  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.
