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Infrastructure Magnetism
Magnetism

Infrastructure - Magnetism

Investigations of various aspects of magnetism can be conducted on all neutron-scattering instruments at OPAL.

Neutron scattering instruments

To name just some of the principal experiments, magnetic structures can be determined from powder samples on ECHIDNA and WOMBAT, from single-crystal samples on KOALA, KOWARI and WOMBAT, and from epitaxial samples on TAIPAN, magnetic excitations can be traced in single-crystal samples on TAIPAN or in powder samples on PELICAN, flux-line lattices and magnetic correlation lengths can be studied on QUOKKA, and magnetic depth profiles in thin-film structures determined by reflectometry on PLATYPUS.

Polarised neutrons 

Polarising station
The 3He polarising station atop the guide bunker

The sensitivity of neutron scattering to magnetism can be considerably enhanced if the incident neutron beam is polarised, i.e. the neutron spins are all aligned parallel. This is currently achieved using supermirrors on PELICAN, PLATYPUS, and QUOKKA. Soon, polarisation via 3He spin filters will be possible on all instruments, offering the particular advantage of wavelength- and angle-independent spin polarisation or spin analysis.

One application amongst many will be the study of the distribution of unpaired electrons around a magnetic atom, by direct determination of the spin density by the flipping-ratio technique using single crystals. The experimental observations are analysed using methods similar to those for X-ray charge density. Another tantalising possibility is imaging of magnetic fields and magnetic domains on the neutron radiography/tomography/imaging instrument, DINGO, due to enter the user program this year.

Sample environment

Magnetism in the increasingly more complex materials of interest imposes severe demands on the neutron experiment. It is rare that magnetism can be observed or characterised correctly under ambient conditions. Molecular compounds that order magnetically in three dimensions, often only do so at very low temperatures. The suite of available cryostats is growing with the possibility to provide temperatures routinely down to 0.25 K on most instruments.

To study the magnetism in those materials that do not order spontaneously requires cryomagnets, either to investigate metamagnetic transitions, or to determine the distribution of spin density. Both vertical-field and horizontal-field cryomagnets are available, although their sizes do limit where they may be used. Pressure is another useful variable since the magnetic behaviour is closely linked to the atomic bond lengths. Click here for the current list of available sample environments.

Magnetometry

Complementary property measurements are essential to guide the strategy of the neutron experiment. A vibrating-sample magnetometer will eventually be available at ANSTO for collaborative projects and radioactive samples.

Modelling

Some modelling support for magnetic studies is available, although our resources are limited. Calculation of spin-density distributions to compare with or extend observed data is relatively straightforward. For inelastic data, only interaction constants can be calculated with any confidence.