Thank you guys.
Well, I would like to rotate the sample in 1 degree steps. In each degree step, I stop the motor and then perform a resistivity measurement in the sample.
Someone said something about a dc motor, I did look for some small ones, and I found one with an encoder. The max speed is 100 RPM. In my case, to obtain 1 degree resolution, I think the motor should have low rpm, but even 100 rpm would give 600 degrees/sec ou 1 dg every 1.67 msec, in other words, I had start and stop the motor within 1.67 ms !!
Back to step motors: I've been trying the 28byj-48, but the main problem is the motor displacement is not reproducible when it rotates back and forth sometimes.