Effective spin quantum numbers in iron, cobalt and nickel

The known macroscopic data for the temperature dependence of the normalized spontaneous magnetization, m(s)(T), of iron, cobalt and nickel are reconsidered and compared with our normalized zero-field hyperfine field data, h(hf)(T), obtained by Mossbauer spectroscopy or nuclear magnetic resonance. It is observed that for all three metals h(hf)(T) is not proportional to m(s)(T) and systematically smaller than m(s)(T). This is attributed to smaller magnetic interactions in the domain walls compared to the volume of the domains. The temperature dependence of m(s)(T) and h(hf)(T) can be described in the whole range 0 0 but S = 1 for higher temperatures including the paramagnetic phase. For iron and cobalt the corresponding values are S = 3/2 and 2. (C) 2003 Elsevier B.V. All rights reserved

Crystallographic data collection using a 0.22% bandwidth multilayer

To bridge the gap between traditional multilayer and crystal optics a high-resolution multilayer monochromator with a bandwidth of 0.22% has been designed and installed on a bending-magnet beamline (F3) at the Cornell High Energy Synchrotron Source (CHESS) to provide an unfocused monochromatic X-ray beam for protein crystallography experiments. Crystallographic data of excellent quality from a medium-sized protein, Concanavalin A, were collected and processed using standard Crystallographic programs. The data were successfully used for a structure solution and refinement. The flux from the multilayer monochromator is enhanced, relative to that from a flat Si(111) monochromator, by a factor of 5; consequently, data collection is faster and/or smaller samples may be used. At the same time, the bandwidth is narrow enough to avoid streaked spots. This experiment suggests that multilayer optics may play a valuable role in satisfying the demands of the structural biology community for rapid X-ray data collection, particularly at under-utilized bending-magnet beamlines. © 2005 International Union of Crystallography Printed in Great Britain - all rights reserved.link_to_subscribed_fulltex

On the failure of the Bloch-Kubo-Dyson spin wave theory

Further investigations on magnets with pure spin moments are presented showing that the temperature dependence of the order parameter can accurately be described over a large temperature range by a single T-epsilon power term. The exponent epsilon is found to be independent of the spin order type but it depends on whether the spin quantum number is integral or half-integral and, of course, on the dimensionality of the magnetic interactions. The six empirical spin wave exponents epsilon defined in this way are 9/2, 2 and 3 for isotropic, anisotropic and axial interactions and integral spin quantum number but 2, 3/2 and 5/2 for isotropic, anisotropic and axial interactions and half-integral spin quantum number. Thermodynamic crossover between neighbouring exponents is frequently observed: antiferromagnetic NiO having S = 1 is cubic above TN but undergoes a progressive trigonal lattice distortion with decreasing temperature. For this material a crossover from isotropic (epsilon = 9/2) to anisotropic (epsilon = 2) interactions is observed as a function of decreasing temperature (see Table I). The hexagonal ferromagnet gadolinium having S = 7/2 exhibits a crossover from epsilon = 3/2 to epsilon = 5/2 indicative for a gradual change from anisotropic to predominantly axial interactions with decreasing temperature. Also the itinerant ferromagnets Fe, Ni and Co show the same exponents e as insulators with S = 1/2. While cubic Fe and Ni exhibit epsilon = 2 the same crossover from epsilon = 3/2 to epsilon = 5/2 as for hexagonal Gd is observed for hcp cobalt

An ionothermally prepared S=1/2 vanadium oxyfluoride kagome lattice

Frustrated magnetic lattices offer the possibility of many exotic ground states that are of great fundamental importance. Of particular significance is the hunt for frustrated spin-1/2 networks as candidates for quantum spin liquids, which would have exciting and unusual magnetic properties at low temperatures. The few reported candidate materials have all been based on d9 ions. Here, we report the ionothermal synthesis of [NH4]2[C7H14N][V7O6F18], an inorganic-organic hybrid solid that contains a S = 1/2 kagome network of d1 V4+ ions. The compound exhibits a high degree of magnetic frustration, with significant antiferromagnetic interactions but no long-range magnetic order or spin-freezing above 2 K, and appears to be an excellent candidate for realizing a quantum spin liquid ground state in a spin-1/2 kagome network.PreprintPostprintPeer reviewe