Spin-spin relaxation and magnetic resonances in two copper tutton salts

Abstract

The real and imaginary components χ′ and χ″ of the paramagnetic susceptibilities of CuCS 2(SO 4) 2. 6H 2O and Cu(NH 4) 2(SO 4) 2. 6H 2O have been studied at frequencies of 0.41, 1.21, 1.76 and 3.2 GHz. The powdered samples were placed in transmission cavities (cf. Fig. 1 and 2) and nearly all measurements have been carried out at 20°K. The accuracy of the absolute values is of the order of 5%. The exchange interaction in the Cs Tutton salt is small while in the NH 4 Tutton salt (Table IV) it is of the same order as the dipole-dipole interaction. This leads to a very different magnetic behaviour as may be seen from the figures 4–9, where χ″ is given in perpendicular magnetic fields, and χ″ as well as χ′ in parallel fields. In the figures 10–13 the data obtained in some of the fields have been plotted as a function of the frequency. In zero field (Fig. 10 and 11) reasonably good agreement with Gauss curves is found for the Cs Tutton salt. The high top value of χ″ and the negative values of χ′ at high frequencies are striking. A line shape function (Form. 6) for intermediate cases having the Gauss and Lorentz shapes as the two limits is proposed. The results for the NH 4 Tutton salt fit well with the shape function which is expected to apply in view of the ratio of exchange to dipole-dipole interaction derived from the specific heat of the spin system. The decrease of width of the χ″ curve in a perpendicular field due to truncation of the Hamiltonian of dipole-dipole interaction is observed for the Cs Tutton salt as well as a weak absorption band at half the field H L of the Larmor resonance (Fig. 4). The exchange narrowing of the Larmor resonance in the NH 4 Tutton salt is observed and, in accordance with the 10/3 factor, the width is found to decrease with increasing field (Fig. 7). In large parallel fields marked resonances at the Larmor frequency and the double Larmor frequency (at constant frequency occuring at the Larmor field and half the Larmor field) are observed in the Cs Tutton salt (Fig. 5, 6 and 13). The absolute values, as well as the ratio of the intensities agree satisfactorily with the formulae taking the hyperfine interaction of the copper nuclei into account, which have been developed by Dr. Caspers in his appendix. On the whole good agreement is found with the theoretical expectations a great part of which may be found in the appendix mentioned.