Meeting on diffraction ('99)

Diffraction with Incident Beam Polarisation on OSIRIS

The long wavelength diffraction component of the OSIRIS Project has been constructed and is now operational. The next step is to add incident beam polarisation and this will be established during this winter.

 We therefore felt it was timely to hold a meeting to address the possibilities offered by OSIRIS in the area of neutron diffraction with polarised incident beams. Our intention in the meeting was to have experts in the technique describe the possibilities offered by the new instrument during the morning session, and experts in the scientific applications describe its potential use during the afternoon. The underlying aim was to explore the scientific possibilities offered by this new phase of the instrument, and to identify, alert and listen to the user base. In this way the instrument will be most likely to be fully utilised when available.

The workshop kicked off in an upbeat mood with a warm welcome from Dr Uschi Steigenberger, head of ISIS Inelastic and Support Division, reminding us of the history and development of The OSIRIS Project. Prof. Otto Schärpf from Munich Technical University, followed it with a presentation which reminded us of the essential Blume’s formulae involved in polarised neutron scattering. His theoretical introduction gave us an insight of the broad avenues that will be available with OSIRIS when polarisation becomes available to the users. It was followed by a description of the instrument by its Instrument Scientist Ken Andersen, which touched the technical aspects, problems, solutions and possible avenues for the future. Examples from recent experiments showed the power and uniqueness of the instrument, even before the polarisation option becomes available. Prof. Otto Schärpf closed this morning session with a second talk. His dissertation on “What is needed (Instrumentation)” to achieve what Blume’s equations offers, gave an explanation of the elements that are necessary to achieve incident beam polarisation. He touched in detail the different types of spin-flippers with its advantages and disadvantages and the necessary corrections and their importance. Finally Prof. Schärpf told us about the common pitfalls that may occur when working with remanent supermirrors.

The afternoon session concentrated on the possible applications of the methodology to condensed matter physics. The session started with a general overview of the “Key Problems in Magnetism” by Prof. Andrew Harrison from The University of Edinburgh, delving into several topics like the rational design of magnetic materials, modelling of magnetic systems, strongly correlated electronic systems, combination of low dimensionality and frustration like the Kagomé lattice and the importance of subtracting the Bragg peaks from the diffraction pattern to obtain the diffuse component. According to Dr Harrison, OSIRIS is well positioned to tackle these topics now and specially when the high resolution at long wavelength is combined with the polarisation option. This general overview was followed by an exciting talk on “Diffuse Magnetic Scattering of Polarised Neutrons” by Prof. Bob Cywinski of the University of St. Andrews. His talk started by giving the reasons to measure the diffuse scattering. Whilst the elastic scattering can only provide information of the mean magnetic order within a sample, diffuse scattering gives information on the spatial variation around this mean magnetic moment and therefore information on the sensitivity of the magnetic order to the local atomic environment. Very interesting examples were shown, like the effect of defects in ferromagnetic binary alloys. To understand their properties and to match the advances in computational and theoretical methods, high quality data will be necessary and in Prof. Cywinski’s opinion OSIRIS has a considerable potential for these applications.

The subject of the last presentation was “Spin and Charge Densities” by Prof. Philip Reynolds from The Australian National University. Spin and charge densities are fundamental quantities, since they are directly connected to Quantum Mechanics and therefore provide an ideal language for communicating with theoreticians. In his talk he emphasised the two ways of obtaining these quantities “the old way” which includes: x-ray measurements, polarised neutron diffraction using large single crystals and neutron diffraction. And “the new OSIRIS way”: synchrotron and polarised neutron diffraction on small amount of powder. He also delved into the exciting subject of separation of coherent and spin incoherent contributions by using only incident beam polarisation via polarisation of the nuclei. This would allow proton mapping in diamagnetic samples, paramagnets and complex materials; spin mapping of large molecular clusters and the important removal of the incoherent background in structural determination.

In summary, the workshop surpassed all our previsions and there was an extremely powerful exchange of ideas between the condensed matter scientists and the instrumental scientists that is already starting to be fruitful.

David Martín

1st December 1999

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