The AANSS is a great mix of formality and informality, quality science in a relaxed atmosphere. Anyone who has or might or ought to use neutron scattering in their work (and isn’t that all of us, really?) is invited. And here’s a trick: Registration is $50 cheaper for ANBUG members but ANBUG membership is free! So join up!
More years ago than I like to recall, I helped out with a study of the magnetic structure of TbNiAl4. The first paper we did on that goes back to about 2006. I did a refinement of the neutron diffraction data, and I got some things right, but what was not apparent — partly due to the limitations of powder diffraction and partly due to the limitations of the user (me) — was the subtlety and complexity of the real magnetic structure.
Fortunately, PhD Scholar Reyner White has now done it properly, with better data. He found that the propagation vector I determined was pretty good, but has been able to show that the magnetic structure shows an “`elliptical helix’ type structure in which the moments rotate in the ab-plane as one moves along the c-axis”, which is far more concrete — I showed that it was incommensurate, and I found the propagation vector, but exactly what it was that was incommensurate I was wily enough not to say.
It sure is nice to work with good people who take the time to do things well.
An article referred to before has been officially published. It is called ‘Chemical and magnetic ordering in Fe0.5Ni0.5PS3‘ and is available (subscription needed, sorry) at http://link.springer.com/article/10.1007%2Fs10751-014-1108-6.
D. J. Goossens, G. A. Stewart, W. T. Lee, A. J. Studer, ‘Chemical and magnetic ordering in Fe0.5Ni0.5PS3‘, Hyperfine Interactions, April 2015, Volume 231, Issue 1-3, pp 37-44.
What’s it about? Well…
Many thanks to Glen and Hal Lee.
Before you can read or write stories you must learn spelling and grammar; before you can play a sonata on the piano you must learn scales, harmony, and musical notation; and before you can go into a laboratory and make an intelligent stab at discovering something new, there is a lot of dull, hard work to be done. No one escapes that.
Francis Bitter, Magnets: The Education of a Physicist
Maybe eighteen months or even two years ago I notice that the conference on Hyperfine Interactions and Nuclear Quadrupole Interactions (HFINQI) was coming to Canberra. I always take a close look at conferences that come to Canberra, because I work there. At the time I was working on a project studying the properties of some magnetically unusual materials, the MPS3 family of compounds. In particular, we were looking at ones that contained 50:50 ratios of two different magnetic transition metals on the M site, and we had found some interesting behaviour in Fe0.5Ni0.5PS3. This compound seems to show time-dependent magnetic properties suggesting glassiness and disorder.
Now, iron-containing materials can be studied using Mössbauer spectroscopy, which is sensitive to the crystal and magnetic environments of the Fe atoms, in this case Fe2+ ions.
My students and I had studied the materials using magnetometry and neutron diffraction, but if I wanted to put the work in to HFINQI I needed a hyperfine technique, which Mössbauer is. I contacted Glen Stewart at PEMS at UNSW Canberra, who is an expert in Mössbauer and we talked over an experiment. In the end, Glen collected some lovely data and was able to fit it with a really nice model, and here’s a picture:
We find that the time-dependence is not apparent in the Mössbauer, which is not surprising as it is a slow technique. We also find that the Fe environments are not random, which is perhaps the most significant result, as it will relate to the interactions between magnetic species and so to the magnetic properties.
Now, while putting a paper into HFINQI prompted the Mössbauer work, which is a backwards way of choosing what science to do, it was by no means a silly experiment to do, as the site-symmetry information available from the technique complements the crystal symmetry information from neutron diffraction, and has indeed enhanced our understanding of the compound.
The paper is available (paywalled, I’m sorry to say) at http://dx.doi.org/10.1007/s10751-014-1108-6.
Chemical and magnetic ordering in Fe0.5Ni0.5PS3 by D.J.Goossens · G.A.Stewart · W.T.Lee · A.J.Studer, Hyperfine Interactions (full reference not yet available as of Dec 27, 2014).
Last year Fred Marlton did his Honours in Chemistry at ANU, and this year the little paper we wrote came out. It was more done to give Fred a sense of the publishing process, and communicating science in that way, but it does have a few handy results in it as well, ones that could save time and effort for people synthesising compounds from the family LnxY1−xMnO3. The paper is published in Zeitschrift für Naturforschung B, which is a venerable journal with, based on this experience, an excellent editorial process. The abstract can be viewed here.
In essence the title says it all; we used microwave sintering to make the samples, which saved an order of magnitude in time and electricity, and allowed Fred to quickly map out the whole phase diagram describing where these compounds form.
What we found was that solid-state microwave synthesis allows manufacture of high-quality samples in hours rather than days. The resulting phase diagram accords well with results from the literature, and from calculations based on the Goldschmidt tolerance factor for the stability of perovskite structures, suggesting that the transformation from hexagonal to perovskite with doping is driven essentially by ion sizes. Some results concerning the microwave synthesis of BaLnInO4 compounds, where Ln is a lanthanide, are noted. Microwave sintering of BaNdInO4 yields single-phase samples where conventional sintering does not.
Magneticians sometimes want to plot curves of spontaneous magnetization as a function of temperature for magnetically ordered materials, usually to compare to experimental results from magnetic neutron scattering or magnetometry. These curves are based on the Brillouin function. Their shape depends on the total angular momentum, J or sometimes S. They look something like this:
All this program, found here at some silly website does is take the J from the user and write out the numbers for the plot. The actually machinery is taken from M I Darby 1967 Br. J. Appl. Phys. 18 1415 doi:10.1088/0508-3443/18/10/307. And it is this paper that should be referenced, or in fact used directly.
And, yes, I did draw the graph in Excel. Ugly but good enough…
A very interesting field within solid state science these days is magnetocaloric materials (I could put a link here but you might as well just do a quick search). To some extent a lot of magnetic materials are magnetocaloric. Essentially, since application of a field increases the degree of order of the material (since it makes the magnetic atoms become more aligned), it reduces the entropy and therefore (for an isolated system) causes the temperature to go up, since energy can’t get in or out.
Say the material starts off isolated but at room temperature, then heats up on application of the field, then we let it get into contact with the air. It will lose heat to the air and reach air temperature. Then, if we turn off the field adiabatically (i.e., after again isolating the system) some of the ‘temperature’ energy will go into disordering the magnetic moments of the atoms. Since again energy is conserved, this means the temperature goes down. This cooled magnetic material can then be used to cool a load — say, a refrigerator full of beer.
Advantages of this over conventional gas refrigeration include the lack of moving parts — essentially, it can be done by turning off and on magnetic fields — and the lack of nasty gases that might escape. Potentially there are robustness benefits too. The only problem is as yet we don’t have materials whose performance is good enough at everyday temperatures. Magnetic cooling has been used — and for a long time — by scientists trying to do low temperature physics.
TbNiAl4 is interesting as it is an inverse magnetocaloric material. It cools down when we apply a field to it. This implies that the application of a field increases the entropy. So we wanted to know what was going on. This we found out, and it was published a little while ago in Physical Review B. (And also here.) The first key piece of information is in the magnetisation as a function of applied magnetic field. That looks like this:
So there are a couple of interesting things here. First, as the field is increased (solid line) we get a sudden jump at about 6T (Tesla); the material is switching from antiferromagnetic to ferromagnetic. But clearly not all the magnetic moment on the Tb is involved, because a Tb3+ ion has over 9μB (Bohr magnetons,the unit of atomic magnetism) and at 6T we are getting about 3μB. Once we get to about 9T the moment increases again. So some of the moment gets aligned at 6T, some needs more field.
Then, when we go down in field we have to go all the way down to about 3T before the moments ‘unalign’ — we get a different result going forwards compared to backwards. This is hysteresis and means a first order (sudden) phase transition. So what’s going on?
Well, the best probe of magnetic ordering is neutron diffraction, so we went to Wombat at ANSTO and did some experiments. We had a single crystal of TbNiAl4 so we used Wombat to map reciprocal space (the diffraction pattern — where the neutrons go after they scatter off the material) at a range of fields and temperatures. We get a three-dimensional map of the scattering, something like this:
And from this what we found was that at low field the material shows a commensurate antiferromagnetic order, that is,the pattern of atomic magnetic moments repeats over a distance that can be measured in terms of a simple number of repeats of the crystal unit cell, and the moments are arranged such that they cancel each other out and the overall moment (magnetisation) of the sample is small. But as the field passes 6T, it switches into a state where: (1) A component of each moment aligns with the field, giving the ferromagnetic response (2) Some of the moment retains the antiferromagnetic order but adds a small incommensurate component. This means that the repeat is not a simple number of unit cells, and also means the degrees of freedom of the system have changed. It takes more parameters to describe the ordering at the higher field. So as we increase field, we get a rapid increase in overall moment plus enter a more complicated state. There is an entropy change associated with this, and it is this that gives the inverse magnetocaloric effect. There is no ‘disorder’ as such.
This work was driven by Dr Wayne Hutchison at PEMS at UNSW, and I am very grateful to have been part of this collaboration.