The Claude Bernard Lecture, 1986 Ordering nuclear spins

Abstract

It has been understood since the time of Ampère that magnetism in matter can be explained by the existence in its midst of elementary magnetic moments, tiny magnets, later shown to result from the elec­tronic structure of atoms. Ferromagnetism, a state of matter where these elementary moments order themselves parallel to each other, has been known for centuries, antiferromagnetism, a more complicated arrange­ment, for over half a century. These ordered arrangements arise because of the forces that these elementary magnets exert on their neighbours and can be destroyed by thermal agitation if the temperature of the sample exceeds a critical value, the transition temperature. Atomic nuclei also have magnetic moments, three to four orders of magnitude smaller than the electronic moments and give rise to nuclear magnetism, studied by the methods of NMR, whose applications to various fields of science, technology and nowadays medicine, are widely known. On the other hand, nuclear ferromagnetism and antiferromagnetism never occur in Nature because they require fantastically low tempera­tures (of the order of a microkelvin), which can only be produced by sophisticated means in highly specialized laboratories. This lecture is devoted to a description of the methods for producing, observing and describing such exotic states.