I am studying Giant Magnetoresistance (GMR) and I'm having difficulty reconciling why the GMR effect is isotropic; the resistance of the material drastically drops when exposed to a North oriented magnetic field, and equally so for a South oriented magnetic field. What I know is that, in a typical GMR multilayer material, a very thin non-magnetic metal (called the conductor layer, or spacer layer) is sandwiched between two ferromagnetic metal layers. The two ferromagnetic layers are manufactured such that their electrons have spins that are anti-parallel to each other. Also, one layer is "pinned" such that an external magnetic field won't change the magnetic moment of it.
This causes an electron in the conduction layer to experience strong scattering effects, which means the material is highly resistive.
When exposed to an external magnetic field (North or South), the "unpinned" layer's electron spins becomes oriented to it, such that both the ferromagnetic layers have parallel magnetic moments.
This causes an electron in the conduction layer to experience less scattering effects, meaning the material is suddenly much less resistive.
This is the GMR effect.
What I don't understand:
How come both a North or South magnetic field equally cause the same parallel orientation (with respect to the pinned ferromagnetic layer) of the unpinned ferromagnetic layer's electron spins?
I would expect one field direction (say, North) to cause the orientation to be parallel, and the other (say, South) to cause the orientation to be more anti-parallel (or unchanged).
Edit: Perhaps I'm confusing spin valves with generic GMR effects. See the below image from this excellent video: https://youtu.be/cID4fKraWkE?t=1143