The 'tube-transistor' enigma by MC carts?

Raul indicates "bobbin" is synonymous with "armature", just different terms for the "thing" they wind the coil-wire around. Is "core" another synonym?
I ask because there is also the "air core" design, where the coil-wire is wound around — nothing. Thus there is no interaction between the different materials, hysteresis, or "magnetization" of an iron core.
As it was explained to me, the "air core" is wound around something of course, it must be. They use — Ice. The wire is wound around a sliver of ice of the right shape, which then melts away, leaving no core/bobbin/armature at all. Hence "air core". This must be done in a deep-freezer, of the walk-in kind.Fidelity Research used this method in some of their models, as it was explained to me by a FR aficionado I knew many years ago. Maybe he was wrong, maybe nuts — but my FR MC-201 advertises its air-core as a feature that offers lighter moving mass and lack of the interactions mentioned above.
Do any of you know more about this?
I’d like to think he wasn’t crazy. I bought several FR cartridges from him, because of their sound of course, not the ice-theory.

@bimasta Here is some info translated from Japanese about Air Core Coil of Fidelity-Research PMC-3 :

" Air core coil is adopted as coil, weighing is realized compared to ordinary coil part made of magnetic material as core material, and magnetostriction caused by hysteresis curve of core material is expelled. In addition, by adopting a new special lightweight resin coil bobbin, the total weight of the oscillator is reduced to about 1/3, and the tracking ability is improved.... Samarium cobalt magnet is adopted as a magnetic circuit. This material has an ideal characteristic that it has higher holding power compared with ordinary Alnico type and it is hard to self-demagnetize. Moreover, the yoke part can also be downsized, the cartridge’s own weight is drastically reduced, and the initial characteristics can be kept for a long time.... Precision cutting is applied to the yoke to make the entire magnetic circuit close to a circular shape. As a result, effective magnetic path length can be shortened, size can be reduced, and magnetic flux leakage at the corner portion is reduced to achieve high efficiency.... A 0.04 mm diameter net flow line is adopted as the coil. "

The trade off with an air core versus an iron core or other variant is that you need more turns of wire to achieve the same energy out. So this adds resistance.

Dear @bimasta: Always exist trade-offs. I'm not  cartridge designer but maybe the best is to build cartridge coreless and yoke-less like the Yamaha X-1s. I own it and is truly good performer even that's a vintage one.

R.