For your situation, the diode can be just about anything, so long as you don't exceed the reverse breakdown. If your coil voltage is 12V, then find one around 15V or better. A little 1N4148 will handle 100V in reverse, and can handle 450 mA of peak fwd current (200 mA continuous). Since you are using the diode to let the current in the coil slowly drop (rather than stop instantly), the diode will initially see your steady-state current of 200 mA. However, that will only last for a short time before the magnetic field in the coil collapses, and the current beings to drop to zero.