In most of the spinel ferrites like Ni-Zn, Mn-Zn grain size decreases with increasing Aluminum concentration. On what parameters the grain size depends.
Could you please specify a little further, how you produce your spinel ferrites ? For example, if they are produced from the melt, the action of Al may be that it acts as seed compound (for example by capturing the oxygen traces) and like that during solidication, you end up with more crystals growing and a finer grain structure.