Assuming that the Atmel Vdd is 5V, this will work:
Note that any NFET with a Gate Threshold of <3V, with a Drain current rating of >1A, a drain voltage of >20V, and an on-resistance < 0.1Ω will work.
The 10k resistor is to prevent the gate of NFet floating while the Atmel is booting up and the Port Pin is yet undefined (or defaults to an input). The 10k shouldn't load the output of the Atmel while it is high.
The minimum pulse width that the solenoid can be driven on is effected by the snubber diode. Look at the plot for V(D) Green trace. Note that the diode conducts and V(D) is one diode drop above the 12V rail for the duration it takes the coil current to decay to zero. The coil inductance of 50mH is a WAG, but gives reasonable time constants consistent with what I have measured...