Special Magnetic Fields are a particular part of the sidequests that allow you to evolve certain Pokemon. The Pokemon Vikavolt, Probopass and Magnezone can only be aquired by evolving their pre-evolutions during these parts of the sidequests.
Special Magnetic Fields can be found on numbers 1176 1177 1178 1179 1421 1422 1423 1424 1425 1426 1427 1428 1429 1587 1588 1589 1850 1851 1852 1853 1854 1855 1856 1857 1858 1859.