Previous measurements on CdS (I) have shown a temperature shift in the single electron spin resonance (ESR) impurity band line. At 34 GHz this shift was found to be: dg= - 2.5 x 10ˉ ⁴/°K dT in going from 4.2°K to 1.7°K. This thesis presents the construction and testing of a metal temperature controlled dewar for the purpose of measuring the shift over the temperature range from 40°K to 1.7°K using an X-band (9 GHz)spectrometer. The signal was observed between 1.7°K and 4.2°K but rapidly broadened and could not be seen at higher temperatures. Since the lowest attainable temperature of the metal dewar was around 5°K, it could not be used as planned. Further studies using glass dewars proved that the g-shift at 9 GHz was too small, to be measured. The theory for the g-shift is discussed and the performance of the dewar evaluated. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

A cryostat has been built for cooling specimens to temperatures of the order of a hundredth of a Kelvin by thermal contact with an adiabatically demagnetized paramagnetic salt pill. The apparatus was designed for performing nuclear orientation experiments. This thesis describes the construction of the apparatus and experimental tests studying the nuclear orientation of (60)Co in an iron plate. The paramagnetic salt used was chromium potassium alum in an alum-glycerine slurry. In addition to the chrome alum pill a guard pill of manganous ammonium sulphate was used between the alum pill and the 1K helium bath. The pills were supported and thermally isolated by German silver spacers. A copper heat link was embedded in the alum-glycerine slurry and soldered to the specimen to provide thermal contact. Several heat links were used ranging from a bundle of five thousand copper wires to a copper foil "concertina" arrangement. A Ventron niobium-titanium superconducting solenoid which produced fields up to 48 kilogauss was used for the magnetic cooling. A superconducting polarizing solenoid was used to magnetically saturate the polycrystalline iron plate. Anisotropies in the gamma radiation intensity from (60)Co of 7 to 11 per cent corresponding to temperatures of 37 to 45 m K were observed. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

The electronic g-tensor in antimony doped cadmium sulphide has been measured at 1.1°K. A sample doped to a room temperature resistivity of 3.3 ohm-cm exhibited an anisotropic g-tensor with g// = 1.788 and g⊥ = 1.770. A single asymmetric line was observed. The asymmetry of this line was found to decrease with decreasing incident power. Further experiments to study this line shape are indicated. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

The ESR g-value and susceptibility measurements for DMTM(TCNQ)₂ have been studied as a function of angle made by crystal with magnetic field and temperature. The angular dependence of g-value is fitted to g² = α+βcos2θ - γsin2θ for three orthogonal directions of crystal rotation. The principal g-values are close to those expected for TCNQ compounds: g₁ = 2.0034, g₂ = 2.0030, g₃ = 2.0024. The susceptibility as a function of temperature agrees with bulk susceptibility measurements except that the maximum position occurs at about 30 K. The results of Oostra et al. for bulk susceptibility showed a maximum at around 50 K. The phase transition reported by Visser et al. at 272 K is observed in the ESR data as a 15% decrease in susceptibility. The linewidth is remarkably anisotropic typical of TCNQ salts. The phase transition study is done for two orientations of the crystal with the magnet field. In one of the orientations the linewidth narrows from 0.15 to 0.11 gauss and in the other it narrows from 0.24 to 0.18 gauss. In the former case there is a growth of a second line due to the twinned stack; transformation twinning takes place at the phase transition. A small level-crossing interaction is inferred from the change in relative intensities of the lines near the crossover. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

The studies presented in this thesis are in two areas. An investigation of aromatic nitrene molecules using EPR spectroscopy was undertaken. This revealed a general tendency for these molecules to exhibit two forms when trapped in a crystalline lattice. This is qualitatively interpreted as a perturbing effect of nitrogen evolved during generation from the azide. Single crystal experiments were made on 2,4,6-tribromophenyl nitrene, which exhibited an unusually large anisotropy in the zero field splitting, implying a bent structure. Temperature variation of several aromatic nitrene zero field splittings were made. The variation was interpreted as thermal averaging amongst states possessing different zero field parameters. The nature of these states however, could not be inferred. The variation is compared with similar studies. A computer program to least squares fit EPR angular data to a spin Hamiltonian has been written. This programme was used to obtain a description of the angularly dependent EPR data from an aromatic nitrene, and also for a radical species with a large hyperfine interaction. / Science, Faculty of / Chemistry, Department of / Graduate

Two phase transitions have been found in DEM(TCNQ)₂ at 400(3) K and at 442(6) K to 453(6) K using ESR and measurements of the microwave conductivity. These temperatures are less than the values of 415 K and 483 K previously obtained from temperature dependent Guinier measurements. Below 400(3) K two ESR lines, I and II are observed corresponding to the two stacks, B and A, in DEM(TCNQ)₂. Above 400(3) K and below 453(6) K only one line remains with the same g value as line I. The angular dependence of the g value was fitted to g[sub=‖]² = g[sub=⊥] ²cos2θ + g²sinθ and values of g[sub=⊥] = 2.003551(14) and g[sub=‖] = 2.00273015) were obtained. Between 298 K and 442(6) K the conductivity was that of a semiconductor with an exitation energy E[sub=0] = 0.385(52) e.v. The -4k[sub=⌐] phase transition is postulated at 447(9) K. The phase transition at 400(3) K is due to a transfer of spin density from stack A to stack B, and has no effect on the total spin susceptibility or on the conductivity to within the experimental error. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

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