First of all, I think you mean 40mA, not 40mV).
Second, we need a bit more information -- like:
- What kind of motor?
a. AC or DC?
b. What voltage do you plan to run it at? [Similar to the question "what voltage is it rated to run at", but my question spans the scope of intentional abuse ;D )
c. What Kind of maximum current can we expect (or, how about a description of the actual application)? - What PWM frequency? This will have an impact on whether or not an Arduino output can directly drive the MOSFET [i.e. charge/discharge the Gate capacitance rapidly enough -- the more current that needs to be switched, the greater the gate capacitance tends to be, and at higher PWM frequencies, you might need a gate driver to do the job.
- Will this need to be powered via a battery, or some external mains-based supply, or ???
- What sort of duty cycle (in other words, will the motor be on for extended periods, or only brief periods, or some random/chaotic intermittence? Will this be a highly predictable duty cycle, or can some incongruence be expected?
- Will this just be a fun little project, or is this destined for production?