ANSTO's research capabilities, led by the OPAL nuclear research reactor and associated instruments provide access to users investigating areas as diverse as materials, life sciences, climate change and mining/engineering.
Magnetism
Neutron scattering applied to understanding magnetic structures and interactions.
"Magnetism is the Killer App for neutron scattering" (Introduction to separate presentations by Thomas Brückel, IFF, and Bruce Gaulin, McMaster University)
As an experimental tool for the study of magnetism, neutron scattering is without equal in its range of applications. Although the neutron has no charge, it does have a magnetic moment, which can interact with the electronic magnetic moments of a crystalline material by diffraction to probe the magnetic structure, by inelastic scattering to probe the magnetic interactions, and even interact with the nuclear moments to add a further dimension to structural studies.
Bulk magnetic properties are strongly linked to the interactions between the moments at the atomic level, and therefore neutron scattering can play a key role in developing better magnets for technological applications.
Resources
 |
 |
|||
| Publications | Highlights | Facilities | Project members | Collaborators |
Latest News
|
| Latest News Relating to Magnetism |
|
Group member Frank Klose will give an invited talk entitled "Combining (Polarised) Neutron and Synchrotron Scattering Methods for Investigations on Magnetic Thin Film Nanostructures" at the 8th Pacific Rim International Conference on Advanced Materials and Processing (PRICM-8), in Hawaii, USA, 4-9 August 2013. |
Highlights
| Recent magnetism highlights include: |
Principal Themes
The principal themes addressed within the project are:
Transition-metal Magnetism
Many transition-metal elements and compounds are strongly ferromagnetic at room temperature. Beginning with the archetypical magnetite or lodestone, transition metals have long formed the basis of most magnets for technological applications, thanks to the high crustal abundance of most transition-metal elements and their high variability of physical properties in stoichiometric combinations or by alloying. Colossal magnetoresistance, where the electrical resistance is changed dramatically in the presence of a magnetic field, is observed in some (mostly Mn-based) perovskite oxides.
Rare-earth Magnetism
Rare-earth elements are ferromagnetic but with Curie temperatures below room temperature. However they can form compounds with transition metals, and many of these compounds have Curie temperatures above room temperature. In addition the crystalline structures often have high magnetic anisotropy, which combined with the large atomic moments yields very strong permanent magnets. At present Nd2Fe14B is the strongest known permanent magnet. The current dwindling supply of rare-earth metals has intensified the search for alternate compounds and physical forms with smaller amounts of rare-earth metals, and neutrons have long played an essential role in guiding this search.
Actinide Magnetism
The unpaired electrons in the actinide series are the 5f electrons which often form into bands rather than exist as localised electrons. In addition there is a large inherent orbital moment which leads to strong spin-orbit interaction, comparable in magnitude to the bandwidth. Both properties lead to an extreme diversity of magnetic features in the actinide elements, and actinide-element-based alloys and compounds, ranging from itinerant magnetic compounds, through the exotic heavy-fermion materials, to fully localised compounds. Neutron scattering is the tool best suited to determine the degree of electronic hybridisation which is a key parameter in the characterisation of actinides.
While radioactivity and the associated health and security issues may limit the technological use of actinides and their compounds, the diversity of magnetic properties makes the series an excellent playground in which to test theories.
Magnetism in thin films and multilayers
|
|
|
Chemically ordered/disordered bilayers of FePt3 show antiferromagnetic/ferromagnetic order |
Thin film structures, grown for example by sputtering deposition, chemical-vapour deposition or molecular-beam epitaxy (MBE), underpin the present-day electronics industry. It is also possible to produce magnetic multilayers that can be used in devices that exploit the spin rather than the charge of the electron. This new field of magneto-electronics or spintronics is revolutionising the electronics industry.
Already, spin-valve devices, based on multilayers that show giant magnetoresistance, are employed as read heads, and have led to dramatic improvements in data-storage densities. Semiconductor-based spintronic devices can combine storage, detection, logic and communication capabilities on a single chip to produce a multifunctional device that could replace several components in present devices. Diluted magnetic semiconductors (e.g. Ga1-xMnxAs) can be associated reasonably easily with non-magnetic semiconductors for electrical injection or detection of spin-polarised electrons.
At the level of basic science, thin-film growth may produce structures that are stable at lower temperatures than they are in the bulk, by use of an appropriate structural template as the substrate. Our understanding of the magnetism of matter is such that it is unlikely that new bulk alloys or compounds with significantly improved intrinsic properties may be discovered. Tailoring the magnetic properties at the nanoscale however offers much more perspective because the scale is comparable to the ferromagnetic interaction length. By alternating layers of metals with different magnetic properties, we can explore fundamental questions such as how magnetic order propagates across thin layers.
The magnetic coupling mechanism between the layers is often intricate, and can differ dramatically for small differences in the layer thicknesses. Artificial composite multiferroics can be constructed by multilayers of magnetic and ferroelectric materials, to produce, for example, tiny magnetic sensors. Neutron diffraction and specular neutron reflectivity are very well suited to investigate the coupling mechanism. Such studies are possible on epitaxial structures on the triple-axis spectrometer TAIPAN and on both epitaxial and polycrystalline thin-film structures on the reflectometer PLATYPUS.
For basic research, the structures are usually produced as repeated bilayers to produce a structure about 500-1000 Å thick. Structural characterisation of the repeated bilayer is conveniently made by three-axis X-ray diffraction. It is important to characterise the roughness of the interfaces, particularly since the preparation of multilayers has advanced to the point that multilayered structures with layers just two or three atomic planes thick may be produced.
Molecular magnetism
Like epitaxy of metals, molecular engineering of organic compounds offers far more possibilities than traditional inorganic and metallic compounds to design magnets, and other smart materials, such as those with non-linear optical behaviour, with the desired physical properties for particular applications (density, maleability, transparency, solubility, photo-responsiveness, electrical conductivity, bio-compatibility, environment-friendliness, low-temperature fabrication, etc.). Magnetic molecules are also monodispersive, and can be diluted and organised using supramolecular techniques.
A major goal in molecular magnetism is to find molecular ferromagnets, but only a handful of purely organic ferromagnets have been found thus far, the first in 1991. As a consequence, investigation of molecular antiferromagnets is also important, in part to understand the laws governing the type of molecular magnetic ordering, including the role played by structural packing.
Single-crystal diffraction is usually preferred over powder diffraction for investigation of the crystallographic and magnetic structures of molecular materials because of the weakness of the magnetic signal, or the presence of hydrogen, which gives a large incoherent background.
Multiferroics
|
|
|
Antiferromagnetic ordering in LuFe2O4 stabilises ferroelectric charge ordering |
Multiferroics are materials which exhibit more than one ferroic order parameter (most commonly two of ferromagnetism, ferroelectricity, ferrelasticity), usually, but not necessarily, coupled, in a single phase. The coupling of the order parameters lends itself to novel device applications, where the form of the magnetic phase may be controlled by an applied electric field, or the electric polarisation controlled by an applied magnetic field.
In type-I multiferroics, ferroelectricity and magnetism have different sources, appear largely independently of one another, though there is some coupling between them, and ferroelectricity typically appears at higher temperatures than magnetism. In the more recently discovered type-II multiferroics, magnetism causes ferroelectricity, which implies a strong coupling between the two.
Superconductivity
Magnetism and superconductivity are generally thought to be incompatible with each other. According to the Bardeen-Cooper-Schreiffer theory of superconductivity, electrons with opposite spins form pairs that can move through a material without resistance. A magnetic field can destroy superconductivity in two ways: by breaking up the electron pair, or by trying to make both of the electron spins point in the same direction.
These effects also limit how much current can flow through the superconductor because of the disruptive effect of the magnetic field produced by the current itself. Despite the known exceptions to this rule, a thorough characterisation of superconductivity is closely linked to characterisation of the magnetic behaviour. Studies of flux-line lattices in superconductors is a particularly active field in neutron scattering.
Facilities
|
|
Magnetic structures can be determined from epitaxial sampl on the Taipan |
Neutron-scattering instruments
Investigations of various aspects of magnetism can be conducted on all neutron-scattering instruments at OPAL. 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
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 come into operation 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.
Project members
Project manager
Most researchers at the Bragg Institute have some interest in magnetism. The following are those who devote a significant part of their research time to magnetism:
Scientists
-
Dr Jason Gardner (funded by NSRRC, Taiwan)
-
Prof Michael James (presently seconded to the Australian Synchrotron)
-
Dr Clemens Ulrich (jointly with the University of New South Wales)
Post-doctoral researchers
-
Dr Sara Callori (jointly with the University of New South Wales)
-
Dr Narendirekumar Narayanan (jointly with the University of New South Wales)
-
Dr Jianli Wang (jointly with the University of Wollongong)
Thesis students
-
Joel Bertinshaw (affiliated with the University of New South Wales)
-
Grace Causer (affiliated with the University of Wollongong)
-
David Cortie (affiliated with the University of Wollongong)
Past members
-
Dr Maciej Bartkowiak (affiliated with the University of New South Wales)
-
Dr Sebastian Brück (jointly with the University of New South Wales)
-
Dr Wojciech Miiller (jointly with the University of Sydney)
-
Dr Pauline Rovillain (jointly with the University of New South Wales)
-
Dr Thomas Saerbeck (affiliated with the University of Western Australia)
Principal collaborators
These are the organisations or researchers with whom we have formal links (joint appointments, shared grants, contractual access to complementary equipment, ...) or individual researchers who have published at least five magnetism-related papers with us in the past two years.
-
CSIRO Materials Science and Engineering, Lindfield: Dr Stephen Collocott
-
Institute of Materials & Engineering Sciences, ANSTO: Dr Gordon Thorogood
-
Institute of Superconducting and Electronic Materials (ISEM), University of Wollongong: Prof Shi Xue Dou, Prof Xaiolin Wang, Dr Rong Zeng
-
Research School of Chemistry, Australian National University: Dr Darren Goossens
-
School of Chemistry, University of New South Wales, Kensington: A/Prof John Stride
-
School of Chemistry, University of Sydney: Prof Brendan Kennedy, A/Prof Chris Ling
-
School of Physical, Environmental and Mathematical Sciences, University of New South Wales, Canberra: Prof Stewart Campbell, Prof Seán Cadogan, Dr Wayne Hutchison, Dr Annemieke Mulders
-
School of Materials Science and Engineering, University of New South Wales, Kensington: Prof Sean Li