It needs to be flexible, but not so ductile that the magnetic fields produced deform it under the associated stresses, so preferably 'rigid' in one or two dimensions and flexible in the others. You then need to fixate it after you've made your assembly otherwise the stresses will rip it all to bits.

This is why the copper wire in an electric motor is potted: the wire is what the forces in the motor act on, if the wire isn't rigidly fixed it will vibrate (and reduce efficiency) and that vibration will eventually wear through the insulation causing a short in the motor.