In a PM you asked if you would be better served by a raw iron or soft ferrite core. Wanted to share my answer here for others.
I’d use ferrite. There’s three factors affecting your choice: permeability, coercivity, availability
Magnetic permeability is the mu variable you often see in formulas relating to coils. More permeability means a stronger field will be induced within the implant. Ferrite generally has a larger permeability than raw iron (unless the iron was annealed in the presence of an external field).
Coercivity communicates how difficult it is for an external field to realign the magnetic domains within a material. For a permanent magnet, you would want high coercivity (hard to change). For the core material in a transformer which has to alternate frequently, you would want low coercivity (easy to change). Since the coil in implants is essentially one side of a transformer (the other being the transmitting device) you definitely want a low coercivity material. Raw iron has a slightly lower coercivity than soft ferrite, but they’re both miniscule and 55Hz is an extremely low frequency for this type of consideration, so it shouldn’t matter.
Ferrite is a ceramic material constructed from rust and trace other metals like nickel, cobalt, and aluminum. It’s extremely abundant and cheap. It is usually not electrically conductive or chemically reactive.
Raw iron on the other hand can be more difficult to come by. It’s very reactive with oxygen, so it usually comes in the form of mild steel which has a few percentage points of carbon in it’s makeup. Steel is very different magnetically. Both iron and steel are also electrically conductive.