Ionic size effect on the spin gap nature of SrCu₂(TeO₃)₂Cl₂

By means of magnetic susceptibility measurements, we report the effects of partial substitution of Br onto the Cl sites of SrCu₂(TeO₃)₂Cl₂. This material has been a subject of current interest due to indications of a spin gap behavior. However, the magnetic spin couplings responsible for the spin gap nature remain unclear. For each composition of SrCu₂(TeO₃)₂(Cl_1-xBr_x)₂, the temperature-dependent susceptibility exhibits a character of low-dimensional magnetism with a broad maximum at T _max. The magnitude of T _max slightly shifts to higher temperatures with increasing Br concentration. The experimental data can be well fitted to a coupled spin dimer model, yielding an increasing trend for the spin gap size from 172 to 188 K. This indicates that the substitution of Br ions has the effect of reducing the distance between Cu(1) and Cu(2) ions, leading to a stronger antiferromagnetic spin interaction. On the other hand, the variation of the interdimer coupling is marginal, indicative of substituting Br ions having little effect on the distance changes of Cu(1)-Cu(1) and Cu(2)-Cu(2) in SrCu₂(TeO₃)₂Cl₂.