[1] It is known that second-order magnetic phase transition, the transition between ferromagnetic (ferrimagnetic) and paramagnetic states of the material at the Curie temperature, is accompanied by a sharp (theoretically infinite) enhancement of the magnetic susceptibility. A second-order magnetic phase transition within the Earth (usually at mid-crustal depths, depending on geothermal conditions and on the type of magnetic material) is assumed to produce extremely high susceptibility zones of a thickness of a few hundreds of meters. Such strongly magnetized zones may be sources of well-known but not-yet explained geomagnetic anomalies, and at the same time, they may produce complicated electrical conductivity anomalies, as well. The second-order magnetic phase transition should be taken into account as one of the possible sources of geomagnetic and magnetotelluric anomalies.

George P. Rédei: Encyclopedic Dictionary of Genetics, Genomics, and Proteomics. 2nd Edition, 1392 pages, A John Wiley and Sons, Inc. Publication, Hoboken, NJ, USA 2003