Pre-History of Magnetic Neutron Scattering

Prior to the development of neutron scattering techniques, the only known magnetic ordering of materials without current flowing in them was ferromagnetism, because of its strong bulk effects. Attraction between pieces of ferromagnetic "lodestone" had been known since the ancient Greeks and Chinese. The first practical application of ferromagnets was in magnetic compasses, invented in China around 200 BC. The discovery of antiferromagnetism had to wait for magnetic neutron diffraction.

Roentgen discovered X-rays in 1895, and the Braggs began the development of X-ray diffraction for studying crystal structures in 1912. The X-ray sources available, however, were too weak to allow detection of magnetic X-ray scattering, which is very weak. That was only observed after >synchrotron light sources< came into regular use.

Néel first suggested the possibility of antiferromagnetic ordering of magnetic moments in solids in 1930. There was just no way to convincingly detect it at the time.

Chadwick discovered neutrons in 1932, but initally the only sources of neutrons were certain radioactive decays, which produced relatively high-energy neutrons. In the Manhattan Project during the Second World War, on the way to building the nuclear-fission bombs dropped on Hiroshima and Nagasaki, scientists led by Fermi built the first nuclear reactor. One of the goals in this was to prove that the energetic "fast" neutrons emitted in the nuclear fission of one Uranium nucleus could trigger fission other of other nearby Uranium nuclei, leading to a chain reaction and major release of energy. In a reactor, they wanted a controlled, stable reaction (rather than the explosive run-away of a bomb), and they found the easiest way to do this involved including moderator material inside the reactor to slow the fast neutrons down by collision-and-recoil so that in the controlled chain-reaction state there was a gas of neutrons in the reactor with an effective temperature the same as the true temperature of the moderator. These are called thermal neutrons, and they are the type suitable for diffraction from material lattices.

After the WWII, nuclear reactors began to become available for civilian research, and immediately in 1945, Wollan began to investigate the use of neutron diffraction for the study of crystal lattice structure. Working with him, Shull published the neutron diffraction spectrum from an antiferromagnetic ordering structure (of manganese oxide, MnO) in 1951. To see this described on the Oak Ridge Lab history site >click here<. With this, the study of antiferromagnetic ordering structures with neutron scattering began.

dnoakes@vsu.edu
May 2004