Neutron Scattering 1968-1976

Neutron scattering shares many characteristics with the more familiar technique of x-ray scattering.^1,2 Both are immensely useful for determining the precise spatial arrangement of atoms in crystals, which, in turn, tells us much about the structure, bonding, and function of the molecules forming the crystal. However there are some differences between the two methods, making, them complementary. In x-ray scattering, the x-ray photons are scattered primarily by the electrons bound into atoms making up the crystal. Therefore, heavier atoms, with more electrons, scatter x-rays more strongly than light atoms. Consequently, x-ray scattering is a poor choice for locating light atoms such as hydrogen atoms. Since hydrogen atoms comprise a major fraction of many important compounds (especially in biology), another method must be found. In neutron scattering, the primary scattering center is the nucleus, so that we are not subject to the constraint favoring heavy atoms for x-ray scattering. Neutron scattering is very useful for "finding" hydrogen atoms. (Neutron scattering cross sections do not vary regularly across the periodic table.) In addition, the neutron possesses a magnetic moment, which means that neutrons can interact with atoms that have permanent electronic magnetic moments: atoms with unpaired electrons can act as scattering centers for neutrons. Because magnetic scattering is sensitive to the magnitude and orientation of magnetic moments, and to the spatial distribution of the electrons giving rise to the magnetic moment, it is a sensitive probe of magnetic structure and properties. Therefore, neutron scattering becomes an important tool for measurements involving magnetic phenomena. This is a major theme of the entire history of neutron scattering research in the Brookhaven Chemistry Department. The High-Flux Beam Reactor (HFBR) began operation in 1965. Its many contributions to science are summarized here. During the period 1968-1976, the HFBR was the premier instrument for neutron scattering in the world, and research performed in the Chemistry Department reflected that fact.^1,2

1. "Neutron Diffraction" J.M. Hastings and L.M. Corliss, in Physical Methods of Chemistry, A. Weissberger, ed., 3rd edition, Interscience, London (1960).

2. "Neutron Scattering" J.M. Hastings and W.C. Hamilton, in Physical Methods of Chemistry, A. Weissberger and B. Rossiter, eds., 4th edition, Interscience, London (1969).

The great increase in neutron flux due to the HFBR enabled a major shift in direction of the program to the study of “second order” phase transitions. RbMnF₃, an ideal three dimensional cubic antiferromagnet undergoes a second order transition at the Néel temperature (the "critical temperature" analogous to the Curie temperature in a ferromagnet) at which point the alternating spin ordering goes to zero and the two oppositely oriented phases become indistinguishable. A major effort³ was devoted to the study of the static (equilibrium) and dynamic behavior of the spin system in the neighborhood of the Néel temperature, the so-called critical region. In the critical region various properties of the system such as susceptibilities, range of correlations, sublattice or “staggered” magnetization either diverge or go to zero as the critical temperature, T_N., is reached. This behavior is characterized by power laws of the form (T-T_N)^X. Scaling theory predicts relations between these exponents and these were verified in detail. At the time a theory of "dynamic scaling" was proposed to deal with the dynamic or non-equilibrium behavior in the critical region. Measurements of the neutron inelastic magnetic scattering of RbMnF₃ were analyzed in terms of this theory and were in very good agreement with the predictions of dynamic scaling. The coupling to the staggered magnetization of an antiferromagnet is provided by a "staggered magnetic field" which was thought to be difficult, if not impossible, to produce experimentally. Dysprosium aluminum garnet (DAG), an exhaustively studied example of an Ising antiferromagnet was thought to possibly exhibit a tricritical point in the presence of an external magnetic field. While trying to determine whether this was true the rather surprising result^4,5 that an external field in the [111] direction of DAG couples not only to the magnetization, but also to the staggered magnetization was observed. The conditions for the production of this "induced staggered field" were determined.

3. “Quantitative Analysis of Inelastic Scattering in Two-Crystal and Three-Crystal Neutron Spectrometry; Critical Scattering from RbMnF₃" A. Tucciarone, H.Y. Lau, L.M. Corliss, A. Delapalme and J.M. Hastings, Phys. Rev. B4 3206 (1971).

4. "Observation of an Antiferromagnet in an Induced Staggered Magnet Field: Dysprosium Aluminum Garnet near the Tricritical Point" M. Blume, L.M. Corliss, J.M. Hastings, and E. Schiller, Phys. Rev. Lett. 32 544 (1974)

5. "Induced Staggered Magnetic Fields in Antiferromagnets" R. Alben, M. Blume, L.M. Corliss and J.M.Hastings, Phys. Rev. B11 295 (1975)

Last Modified: January 31, 2008