Previous results from the authors~2 concerning the rising of a tiny photon anomalous paramagnetic moment μ_γ, that is due to its interaction with a magnetized virtual electronpositron background, are complemented and discussed. It is shown that in the region beyond the first threshold, where photons may decay into electronpositron pairs, for magnetic fields large enough, the vacuum becomes unstable and decays also into electronpositron pairs.
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