Using a common microwave
Q: Is it permissible for me to use the same microwave, at the workplace, that my non-muslim colleagues use to heat their food?
The possibility of the remnants of Non-Halaal foods that would be found on the inner walls of the microwave poses the threat of contamination of your meal.
However, in such constrained circumstances, kindly ensure that your meal is placed in a covered plastic or glass container wherein the risk of contamination with Non-Halaal food is minimised. Also ensure that the microwave is cleaned daily. It would be best if you could use the microwave after it is cleaned and before non-Halaal foods are warmed therein.
And All?h Knows best!