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Proton magnetic resonance in paramagnetic and antiferromagnetic CoCl₂·6H₂O Sawatzky, Erich


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.

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