Magnetic behavior of halobis(diethyldiselenocarbamato)iron(III): Interactions, anisotropy, and three-dimensionalXYferromagnetism

Abstract

The chloro, bromo, and iodo derivatives of the complex specified in the title have been examined by M\"ossbauer spectroscopy and by magnetic susceptibility and magnetization measurements. All derivatives have orbital singlet, spin quartet ground states. M\"ossbauer spectra are consistent with positive-$D$ crystal-field parameters in each case. Cooperative transitions are observed in the chloro and bromo derivatives at 3.5\ifmmode\pm\else\textpm\fi{}0.05 and 2.7\ifmmode\pm\else\textpm\fi{}0.05 K, respectively. Below these temperatures the M\"ossbauer spectrum consists of a magnetic hfs component together with an unsplit quadrupole doublet whose intensity, relative to the magnetic hfs, decreases with decreasing temperature. A table of M\"ossbauer hyperfine parameters is presented. The principal crystal susceptibilities of the chloro and bromo derivatives have been measured over an extended temperature range, and are found to be along the [101], ($\overline{1}01$)-normal, and [010] directions. Temperature- and field-dependent magnetization measurements along these directions are also exhibited. The anisotropy in the susceptibility and the magnetization is very large, and of $\mathrm{XY}$ character. Ferromagnetic ordering occurs at 3.463\ifmmode\pm\else\textpm\fi{}0.01 and at 2.690\ifmmode\pm\else\textpm\fi{}0.01 K in the chloro and bromo derivatives, respectively. Analysis of the susceptibility data for these two systems leads to a determination of zero-field splitting parameters, $g$ values, mean exchange interactions, and anisotropy fields. An extensive table of results is displayed, and comparison is made with the corresponding series of halobis(diethyldithiocarbamato)iron(III) complexes. The temperature dependence of the reduced magnetization in the chloro and bromo derivatives, as derived from M\"ossbauer and magnetization measurements, is also exhibited. All the available evidence points to the conclusion that these systems are classifiable as three-dimensional $\mathrm{XY}$ ferromagnets, and that they appear to be the first known examples of this type of magnetic model system. Their $\mathrm{XY}$ character is shown to be a logical consequence of the anisotropic ground-state properties following from the observed zero-field splitting parameters. Susceptibility measurements on a powder sample of the iodo derivative reveal significant antiferromagnetic exchange interactions and permit a tentative determination of probable zero-field splitting parameters to be made. Rather curiously, this system does not order down to temperatures as low as 1.3 K. For all systems discussed here, a comparison is made between observed ordering temperatures and those predicted by available theory on the basis of measured exchange and anisotropy parameters. The relative strength of the exchange interactions in the selenocarbamato and the thiocarbamato series is also considered, as are apparent trends in the ligand dependence of the zero-field splitting parameters.