Work on the iron halides FeX2 has led to the models of A-type antiferromagnetism and the tricritical point in magnetic systems. Here we wish to carry out decoupling experiments to study the effect of the first and second-order phase transitions on the muon spin relaxation. FeCl2 is the simpler of the two, with a tricritical point at 0.8T and 20K. Below the tricritical point there is suggested to be a field-induced Griffiths phase between the A-type antiferromagnetic and saturated ferromagnetic phases. FeBr2 has a much more complex magnetic phase diagram and some of these phases remain poorly understood. While the internal magnetic fields in these materials are likely to be outside the range that can be measured at ISIS, the fields needed to access the interesting areas of the phase diagrams are sufficient to provide significant decoupling and ensure that the spin dynamics can be probed.