The impact of 400 keV $Ar^+$ ion irradiation on the magnetic and electrical properties of in-plane magnetized magnetic tunnel junction (MTJ) stacks was investigated by ferromagnetic resonance, vibrating sample magnetometry and current-in-plane tunneling techniques. The irradiation-induced changes of the magnetic anisotropy, coupling energies and tunnel magnetoresistance (TMR) exhibited a correlated dependence on the ion fluence, which allowed us to distinguish between two irradiation regimes. In the low-fluence regime, ${Phi} < 10^{14} cm^{-2}$, the parameters required for having a functioning MTJ were preserved: the anisotropy of the FeCoB free layer (FL) was weakly modulated following a small decrease in the saturation magnetization $M_S$; the TMR decreased continuously; the interlayer exchange coupling (IEC) and the exchange bias (EB) decreased slightly. In the high-fluence regime, ${Phi} > 10^{14} cm^{-2}$, the MTJ was rendered inoperative: the modulation of the FL anisotropy was strong, caused by a strong decrease in $M_S$, ascribed to a high degree of interface intermixing between the FL and the Ta capping; the EB and IEC were also lost, likely due to intermixing of the layers composing the synthetic antiferromagnet; and the TMR vanished due to the irradiation-induced deterioration of the MgO barrier and MgO/FeCoB interfaces. We demonstrate that the layers surrounding the FL play a decisive role in determining the trend of the magnetic anisotropy evolution resulting from the irradiation, and that an ion-fluence window exists where such a modulation of magnetic anisotropy can occur, while not losing the TMR or the magnetic configuration of the MTJ.
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