Magnetic structure and glassiness in…
So, the latest paper to come out is ‘Magnetic structure and glassiness in Fe0.5Ni0.5PS3‘, in Journal of Magnetism and Magnetic Materials 334 (2013) 82–86 (http://dx.doi.org/10.1016/j.jmmm.2013.01.023).
The science is I think interesting. We find that by mixing Ni and Fe on the metal site we induce a time-dependence in the magnetic behaviour. That is to say that the system likes to order antiferromagnetically, with the magnetic moments on the metal sites cancelling each other out, but that this ordering is quite slow. It takes hours for the magnetic moments to reach their final arrangement.
This shows up in the magnetic susceptibility in the way that the susceptibility — how strongly the material reacts to an applied magnetic field, basically — depends on whether you are warming the sample up or cooling it down. The picture below shows that — as we heat up the cusp in the (admittedly noisy) data comes in at about 140K, but on cooling it comes in at about 90K. So as we cool down from the paramagnetic high temperature state, the system remains paramagnetic till 90K, where if we heat up from the low temperature magnetically ordered state, the system stays ordered up to 140K. If we are cooling and we get to, say, 100K then stop cooling and just measure as a function of time, we get a nice stretched exponential decay of the magnetisation until it reaches the value it would have on the lower curve. that is, it follows the red line, and take about three hours to get there. That means that the lower curve — what we measure on heating — is measuring the true equilibrium state.
One of the other nice things about this paper is that two of the authors — S. Brazier-Hollins and D.R. James — were project students rather than full academics or Ph.D. scholars. Their work on the project was enthusiastic and of high quality, and it was really nice to pull a range of disparate contributions together and see it in print.