A para-magnetic core was a type of planetary core.
In 2377, The Doctor, acting as the ECH, hid USS Voyager on a moon with a para-magnetic core that masked the energy signature of the vessel. (VOY: "Workforce, Part II")
The Star Trek Encyclopedia, 4th ed., vol. 2, p. 123 defined "paramagnetic core" as a, "Geological term describing the center of a planet or moon that is similar to one that creates a magnetic field."