Coil trembling/shaking with IRF520-520N MOSFET
Summarizing the comments:
It would appear you're getting ringing in your circuit because the mosfet turning on is allowing your inductor to sink so much current that your supply rail is seeing a voltage droop. This should be solvable using whatRoughBeast's method of slowing down the inrush current of the mosfet by putting a cap on the gate of the mosfet to ground or Vdd. Either way will cause the mosfet to turn on slower preventing the supply rail from seeing voltage droop. Choose a capacitor big enough to slow down the rise time but small enough so that you can turn the inductor on and off quickly. It should be in the 10nF range probably.

Alternately, you could stiffen the power supply rail by adding a decoupling/bypass cap between the power rail and ground. Add this capacitor as close to the resistor going to the mosfet gate as possible. This capacitor should be much bigger than the one used for the previous solution since it's trying to stiffen the power rail. It should maybe be around 10uF.