Watch me fail at a tech request, tried to make this make sense.
Artificial Soul: Basically, it's a pill/capsule, that when taken acts as an artificial soul. It has just enough magic of the user to give off a magic signature to a mage if they were to relieve the person of it, but not enough to sense or detect with technology. Only if the person actually gets a hold of it, can the magic part be detected. As soon as it's taken, it activates and stays in the mages esophagus, implanting its self there. Once it activates it takes on the appearance of a real soul and attracts all the Psi within the body to that point, acting as a soul as well, but with Psi instead of magic.
Therefore, it acts like a soul, but with Psi not magic. Since Psi is very much like magic in attacks, teleportation, and such, this gives it the illusion of a real soul.
Since it's not magic, AM field would not effect it, and it cannot be detected unless forcefully taken out. A mage can opt to use it with their real soul hidden somewhere, or they can use it as well as their real soul.