Slow rise time on MOSFET
The MOSFET drain has a lot of capacitance. The diode adds a bit, as does your 'scope probe. You should use a much lower resistance than 10K to simulate the motor. 

Also, the 1N4007 is unsuitable for 50kHz, the reverse recovery time is of the order of 5-10usec (unspecified on datasheets). Use a Schottky diode such as 1N5819 for a low voltage motor (for higher voltages than about 30V-50V you can use an ultrafast silicon rectifier rather than a higher voltage Schottky) .