We intend the measure the neutron reflectivity of a purely magnetic interface generated at a ferromagnetic/antiferromagnet phase boundary within an FeRh epilayer, a material that undergoes a first order transition between these two phases. The layer will have a doping gradient such that the phase transition temperature varies throughout its height, and at room temperature will occur roughly in the middle of the layer. The phase boundary, and hence the purely magnetic interface, will move up or down in response to temperature changes or applied fields. PNR is the ideal technique to measure the depth of this purely magnetic interface and hence its motion in response to these stimuli. Nanoscale magnetic field or temperature sensors based on this material can be envisaged.