Spin-spin relaxations and magnetic resonances in a series of hydrated paramagnetic salts

Abstract

The real and imaginary components χ′ and χ″ of the paramagnetic susceptibilities of a series of powdered hydrated salts have been measured at frequencies between 0.18 and 3.2 GHz. Not all frequencies were applied to all substances. The measurements were carried out in perpendicular and parallel fields up to 8.2 or 9.5 kilooersted as well as in zero field. Nearly all measurements were performed at 20°K and no influence of spin-lattice relaxation was found. In Gd2(SO4)3.8H2O, CrK(SO4)2.12H2O, Mn(NH4)2(SO4)2.6H2O and FeNH4(SO4)2. .12H2O internal electric fields cause splittings which dominate over the dipole-dipole interaction or are at least of the same order of magnitude. Apart from a few indications in the chromic alum, the frequencies were not sufficiently high to resolve the structure of the energy levels. In perpendicular fields a clear satellite at zero frequency was found in some cases (fig. 3–5 and table III). In high parallel fields Kronig-Bouwkamp relaxations were found (figs. 3-2 and 5-2). In MnCl2.4H2O, CuSO4.5H2O and CuCl2.2H2O exchange interaction is dominant. In perpendicular fields and in zero field Lorentz and Debije shapes (figs. 8-2 and 9-1) were found respectively. The available theoretical expressions for the exchange-narrowed line widths only permit a rough comparison with the observed widths. In the case of CuSO4.5H2O there is a clear discrepancy, which may be due to the marked difference between the two copper sub-lattices, to which Miedema, Van Kempen, Haseda and Huiskamp attracted attention in connection with their observations below 1°K. The available parallel fields were not sufficiently high to permit observation of the Kronig-Bouwkamp relaxations. In agreement with the expectation, no parallel-field resonances were observed. While Shaposhnikov's prediction for the dependence of χ″ at low frequencies on a parallel field gives in some cases satisfactory agreement with the data, this is not so in the presence of third relaxation, Kronig-Bouwkamp relaxation or when there is a strong exchange interaction.