found to increase monotonically with antimony composition. No electronic contribution to the elastic constants can be observed. The temperature dependences of the elastic constants are independent of the antimony concentration. The elastic constants of bismuth doped with tellurium (0-0.4 at.% Te) have been measured between 4.2 K and room temperature. Changes in some of the elastic moduli have been attributed to electronic contributions, the magnitudes of which are in reasonable agreement with the theoretical predictions. The temperature variation of the ultrasonic velocities, which are independent of tellurium composition, have been explained on the basis of temperature dependent deformation potentials.

The work described in this thesis is mainly concerned with the electrical and optical properties of three types of CdS device namely: 1. Photoconductive cells with two ohmic contacts 2. Schottky diodes with one ohmic contact, and 3. P-n heterojunctions made by the heat treatment of CdS crystals carrying deposited layers of copper. Since a high stability in use is essential for successful application of CdS photoconductors and solar cells, one of the main aims of the research has been to investigate the optically induced processes which lead to the degradation of the efficiency of CdS photosensitive devices. Using the technique of thermally stimulated currents on CdS : C1 crystals doped with monovalent and bivalent copper ions, we have shown that optically induced degradation of photoconductivity in CdS : C1 rods doped with Cu(^++) ions occurs in conjunction with a reduction in the density of shallow traps, and the simultaneous formation of deep traps. In contrast, optical illumination does not have any deleterious effect on CdS : C1 samples doped with Cu ions. With the Schottky diodes, the resistivity of the sample was found to play a significant role. The effects of ageing on such properties as the short circuit photoresponse, the C-V characteristics and the I-V characteristics of the diodes were investigated. It is shown that during ageing an increase in the thickness of the interfacial layer occurs in conjunction with a decrease in the uncompensated donor density. Devices of the third type, i.e. p-n heterojunctions were produced by heating crystals of CdS carrying layers of copper metal. Optical and electrical measurements showed that the heat treatment of a Cu-CdS contact leads to the formation of a layer Cu(_2-x)S which is a mixture of djurleite and chalcocite.

The constant K(_4) for the hexagonal magnetocrystalline anisotropy of the basal plane has been measured at various temperatures between 77K and 180K for alloys of Terbium and Scandium of compositions. Tb(_x)(^so)(_1-x) where x = 0.695, 0.825 and 0.89 using a torque magnetometer with automatic balancing and recording. Simple methods were devised for setting up the instrument and for analysing torque curves so as to correct for shearing and distortions introduced by rotational hysteresis. Instability in the magnetometer was cured by increasing the speed of response. Three methods of calibratior were compared. The variation of K(_4) with temperature for the Tb/So alloys indicates that the anisobropy is of single ion origin and due to the second order magnetostriction of hexagonal symmetry. Analysis of the results of P.K. Ely ( 1967, Ph.D. thesis, University of Durham, unpublished) leads to the same conclusion for pure Terbium. Estimates of at absolute zero indicate that the hexagonal anisotropy contributes significantly to the driving energy to ferromagnetism in pure Terbium end the Terbium rich alloys0Reported changes in the easy axis of Dysprosium have been investigated. Those below 80K are associated with hysteresis in the movement of the domain walls and are not reproducible. Those above 130K may be explained by the existence of fan-like spin structures but could also be due to the uncertainty associated with determinations of easy axis using a torque magnetometer with an unsaturated specimen.

An experimental investigation of the galvanomagnetic effects in a particular A7 structure material and a theoretical study of the symmetry properties of the transport tensors are presented. For the experimental study, arsenic-antimony alloy single crystals have been grown at the minimum melting point composition (25.5 at. % A8) where the solidus and liquids touch on the phase diagram. Dislocation etch pit studies have been made on the (111) cleavage faces. Measurements have been made between 1.5K and 300K of the twelve components that define the low-field magnetoresistivity tensor and of the orientation dependence of the tensor components P (_11) (B (_1), B (_2), 0), P (_11) (B (_1), 0 B (_3)) and P (_21) (B (_1), 0, B (_3)). A least-mean-squares fit to the data has been used to obtain the model parameters for a two band, multivalley, ellipsoidal Fermi surface. The alloy model parameters are compared and contrasted with those of the parent elements. The alloy is semi metallic. In the theoretical work, the forms of the magnetic field dependent transport tensors are established for all 32 crystallographic point groups. A formulation of galvanomagnetic and thermomagnetic effects based on the separation of the tensor components into “even” and “odd” functions of the applied magnetic field is given. It is shown that the Umkehr effect is a natural result of the anisotropy of crystals.

Density functional theory (DFT) is a method of effectively solving the many-electron Schrödinger equation, enabling the properties of condensed matter systems to be calculated from first principles. With the commonly used local density approximation (LDA), and generalised gradient approximations (GGAs), to the exchange correlation functional, it is currently possible to perform calculations on systems containing several hundred atoms. The accuracy of such calculations depends on the system under study and on which particular properties one wishes to calculate. The use of more advanced functionals has the potential to improve accuracy, at the expense of greater computational demand. In this work we use the LDA to calculate certain properties of GaN, such as geometry, band structure, and surface properties, including the reconstruction of GaN surfaces under the presence of hydrogen. We then describe our computational implementation of advanced density functionals, including screened exchange (sX-LDA), Hartree-Fock (HF), and exact exchange (EXX), within an efficient, fully parallel, plane wave code. The implementation of sX-LDA and HF is used to calculate band structure properties of Si, GaN, and other simple semiconductors, and it is found that sX-LDA can improve results significantly beyond the LDA. We also derive and implement the theory that allows one to calculate directly the contribution to the stress tensor from exchange and correlation when using these functionals, and demonstrate this with some simple test cases. Finally, we introduce some new theoretical ideas that may pave the way for yet more accurate density functionals in the future.

This thesis investigates the properties of colossal magnetoresistive perovskite manganites from a theoretical and experimental point of view. In light of the failings of the double-exchange mechanism which has traditionally been used to explain the physics of colossal magnetoresistive manganites, we investigate the properties of these materials using the current carrier density collapse (CCDC) theory. We show that the CCDC theory can adequately explain the resistivity of perovskite manganites when subjected to zero external magnetic field and when in the presence of an applied magnetic field, thus accounting for the large negative magnetoresistance in these materials. Also the theory in conjunction with a generic phase seperation model allows us to explain the resistivity and for the first time, specific heat in disordered manganites. Experimental investigations into La0.75 Sr0.25 MnO3 have been undertaken. Homemade samples of La0.75 Sr0.25 MnO3 have been produced. Different annealing treatments have allowed us to produce samples with different crystallite sizes, as determined by Rietveld refinement. The effects of crystallite size upon the resistivity, magnetoresistance, magnetic properties and specific heat have been investigated. The resistivity, magnetoresistance and magnetic properties are in qualitative agreement with the CCDC theory, and the specific heat data are seen to be in quantitative agreement with the CCDC theory. We find no results that are in contradiction to the CCDC theory.

An experimental investigation of the structural and magnetic properties of Iron based ferromagnetic shape memory alloys Pd57In25Fe18, Ti50Pd40Fe10, Ti50Pd35Fe15, FeMnSi, Fe66.7Mn26.8Si6.5 and Fe57.4Mn35Si7.6 is reported. Magnetisation measurements and high resolution powder neutron diffraction measurements were used to characterise the structural properties of each alloy. The parent phase of the Pd57In25Fe18 specimen has a FCC unit cell, space group Fm3m and lattice parameter a=6.293 ± 0.006Å. The Fe atoms are essentially equally distributed on to the 4a and 4b sites with the Pd occupying the 8c site. On cooling the sample transforms into a tetragonal structure with I 4 / mmm space group and cell parameters a=4.097 ± 0.004Å and c=7.289 ± 0.007Å. Magnetisation measurements performed on the specimen found Tc=140 ± 10K and a moment per formula unit of 1.39 ± 0.03μB. The inverse susceptibility has a Curie-Weiss variation, which yields a paramagnetic Curie temperature of 159 ± 10K, an effective paramagnetic moment of 6.93 ± 0.06μB and a paramagnetic moment μp of 2.86 ± 0.03μB which is close to that generally observed for Fe on an FCC lattice. Neutron diffraction measurements performed on Ti50Pd50-xFex compounds have confirmed the parent phase to be a cubic B2-type structure with the Pm3m space group with lattice parameters a=3.134 ± 0.007Å when x=10 and a=3.118 ± 0.009Å when x=15. On cooling, the martensitic phase transformation occurs and the alloys transform into an orthorhombic B19 structure ( Pmma ) with a 3-fold modulation. The lattice parameters were determined as a=13.853 ± 0.001Å, b=2.872 ± 0.001Å and c=4.513 ± 0.002Å when x=10 and a=13.465 ± 0.002Å, b=2.923 ± 0.002Å and c=4.526± 0.001Å when x=15. The Curie temperature has been estimated using the Brillouin J=1 function as Tc~ 470 ± 10K when x=10 and Tc~523 ± 10K when x=15. For the FeMnSi specimen, the stoichiometry of alloys is consistent with that of C1b compounds. The F 43m space group was employed for the cubic phase with a lattice parameter a=5.684 ± 0.008Å. On cooling the material transforms into a hexagonal martensite phase with 3 P6 /mcm II space group and lattice parameters, a=6.867 ± 0.001Å and c=4.772 ± 0.002Å. In the Fe57.4Mn35Si7.6 compound there is no structural phase transition, the system orders in a FCC unit cell with space group Fm3m and lattice parameters a=3.616 ± 0.007Å. In the Fe66.7Mn26.7Si6.5 case at 500K the alloy orders in a FCC unit cell with space group Fm3m and lattice parameters a=3.611 ± 0.001Å. On cooling, the material transforms into a HCP cell with the space group 3 P6 / mmc and the lattice parameters a=2.540 ± 0.002Å and c=4.108 ± 0.001Å. In the FeMnSi alloy below 69K there is the onset of an incommensurate phase down to 5K, all the Fe-Mn-Si based alloys order with a type-1 FCC antiferromagnetic structure. In the case of FeMnSi, TN=69 ± 3K and for the Fe57.4Mn35Si7.6 and Fe66.7Mn26.8Si6.5 specimens TN has been estimated using the Brillouin J=2 function, to be TN~389 ± 10K and 315 ± 10K respectively. From neutron measurements, at 5K a magnetic moment of 2.69 ± 0.04μB per site and 2.20 ± 0.05μB per site was calculated for the Fe57.4Mn35Si7.6 and Fe66.7Mn26.8Si6.5 specimens respectively.