Spin gap formation in the quantum spin systems TiOX, X = Cl and Br

Abstract

In the layered quantum spin systems TiOCl and TiOBr, the magnetic susceptibility shows a very weak temperature-dependence at high temperatures and transition-induced phenomena at low temperatures. There is a clear connection between the observed transition temperatures and the distortion of the octahedra and the layer separation. Band structure calculations point to a relation between the local coordinations and the dimensionality of the magnetic properties. While from magnetic Raman scattering only a small decrease of the magnetic exchange by -5 to 10% is derived comparing TiOCl with TiOBr, the temperature dependence of the magnetic susceptibility favours a much bigger change.