Samarium

Samarium-cobalt was the first rare-earth magnet, commercialised in the 1970s, and although neodymium beat it on strength it has never been displaced where heat matters: SmCo holds its magnetism past 300 °C and barely corrodes, so it goes into aerospace actuators, missile guidance and turbopumps. The other samarium story is nuclear. Samarium-149 has a thermal neutron capture cross-section around 41000 barns, so it builds up as a fission product and quietly poisons a reactor core; operators budget for it explicitly. The element was also the first ever named after a person, indirectly — the mineral samarskite honoured the Russian mining official Vasili Samarsky-Bykhovets.