Does anyone have a good reference on the physics behind the "single point iron receiver"? It seems similar to a crystal detector, but the description here sugge

Does anyone have a good reference on the physics behind the "single point iron receiver"? It seems similar to a crystal detector, but the description here suggests that both the needle and the "crystal" are the same material (iron). My guess would be that the non-linear effect is due to iron oxides on the surface.