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Charge states and lattice sites of Fe ions in virgin and Mn-doped Al _x Ga _1− _x N samples were investigated using ^57 Fe emission Mössbauer spectroscopy following radioactive ^57 Mn ^+ ion implantation at ISOLDE, CERN. In the undoped Al _x Ga _1− _x N, Fe ^2+ on Al/Ga sites associated with nitrogen vacancies and Fe ^3+ on substitutional Al/Ga sites are identified. With Mn doping, the contribution of Fe ^3+ is considerably reduced and replaced instead by a corresponding emergence of a single-line-like component consistent with Fe ^4+ on Al/Ga sites. Density functional theory calculations confirm the Fe ^4+ charge state as stabilised by the presence of substitutional Mn ^2+ in its vicinity. The completely filled spin up orbitals in Mn ^2+ (3d ^5 ) are expected to enhance magnetic exchange interactions. The population of the Fe ^4+ state is less pronounced at high Al concentration in Al _x Ga _1− _x N:Mn, a behaviour attributable to hybridisation effects of 3d states to the semiconductor bands which weakens with increasing (decreasing) Al (Ga) content. Our results demonstrate that co-doping promotes the co-existence of unusual charge states of Fe ^4+ and Mn ^2+ , whereas their trivalent charge states prevail with either transition metal incorporated independently in III-nitrides. Co-doping thus opens up a new avenue for tailoring novel magnetic properties in doped semiconductors.