negative power in hspice simulation working with MTJ

I simulated a simple gate(CMOS Not gate), and for once I put a resistor on the path to the ground and once again I put a MTJ on the path to the ground and measured power in both cases.
In the first case, we have a positive power in MicroWat and it is reasonable,
But in the second case,we have a negative power, and I dont't understand that where the problem is.