Guys... the datasheet for this part shows that at 48V D-S and ~7A this part can only safely operate for no longer than 2 milliseconds before the junction overheats. Look at the SOA curve for this. It definately cannot be operated in a DC sense.. So, is this application pulsing the magnet on / off?
The part of the SOA curve is the power limit (slanted curve) and is strongly related to the ability for the FET to dissipate heat. Even though the current and voltages are under their maximum limits, is the power? The way to find out is to do the thermal calculations and check the SOA curve.
When your part dies is it hot? It seems like their is evidence that your problem is not related to the voltage spikes on the inductor. Maybe your issue is the power dissipation as I've discussed. How long does it operate with 48V and ~7A? What is the thermal resistance of the heat sink that it is on?
You may need a much much bigger MOSFET or parallel several devices if this is in fact your issue (which I'm not totally convinced it is yet) - But nevertheless, do not ignore the power operating conditions.