||Neutron Scattering Studies of the Single Crystal A-site Spinel CoAl2O4
||STEPHEN E. NAGLER
|Neutron Scattering Science Division, Oak Ridge National Laboratory
Adjunct Professor of Physics, University of Tennessee
Stephen E. Nagler received his B.S., M.S., and Ph.D. degrees in physics from the University of Toronto. He joined the former Solid State Division’s Neutron Scattering Group in 1995 and served as group leader for Neutron Spectrometry from 1996 to 2005. Nagler served as the interim director of the High Flux Isotope Reactor Center for Neutron Scattering in 2005–2006, guiding the center through a critical DOE review while ensuring research productivity, successful instrument upgrades, and integration of neutron scattering at the HFIR and at the Spallation Neutron Source. In 2006, he was named chief scientist for the new Neutron Scattering Science Division. Nagler is internationally known as a leader in the investigation of magnetic excitations and quantum critical behavior in materials and published many papers in Nature Physics, Phys. Rev. Lett.
The material CoAl2O4 in particular has been put forth as a promising candidate to demonstrate novel ‘spiral spin liquid’ and order-by-disorder physics (Bergman et al., Nature Physics 3, 487 (2007)). This presentation discusses neutron scattering results on single-crystal CoAl2O4 , including elastic scattering and inelastic scattering using both triple axis and time-of-flight methods. At high temperatures the elastic signal shows strong Lorentzian diffuse scattering about specific Bragg positions that would normally be indicative of collinear antiferromagnetic order. At T* ~ 6 K, a temperature that has been associated in the past with an anomalous spin glass transition, one observes a qualitative change in the scattering lineshape. The data can be interpreted as evidence for a first order phase transition to an ordered state, albeit with the spatial extent of long range order limited by kinetics. Below T*, inelastic scattering reveals well-defined dispersive excitations, with a small spin gap at the magnetic zone center.
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