You're simply wrong. Again. The rotor windings of induction machines are usually copper or aluminum, the stator windings are usually copper.
As for the rotors and stators themselves, they are good old iron/steel laminates and will hold some magnetism just fine (they're laminates to cut down on the eddy currents). If the rotors themselves would be made out of copper the motor would not function at all, it needs a ferromagnetic part there to focus the field.
Source: have done a number of induction motor to windmill conversions. Also: have you ever tried any of this or are you coming at this from a theoretical angle?
Well, there was that time when I was getting an EE degree where I asked my drives professor if I could use an induction motor as a generator, and that's the response he gave.
With all respect for your professor, you could of course simply try rather than echoing words from long ago.
Fortunately not everybody will take 'it can't be done' as the final answer :) It's not what can't be done in theory that matters but what can be done in practice.
As for the rotors and stators themselves, they are good old iron/steel laminates and will hold some magnetism just fine (they're laminates to cut down on the eddy currents). If the rotors themselves would be made out of copper the motor would not function at all, it needs a ferromagnetic part there to focus the field.
Source: have done a number of induction motor to windmill conversions. Also: have you ever tried any of this or are you coming at this from a theoretical angle?
Here is some good info for you:
http://marineengineeringonline.com/tag/residual-magnetism/