We intend to study a new class of materials known as 3D topological insulators. The electronic states in these materials have a non-trivial topology, different to almost all other known materials, and at the interface between these materials and 'ordinary' matter many exotic and novel phenomena manifest themselves. In particular they must have gapless surface states. However, to access much of the new physics of 3D topological insulators it is necessary to open an energy gap in their surface states. The most promising route for this is by inducing magnetic order. We have already studied the effect of doping to induce ferromagnetism in these materials. Here we intend to use polarised neutron reflectometry to spatially resolve the magnetism induced by proximity with a neighbouring ferromagnetic insulator.