The work reported here is a detailed study of the proton magnetic resonance in single crystals of CoCl₂∙6H₂O. This substance is paramagnetic at high temperatures and becomes antiferromagnetic at about 2.25°K. The proton resonance frequency is a measure of the total magnetic field at the positions of the protons, which is the vector sum of the applied magnetic field with the internal field produced by the surrounding magnetic ions. At room temperature a single line about 6 gauss wide is observed. This line splits into a number of components at liquid helium temperatures. The position and number of lines strongly depend on temperature and on the direction of the externally applied magnetic field. The maximum overall splitting at 4.2°K is about 150 gauss in a field of 5000 gauss. At, 2.1°K the maximum splitting observed is about 2500 gauss. From the resonance lines in the paramagnetic phase it was possible to calculate the direction cosines of one proton-proton vector. The resonance spectra in both phases were found to agree well with the theory predicting the positions of the resonance lines and their dependence on crystal orientation. The transition temperature T[subscript N] was measured as a function of applied field and crystal orientation using the proton resonance lines, since they are very sensitive functions of temperature near T[subscript N]. T [subscript N] is found to be a complicated function of the applied field and crystal orientation, which cannot be described by T[subscript N](H,) = T(0) - const. H², as predicted by the Weiss Molecular field theory. The transition takes place over a temperature region of about 10[power -2] °K, and effects due to short range order are observed just above T[subscript N]. The magnetic susceptibility in zero field was measured along the preferred axis of antiferromagnetic alignment. This, together with specific heat data from published literature, was used to show a mutual consistency between thermodynamic variables and T[subscript N] obtained by NMR. The sublattice magnetization in the antiferromagnetic phase was measured as a function of temperature. It is found to depend logarithmically on T[subscript N] - T, but is independent of applied field and crystal orientation. Further experiments are suggested, which would add greatly to the understanding of the magnetic behaviour of CoCl₂∙6H₂0. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

Standard radio-frequency nuclear resonance spectroscopy techniques have been applied to study the fine structure of the proton magnetic resonance absorption line in single crystals of CoCl₂.6H₂O. Cobaltous Chloride is a paramagnetic crystal at high temperatures and becomes antiferromagnetic at about 2.29°K. The position and number of lines strongly depend on temperature and on the direction of the externally applied magnetic field. Fewer lines than the theoretical number of twenty-four were always observed. At room temperature the proton resonance at 12 Mc/sec. in a field of 2.82 K gauss consists of a single line about six gauss wide. A splitting of this line into a maximum of six components has been observed at liquid helium temperature. The maximum overall separation at 4.2°K is about 110 gauss. For each direction of the externally applied magnetic field the separation between the lines increases with decreasing temperature. The transition temperature is measured and effects due to short-range order above the transition are observed. Theoretical formulae for the positions of the component lines are developed by considering the two-proton spin system within a water molecule of hydration immersed in the homogeneous external field [formula omitted] H and the inhomogeneous time- averaged field of the cobalt ions. Measurements in the antiferromagnetic state have been partially completed. / Science, Faculty of / Physics and Astronomy, Department of / Graduate