Anisotropic magnetic exchange observed by electron spin resonance in hexaammineruthenium(III) chloride sulfate trihydrate

Abstract

The orthorhombic crystal structures of [M^III(NH₃)₆]Cl[SO₄]·3H₂O for M = Ru at 295 K and M = Ru or Co at 92 K have been determined by X-ray diffraction. Single-crystal ESR experiments on a ruthenium doped cobalt crystal at 77 K are interpreted simply in terms of a g tensor with principal values 2.005(6), 2.123(4) and 1.629(4) aligned almost along the a, b and c axes. This can be parameterised by a crystal-field model, and its orientation understood in terms of the crystal structure. However the pure ruthenium salt, unlike the ruthenium doped cobalt salt, shows unusual structure in its ESR spectra. The structure is extremely broad in energy, comparable with the microwave energy, but with a number of distinct peaks in the resonance which change dramatically with crystal orientation. The broadened spectra result from magnetic exchange between nearest neighbours occupying a two-dimensional network in the crystal. Since the exchange energies are greater than differences in single-ion energy terms, it is only the anisotropy in the magnetic exchange that is observed. It has principal values –0.18(2), 0.070(2) and 0.102(3) cm^–1(or all signs reversed), with principal axes approximately along a, b and c. It is highly unusual to observe structure due to such large magnetic exchange in ESR studies of magnetically concentrated materials.