Now, how could a magnetic field form there? If we have friction in on a dielectric (i.e. a bad conductor) we tend to get so called static electricity. if there is a good conductor short circuiting the dielectric (i.e. iron core) the energy discharges.
To see what happens when electricity discharges take a compass, a piece of wire and a battery. Place the compass on the wire and connect the wire (for a few seconds) and see what the compass needle does.
That model is pretty much oversimplified but could get you on the right track.
I ended up looking into torque converters in automatic transmissions (fluid couplings) and found once initial inertia is overcome the system can operate with a high degree (95%) of efficiency. Which sort of answers my Q about inner core, outer core and crust and suggests no need for the magnetic field to play a role in earth rotation..